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Gas chromatography in environmental analysis driscoll 2004 l


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This is a paper that I wrote a number of years ago that was published by Wiley in The Handbook of Environmental Analysis

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Gas chromatography in environmental analysis driscoll 2004 l

  1. 1. Environmental Instrumentation Handbook Dr. Jack Driscoll, PID Analyzers, LLC Gas Chromatography in Environmental Analysis1.0 Introduction stationary phase simultaneously at the injector but move along at different rates. The lower the vapor pressure of the This chapter is written from the compound (higher boiling point), the longerperspective of an environmental the compound will remain in the stationaryprofessional. It focuses on those aspects of phase. The time that each compoundGas Chromatography that aid in the remains in the stationary phase depends onselection of instrumentation and columns for two factors: the vapor pressure of theboth field and laboratory methods. In compound and its solubility in the stationaryaddition, it should be a useful resource for phase. These compounds are then detectedanyone who is responsible for interpreting at the end of the column. A plot of the outputdata collected in the field. of the detector response versus time is We start by describing the theory of termed a chromatogram.gas chromatography (GC), then we discuss Elution times may be reduced bythe selection of columns, gas solid and gas increasing the temperature of the GC oven.liquid chromatography, phases, packed and GC’s can be run isothermally (constantcapillary columns. This material is intended temperature) to separate a narrow boilingto provide the reader with sufficient range of solutes. If the separation of low andinformation to select a proper column for high boiling compounds is necessary,analysis of a particular site or a difficult temperature programming (linear increasesample. In the next section, we describe the of column temperature) is used.hardware required for GC. The fourth The Retention time is defined as thesection describes the need for good time measured from the start of injection totemperature control even for field GC’s. the peak maximum and can be used to The GC detectors that are identify resolved components in mixtures.described include the photoionization The times measured as RT1, RT2, RT3detector (PID), the flame ionization detector shown in Fig. 1 would be the retention times(FID), the thermal conductivity detector for components A1-A3. The retention time is(TCD), the electron capture detector (ECD), characteristic for a compound and thethe far UV absorbance detector (FUV) and stationary phase at a given temperature andthe flame photometric detector (FPD). For is used for identification when the mixture ofeach detector, the theory of operation, the compounds is completely resolved. Torange, detection limits and characteristics confirm that a particular component isare described. Individual species can be present requires the identification on twomeasured from ppt to % levels with either a columns with different polarities of stationaryspecific or universal type detector. phases. Some environmental methods allow Finally, we discuss the analysis of confirmation of compound identity byvolatile organic (VOC) and semivolatile comparing both retention times and detector(SVOC) compounds, dual detectors response factors with known standards.(PID/FID), headspace, as well as Instruments that are configured for eitherconcentrators for GC’s that can be used for dual columns with a single detector, or amonitoring low or sub ppb levels of toxic single column with dual detectors (PID/FID)species at the fenceline. can combine analysis and confirmation in a single run.2.0 Gas Chromatography Theory Gas chromatography (GC) is amethod of continuous chemical separationof one or more individual compoundsbetween two phases. One phase remainsfixed (stationary phase); the other, themobile phase (carrier gas), flows over thestationary phase. The components enter theDriscoll, John 1Environmental Instrumentation HandbookWiley (2004) CH 8
