CRC Book Chapter 5 Draft Appendix L Surface-enhanced Raman detection of chemical agents in water Steven Christesen, Kevin Spencer, Stuart Farquharson, Frank Inscore, Kristina Gonser, and Jason Guicheteau I. INTRODUCTION In 1994 and 1995, sarin gas was released in the Japanese cities of Matsumoto and Tokyo,1 respectively bymembers of the AUM Shinrikyo religious cult. Later in 1995, the same group attempted to poison commuters inTokyo with cyanide.1,2 Fortunately, the cyanide producing devices were discovered before they could be used.Although unsophisticated by military standards, the two sarin attacks resulted in 19 deaths and almost 6000 peopleinjured. More than 30 deliberate chemical releases were reported in Japan in 1998 alone causing more deaths andinjuries. The agent used in the Tokyo and Matsumoto attacks (sarin or GB) is one of a class of toxic organophosphorusnerve agents that include soman (GD), tabun (GA), cyclo-sarin GF and VX (no common name). These nerve agentsare particularly toxic and have LD50’s ranging from 24 milligrams per kilogram of body weight for sarin to 0.07mg/kg for VX, where the LD50 is defined as the lethal dose of a liquid agent causing death in 50% of a givenpopulation via percutaneous liquid exposure on bare skin.3 Organophosphorus nerve agents are toxic because theybind to acetylcholinesterase enzymes (AChE) thereby inactivating them and allowing the neurotransmitteracetylcholine to accumulate at synapses. Symptoms of nerve agent poisoning include twitching, pinpointed pupils,convulsions, coma, and eventually death if the level of exposure is high enough. The threat of terrorism within the United States became a reality in 2001 with the attacks on the Pentagonbuilding and the World Trade Center towers. The mailing of anthrax, shortly thereafter, and the salmonellapoisonings of salad bars in Oregon by the followers of Bhagwan Shree Rajneesh are just two examples of biologicalterrorism in the US. The variety of terrorist attacks and the widespread use of GA, GB, and GF in the Iraq/Iran warsuggest that chemical warfare agents must be considered as a potential weapon against the US. Although themajority of these incidents have involved vapor or aerosol dispersal of the agent, the threat to water supplies isobvious and techniques for quickly and accurately identifying the threat are needed. The Army’s water quality standards for allowable chemical agent contamination levels in drinking water arepublished in the Department of the Army’s Technical Bulletin TB MED 577 Sanitary Control and Surveillance ofField Water Supplies.4 This document is in the process of being updated to include new standards for chemical agentcontamination. The new standards are expected to conform generally to the recommendations of the NationalAcademy of Sciences (NAS) Subcommittee on Guidelines for Military Field Drinking-Water Quality,5 which areshown in Table 1. In the case of nerve agents, these limits are based on a modeled 25% inhibition of AChE, andrepresent a “no observed adverse effect level” (NOAEL). Another class of chemical agents is the vesicants that include sulfur mustard (HD), three variations of nitrogenmustard (HN-1, HN-2, and HN-3), and lewisite (L). The ingestion of low concentrations of HD in water is expectedto result in gastrointestinal irritation. Based on toxicity studies on rats, the military guideline limits were set at 47µg/L and 140 µg/L for water consumption rates of 15 and 5 liters/day, respectively. Although used in World War I,and by the Aum Shinrikyo terroists, hydrogen cyanide is not considered a militarily significant agent due to its highvolatility and rapid detoxification by humans. Environmental cyanide contamination of water, however, can occuras the result of industrial processes such as electroplating and metal polishing. Cyanide is not nearly as toxic as thenerve agents or mustard, but its ready availability lends itself to possible use by terrorists to poison water supplies. The military’s M272 water testing kit, first fielded in 1984, is currently used to detect and identify chemicalagents in treated and source water. Agents are detected via color changing reactions with sensitivities of 0.02 mg/Lfor nerve agents, 2 mg/L for mustard and lewisite, and 20 mg/L for cyanide.6 In general, the M272 is very sensitiveand meets the current requirements listed in TB MED 577, but it is not sufficiently sensitive to ensure that watermeets the recommended water quality standards shown in Table 1. In addition, the vials and chemicals used in thekit are not easily manipulated when wearing a protective suit, mask, and gloves. In an effort to meet military and national security needs for detecting chemical agents in water,sophisticated laboratory methods have been investigated with reasonable success. More than a decade ago, Black etal. demonstrated the ability of combining gas chromatography with mass spectrometry detection (GC/MS) tomeasure sarin and mustard.7 Sega at al. used GC with a phosphorous-selective flame ionization detector to analyzenerve agent hydrolysis products in groundwater,8 while several researchers used capillary electrophoresis (CE) tomeasure chemical warfare agents and their hydrolysis products.9, 10, 11 The sensitivity of these techniques hasimproved by two orders of magnitude from 1 mg/L to 0.01 mg/L in 10 years. A comprehensive development of 1
CRC Book Chapter 5 Draftthese techniques was undertaken by Creasy et al. in analyzing chemical weapon decontamination waste from theJohnston Atoll.12,13 These researchers used GC/MS for nerve agents, GC coupled atomic emission detection forarsenic compounds, LC/MS for mustard compounds, and CE with ultraviolet absorption detection for alkylphosphonic acids. Detection limits of 0.02 and 0.140 mg/L were reported for nerve agents and mustard,respectively. Detection of the alkyl phosphonic acids have proven more difficult, and Liu, Hu and Xie recently usedGC/MS to detect mg/L concentrations of these degradation products.14 However, they concede that all of theseseparation methods require extraction, derivatization, and repeated column calibration, making them labor intensive,time consuming (typically 30 to 60 minutes), and less than desirable for field use. Another variant of theseseparation/mass detection technologies is ion mobility spectrometry (IMS).15 This technology has been successfullydeveloped to measure explosives in air samples, and commercial products can be found at most airports.16 Eicemanet al. have investigated the ability of IMS to measure organophosphorous compounds in air,17 while Steiner et al.have investigated IMS to measure chemical agent simulants in water.18 In the latter case, electrospray ionizationwas coupled to the sample entry point of an IMS, and a time-of-flight MS was added as an orthogonal detector.Water samples spiked with 10 mg/L diisopropylmethylphosponate and thiodiglycol could be measured in 1-min,once sample pretreatment was accomplished. It is worth noting that with proper care these MS-based technologiesare likely to detect chemical agents with virtually no false-positives, but detection limits are still insufficient by 1 to2 orders of magnitude in the case of nerve agents and their hydrolysis products. Raman spectroscopy, using both NIR19 and UV20 laser excitation, has found military application for thedetection and identification of chemical agents. The highly selective nature of the Raman spectrum makes thistechnology ideal for non-intrusive and/or short range remote detection of highly concentrated samples. But the tracelevel detection sensitivity required for chemical agents in water is beyond the capabilities of normal Ramanspectroscopy. Improved sensitivity, however, can be achieved with surface-enhanced Raman spectroscopy (SERS).Work by Vo-Dinh on the SERS detection of organophosphorus insecticides21 and organophosphonate vapors22 usingsilver coated spheres and oxidized silver foils, respectively, demonstrated the potential efficacy of SERS for nerveagent detection. More recently, two different SERS substrates, one produced by electrochemical roughening ofsilver or gold foils23 and one produced by gold- or silver-doped sol-gels24,25 were used to measure the primaryhydrolysis of nerve agents, methyl phosphonic acid, with detection limits of 50 to 100 µg/L, respectively. In orderto develop a SERS sensor for detecting chemical agents in water, however, sensitivity and reproducibility need to bedemonstrated for actual chemical agents in all classes. To this end, results on the detection of VX, HD, CN, andtheir hydrolysis products using SERS these substrates are presented. II. EXPERIMENTALA. SERS Substrates 1. Electrochemically Roughened Silver Foils The electrochemically roughened silver substrate foils (EIC Laboratories, Inc., Norwood, MA) were prepared asdescribed in Reference 26 from silver foil coupons (Surepure) cut to a size of ¼” x ½” on a dedicated jeweler’s saw.The cut edges and faces were smoothed to mirror flatness using 0.3 µm Al2O3 polish then washed and stored in 0.1M KOH solution prior to electrochemical roughening in 0.1 M KCl using a platinum gauze counter electrode.Roughening was accomplished by cycling 20 times at a sweep rate of 10 mV/s with upper and lower limits of 0.25and -0.6V versus silver/silver chloride, respectively. The silver substrate foils were electrochemically cleaned atcathodic potential to remove chemical impurities and then soaked overnight in water to remove excess chloride ion.These foils were then electrochemically cycled in sodium hydroxide to create a thick hydroxide layer, whichstabilizes the silver chemistry and reduces its propensity for oxidation. The gold and silver foils were shipped to USArmy Chemical Biological Center (ECBC) for agent testing and generally used within a week of manufacture. Allmeasurements were made by either dipping the foil directly into the solution or by spotting the foil with less than 10µL of the analyte in water. In both cases, the sample was allowed to dry on the substrate foil before recording theSER spectrum. 2. Silver- or Gold-Doped Sol-Gels The sol-gel coated vials were prepared by Real-Time Analyzers, Inc. (RTA, Middletown, CT) using procedurespreviously published by Farquharson et al.27 Silver-doped sol-gels were formed by adding ammonium hydroxide toa solution of silver nitrate, tetramethyl orthosilicate (TMOS), and methanol. Gold-doped sol-gels were coated by 2