  2. 2. Gas Chromatography Chapter 8 compounds in boiling point order. When polar functional groups are added in stationary phase. The Number of theoretical plates is a term taken from chemical engineering originally used to describe the efficiency of a distillation apparatus. This theory was applied to columns in gas chromatography to describe the efficiency (separation ability) of a column. Separation occurs as a result of continuous movement between the stationary phase and the mobile phase Clearly, the larger the number Figure 1. of plates, the greater the resolving power of the column. 2.1 Column Selection- Number of theoretical plates (n) is 2 given by: n = 5.545 (t/w) Where t= retention time; w = peak There are a large number of GC half hightpackings available. Each of these exhibit Number of effective theoreticalspecific retention characteristics for specific plates (Neff) is given by: Neff = 5.545 (t/w)2compounds. Many times, a better separationis obtained more easily by changing the t´= adjusted retention time = t – tm ;liquid phase than by increasing the length of tm = retention time for inert peak likethe column. A properly made capillary methanecolumn of 5M in length will have about Height equivalent to a theoretical plate (h)12,000-15,000 plates effective plates, more is given by:than 100 times the resolving power of a h = L/nshort packed column. Interesting enough, Where L= length of columnwith all of the developments in capillarycolumn technology, at a recent symposium(1), one researcher was still talking about 2.1.1 Gas Solid chromatography-the utility of short, packed columns at GSCambient temperature. With the minimum ofseparating power (efficiency), many peaks Solid packings are generally used tocould still be unresolved under a single separate gases and compounds with boilingpeak. A comparison of packed and capillary points below propane. Polymers which arecolumns is shown in Table I. derivatives of styrene divinyl benzene, cross linked acrylic ester, cross linked polystyrene etc. are small particles with pores and Table I variable surface areas. These porousComparison of Packed and Capillary polymers are available in a variety ofColumns polarities for specific separations of low molecular weight compounds (methane, Parameter Packed Capillary ethane, ethylene, H2S). It would be difficultLength 1.5-6 5-100 to analyze ethylene (gas) and benzene (very(meters) long retention time) on these porousID (mm) 2-4 0.15-0.53 polymers and similarly, it is difficult toFlowrate 10-60 0.5-30 analyze ethylene on a short (5M) capillary(ml/min) column since it would be an unretainedTotal plates 5000 (2 M) 75,000 (would elute very quickly) compound.(length M) (25M) Zeolytes or molecular sieves thatFilm thickness 1-10 0.1-5 employ size exclusion for separation.(µ) Certain molecules that are small enough to enter the pores exist the stationary phase exit the stationary phase later than larger Methyl silicone stationary phase is molecules that cannot enter the poresconsidered non-polar generally elutingDriscoll, John N. 2Environmental InstrumentationWiley ( 2004) Ch 8
  3. 3. Gas Chromatography Chapter 8readily. These phases are commonly used packed or capillary column for a variety offor the separation of permanent gases analytes. The terminology in Table II is the(including O2 and N2). of Ohio Valley Specialities (OV). These silicone phases in order of polarity are least2.1.2 Gas Liquid chromatography- polar (OV1, OV101), medium polarityGLC (OV1701), and most polar (OV275). Their Columns with liquid stationary composition is as follows:phase are generally used to separatecompounds with boiling points above Table IIpropane (C H ). More that 70% of all List of GC Applications and Column 3 8 Phasesseparations in gas chromatography can beaccomplished with a methyl silicone liquid Applications Column Phasesphase (OV1, OV101, SE30). However, there Alcohols Carbowax 20M, OV1701are more than 1000 packed column liquid Aldehydes Carbowax 20M, OV1,phases available attesting to their versatility SE30for specific separations. Amines OV54 Aromatic HC Carbowax 20M2.1.3 Types of column phases Dioxins OV54 Glycols Carbowax 20M, OV1701 The stationary phase is most Halogenated HC OV54, OV1701influential column parameter since it Ketones OV1, OV54determines the final resolution and has an PAHs OV54, OV1701influence on the sample capacity and the PCBs OV54, OV1701analysis time. The most important thing to Pesticidesremember is that “likes dissolve likes". Triazine herbicides OV351, OV225 EPA 608 OV54, OV1701Separate non polar compounds on a non Phenolspolar column and polar compounds on a Free OV1, OV225polar column. In Fig. 2, the range of polarity Acetylated OV54, OV1701of a group of organic compounds is Solvents OV54, OV1701compared with the polarity of differentphases. Many of the phases used in packed columns are also used in capillary columns with much greater effect on the latter. In Fig. 3 is a schematic representation of packed and capillary columns. A comparison of the separation of packed and capillary columns is given in Fig. 4. Note that a significantly larger number of peaks are detected with the capillary column.Fig. 2 Polarity of Compounds andPhasesIn other words, if one has non polarhydrocarbons to separate, use a non polarphase like (SE30, NBW30); with more polarcompounds like alcohols, esters use a polarphase like carbowax, etc. The data in TableII lists the optimum liquid phases on aDriscoll, John N. 3Environmental InstrumentationWiley (2004) Ch 8
  4. 4. Gas Chromatography Chapter 88Fig. 3 Schematic Representation of was a problem but these columns were more inert than stainless steel. Fused silica was also used as column material but it was more difficult to work with a flame than glass and was easily broken. The coatings (stationary phase) on the columns were adsorbed but not bonded. In 1979, Dandeneau (2) described a new type of fused silica capillary column that had a coating of polyimide on the outside which made the column relatively flexible. At the same time, the use of bonded (to the fused silica) stationary phases increased dramatically because of the longer lifetime, inertness, and reduced bleed. This created a surge in the use of capillary columns, particularly in the US. Some years ago, glass lined stainless steel columns were introduced. These again improved the utility of capillary columns, particularly in the field. Capillary columns have a highPacked and Capillary Columns resolution (3000 plates/meter) and vary from 5-100 meters in length. The liquid phases (polar or non-polar) are bonded to the fused silica. The columns can be made of fused silica (coated with polyimide so that they are flexible or stainless steel (lined with fused silica). Column diameters can be 0.53, 0.32, 0.20 or 0.15 mm. Capillary columns can also be packed with porous polymers (bonded to the fused silica) to form highly efficient PLOT columns for separation of low molecular weight compounds or fixed gases. A comparison of columns and their characteristics is given in Fig. 5.Fig. 4 Chromatogram of Packed andCapillary Columns Fig. 5 Comparison of column types2.2 Capillary columns Packed columns have a relativelyCapillary columns were first used in gas high sample capacity (difficult to overloadchromatography during the nineteen sixties. column) because the liquid (stationary)The early columns were long (50 meters), phase coating is quite high compared tonarrow bore, stainless steel or soft (soda capillary columns. With bonded capillarylime) glass tubing. With the latter, breakage columns, the film thickness of the stationaryDriscoll, John N. 4Environmental InstrumentationWiley (2004) Ch 8
  5. 5. Gas Chromatography Chapter 8phase can be controlled. A thin layer of shown in Figure 7. The GC consists of thestationary phase will provide a faster column following components:that is better for high molecular weightcompounds. Here, one has to consider thecapacity factor, k, which is ratio of the time the solute Fig. 7 Schematic of a Gasspends in the stationary and mobile phases. Chromatograph k = (t – tm / tm) = t/tm 3.1 Injector-There are a number of factors that effect the A sample is introduced into thecolumn performance these include: heated injector, where it is Inner diameter (ID)- the smaller the vaporized and carried on to theID, the higher the efficiency and the shorter column via a liquid or gas syringe,the analysis time liquid or gas valve, concentrator, Film thickness-The higher the FT, purge & trap, etc. the greater the capacity; the higher 3.2 Packed Columns- the film thickness, the longer the 1/4”, 1/8” or 1/16” (micropack) 2-3 analysis time; thickness ranges from meters in length- 300-500 plates per meter- 0.1-5um packing material: porous polymer, liquid Length- Increasing the length will phase (1-3%) on diatomite increase resolution, the analysis 3.3 Capillary columns- time and the capacity 0.53, 0.32, 0.20 , 0.15 mm column withThe effect of these parameters is shown in liquid phase bonded to the fused silica;Fig. 6. available in fused silica lined stainless steel with the liquid phase bonded to the silica; efficiency≈ 1000-3000 plates/M with typical length 15-30 M 3.4 Carrier gas- mobile phase that is used to move the components through the column to the detector; note that the high sensitivity detectors (PID, FID, ECD, FUV, FPD) require high purity carrier; the ECD requires that oxygen and water be eliminated (trap is usually required) from the carrier since these species can absorb electrons and effect sensitivity 3.5 Oven-Fig. 6 Effect of Film thickness and Other