CRC Book Chapter 5 Draftadding nitric acid to a solution of gold tetrachloride, TMOS and methanol. The two precursor solutions wereprepared, mixed, and transferred to 2-ml glass vials, dried and heated. After sol-gel formation, the incorporatedmetal ions were reduced with dilute sodium borohydride (1 mg/ml), followed by a water wash to remove residualreducing agent. The sol-gel vials were produced at RTA and shipped to ECBC for testing. In all cases, the vialswere filled with the desired agent in solution, capped, and then the SER spectrum was recorded.B. Sample Preparation Stock solutions of HD, VX, and EA2192 were prepared at the ECBC by using Chemical Agent AnalyticalReference Material (CASARM)-Grade neat agent (95+ % purity). Cyanide solutions were made by dissolving KCN(Aldrich) in DI water. For the measurements using the roughened metal foils, the samples were prepared bydissolving the neat agent in distilled, deionized (DI) water at approximately 1 mg/ml and making serial,volumetrically dilutions to achieve the lower concentrations. For the measurements using silver- and gold-dopedsol-gel coated vials the neat agents were dissolved in 2-propanol prior to the addition of DI water. In this case,samples of several concentrations were prepared just prior to the start of each test. The percentage of 2-propanolwas kept constant throughout the volumetric dilution series. Typically, the HD and VX samples contained 1.1 %and 2.0 % 2-propanol, respectively.C. Instrumentation Tests were preformed using two different Raman instruments. For the measurements with roughened silverfoils, a dispersive Raman spectrometer, comprised of a 785 nm diode laser (300 mW), an echelle spectrograph, anda TE cooled CCD camera operating at approximately 45°C below ambient (model RS2000, InPhotonics, Norwood,MA), was used to acquire 1 cm-1 resolution spectra.19 The laser was fiber optically coupled to a sample chamberbox using a RamanProbeTM (InPhotonics). The probe both conveyed the laser light to the sample and served as anoptical filtering device to remove background signals arising in the fiber optic cable. For measurements using sol-gel coated vials, a Fourier transform Raman spectrometer (model IRA-785, Real-Time Analyzers) equipped with a silicon photo-avalanche detector (Perkin Elmer model C30902S, Stamford, CT)was used to acquire spectra at a resolution of 8 cm-1. A 785 nm diode laser (model 785-600, Process Instruments,Salt Lake City, UT) delivered approximately100 mW of power to the sample through a 30 foot fiber optic cable,such that the instrument could be located outside the lab for added safety. The sample system consisted of an XYpositioning stage (Conix Research, Springfield, OR) on which the vials were mounted horizontally just inside thefocal point of an f/0.7 aspheric lens. This lens and the other optics within the fiber optic probe have been previouslydescribed.27 III. RESULTS AND DISCUSSIONA. HD and TDGSulfur mustard (bis(2-chloroethyl) sulfide) hydrolyzes to form mustard chlorohydrin (2-chloroethyl 2-hydroxyethylsulfide), which then hydrolyses to thiodiglycol (TDG). These reactions progress through cyclic sulfonium ionintermediates as shown in Figure 1. Although HD rapidly hydrolyzes to TDG (Table 2), the rate is limited by itslow solubility and solvation rate.28 HD hydrolysis approaches an SN1 mechanism via the pathway shown in Figure 1only when predissolved in organic solvent and at low concentrations (less than 0.001 M or 160 mg/L).29 At theinterface of the HD droplet with water, the sulfonium ions can also interact with TDG to form stable sulfonium salts,such as HD-TDG, CH-TDG, and H-2TDG (Figure 2). The presence of multiple stable HD conformers and theformation of the sulfonium salts complicate the analysis of the SER spectra of sulfur mustard in treated water. The SER spectra of HD in water are dominated by a broad peak with a maximum intensity at 620 cm-1, for bothelectrochemically roughened silver foils and silver-doped sol-gel coated vials, and at 610 cm-1 for gold-doped sol-gel vials. This broad peak aligns with the series of peaks observed between 600 and 800 cm-1 in the normal Ramanspectrum of HD and can be ascribed to the C-S and C-Cl stretching modes (Figure 3).30,31,32 At least two additionalpeaks appear in the spectra at 1003 and 1292 cm-1 for the silver foils, and at 1007 and 1290 cm-1 for the gold-dopedsol-gels. Based on the Raman peaks at 1038 and 1292 cm-1, these peaks are assigned to a C-C stretching mode and aCH2 bend.33 The shift in the C-C stretch is also observed in the C-S or C-Cl stretch. These shifts are consistent with thosereported in the literature by Joo et al.34 for diethyl sulfide (DES) on silver. They report an approximate 20 cm-1 shiftof the C-S stretching modes of the different conformers, and suggest that this is due to DES binding to the surface 3
CRC Book Chapter 5 Draftvia the sulfur atom. The general shift here suggests that it is likely that HD also binds via the sulfur atom. Thesilver foil spectra also contain a peak at 722 cm-1, which is absent or at least weak in both sol-gel spectra. There areat least four possible explanations as to the source of this peak. 1) In the case of the silver foils, the samples weremeasured dried, while the sol-gels were measured in solution, and the 722 cm-1 may represent a different conformeron the surface. 2) The HD was dissolved in water for the foils, and in a combination water and organic solvent forthe sol-gel vials. The latter co-solvent should dramatically increase solvation, expedite hydrolysis and the removalof the terminal chlorines. If so, then the 722 cm-1 peak may be assigned to a C-Cl stretching mode. This issupported by the fact that thiodiglycol does not have this peak, at least under some measurement conditions. 3) The722 cm-1 peak could be due to the formation of one of the sulfonium salts (Figure 2), but these salts are more likelyto contribute new peaks to the sol-gel spectra, not the foil spectra. 4) It was also noted that this peak was moreintense than the 620 cm-1 peak in some foil measurements, and it may alternatively be assigned to a photo-degradation product. This is supported by the generation of a peak at the same frequency in photo-degraded TDG.35Furthermore, at high laser powers and long exposures (several minutes) additional peaks (e.g. 668 cm-1) alsoappeared in HD SER spectra using the gold-doped vials, supporting the assignment of the 722 cm-1 peak to photo-degradation. HD was consistently measured at approximately 50 mg/L using the silver foils (e.g. Figure 3C), andoccasionally at 100 mg/L for the silver-doped sol-gels. Both substrates provided repeatable measurements of 1 g/Lfor HD (e.g. Figure 3B). However, the reproducibility of the SER signal intensity for HD was highly variable at 777mg/L using the silver foils, but considerably better at 1000 mg/L using the silver-doped sol-gels.36 It is not clear,however, how much of the variability in the former is the result of non-reproducibility in the substrate or the dryingprocess, or how much might be explained by the complex and changing mixture of mustard hydrolysis products. The SER spectrum of thiodiglycol was only measured using silver-doped sol-gels. Initial measurements at 1000mg/mL yielded a high quality spectrum that was consistent with SERS of HD and the Raman spectrum of TDG(Figure 4). The SER spectrum is dominated by three peaks at 627, 715, and 1008 cm-1, while the Raman spectrum isdominated by peaks at 652 and 1005 cm-1, which can be assigned to S-C and C-C stretching modes, respectively.Although the 715 cm-1 peak could correspond to either of two weak Raman peaks at 680 and 728 cm-1, it was absentin recent SER measurements of TDG in a flowing stream.35 Here, measurements at low laser powers (15 mW,Figure 4B) confirm that this peak, as well as the 1008 cm-1 peak, are likely due to photo-degradation. Thisconclusion also supports the assignment of the 722 cm-1 peak in the HD SER spectra to a TDG photo-degradationproduct.B. VX, EA2192, and EMPA VX (ethyl S-2-diisopropylamino ethyl methylphosphonothioate) hydrolyzes in distilled water via the pathwaysshown in Figure 537,38,39 with a half-life of greater than 3 days.40 The hydrolysis product EA2192 is itself highlytoxic and much more stable in water, although it will eventually hydrolyze to 2-(diisopropylamino) ethanethiol(DIASH). The ethyl methylphosphonic acid (EMPA) product of Reaction Pathway 1 can also undergo furtherhydrolysis to methylphosphonic acid (MPA). Both the silver foils and sol-gels produced SER spectra that had manyspectra features in common with each other and the normal Raman spectrum (Figure 6). In particular, at least threeoverlapping peaks are seen in all three spectra between 425 and 575 cm-1. Farquharson et al. have assigned thepeaks at 460, 485, and ~530 cm-1 to a POn bending mode, an NC3 stretching mode, and a POnS bending mode,respectively (Table 3).41 Neither substrate provided good sensitivity for VX. The sol-gels provided goodreproducibility at 1000 mg/L (Figure 6B), but only occasionally was a spectrum observed at 100 mg/L, whereas thefoils produced spectra at 100 mg/L on most attempts (Figure 6A). In the case of the latter, a large backgroundcontribution above 800 cm-1, was removed by baseline correction. EA2192 (ethyl S-2-diisopropylamino methylphosphonothioate) the primary hydrolysis product of VX followingReaction Pathway 2 (Figure 5) produced very similar SER spectra using silver foils or silver-doped sol-gels (Figure7). In fact, a high quality spectrum is obtained using the foils at 113 mg/L with peaks at 481, 584, 622, 700, 743,780, 810, 830, 942, 974, 1040, 1120, 1366, 1442, and 1461 cm-1. Many of these peaks have been assignedpreviously (Table 3).41 The SER spectra are however, considerably different than the Raman spectrum of EA2192(Figure 7C). Most notably, the 1055 and 1185 cm-1 peaks, assigned to a PO2S stretch and NC stretch, in the Ramanspectrum are absent in the SER spectra. It is surprising that the PO2S stretch is not SER-active, and its assignmentmay be in doubt. It is also worth noting that the most intense peak in the SER spectra at 942 cm-1, assigned to anNC3 stretch, also dominated the SER spectrum of DIASH.41 4