Isothermal or temperature programmed Parameters on column Performance heated device; the higher the temp. the shorter the retention time; good temperatureIn summary, the selection of a column stability ,+/- 0.1-0.2°C ,is required (seeinvolves a number of tradeoffs and specific section 4)knowledge of the compounds to be 3.6 Detector-analyzed, Produces a response proportional to component that is separated by column.3.0 GC hardware Detectors may include a photoionization detector (PID), flame ionization detectorA schematic of the typical GC hardware is (FID), thermal conductivity detector (TCD), electron capture detector (ECD), flame photometric detector (FPD) or far UV absorbance detector (FUV) 3.7 Amplifier- Receives an output from a detector (typically picoamps for an ionization detector) and 5dbook of Environmental Instruments
  6. 6. Gas Chromatography Chapteramplifies it so that the signal can be In addition to the temperaturedetected by a recorder or integrator dependence of K, the ordinary diffusion3.8 Integrator- takes signal from amplifier coefficient (D) has a temperatureand produces an output (chromatogram) dependency as does the term (td), theand peak height or area (used for lifetime in the stationary phase. The latterquantitation). If we note Fig. 1, The height of can be approximated through the Arrheniusthe peak measured from the baseline to the equation (4).peak maximum and the area which is Retention time is defined as the timedetermined by integrating the area from injection to the peak maximum and canunderneath the peak are proportional to be used to identify resolved components inconcentration. Generally integrators will mixtures. Since the retention times are usedprovide both area and height values. At low to identify the species of interest, a shift inconcentrations with packed columns, peak temperature could lead to the wrong speciesheight may provide a better value. being identified, particularly in a complex mixture. Ambient temperatures, as anyone4.0 Temperature Control knows, are anything but constant.Many of the portable gas chromatographs of If the separation of low and highthe nineteen eighties were typically ambient boiling compounds is required, temperaturetemperature instruments with no programming (linear increase of columntemperature control and short temperature) is needed.chromatographic columns. Even today, The difficulty with temperaturesome of these portable instruments do not control is that it takes power to maintain thehave very good temperature control. The temperature and the higher the temperatureproblem with these instruments was that a the greater the power consumption. Thus, inchange of just a few degrees centigrade in the design of field portable GCs, much ofthe temperature of the column can result in the flexibility is lost if battery operation is thea significant change in the retention time of most important criteria. Alternatives tothe species of interest. The chromatographic internal batteries are generators and bat-separation depends upon solute`s (material teries in vehicles. the GC311 has beenbeing analyzed) partitioning between the designed to operate from generators orstationary and mobile phase. This controls vehicle batteries. Using these alternativethe efficiency or separating power of the methods, for the HNU GC311, one has tocolumn. Temperature control is very make few concessions in performance ofimportant, therefore we have added this the instrument.section. In the nineteen eighties, a number Giddings (3) has developed an of portable GCs were introduced thatexpression for the efficiency or plate height employed temperature control and, for the(H) as follows: first employed capillary columns. Since H = 2D/v + dp + 2R (1-R) vtt resolution is proportional to column lengthwhere: D is the ordinary diffusion considerably better performance can becoefficient; obtained with a 5 meter column than a 0.3 R is the ratio of zone velocity to meter column. The longer the column themobile phase velocity; better the separation. Some of the portable (td) is the lifetime in the stationary GCs maintain temperatures of only 50oC (tophase; minimize power consumption) and are dp is the particle diameter; and limited in the variety of species that can be v is the velocity. analyzed. The partition or distribution constant(K) has a temperature coefficient (related to 5.0 GC DetectorsR) which is given by: The detectors selected for this section include the most popular detectors K = k (e -dH/RT) for field work. We have not included the Where: k is a constant; dH is the mass selective detector (MSD) in thisenthalpy of sorption; R is the ideal gas section.constant; and T is the absolute temperature.Driscoll, John N. 6Environmental InstrumentationWiley (2004) Ch 8