CRC Book Chapter 5 Draft The silver foils provided reasonable sensitivity for EA2192 and spectra were obtained at a number ofconcentrations to as low as 33 mg/L. The silver-doped sol-gels successfully measured 1000 mg/L, but no lowerconcentrations were tried. The SER spectra for EMPA are more interesting (Figure 8), in that both the sol-gel and foil spectra appear tocontain photo-degradation products. In the case of silver-doped sol-gels, a spectrum can be obtained that has manyof the same features as found in the normal Raman spectrum, as long as low laser powers are used (30 mW, Figure8B). Peaks occur at 505, 730, 792, 893, 1047, 1098, 1293, 1420, and 1454 cm-1 in both spectra, which all have beenassigned.41 The primary difference between the two spectra is that a second intense peak occurs at 745 cm-1, albeitthe normal Raman spectrum contains a shoulder at close to this frequency. As soon as the laser power is increasedto 100 mW, this peak increases in intensity substantially, indicating photo-degradation (Figure 8A). This effect iseven more dramatic for the silver foils, where the this peak completely replaces the 730 cm-1 peak, especially at lowconcentrations (100 µg/L, Figure 8D). Additional peaks also grow in at 610, 915, and 955 cm-1. It is reasonable toassign the 745 cm-1 peak to the formation of methyl phosphonic acid, since it has a dominant peak at 755 cm-1, butMPA does not contain the latter three peaks, and it is unclear as to composition of the degradation product. It isclear from the data that EMPA degrades rapidly in the presence of silver and laser irradiation.C. Cyanide Hydrogen cyanide (AC) is a highly volatile liquid (boiling point of 25.7 °C) belonging to a class of chemicalagents known as blood agents. It has commercial uses in the extraction of gold from ore and in the manufacture ofother chemicals, such as acrylonitrile and methyl methacrylate. AC is highly soluble in water and hydrolyzes slowlyto ammonia and formic acid. Hydrogen cyanide and its potassium and sodium salts release toxic free cyanide (CN)when dissolved in weak acids such as water. Cyanide’s toxicity results from its attack on the enzyme cytochromeoxidase thereby preventing cell respiration and the normal transfer of oxygen from the blood to body tissues. Research in the past focused on the gold mining and textile industries’ concerns over cyanide leakage into thegroundwater and the EPA’s limit of 1 part per million or less of CN in detoxified industrial waste. The majority ofthese SERS techniques not only yield detection limits below the EPA mandate but well into the low part per billionrange. Detection of cyanide has also become somewhat of a standard assessment of a SERS substrate’s capabilityand sensitivity. SERS applications utilizing gold nanostructures,42,43,44 silver electrodes,45,46,47,48,49 sol-gels,25,50 andothers51,52 have reported great success at detecting CN in a variety of forms. The success can be attributed to thehigh binding efficiency of –(C≡N) to the metal surface resulting in a distinct band between 2120-2150 whoseposition is dependent on pH and concentration.25,42 Both silver- and gold-doped sol-gel vials have been used to measure cyanide in water to 1 mg/L and below(Figure 9A). In fact very reproducible measurements have been made at 10 mg/L. Both electrochemicallyroughened silver and gold foils have successfully been used to measure low concentrations of cyanide, and in thecase of the latter, consistent measurements down to 20 µg/L. The SER signal strength is highly variable, however,as seen for cyanide in water at 200 µg/L (Figure 9B). Data collected on the same day (but different gold foils) weremore consistent than measurements on different days, indicating that the substrates may be changing in storage orshipment. This is illustrated by a fit of the data from the two days by a Langmuir adsorption isotherm (Figure 10).This isotherm can be used to describe both physical and chemical adsorption:53 Kc θ= (1) Kc + 1 The cyanide concentration is given by c, the fractional surface coverage by θ , and the adsorption equilibriumconstant by K. The fractional coverage is calculated as the ratio of the SERS peak intensity divided by the intensityat full surface coverage (I/Imax). In this analysis, both K and Imax were fit using a nonlinear regression technique(DataFit, Oakdale Engineering, Oakdale, PA). The calculated adsorption equilibrium constants for the two days area factor of 10 different (0.4 L/mg vs. 0.04 L/mg). IV. CONCLUSIONS The potential sensitivity and selectivity of SERS coupled with the lack of strong water interference make it anattractive technique for chemical detection in aqueous solution. Two very different SERS-active substrates, 5
CRC Book Chapter 5 Draftelectrochemically roughened gold and silver foils, and gold- and silver-doped sol-gels both proved capable ofmeasuring sulfur mustard, VX, cyanide and several hydrolysis products of these chemical agents. However, only inthe case of cyanide was sensitivity sufficient for a SERS-based chemical agent water monitor. Substantialimprovements in sensitivity are required for the other agents. Furthermore, low laser powers are required tominimize photo-degradation. Finally, the ability to manufacture substrates that yield reproducible results remainselusive. Nevertheless, the detection limits for some of the phosphonic acid nerve agent hydrolysis products andcyanide show promise. Efforts to improve sensitivity and reproducibility will continue to be pursued.Table 1. Recommended Field Drinking Water Guidelines CONSUMPTION RATE Chemical Agent 5 L/day 15 L/day Cyanide (µg/L) 6000 2000 Sulfur mustard (µg/L) 140.0 47.0 Nerve agents (µg/L) GA 70 22.5 GB 13.8 4.6 GD 6.0 2.0 VX 7.5 2.5Table 2. Hydrolysis half-lives and solubilities of chemical agents and their primary hydrolysis products. Chemical Water Solubility (at Hydrolysis ½ life Agent 25°C) Sarin (GB) 39 hr (pH 7) completely miscible IMPA stable but can hydrolyze to MPA 4.8 g/L MPA very stable >1000 g/L VX >3 days (pH 7) 150 g/L EA2192 > 10 x VX ∞ sol. DIASH stable ca. 1000 g/L EMPA >8 days 180 g/L MPA very stable >1000 g/L HD 5 min 0.648 g/L* Mustard 3 min** Chlorohydrin TDG stable 6900 g/L*Seidell A. 1941. Solubilities of organic compounds. A compilation of quantitative solubility data from theperiodical literature. Vol. 11, 3rd Edition. New York: D. Van Nostrand Company, Inc. 241-242.**Ogston, A. G.; Holiday, E. R.; Philpot, J. St. L.; Stocken, L. A., Trans. Faraday Soc., 1948, 44, 45-52. 6
CRC Book Chapter 5 DraftTable 3. Tentative vibrational mode assignments for EA2192 and VX:41 Normal Raman, Sol-gel SER, andRoughened Ag SER. EA2192 VX Sol-Gel Roughened Sol-Gel Roughened NR NR Tentative Assignments SER* Ag SER SER* Ag SER 386 387 372 376 SPO bend 418 413 CC or CN bend 453 456 461 458 441 POn bend 484 481 481 484 484 487 NC3 breathing 499 POn bend 513 526 523 528 539 532 POn(S) bend 587 584 586 NCn bend 645 623 667 622 PSC bend CSH bend 693 700 696 682 CS stretch 732 735 743 744 731 735 PC stretch + backbone (CPOCC) 769 769 771 PC stretch and/or backbone 814 811 811 805 SC stretch + NC3 breathing 831 830 830 836 820 863 856 CH3 bend 905 891 883 891 885 889 OPC stretch / CCN stretch 925 947 939 943 931 939 940 NC3 stretch 966 971 975 965 POn stretch 1010 1006 1015 1006 1009 POn or CH3 bend 1043 1040 1029 1031 SCCN bend 1054 PO2(S) stretch 1100 1100 1101 1096 1098 OC or CC stretch 1132 1125 1128 1121 1125 NC stretch 1183 1183 1170 NC stretch 1219 1214 1220 NC stretch 1229 1228 1237 CH2 bend 1306 1300 1300 1301 CH3 bend 1329 1327 1329 1343 CN bend + CC bend 1366 1365 1369 1366 1399 1399 1394 1400 CH3 bend / NC3 stretch 1418 CH3 bend 1427 1423 1443 1439 1439 CH2 bend 1451 1446 CHn bend 1460 1464 1463 1462 1462 1461 CHn bend 1493 7
CRC Book Chapter 5 Draft + S H2O S H2O SCl Cl Cl + Cl- Cl OH + HCl I Mustard HD Chlorohydrin CH + S H2O S - H2O SCl OH HO + Cl HO OH + HCl II TDGFigure 1. HD hydrolysis pathway. S SCl S+ HO S+ HO OH HO OH H-TDG CH-TDG OH S S + S+ HO HO OH H-2TDGFigure 2. Sulfonium salts produced in reaction of I and II (Figure 1) with TDG. 8
CRC Book Chapter 5 DraftFigure 3. SERS of HD using A) gold-doped sol-gel, B) silver-doped sol-gel, and C) electrochemically roughenedsilver foil. D) Raman spectrum of HD. Conditions: A) and B) 1 g/L in isopropanol/water, 100 mW of 785 nm, 1-min, C) 0.777 g/L in distilled water, 100 mW of 785 nm, 0.5-min, D) neat HD, 300 mW of 785 nm, 1-min.Figure 4. SERS of TDG using silver-doped sol-gels using A) 100 and B) 15 mW of 785 nm excitation, and C)Raman of neat TDG. Conditions: A) and B) 1 g/L, and C) 300 mW. 9
CRC Book Chapter 5 Draft OH 1 H N + O P O S 34%-37% DIASH EMPA O S O P N H2O 2 S P N O + EtOH 42%-50% O VX H EA2192 3 S- N ~10% O P O + HO O-ethyl 2-(diisopropylamino) methylphosphonothioate ethanolFigure 5. VX hydrolysis pathways.Figure 6. SERS of VX using A) roughened silver foil and B) silver-doped sol-gel, and C) Raman spectrum of neatVX. Conditions: A) 1 g/mL B) 129 mg/L. 10
CRC Book Chapter 5 DraftFigure 7. SERS of EA2192 using A) silver-doped sol-gel vial and B) roughened silver foil, and C) Raman spectrumof solid EA2192. Conditions: A) 1 g/L, B) 113 mg/L. The substrate was dipped into the solution for 2 minutes priorto collecting a 0.5 min spectrum.Figure 8. SERS of EMPA in silver-doped-sol-gel vials using A) 100 and B) 30 mW of 785 nm excitiation. SERS ofEMPA on electrochemically roughened silver foils for D) 0.1 and E) 1 mg/L samples. Raman spectra of neat EMPAin C) and E) for comparison. Conditions A) and B) 1 g/L, ... 11
CRC Book Chapter 5 DraftFigure 9. SER spectra of sodium cyanide A) in three different silver-doped sol-gel coated vials at 1, 10, and 100mg/L and B) on four different roughened gold foils all at 0.2 mg/L. Conditions: A) 100 mw 785 nm, 1-min, 8, B)300 mw 785 nm, 0.5-min, 1. The top two traces were measured on the same day, as were the bottom two traces. 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 1 10 100 1000 10000 100000 1000000 Concentration (µg/L)Figure 10. Plot of surface coverage (θ) vs. concentration for CN on electrochemically roughened gold. Thediamonds and open circles are results from two different days, and each point represents data from a differentsubstrate. The solid line and dashed line are fits of the data to the Langmuir adsorption isotherm and have R2 valuesof 0.971 and 0.944, respectively. 12