  7. 7. Gas Chromatography Chapter 85.1 PID Fig. 8 Schematic Representation of theIn 1976, the first commercial PID was PIDdescribed by HNU Systems, Inc. (5). Theprocess of ionization which occurs when a In terms of sensitivity, the PID is from 10-photon of sufficient energy is absorbed by a 100 times more sensitive than the FIDmolecule that results in the formation of an making this detector ideal for environmentalion plus and electron: applications. This results from the higher ionization efficiency of the PID. The R + hν ⇒ R+ + e - apparent ionization efficiency of the PID iswhere: approximately 10-4 while that of the FID is R = an ionizable species 10-5. hν= a photon with sufficient energy Some characteristics of the PID areto ionize species R given in Table III. The sensitivity of the PID In the ion chamber, the ions (R+) response with the structure of organicformed by absorption of the UV photons are compounds (6) is given in Table IV.collected by applying a positive potential toaccelerating electrode and measuring the Table IIIcurrent at the collection electrode. A PID PID Characteristicsconsists of an ion chamber, a UV lamp with Sensitivity increases as the carbon numbersufficient energy to ionize organic and increases (carbon counter) For 10.2 eV lamp, responds to carboninorganic compounds, a voltage source for aliphatic compounds > C4, all olefins and allthe accelerating electrode and an amplifier aromaticscapable of measuring down to one picoamp The PID also responds to inorganicfull scale. A schematic of a PID is shown in compounds such as H2S, NH3, Br2, I2, PH3,Fig. 8. A list of ionization potentials is given AsH3, e.g. any compound with an ionizationin the Chapter on Photoionization. potential of < 10.6 eV The PID is a concentration sensitive The PID is more sensitive than the FID; 40 xdetector (sample is not destroyed) where more sensitive for aromatics, 20 times forthe sensitivity is increased as the flowrate is olefins, & 10 times for alkanes > C6reduced. Thus, the sensitivity can be Non destructive detector; other detectorsimproved by operating the PID at lower can be run downstreamflowrates, however, one must have sufficient Only carrier gas (prepurified nitrogen or helium) is required for operationflow to sweep the sample through the PID. Concentration sensitive detector CPID ∝ 1/Fwhere: C= concentration F= flowrate of the carrier gas Table IV PID Sensitivity for Organic Compounds Sensitivity increases as carbon number increases For n-alkanes, SM= 0.715n-0.457 where SM = molar sensitivity relative to benzene (benzene= 1.0) and n = carbon number Sensitivity for alkanes < alkenes < aromatics Sensitivity for alkanes < alcohols ≤ esters < aldehydes < ketones Sensitivity for cyclic compounds > non cyclic compounds Sensitivity for branched compounds > non branched compounds Sensitivity for fluorine substituted < chlorine substituted < bromine substituted < iodine substitutedDriscoll, John N. 7Environmental InstrumentationWiley (2004) Ch 8
  8. 8. Gas Chromatography Chapter 8For substituted benzenes, ring activators The FID is a mass sensitive(electron releasing groups) increase detector, the output of which is directlysensitivity proportional to the ratio of the compound’sFor substituted benzenes, ring deactivators ( carbon mass to the total compound mass.electron withdrawing groups) decrease Thus, the sample is destroyed in the flame.sensitivity (exception: halogenated benzenes) Some characteristics of the FID are as listed in Table VI. For soil and water samples thatinvolve solvent extraction, there are anumber of solvents that can be used to 5.3 ECDproduce a small or negative response withthe PID. These are shown in Table V. The The ECD consists of anadvantage of these solvents is that many of accelerating and collection electrode as wellthe volatile hydrocarbons can still be as a radioactive source. The source, 63Ni, isdetected since the solvent peak is like an a beta (electron) emitter and produces aunretained compound and elutes very high background level of free electrons inquickly. The FID, for example, does not the carrier gas. Any compounds that enterrespond to carbon disulfide. This can be the detector which are electron absorbingused for a similar purpose but a hood will beneeded to minimize odor problems. Table VI FID Characteristics Table V Sensitivity increases as the carbon number PID Response with Various Solvents increases (carbon counter) Sensitivity to substituted species depends on Solvent Ionization Response the mass of carbon present and the ability to Potential (eV) break the carbon bondsWater 12.35 Negative The FID is most sensitive to hydrocarbons peak Detector is destructive since sample isMethanol 10.85 Negative burned peak Requires the use of zero grade (high purity)Chloroform 11.42 Negative hydrogen and air to produce the flame peakCarbon 11.47 Negativetetrachloride peak reduce the background level of freeAcetonitrile 12.22 Negative peak electrons and there is a resultant drop in thePentane 10.35 Small positive current which is measured by an peak electrometer. The newer type of electronics (pulsed constant current) have improved the performance dramatically increasing the5.2 FID linear response from 102 to >105. With no The process of ionization which sample, the pulse frequency is low. Whenoccurs in organic compounds when the electron absorbing compound passescarbon- carbon bond is broken via a thermal through the detector, the frequencyprocess in the flame that results in the increases to compensate the current loss toformation of carbon ions. These ions are the sample. The concentration is thencollected in the flame by applying a positive proportional to the pulse frequency.potential to the FID jet and the ions are Earlier ECD’s (with DC electronics)pushed to the collection electrode where the had problems with saturation of the currentcurrent is measured. The response (current) and subsequent reduction of the linearis proportional to the concentration and is range of the detector to just over 100. Themeasured with an electrometer/amplifier. An most sensitive compounds for this areFID consists of a combustion/ion chamber, a chlorinated hydrocarbons which haveflame, a voltage source for the accelerating sensitivities as low as 0.1 ppb of lindane.electrode (usually applied to the jet) and anamplifier capable of measuring down to one 5.4 TCDto five picoamperes full scale.Driscoll, John N. 8Environmental InstrumentationWiley (2004) Ch 8
  9. 9. Gas Chromatography Chapter 8 Measures difference between the detector uses rare earth filters instead ofthermal transfer characteristics of the gas interference filters for S & P to improveand a reference gas, generally helium but detection limits and eliminate some of thehydrogen or nitrogen can be used deficiencies of interference filters (7).depending on the application. The sample Detection limits in the 5 pg and 20 pg rangeand reference filaments are two legs of a for P and S respectively.Wheatstone Bridge. A constant current isapplied with a resultant in a rise in filament 5.6 FUVtemperature. As the sample passes through Most organic and inorganic speciesthe detector, the resistance changes as the absorb strongly in the far UV. Notablereference gas is replaced by the sample exceptions are the inert gases, helium andwhich has a lower thermal conductivity. The nitrogen which absorb very weakly in thisthermal conductivities for a number of region. Certain diatomic species such as O2compounds are given in Table VII. This which have low absorption in the region ofdifference in the lamp energy (124 nm) will have a poor response but low ppm levels can still be Table VII detected.Thermal Conductivities for Selected The far UV detector is relatively newCompounds to gas chromatography (compared to other GC detectors) since it was introduced by Component Thermal HNU Systems in 1984. It is frequently Conductivity* compared with the thermal conductivity Acetylene 0.78 detector since it will respond to any Ammonia 0.90 compound that absorbs in the far or vacuum Butane 0.68 UV. The latter name is a misnomer since Carbon 0.55 with a carrier gas flowing through the cell, a dioxide Chlorine 0.32 vacuum is not needed. Thus, the detector Ethane 0.75 has a response that is nearly universal, a helium 5.97 low dead volume (40 µl), and a fast Hydrogen 7.15 electrometer time constant. The primary Methane 1.25 emission from this lamp is the 124 mn line. Sulfur 0.35 Although there are visible lines from this dioxide lamp, the photodiode is unresponsive to any Xenon 0.21 long wavelength UV or visible emissions * relative to air 0°C and only the absorption at 124 nm needs to be considered (8) for the absorption process.resistance is proportional to the The minimum detection limits forconcentration. The response is universal organic compounds, oxygen, water, andsince the detector responds to any inorganic compounds are in the range fromcompound that conducts heat. The minimum 0.1 to 10 ppm. A summary of the detectiondetection limit is in the 100-200 