CRC Book Chapter 5 Draft References1 Global Proliferation of Weapons of Mass Destruction: A Case Study on the Aum Shinrikyo, The Senate Government Affairs Permanent Subcommittee on Investigations, October 31, 1995 Staff Statement, http://www.fas.org/irp/congress/1995_rpt/aum/index.html,2 The Japan Times Online Tuesday July 18, 2000. http://www.japantimes.co.jp/cgi- bin/getarticle.pl5?nn20000718a1.htm.3 Committee on Toxicology. Review of Acute Human-Toxicity Estimates for Selected Chemical-Warfare Agents, Nat. Acad. Press (Washington, D.C.) 1997.4 http://chppm-www.apgea.army.mil/documents/TBMEDS/TBMED577.pdf5 Committee on Toxicology. Guidelines for Chemical Warfare Agents in Military Field Drinking Water, Nat. Acad. Press (Washington, D.C.) 19956 McKone, T.E., Huey, B.M., Downing, E., and Duffy, L.M., Strategies to Protect the Health of Deployed U.S. Forces: Detecting, Characterizing, and Documenting Exposures, Division of Military Science and Technology and Board on Environmental Studies and Toxicology, National Research Council, NATIONAL ACADEMY PRESS, Washington, D.C.,207, 2000. http://www.nap.edu/books/0309068754/html.7 . R.M. Black, R.J. Clarke, R.W. Read, M.T. Reid: J. Chromat. 662, 301 (1994)8 . G.A. Sega, B.A. Tomkins, W.H. Griest: J Chromat. A 790, 143 (1997)9 . S.A. Oehrle, P.C. Bossle: J. Chromat. A 692, 247 (1995)10 . J.E. Melanson, B.L-Y. Wong, C.A. Boulet, C.A. Lucy: J. Chromat. A 920, 359 (2001)11 . J. Wang, M. Pumera, G.E. Collins, A. Mulchandani: Anal. Chem. 74, 6121 (2002)12 . W. Creasy, M. Brickhouse, K. Morrissey, J. Stuff, R. Cheicante, J. Ruth, J. Mays, B. Williams, R. O’Connor, H. Durst: Environ. Sci. Technol. 33, 2157 (1999)13 . W.R. Creasy: Am. Soc. Mass. Spectrom. 10, 440 (1999)14 . Q. Liu, X. Hu, J. Xie: Anal. Chim. Acta 512, 93 (2004)15 . G.A. Eiceman, Z. Caras: Ion Mobility Spectrometry. (CRC Press, 1994)16 . See products from Smiths Detection, Bruker Daltronics, etc.17 . N. Krylova, E. Krylov, G.A. Eiceman: J. Phys. Chem. 107, 3648 (2003)18 . W.E. Steiner, B.H. Clowers, L.M. Matz,W.F. Siems, H.H. Hill Jr.: Anal. Chem. 74, 4343 (2002)19 Christesen, S., MacIver, B., Procell, L., Sorrick, D., Carrabba, M., and Bello, J., Non-intrusive analysis of chemical agent identification sets (CAIS) using a portable fiber-optic Raman spectrometer, Appl. Spectr., 53, 850, 1999.20 Sedlacek III, A.J., Christesen, S.D., Chyba, T., and Ponsardin, P., Application of UV-Raman spectroscopy to the detection of chemical and biological threats, in Proc. SPIE Vol. 5269, Sedlacek III, A.J, Christesen, S.D., Colton, R. and Vo-Dinh, T., Eds., SPIE, Bellingham, Washington, 2004, 23.21 Alak, A.M. and Vo-Dinh, T., Surface-enhanced Raman spectrometry of organophosphorus chemical agents, Anal. Chem., 59, 2149, 1987.22 Taranenko, N., Alarie, J-P., Stokes, D.L., VoDinh, T., Surface-enhanced Raman detection of nerve agent simulant (DMMP and DIMP) vapor on electrochemically prepared silver oxide substrates, J. Raman Spectrosc., 27, 379, 1996.23 Spencer, K., Sylvia, J., Clauson, S. Janni, J., Surface-enhanced Raman as a water monitor for warfare agents, in Proc. SPIE Vol. 4577, Christesen, S.D. and Sedlacek III, A.J, Eds., SPIE, Bellingham, Washington, 2002, 15824 Farquharson, S., P. Maksymiuk, K. Ong, and S. Christesen, Chemical agent identification by surface-enhanced Raman spectroscopy, in Proc. SPIE Vol. 4577, Christesen, S.D. and Sedlacek III, A.J, Eds., SPIE, Bellingham, Washington, 2002, 166.25 Farquharson, S., Gift, A., Maksymiuk, P., Inscore, F. E., and Smith, W. W., pH dependence of methyl phosphonic acid, dipicolinic acid, and cyanide by surface-enhanced Raman spectroscopy, in Proc. SPIE Vol. 5269, Sedlacek III, A.J, Christesen, S.D., Colton, R. and Vo-Dinh, T., Eds., SPIE, Bellingham, Washington, 2004, 117.26 Spencer, K.M., Sylvia, J.M., Marren, P.J., Bertone, J.F., and Christesen, S.D., Surface-enhanced Raman spectroscopy for homeland defense, in Proc. SPIE Vol. 5269, Sedlacek III, A.J, Christesen, S.D., Colton, R. and Vo-Dinh, T., Eds., SPIE, Bellingham, Washington, 2004, 1.27 Farquharson, S, Gift, A., Maksymiuk, P. and Inscore, F., Appl. Spectrosc. 58, 351 (2004).28 Ogsten, A.G.; Holiday, E.R.; Philpot, J. St. L.; Stocken, L. A., The replacement reactions of b,b-dichlorodiethyl sulphide and of some analogues in aqueous solution: the isolation of b-chloro-b-hydroxydiethyl disulphide, Trans. Faraday Soc., 44, 45, 1948. 13
CRC Book Chapter 5 Draft29 Yang, Y-C., Szafraniec, L.L, Beaudry, W.T., and Ward, J.R., Kinetics and mechanism of the hydrolysis of 2- chloroethyl sulfides, J. Org. Chem., 53, 3293, 1988.30 Sosa, C., Bartlett, R.J., KuBulat, K. and Person, W.B., A theoretical study of the harmonic vibrational frequencies and infrared intensities of XCH2CH2SCH2CH2X and XCH2CH2SH (X=H, Cl), J. Phys. Chem., 93, 577, 1989.31 Donovan, W.H. and Famini, G.R., Conformational analysis of sulfur mustard from molecular mechanics, semiempirical, and ab initio methods, J. Phys. Chem., 98, 3669, 1994.32 Christesen, S.D., Vibrational spectra and assignments of diethyl sulfide, 2-chlorodiethyl sulfide and 2,2’- dichlorodiethyl sulfide, J. Raman Spectrosc., 22, 459, 1991.33 Donovan, W.H. and Famini, G.R., Jensen, J.O., and Hameka, H.F., Phosphorus, Sulfur, and Silicon, 80, 47, 1993.34 Joo, T.H., Kim, K. and Kim, M.S., Surface-enhanced Raman study of organic sulfides adsorbed on silver, J. Mol. Struct., 162, 191, 1987.35 Inscore, F., and Farquharson, S., Detecting hydrolysis products of blister agents in water by surface-enhanced Raman spectroscopy, in Proc. SPIE Vol. 5993, Vo-Dinh, T., Lieberman, R.A., and Gauglitz, G., Eds., SPIE, Bellingham, Washington, 2005, 19.36 Farquharson, S., A. Gift, P. Maksymiuk, F. Inscore, W. Smith, K. Morrisey, and S.D. Christesen, Chemical agent detection by surface-enhanced Raman spectroscopy, in Proc. SPIE Vol. 5269, Sedlacek III, A.J, Christesen, S.D., Colton, R. and Vo-Dinh, T., Eds., SPIE, Bellingham, Washington, 2004, 16.37 www.mitretek.org/home.nsf/homelandsecurity/VX38 Szafraniec, L. J.; Szafraniec, L. L.; Beaudry, W. T.; Ward, J. R., On the Stoichiometry of Phosphonothiolate Ester Hydrolysis, CRDEC-TR-212, July 1990, AD-A25077339 Epstein, J.; Callahan, J. J.; Bauer, V. E., The kinetics and mechanisms of hydrolysis of phophonothiolates in dilute aqueous solution, Phosphorus, 1974, 4, 157-163.40 Yang, Y-C., Chemical detoxification of nerve agent VX, Acc. Chem. Res., 1999, 32, 109-11541 Farquharson, S, Gift, A., Maksymiuk, P. and Inscore, F., Surface-Enhanced Raman Spectra of VX and its Hydrolysis Products, Appl. Spectrosc., 59, 654, 2005.42 Tessier, P. M., Christesen, S. D., Ong, K. K., Clemente, E. M., Lenhoff, A. M., Kaler, E. W., and Velev, O. D., On-line spectroscopic characterization of sodium cyanide with nanostructured gold surface-enhanced Raman spectroscopy substrates, Applied Spectroscopy, 56, 1524, 2002.43 Kuncicky, D. M., Christesen, S. D., and Velev, O. D., Role of the micro- and nanostructure in the performance of SERS substrates assembled from gold nanoparticles, Appl. Spectrosc., 59, 401, 2005.44 Tessier, P. M., Christesen, S. D., Ong, K. K., Clemente, E. M., Lenhoff, A. M., Kaler, E. W., and Velev, O. D., Assembly of gold nanostructured films templated b colloidal crystals and used in surface-enhanced Raman spectroscopy, in Proc. SPIE Vol. 4577, Christesen, S.D. and Sedlacek III, A.J, Eds., SPIE, Bellingham, Washington, 2002, 53.45 Shelton, R. D., Hass, J. W., and Wachter, E. A., Surface-enhanced Raman detection of aqueous cyanide, Appl. Spectrosc., 48, 1007, 1994.46 Mahoney, M. R., and Cooney, R. P., The evidence for stoichiometric silver oxide-cyanide phase in cyanide in cyanide SERS from silver electrodes, Chem. Phys. Lett., 117, 71, 1985.47 Benner, R. E., Dornhaus, R., Chang, R. K., and Laube, B. L., Correlations in the Raman spectra of cyanide complexes adsorbed on silver electrodes with voltammograms, Surf. Sci., 101, 341, 1980.48 Kellogg, D. S. and Pemberton, J. E., Effects of solution conditions on the surface-enhanced Raman scattering of cyanide species at Ag electrodes, J. Phys. Chem., 91, 1120, 1987.49 Billmann, J., Kovacs, G., and Otto, A., Enhanced Raman effect from cyanide adsorbed on a silver electrode, Surf. Sci., 92, 153, 1980.50 Premasiri, W. R., Clarke, R. H., Londhe, S., and Womble, M. E., Determination of cyanide in waste water by low-resolution surface enhanced Raman spectroscopy on sol-gel substrates, J. Raman Spetrosc. 30, 827, 1999.51 Vo-Dinh, T., Surface-enhanced Raman spectroscopy using metallic nanostructures, Trends Anal. Chem., 17, 557, 1998.52 Wachter, E. A., Storey, J. M. E., Sharp, S. L., Carron, K. T., and Jiang, Y., Hybrid substrates for real-time SERS-based chemical sensors, Appl. Spectrosc., 48, 193, 1995.53 Adamson, A.W. and Gast, A.P., Physical Chemistry of Surfaces (Wiley Interscience, New York, 1997), 6th ed., p. 599. 14
Springer Book Kneipp Editor Appendix M Draft 125. Detecting chemical agents and their hydrolysis products in water Stuart Farquharson, Frank E. Inscore and Steve Christesen Real-Time Analyzers, Middletown, CT, 0645725.1 INTRODUCTIONThe use of chemicals as weapons was introduced during World War I. It is estimated that chlorine, phosgene andsulfur-mustard (HD) resulted in an estimated death of 100,000 soldiers and 1 million injuries . Over the next 20years, chemicals designed specifically for warfare were developed; this included the substantially more toxic nerveagents, tabun, sarin, and soman (GA, GB, and GD, respectively). Fortunately, these abhorrent chemicals were notused in WWII, as world leaders feared reprisal attacks on their cities. During the Iran-Iraq war in the 1980s, theIraqis used HD, GA, GB, and GF (cyclo-sarin), and in 1988, Saddam Hussein used mustard and possibly nerveagents in killing several thousand Kurds .In more recent years, chemical agents have been used by terrorists. In Japan, the Aum Shinrikyo religious cultreleased GB within the Tokyo subway system in 1995 . The release of GB in this confined space had devastatingeffects resulting in 12 fatalities and hospitalization of thousands. This event and the mailing of anthrax causingspores through the US Postal System in 2001 demonstrated that deployment of chemical and biological agents donot require sophisticated delivery systems, and a wide range of attack scenarios must be considered. Among thesescenarios is the deliberate poisoning of drinking water. This includes water supplies used in military operations andwater delivered to major cities from reservoirs and through distribution systems. Countering such an attack requiresdetecting poisons in water rapidly, and at very low concentrations.The required detection sensitivity for each agent depends on several factors, such as toxicity and hydrolysis (Table25.1). In the case of cyanide (AC) it known that 4 milligrams per liter of water produces detectable changes inhuman blood chemistry and 8 mg L-1 causes severe, but reversible symptoms . The military has used this andother toxilogical data to set a field drinking water standard (FDWS) for cyanide at 2 mg L-1 . The FDWSrepresents the maximum allowable concentration that is assumed safe when 15 L of water per day is consumed over5 days (expected soldier intake in arid climates). Human toxicity data for the other chemical warfare agents in waterhave, in general, not been determined. The normal route of exposure for chemical warfare agents is inhalation, andmost of the toxicity data is given as the LCt50s , the concentration that is lethal to 50% of an exposed populationas a function of exposure time. In the case of mustard, animal studies along with the inhalation LCt50, the oral lethaldosage of 0.7 mg per kg of body mass (LD50), and modeling studies , have been used to set the FDWS at 0.047mg L-1. Similar analyses of LCt50s and LD50s for GB and VX have been used to set their FDWS at 0.0046 and0.0025 mg L-1, respectively. The FDWS concentrations have also been used by the military to set the minimumdetection requirement for poisons in water.Table 25.1. Military field drinking water standard , lethal exposures and dosages [1,3,5], and water properties forselected chemical warfare agents. Chemical FDWS LCt50 LD50 Water Hydrolysis Hydrolysis 5-day/15L inhalation oral Solubility Half-Life* Product (mg L-1) (mg-min m-3) (mg kg-1) at 25°C HCN (AC) 2 2000 - - CN NaCN 480 g L-1 Mustard (HD) 0.047 900 0.7 0.92 g L-1 2-30 hours TDG Sarin (GB) 0.0046 70 2 completely 20-40 hours IMPA, MPA miscible VX 0.0025 35 0.07 150 g L-1 82 hours DIASH, EMPA, EA2192, MPAIn order to detect these poisons in water, their properties in water must also be considered, i.e. the solubility, rate ofhydrolysis, and hydrolysis products formed. In the case of cyanide, as HCN, KCN, or NaCN, all of these chemicalsare extremely soluble in water (completely miscible, 716, and 480 g L-1, respectively) . In solution the cyanide
Springer Book Kneipp Editor Draft 2ion is formed in equilibrium with the conjugate acid, HCN (Figure 25.1A), according to the Ka of 6.15x10-10 [7 ]. Inthe case of cyanide then it is important to know the pH, if one form of the chemical is to be detected versus theother. For example, if 2 mg L-1 of NaCN is added to water (the FDWS), then 1.25 mg L-1 of CN- and 0.75 mg L-1 ofHCN will be present. A B C DFigure 25.1. Hydrolysis reaction pathways for A) CN, B) HD, C) GB, and D) VX.In the case of sulfur-mustard, the situation is somewhat more complex. It is marginally soluble in water tending toform droplets, and hydrolysis occurs at the droplet surface. This property has made measuring the hydrolysis rateconstant difficult, and half-lives anywhere from 2 to 30 hours are reported . Chemically, the hydrolysis of HDinvolves the sequential replacement of the chlorine atoms by hydroxyl groups through cyclic sulfonium ionintermediates to form thiodiglycol (TDG, Figure 25.1B) . If a median hydrolysis rate is assumed, then earlydetection of poisoned water will require measuring HD, while post-attack or downstream monitoring will requiremeasuring TDG. For sarin, the analysis is more straightforward, since it dissolves readily into water and it is stablefor a day or more. In this case, detecting poisoned water will largely require measuring sarin, while monitoring theattack will require detecting its sequential hydrolysis products, isopropyl methylphosphonic acid (IMPA) and methylphosphonic acid (IMPA, MPA, respectively, Figure 25.1C) [8,10,11]. The other hydrolysis products, hydrofluoricacid and 2-propanol, are too common to provide definitive evidence of water poisoning and their measurementwould be of limited value. VX is reasonably soluble, and like sarin, is fairly persistent with a hydrolysis half-lifegreater than 3 days . Unfortunately, one of its hydrolysis products, known as EA2192, is considered just astoxic as VX, more soluble and more persistent in water . Consequently, detecting the early stages of poisoningwater should focus on measuring VX, while longer term monitoring should focus on EA2192.The earliest technologies developed for CWA detection were based on electrochemical, ionization, or colorimetricanalysis. Examples of the latter include phosgene, M8 and M9 tape, which change color when in contact with asample like pH paper. Although these tapes are easy to use, they are not generally agent specific and suffer from ahigh percentage of false-positives . For example, M8 changes color when in contact with common solvents suchas acetonitrile, ethanol, methanol, or common petroleum products such as brake fluid, lighter fluid, or WD-40 .More rigorous laboratory methods have been successfully developed to detect chemical agents with minimum false-positive responses. More than a decade ago, Black et al. demonstrated the ability of combining gas chromatography
Springer Book Kneipp Editor Draft 3with mass spectrometry detection (GC/MS) to measure sarin and mustard . Sega at al. used GC with aphosphorous-selective flame ionization detector to analyze nerve agent hydrolysis products in groundwater ,while several researchers used capillary electrophoresis (CE) to measure chemical warfare agents and theirhydrolysis products [18,19,20]. The sensitivity of these techniques has improved by two orders of magnitude from 1mg L-1 to 0.01 mg L-1 in 10 years. A comprehensive development of these techniques was undertaken by Creasy etal. in analyzing chemical weapon decontamination waste from the Johnston Atoll [11,21]. These researchers usedGC/MS for nerve agents, GC coupled atomic emission detection for arsenic compounds, LC/MS for mustardcompounds, and CE with ultraviolet absorption detection for alkyl phosphonic acids. Detection limits of 0.02 and0.140 mg L-1 were reported for nerve agents and mustard, respectively. Detection of the alkyl phosphonic acidshave proven more difficult, and Liu, Hu and Xie recently used GC/MS to detect mg L-1 concentrations of thesedegradation products . However, they concede that all of these separation methods require extraction,derivatization, and repeated column calibration, making them labor intensive, time consuming (typically 30 to 60minutes), and less than desirable for field use. Another variant of these separation/mass detection technologies is ionmobility spectrometry (IMS) . This technology has been successfully developed to measure explosives in airsamples, and commercial products can be found at most airports . Eiceman et al. have investigated the ability ofIMS to measure organophosphorous compounds in air , while Steiner et al. have investigated IMS to measurechemical agent simulants in water . In the latter case, electrospray ionization was coupled to the sample entrypoint of an IMS, and a time-of-flight MS was added as an orthogonal detector. Water samples spiked with 10 mgL-1 diisopropylmethylphosponate and thiodiglycol could be measured in 1-min, once sample pretreatment wasaccomplished. It is worth noting that with proper care these MS-based technologies are likely to detect chemicalagents with virtually no false-positives, but detection limits are still insufficient by 1 to 2 orders of magnitude in thecase of nerve agents and their hydrolysis products.More rapid analysis of agents in the solid, liquid and gas phase has been demonstrated by vibrational spectroscopy[27-31]. Hoffland et al. reported infrared absorbance spectra and absolute Raman cross sections for severalchemical agents , while Christesen measured Raman cross sections for sarin, tabun, mustard gas, and VX .Again, however these technologies also have limitations. Raman spectroscopy is simply not a very sensitivetechnique, and detection limits are typically 0.1% (1000 ppm). And infrared spectroscopy would have limited valuein analyzing poisoned water, since the very strong infrared absorption of water would obscure most other chemicalspresent. Nevertheless, efforts to overcome these limitations have been demonstrated. Braue and Pannella quantifiedthe G-series nerve agents (tabun, sarin, and soman) in terms of infrared attenuated total reflectance using a circle-cell .Enormous improvements in sensitivity for Raman spectroscopy can be achieved through surface-enhancement .The interaction of surface plasmon modes of metal particles with target analytes can increase scattering efficiencyby as much 14-orders of magnitude, although 6-orders of magnitude are more common. The details of surface-enhanced Raman spectroscopy (SERS) can be found in the beginning of this book. The utility of SERS to measurechemical agents was first demonstrated by Alak and Vo-Dinh by measuring several organophosphonates assimulants of nerve agents on a silver-coated microsphere substrate . Spencer, et al. used SERS to measurecyanide, MPA, HD and EA2192 on electrochemically roughed gold or silver foils [34,35,36]. However, in all ofthese measurements, the sample needed to be dried on the substrates to obtain the best sensitivity (e.g. 0.05 mg L-1for MPA). More recently, Tessier et al. obtained SERS of 0.04 mg L-1 cyanide in a stream flowing over a substrateformed by a templated self-assembly of gold nanoparticles . However, optimum sensitivity requiredintroduction of an acid wash and the measurements were irreversible.In the past few years, we have also been investigating the ability of SERS to measure chemical agents at 0.001 mgL-1 in water and with sufficient spectral uniqueness to distinguish the agent and its hydrolysis products [38-43]. Inour work, we have developed silver-doped sol-gels as the SERS-active medium. These sol-gels can be coated on theinside walls of glass vials, such that water samples can be added to perform point-analysis, or they can beincorporated into glass capillaries, such that flowing measurements can be performed . Here, both samplingdevices were used to measure and compare SER spectra of AC, HD, VX and several of their hydrolysis products,TDG, EA2192, EMPA, and MPA. In addition, a field-usable Raman analyzer was used to measure 0.01 mg L-1cyanide flowing in water with a detection time of less than 1-min.
Springer Book Kneipp Editor Draft 425.2 EXPERIMENTALSodium cyanide, 2-hydroxyethylethyl sulfide (HEES), 2-chloroethylethyl sulfide (CEES) and methylphosphonicacid (MPA) were purchased from Sigma-Aldrich (St. Louis, MO) and used as received. Ethyl methylphosphonicacid (EMPA), isopropyl methylphosphonic acid (IMPA), 2-(diisopropylamino) ethanethiol (DIASH), andthiodiglycol (TDG, bis(2-hydroxyethyl)sulfide) were purchased from Cerilliant (Round Rock, TX). Highly distilledsulfur mustard (HD, bis(2-chloroethyl)sulfide), isopropyl methylphosphonofluoridate (GB), ethyl S-2-diisopropylamino ethyl methylphosphonothioate (VX), and ethyl S-2-diisopropylamino methylphosphonothioate(EA2192) were obtained at the U.S. Army’s Edgewood Chemical Biological Center (Aberdeen, MD) and measuredon-site. All samples were initially prepared in a chemical hood as 1000 parts-per-million (1 g L-1 or 0.1% byvolume, Environmental Protection Agency definition) in HPLC grade water (Fischer Scientific, Fair Lawn, NJ) or insome cases methanol or ethanol (Sigma-Aldrich) to minimize hydrolysis.Once prepared, the samples were transferred into 2-ml glass vials internally coated with a silver-doped sol-gel(Simple SERS Sample Vials, Real-Time Analyzers, Middletown, CT) or drawn by syringe or pump into 1-mmdiameter glass capillaries filled with the same SERS-active material [45,46,47]. In the case of flow measurements, aperistaltic pump (variable flow mini-pump, Control Co., Friendswood, TX) was used to flow the various cyanidesolutions through a SERS-active capillary at 1 mL min-1. The vials or capillaries were placed on aluminum platesmachined to hold the vials or capillaries on a standard XY positioning stage (Conix Research, Springfield, OR),such that the focal point of an f/0.7 aspheric lens was positioned just inside the glass wall. The probe optics andfiber optic interface have been described previously . SER spectra were collected using a Fourier transformRaman spectrometer equipped with a 785 nm diode laser and a silicon photo-avalanche detector (IRA-785, Real-Time Analyzers). All spectra were nominally collected using 100 mW, 8 cm-1 resolution, and 1-min acquisitiontime, unless otherwise noted. Complete experimental details can be found in Reference 48. For added safety, allsamples were measured in a chemical hood. In the case of actual agents measured at Edgewood, the FT-Ramaninstrument was placed outside the laboratory and 30 foot fiber optic and electrical cables were used to allow remoteSERS measurements and plate manipulation.25.3 RESULTS AND DISCUSSION25.3.1 Cyanide. Sodium cyanide completely dissolves in water forming the ions in equilibrium with the conjugateacid, HCN as described above. Concentrations of 1.0, 0.1, and 0.01 mg L-1 result in CN- concentrations of 0.52,0.016, and 0.00021 mg L-1 as the corresponding pH decreases from just above the pKa of 9.21 at 9.24 to 8.48 and7.54. This is significant in that only CN- appears to interact sufficiently with silver to produce a SER spectrum, andno spectral signal is observed below pH 7 (except on electrodes at specific potential conditions ). The SERspectra of cyanide are dominated by an intense, broad peak at 2100 cm-1 attributed to the C≡N stretch (Figure 25.2).This mode occurs at 2080 cm-1 in Raman spectra of solutions, and the frequency shift in SER spectra is attributed toa strong surface interaction, which is supported by the appearance of a low frequency peak at 135 cm-1 due to a Ag-CN stretch (not shown). It is also observed that as the concentration decreases, the CN peak shifts to 2140 cm-1.This shift has been attribute to the formation of a tetrahedral Ag(CN)32- surface structure , as well as to CNadsorbed to two different surface sites . Alternatively, it has also been suggested that at concentrations near andabove monolayer coverage, the CN- species is forced to adsorb end-on due to crowding, and at lower concentrationsthe molecule can reorient to lie flat. This suggests that the 2100 and 2140 cm-1 peaks correspond to the end-on andflat orientations, respectively. However, a previous concentration study of cyanide on a silver electrode observedthe reverse trend, i.e. greater intensity was observed for the 2100 cm-1 peak at low concentration .Repeated measurements of cyanide in the SERS-active vials consistently allowed measuring 1 mg L-1 (1 ppm), butrarely below this concentration (Figure 25.2A). Nevertheless, this sensitivity is in general sufficient for pointsampling of water supplies. In the case of continuous monitoring of water, the capillaries are a more appropriatesampling format, and they also allowed routine measurements at 0.01 mg L-1 and repeatable measurements at 0.001mg L-1 (1 ppb, Figure 25.2B). Employing this format, a 50 mL volume of 0.01 mg L-1 cyanide solution was flowedat 2.5 mL min-1 through a SERS-active capillary, and spectra were recorded every 20 seconds. As Figure 25.3shows, the cyanide peak was easily discerned as soon as the solution entered the capillary and remained relativelystable over the course of the experiment. It is worth noting, as indicated above, that the SERS peak in Figure 25.3 isin fact due to 210 ng L-1!
Springer Book Kneipp Editor Draft 5 A BFigure 25.2. Surface enhanced Raman spectra of CN in water in silver-doped sol-gel A) coated glass vials and B)filled glass capillaries. All spectra were recorded using 100 mW of 785 nm in 1-min and at a resolution of 8 cm-1.Figure 25.3. 2100 cm-1 peak height measured during continuous flow of a 0.01 mg L-1 (10 ppb) cyanide in water.Surface-enhanced Raman spectra are shown for 1 and 6 min after sample introduction. A 2.5 mL min-1 flow ratewas used and spectra were recorded every 20 sec using 100 mW of 785 nm.
Springer Book Kneipp Editor Draft 625.3.2 HD and CEES. The surface-enhanced Raman spectrum of HD is dominated by a peak at 630 cm-1 with anextended high frequency shoulder composed of at least two peaks evident at 695 and 830 cm-1, as well as amoderately intense peak at 1045 cm-1 (Figure 25.4A). The latter peak is assigned to a CC stretching mode, based onthe assignment for a peak at 1040 cm-1 in the Raman spectrum of HD . The assignment of the 630 cm-1 peak isless straightforward, since the Raman spectrum of HD contains five peaks in this region at 640, 655, 700, 740, and760 cm-1 [40,52]. Theoretical calculations for the Raman spectrum of HD indicate that the first three peaks are dueto CCl stretching modes, and the latter two peaks to CS stretching modes . Based on these calculations, and theexpected interaction between the chlorine atoms and the silver surface, it is reasonable to assign the 630 cm-1 SERSpeak to a CCl mode . However, recent SERS measurements of diethyl sulfide produced a very simple spectrumwith an intense peak at 630 cm-1 [54,55], strongly suggesting CS or CSC stretching modes as the appropriateassignment for this peak . The authors of the theoretical treatment concede that the CCl and CS assignmentscould be reversed . The CS assignment also indicates that HD interacts with the silver surface through the sulfurelectron lone pairs. But, interaction between chlorine and silver is still possible and may be responsible for the 695cm-1 peak. The 830 cm-1 peak is left unassigned. A BFigure 25.4. Surface-enhanced Raman spectra of A) HD in methanol and B) TDG in water. Spectral conditions asin Fig. 25.2, samples were 1 g L-1.The surface-enhanced Raman spectrum of TDG is also dominated by a peak at 630 cm-1 with minor peaks at 820,930, 1210, and 1275 cm-1 (Figure 25.4B). Again, the 630 cm-1 peak is preferably assigned to a CSC stretching modeversus a CCl mode, especially since the chlorines have been replaced by hydroxyl groups. Furthermore, the lack ofa 695 cm-1 peak in the TDG spectrum supports the assignment of this peak in the HD spectrum to a CCl mode. The930, 1210 and 1275 cm-1 SERS peaks are assigned to a CC stretch with CO contribution, and two CH2 deformationmodes (twist, scissors, or wag) based on the assignments for the corresponding peaks at 940, 1230 and 1290 cm-1 inthe Raman spectrum of TDG [52,54 ]. It is worth noting that irradiation at high laser powers or for extended periodsproduces peaks at 715 and 1010 cm-1, which are attributed to a degradation product, such as 2-hydroxy ethanethiol.The SERS of CEES is very similar to HD, dominated by a peak at 630 cm-1 that is accordingly assigned to a CS orCSC stretching mode (Figure 25.5A). This peak also has a high frequency shoulder centered at 690 cm-1, and a thirdpeak appears at 720 cm-1 in this region. Again, these can be assigned to CCl or CS modes. The quality of thisspectrum also reveals weak peaks at 1035, 1285, 1410, and 1445 cm-1. Peaks at 1035, 1285, 1425, and 1440 cm-1
Springer Book Kneipp Editor Draft 7appear in the Raman spectrum of CEES, and the previous peak assignments are used here , i.e. the first peak isassigned to a CC stretch, while the remaining peaks are assigned to various CH2 deformation modes. A BFigure 25.5. Surface-enhanced Raman spectra of A) CEES and B) HEES. Spectral conditions as in Fig. 25.2,samples were 1 g L-1 in methanol.Replacing the chlorine atom of CEES by a hydroxyl group in forming HEES produces SER spectral changesanalogous to those cited above for HD and TDG. Again, the SER spectrum is dominated by an intense peak at 630cm-1 attributed to a CS or CSC stretching mode, and the other CEES peaks in this region, specifically the 720 cm-1peak, disappear (Figure 25.5B). Peaks with modest intensity at 1050 and 1145 cm-1 are assigned to a CC stretchingmode and CH2 deformation, respectively. A new peak at 550 cm-1 is likely due to a skeletal bending mode, such asCSC, SCC, or CCO. Finally, it is worth stating that HD, TDG, CEES, and HEES all produce moderately intensepeaks at 2865 and 2925 cm-1 (not shown), that can be assigned to symmetric and asymmetric CH2 stretching modes.Only a limited number of measurements of HD were performed to evaluate sensitivity, due to the safetyrequirements. HD was repeatedly observed at 1 g L-1 and usually observed at 0.1 g L-1 (100 ppm) in the SERS-active vials  But even at the latter concentration, substantial improvements in sensitivity are required toapproach the required 0.05 mg L-1 (50 ppb) sensitivity. More extensive experiments were performed on HD’shydrolysis product, TDG since this chemical is safely handled in a regular chemical lab. Flowing TDG throughSERS-active capillaries allowed repeatable measurements at 10 mg L-1, and routine measurements at 1 mg L-1 (1ppm) . These SERS measurements of TDG suggest that the required HD sensitivity may be achievable usingthis technique. Similar flowing measurements in capillaries for HD, CEES, and HEES have not been performed.25.3.3 Sarin. SERS measurements of GB have not been made, but its primary hydrolysis products, IMPA andMPA, have been measured using the SERS-active vials. The SERS of IMPA is very similar to its Raman spectrum, which in turn is very similar to the Raman spectrum of sarin . The SER spectrum is dominated by a peakat 715 cm-1 (Figure 25.6A), which is assigned to a PC or PO plus skeletal stretching mode, as is a weak peak at 770cm-1. These assignments are also consistent with a theoretical treatment of the Raman spectrum for sarin .Similarly, a modest peak at 510 cm-1 can be assigned to a PC or PO plus skeletal bending mode. Other SERS peaksof modest intensity occur at 875, 1055, 1415, and 1450 cm-1, and based on the spectral analysis of sarin and theRaman spectrum of IMPA with peaks at 880, 1420, and 1455 cm-1, are assigned to a CCC bend, a PO3 stretch, a CH3bend, and a CH2 rock, respectively.
Springer Book Kneipp Editor Draft 8 A B CFigure 25.6. Surface-enhanced Raman spectra of A) IMPA, B) MPA, and C) EMPA. Spectral conditions as in Fig.25.2, samples were 1 g L-1 in water.MPA has been well characterized by infrared and Raman spectroscopy [58,59], as well as normal coordinateanalysis , and the literature assignments are used here for the SERS of MPA. The SER spectrum is dominatedby a peak at 755 cm-1, which is assigned to the PC symmetric stretch (Figure 25.6B). In comparison to IMPA, it isclear that removing the isopropyl group shifts this frequency substantially (40 cm-1), as the mode becomes a purerPC stretch. Additional peaks with comparatively little intensity occur at 470, 520, 960, 1040, 1300, and 1420 cm-1,and are assigned to a PO3 bending mode, a C-PO3 bending mode, a PO3 stretching mode, another PO3 bendingmode, and two CH3 deformation modes (twisting and rocking).SERS-active vials allowed repeatable measurements of MPA at 10 mg L-1 and routine measurements at 1 mg L-1,and repeatable measurements of IMPA at 100 mg L-1 and routine measurements at 10 mg L-1. Again, however,substantial improvements in sensitivity are required to achieve the minimum requirement of 0.004 mg L-220.127.116.11 VX. The hydrolysis of VX can occur along two pathways (Figure 25.1D) [11,22], either being converted toDIASH and EMPA or EA2192 and ethanol with the former pathway favored four to one. These products alsohydrolyze, and EMPA forms MPA and ethanol, while EA2192 forms DIASH and MPA. Here the SER spectra ofVX, EA2192 and DIASH are compared, while EMPA is compared to IMPA and MPA.The SER spectrum of VX is similar to its Raman spectrum with corresponding peaks at 375, 460, 540, 730, 1095,1300, 1440, and 1460 cm-1 (Figure 25.7A). Since a computer predicted Raman spectrum contains most of themeasured Raman spectral peaks [43,61], it is used to assign the above SERS peaks respectively to an SPO bend, aCH3-P=O bend, a PO2CS wag, an OPC stretch, a CC stretch, and three CHn bends. As previously described forCEES and HD, the 730 cm-1 peak could alternatively be assigned to a CS stretch, but the SER spectra of thesechemicals suggest otherwise.The SER spectrum of EA2192 is somewhat different than VX with the PO modes having limited intensity and theNC3 modes having significant intensity (Figure 25.7B). Specifically, the EA2192 spectrum has moderately intensepeaks at 480, 585, 940, and 1125 cm-1 that can be assigned to an NC3 breathing mode, an NCC bending mode,another NC3 stretching mode, and a NCC stretching mode. Two additional peaks with significant intensity at 695
Springer Book Kneipp Editor Draft 9and 735 cm-1 are assigned to a CS stretching mode and an OPC stretching mode, respectively. Two peaks of modestintensity at 525 and 970 cm-1 are attributed to a PO2S bending mode and a PO2 stretching mode. A B CFigure 25.7. Surface-enhanced Raman spectra of A) VX, B) EA2192, and C) DIASH. Spectral conditions as in Fig.25.2, samples were 1 g L-1 in water.The SER spectrum of DIASH contains most of the NC3 modes cited previously for EA2192 (Figure 25.7C),specifically peaks appear at 480, 585, 940, and 1120 cm-1, and can be assigned as above. Additional peaks at 740,810, and 1030 cm-1, are assigned to CH bending, a combination of SC stretching and NC3 bending, and SCCNbending modes, based on the Raman spectrum of DIASH . A broad peak centered at 695 cm-1 also occurs thathas previously been assigned to an SC stretch, but the frequency and intensity of this mode in the HD and CEESspectra above, makes this assignment less certain.It is worth noting the similarity between the EA2192 and DIASH SER spectra, the principle difference being theaddition of the SCCN bending mode at 1030 cm-1 for the latter. This may simply be due to the fact that bothmolecules interact through the sulfur with the metal surface to similar extents resulting in similar spectra. However,it is also possible that the EA2192 spectrum is of DIASH formed either by hydrolysis or photo-degradation. Sincethe sample was measured within one hour of preparation, and the hydrolysis half-life is on the order of weeks ,the former explanation seems unlikely. Since the peak intensities did not change during these measurements, photo-degradation catalyzed by silver also seems unlikely. Further experiments are required to clarify this point.The SER spectrum of the other hydrolysis product formed from VX, EMPA, is shown in Figure 25.6. It is includedwith MPA and IMPA, the hydrolysis products of GB, for convenient spectral comparison of these structurallysimilar chemicals. The spectrum is dominated by a peak at 745 cm-1 with a substantial low frequency shoulder at725 cm-1. Both are assigned, similarly to IMPA, to PC or PO plus skeletal stretching modes. In fact, virtually all ofthe peaks in the SER spectrum correspond to peaks of similar frequency in the SER spectrum of IMPA, and areassigned as follows: the peaks at 480 and 500 cm-1 to PC or PO plus skeletal bends; 890, 1415, and 1440 cm-1 toCHn deformations; 945 and 1060 cm-1 to POn stretches; and 1095 to a CO or CC stretch. A peak at 1285 cm-1 isassigned to a CHn deformation based on the MPA spectral assignment for a peak at 1300 cm-1.In this series of chemicals VX and EA2192 were routinely measured at 100 mg L-1, and on occasion at 10 mg L-1using the SERS-active vials. Again, however, only a limited number of measurements were attempted. More
Springer Book Kneipp Editor Draft 10extensive measurements of EMPA using the SERS-active capillaries allowed repeatable measurements of 10 mg L-1and routine measurements of 1 mg L-1. No concentration studies of DIASH were undertaken.25.4 ConclusionsThe ability to obtain surface-enhanced Raman spectra of several chemical agents and their hydrolysis products hasbeen demonstrated using silver-doped sol-gels. Two sampling devices, SERS-active vials and capillaries, provideda simple means to measure water samples containing chemical agents. No sample pretreatment was required and allspectra were obtained in 1 minute. It was found that the SER spectra can be used to identify chemical agents byclass. Specifically, cyanide contains a unique peak at 2100 cm-1, HD and CEES both have a unique peak at 630cm-1, while VX has a unique peak at 540 cm-1. In the case of HD and CEES, their hydrolysis products produce verysimilar spectra, and it may be difficult to determine relative concentrations in an aqueous solution. In the case of theVX hydrolysis products, EA2192 and DIASH were spectrally similar, as was IMPA and MPA. However, thereappears to be sufficient differences when comparing entire spectra, such that chemometric approaches might allowsuccessful compositional analysis of aqueous solutions.The SERS-active vials and capillaries provided sufficient sensitivity to measure cyanide below the required 2 mg L-1sensitivity either as a point measurement or as a continuous flowing stream measurement. Measurements of TDGsuggest that the sensitivity requirements for it and HD may be attainable with modest improvements. In contrast,the vials and capillaries did not provide sensitivity sufficient to meet the requirements of VX. In this casesubstantial improvements in sensitivity are required and are being pursued.25.5 AcknowledgementsThe authors are grateful for the support of the U.S. Army (DAAD13-02-C-0015, Joint Service Agent Water Monitorprogram) and the Environmental Protection Agency (EP-D-05-034). The authors would also like to thank Mr.Chetan Shende for sol-gel chemistry development.25.6 References1 S.L. Hoenig: Handbook of Chemical Warfare and Terrorism. (Greenwood Press, 2002) p. 8, 19, 34-632 H. Nozaki, N. Aikawa: Sarin poisoning in Tokyo subway. Lancet 345, 1446 (1995)3 Committee on Toxicology: Review of Acute Human-Toxicity Estimates for Selected Chemical-Warfare Agents. (Nat Acad Press, 1997)4 Committee on Toxicology: Guidelines for Chemical Warfare Agents in Military Field Drinking Water. (Nat Acad Press, 1995)5 T.C. Marrs, R.L. Maynard, F.R. Sidell: Chemical Warfare Agents: Toxicology and Treatment. (John Wiley and Sons, 1996)6 Material Safety Data Sheets, available at www.msds.com7 D.R. Lide, Ed: Handbook of Chemistry and Physics: (CRC Press, 1997) p. 8-438 N.B. Munro, S.S. Talmage, G.D.Griffin, L.C. Waters, A.P. Watson, J.F. King, V. Hauschild: Environ. Health Perspect. 107, 933 (1999)9 A.G. Ogsten, E.R. Holiday, J.St.L.Philpot, L.A. Stocken: Trans. Faraday Soc. 44,45 (1948)10 G. Wagner, Y. Yang: Ind. Eng. Chem. Res. 41, 1925 (2002)11 W. Creasy, M. Brickhouse, K. Morrissey, J. Stuff, R. Cheicante, J. Ruth, J. Mays, B. Williams, R. O’Connor, H. Durst: Environ. Sci. Technol. 33, 2157 (1999)12 Y. Yang: Acc. Chem. Res. 32, 109 (1999)13 Y. Yang, J. Baker, J. Ward: Chem. Rev. 92, 1729 (1992)14 B. Erickson: Anal. Chem. News & Features, 397A (1998)15 Product literature at http://www.wmdetect.com/Library/M8/M8%20Paper.htm16 R.M. Black, R.J. Clarke, R.W. Read, M.T. Reid: J. Chromat. 662, 301 (1994)17 G.A. Sega, B.A. Tomkins, W.H. Griest: J Chromat. A 790, 143 (1997)18 S.A. Oehrle, P.C. Bossle: J. Chromat. A 692, 247 (1995)19 J.E. Melanson, B.L-Y. Wong, C.A. Boulet, C.A. Lucy: J. Chromat. A 920, 359 (2001)20 J. Wang, M. Pumera, G.E. Collins, A. Mulchandani: Anal. Chem. 74, 6121 (2002)
Springer Book Kneipp Editor Draft 1121 W.R. Creasy: Am. Soc. Mass. Spectrom. 10, 440 (1999)22 Q. Liu, X. Hu, J. Xie: Anal. Chim. Acta 512, 93 (2004)23 G.A. Eiceman, Z. Caras: Ion Mobility Spectrometry. (CRC Press, 1994)24 See products from Smiths Detection, Bruker Daltronics, etc.25 N. Krylova, E. Krylov, G.A. Eiceman: J. Phys. Chem. 107, 3648 (2003)26 W.E. Steiner, B.H. Clowers, L.M. Matz,W.F. Siems, H.H. Hill Jr.: Anal. Chem. 74, 4343 (2002)27 L.D. Hoffland, R.J. Piffath, J.B. Bouck: Opt. Eng. 24, 982 (1985)28 S.D. Christesen: Appl. Spectrosc. 42, 318 (1988)29 E.H.J. Braue, M.G. Pannella: Appl. Spectrosc. 44, 1513 (1990)30 C-H. Tseng, C.K. Mann, T.J. Vickers: Appl. Spectrosc. 47, 1767 (1993)31 S. Kanan, C. Tripp: Langmuir 17: 2213 (2001)32 D.L. Jeanmaire, R.P. Van Duyne: J. Electroanal. Chem. 84, 1 (1977)33 A.M. Alak, T. Vo-Dinh: Anal. Chem. 59, 2149 (1987)34 K.M. Spencer, J. Sylvia, S. Clauson, J. Janni: Proc. SPIE 4577,158 (2001)35 S.D. Christesen, M.J. Lochner, M. Ellzy, K.M. Spencer, J. Sylvia, S. Clauson: 23rd Army Sci. Conf. (2002)36 S.D. Christesen, K.M. Spencer, S. Farquharson, F.E. Inscore, K. Gosner, J. Guicheteau: In: S. Farquharson, Ed. Applications of Surface-Enhanced Raman Spectroscopy. (CRC Press, in preparation)37 P. Tessier, S. Christesen, K. Ong, E. Clemente, A. Lenhoff, E. Kaler, O. Velev: Appl. Spectrosc. 56, 1524 (2002)38 Y. Lee, S. Farquharson: Proc. SPIE 4378, 21 (2001)39 S. Farquharson, P. Maksymiuk, K. Ong, S. Christesen: Proc. SPIE 4577, 166 (2001)40 S. Farquharson, A. Gift, P. Maksymiuk, F. Inscore, W. Smith, K. Morrisey, S. Christesen: Proc. SPIE 5269, 16 (2004)41 S. Farquharson, A. Gift, P. Maksymiuk, F. Inscore, W. Smith: Proc. SPIE 5269, 117 (2004)42 F. Inscore, A. Gift, P. Maksymiuk, S. Farquharson: Proc. SPIE 5585, 46 (2004)43 S. Farquharson, A. Gift, P. Maksymiuk, F. Inscore: Appl. Spectrosc. 59, 654 (2005)44 S. Farquharson, P. Maksymiuk: Appl. Spectrosc. 57, 479 (2003)45 S. Farquharson, Y.H. Lee, C. Nelson: U.S. Patent Number 6,623,977 (2003)46 S. Farquharson, P. Maksymiuk: U.S. Patent Numbers 6,943,031 and 6,943,032 (2005)47 S. Farquharson, A. Gift, P. Maksymiuk, F. Inscore: Appl. Spectrosc. 58, 351 (2004)48 F. Inscore, A. Gift, P. Maksymiuk, J. Sperry, S. Farquharson: In: S. Farquharson, Ed. Applications of Surface- Enhanced Raman Spectroscopy. (CRC press, in preparation)49 D. Kellogg, J. Pemberton: J. Phys. Chem. 91, 1120 (1987)50 J. Billmann, G. Kovacs, A. Otto: Surf. Sci. 92,153 (1980)51 C.A. Murray, S. Bodoff: Phys. Rev. B 32,671 (1985)52 S.D. Christesen: J. Raman Spectrosc. 22, 459 (1991)53 C. Sosa, R.J. Bartlett, K. KuBulat, W.B. Person: J. Phys. Chem. 93, 577 (1993)54 F. Inscore, S. Farquharson: J. Raman Spectrosc. (submitted)55 F. Inscore, S. Farquharson: Proc. SPIE 5993, accepted (2005)56 T. Joo, K. Kim, M. Kim: J. Molec. Struct. 16, 191 (1987)57 H. Hameka, J. Jensen: CRDEC-TR-326 (1992)58 R. Nyquist: J. Molec. Struct. 2:123 (1968)59 B.J. Van der Veken, M.A. Herman: J. Molec. Struct. 15, 225 (1973)60 B.J. Van der Veken, M.A. Herman: J. Molec. Struct. 15, 237 (1973)61 H. Hameka, J. Jensen: ERDEC-TR-065 (1993)
Appendix N Detecting hydrolysis products of blister agents in water by surface-enhanced Raman spectroscopy Frank Inscore and Stuart Farquharson Real-Time Analyzers, Middletown, CT, 06457 ABSTRACTProtecting the nation’s drinking water from terrorism, requires microg/L detection of chemical agents and theirhydrolysis products in less than 10 minutes. In an effort to aid military personnel and the public at large, we have beeninvestigating the ability of surface-enhanced Raman spectroscopy (SERS) to detect microgram per liter (part-per-billion)concentrations of chemical agents in water. It is equally important to detect and distinguish the hydrolysis products ofthese agents to eliminate false-positive responses and evaluate the extent of an attack. Previously, we reported the SERspectra of GA, GB, VX and most of their hydrolysis products. Here we extend these studies to include the chemicalagent sulfur-mustard, also known as HD, and its principle hydrolysis product thiodiglycol. We also report initialcontinuous measurements of thiodiglycol flowing through a SERS-active capillary.Keywords: chemical warfare agent detection, CWA, hydrolysis, SERS, Raman spectroscopy 1. INTRODUCTIONThe July 2005 terrorist bombings of the London transit system are a stark reminder that such attacks on the UnitedKingdom and the United States will continue. Countering such attacks requires recognizing likely deployment scenariosand having the required technology to rapidly detect the deployment event. In addition to the expected use of chemicalagents released into the air, terrorists may also poison water supplies with chemical warfare agents (CWAs). TheNational Strategy for Homeland Security designates the Environmental Protection Agency with the task of securing thenations drinking water.1 Presently, the EPA employs several field test kits to monitor drinking water supplies, and gaschromatography coupled with mass spectrometry in supporting laboratories to confirm positive responses.2Unfortunately, these test kits are prone to false-positive responses, and follow-up analysis typically takes a day. This isentirely inadequate for the prevention of widespread illness and potential fatalities.In the past several years we have been investigating the use of surface-enhanced Raman spectroscopy (SERS) to be usedas a field-usable analyzer that can detect chemical agents in water at the required microg/L sensitivity and 10 minutetimeframe.3,4,5,6,7 The expected success of SERS is based on the million-fold or more Raman signal increase obtainedwhen a molecule interacts with surface plasmon modes of metal nanoparticles.8 In the case of cyanide, an industrial-based CWA and methyl phosphonic acid, the final hydrolysis product for the nerve agents, we have measured at orbelow 10 microg/L in one minute.9 The expected success of SERS is also based on the unique set of Raman spectralpeaks associated with the molecular vibrational modes of each molecule. The unique SER spectra should not onlyreduce false-positive responses, but also allow discriminating hydrolysis products of CWAs. This is important, sinceCWAs can hydrolyze rapidly in the presence of water,10 and detection of the hydrolysis products could allowdetermining 1) the state of an attack (ratio of CWA to hydrolysis product(s)), 2) the point of attack initiation, and 3) thecontinued extent and severity of the CWA attack throughout a water distribution system.Previously, we used SERS to measure sarin, tabun, VX, and EA2192, and their respective hydrolysis products.3,4,6,7 Herewe extend these studies to include the chemical warfare agent sulfur-mustard, designated HD, and its primary hydrolysisproduct thiodiglycol (TDG, Figure 1). The physical and chemical properties of this blister agent are well known. It’ssolubility in water is 0.92 g/L with a hydrolysis half-life of 8.5 min (both at 25 C).10 HD has an oral LD50 of 0.7 mg/kgin humans,11 and the military drinking water guideline places the 5-day 5L limit at 100 microg/L.12 TDG is relativelynon-toxic, very water soluble at 690 g/L, and stable in water with a hydrolysis half-life of approximately 6 days.Accordingly, a reasonable sensitivity goal to ensure safe water is placed at 10 microg/L for HD and an equivalent goal tomap HD usage is placed at 10 microg/L for TDG.13 SPIE-2005-5993 19
H2O + 2HClFigure 1. Hydrolysis of bis(2-chloroethyl)sulfide (HD) to bis(2-hydroxyethyl)sulfide (TDG). 2. EXPERIMENTALHighly distilled sulfur mustard, designated HD (bis(2-chloroethyl)sulfide), was measured at the U.S. Army’s EdgewoodChemical Biological Center (Aberdeen, MD). Thiodiglycol, designated TDG here (bis(2-hydroxyethyl)sulfide), waspurchased as an analytical reference material from Cerilliant (Round Rock, TX). TDG was measured at Real-TimeAnalyzers, Inc. (RTA, Middletown, CT). All solvents, including methanol, ethanol, and HPLC water, as well as all sol-gel precursor chemicals including AgNO3, tetramethyl orthosilicate, methyltrimethoxysilane, HNO3 and NaBH4, werepurchased from Sigma-Aldrich (St. Louis, MO). HD samples prepared for SERS analysis consisted of 0.1% v/v HD inmethanol. The methanol was used to minimize hydrolysis. The final concentration is 1000 parts-per-million (ppm, EPAdefinition). TDG samples were prepared for SERS analysis using methanol for static measurements and HPLC gradewater for flow measurements. All HD measurements were performed in SERS-active vials (Simple SERS Sample Vials,RTA),14 while all TDG measurements were performed in SERS-active capillaries (1-mm diameter glass capillaries filledwith silver-doped sol-gels).15,16 In the case of flow measurements, a peristaltic pump (variable flow mini-pump, ControlCo., Friendswood, TX) was used to flow the 1 and 10 ppm TDG samples through a SERS-active capillary at 1 mL permin.The vials or capillaries were mounted horizontally on an XY positioning stage (Conix Research, Springfield, OR), suchthat the focal point of an f/0.7 aspheric lens was positioned just inside the glass wall. The probe optics and fiber opticinterface have been described previously.16 In all cases a 785 nm diode laser was used to deliver ~100 mW of power tothe SERS samples and 300 mW to the Raman samples. A Fourier transform Raman spectrometer equipped with asilicon photo-avalanche detector (RTA, model IRA-785), was used to collect both the RS and SERS at 8 cm-1 resolution. 3. RESULTS AND DISCUSSIONThe surface-enhanced and normal Raman spectra of HD have been measured and are shown in Figure 2. The SERspectrum is dominated by a peak at 630 cm-1 with an extended high frequency shoulder composed of two or more peaks(695, 830 cm-1), as well as a moderately intense peak at 1045 cm-1. It is possible to assign these peaks based on thenormal Raman spectrum of HD, and previous assignments.17 Theoretical studies assigned the 640, 655, 700 cm-1 peaksto C-Cl stretching modes and the 740, and 760 cm-1 peaks to C-S stretching modes. Additional peaks are observed at1040, 1190, 1270, 1295, 1410, 1425, and 1440 cm-1. The first peak is assigned to a C-C stretch, while the remainingpeaks are all CH2 deformation modes (scissors, twists, and wags). Based on these assignments, then only the C-Cl peakmaintains significant intensity in the SER spectrum occurring at 630 cm-1. If the C-Cl assignments are correct, then theSER spectra suggest that the molecule to metal interaction is strongest through the chlorine end groups. Alternatively,the electron lone pairs of the tetrahedrally coordinated sulfur of HD could interact with the silver surface. Consequently,the 630 cm-1 SERS peak could be assigned to CS or CSC stretching modes (see below).18The surface-enhanced and normal Raman spectra of TDG have been measured and are shown in Figure 3. The SERspectrum is dominated by three peaks at 630, 715, and 1010 cm-1 with minor peaks at 400, 820, 930, 1210, 1275, 1410,and 1460 cm-1. Similarly, the Raman spectrum contains two intense peaks at 660 and 1010 cm-1, while moderatelyintense peaks occur at 400, 680 (shoulder), 735, 770, 830, 950, 1040, 1230, 1290, 1420, and 1465 cm-1. In both spectra,the assignment of the peaks near 1000 cm-1 can be confidently assigned to C-C stretching modes, while the peaks from1200 to 1465 cm-1 can be confidently assigned to various CH2 deformation modes. Here, however, it is difficult toassign the 630 cm-1 SERS peak to a C-Cl mode, since the chlorines have been replaced by hydroxyl groups. SPIE-2005-5993 20