ppm. The limits for organic and inorganic compoundsmaximum concentration is 100%. is given in Table VIII.5.5 FPD Table VIII The sample is burned in a hydrogen Detection Limits for the FUV Detectorrich flame which excites sulfur or Compound Detection Limitphosphorus to a low lying electronic level. (ng)This is followed by a resultant relaxation to Sulfur dioxide 0.7the ground state with a corresponding Methane 0.3emission of a blue (S) or green (P) photon. Oxygen 14 Water 3This type of emission is termed Propane 1chemiluminescence. The emission is at 394 Chloroform 5nm for Sulfur and 525 nm for phosphorus. Ethylene 1The S:C selectivity ratio is > 10,000:1. ThisDriscoll, John N. 9Environmental InstrumentationWiley (2004) Ch 8
  10. 10. Gas Chromatography Chapter 8 Hydrogen sulfide 3 sending samples to a laboratory for detailed analysis. It is interesting to note that > 50% of the samples returned to the lab during the A summary of the response and 1980s for the EPA CLP program were norange of the various detectors is shown in detects. This demonstrates just howTable IX. Note that the response ranges important field methods actually are. Thefrom universal (TCD) to selective (FPD for S intermediate Level II analysis was& P) while the detectors span a of a introduced by EPA in order to reduce bothconcentration range of a billion. the time required to start remedial actions and the high costs associated with laboratory analysis. An additional factor was the cost of keeping trained personnel in the field waiting for results (9). Level II measurements involve field analysis with more sophisticated instrumentation (i.e., Table IX portable GC or a GC in a laboratory GC in aSummary of Detector Characteristics trailer) to provide identification (as far as possible) of specific components present.Type Response Carrier Gas Range The final three levels (Levels III-V) PID organic, Nitrogen*, 2 ppb to low % use laboratories located "off site" and inorganic helium*, frequently involve CLP analysis (9). We will hydrogen* FID Organic Nitrogen*, 100 ppb to % not be concerned with these latter helium*, techniques. Of course, a certain percentage hydrogen* of field samples should be returned andFUV Organic, Nitrogen*, 0.1 ppm to low % analyzed by laboratory results with standard inorganic, helium*, fixed hydrogen EPA methods. Semivolatile hydrocarbonsECD Halogenate Argon- 0.1 ppb to 1 ppm do not migrate but may have to be removed d, nitro methane*, as a result of their proximity to a source of cpds. helium, drinking water. The two most serious nitrogen* threats from the volatiles involveTCD Organic, Helium, 200 ppm to 100% inorganic, hydrogen evaporation into the air and migration away fixed from the original source of contaminationFPD sulfur, Nitrogen*, 25 ppb-100 ppm through the soil and into a source of phosphorus helium*, groundwater. Remediation of the hydrogen groundwater to EPA levels may take years.* high purity During the nineteen seventies and eighties, the passage of the Resource, Conservation and Recovery Act (RCRA)6.0 Discussion and Comprehensive Environmental The framework of the EPA Response, Compensation and Recovery Actmethodology involves five levels of (CERCLA or Superfund) expanded the list ofinvestigative screening or analyses. The first chemicals under EPA regulation. This led tolevel (Level I) involves field screening of the development of field screening methodsVOC’s with hand held analyzers (EPA for volatile organics to augment the CLPprotocol specifies a photoionization detector program (10). The portable GC was one oflike HNU Model PI or DL101) and other site the stars of EPAs field screening programscharacterization equipment such as an for the analysis of volatiles (11). In 1988,oxygen meter, explosimeter, radiation EPA published a "Field Methodssurvey equipment and chemical testing Catalogue" (10) that described simplifiedtubes (9). This type of measurement is methods for volatile and semivolatile hydro-described in the Chapter on Photoionization carbons, which had been used for fieldnot here. screening. It is clear that a portable GC or a Level II screening can establish the compact GC for an on-site trailer best meetidentity of the compound(s) and relative the needs for field measurement. A portableconcentrations. In the early to mid nineteen GC, the HNU Model GC311 is shown in Fig.eighties, this was done predominantly by 9 and a compact GC, the HNU Model 321 isDriscoll, John N. 10Handbook of Environmental InstrumentationWiley (2004) Ch 8
  11. 11. Gas Chromatography Chapter 8shown in Fig. 10. Both are capable of VOCs or SVOCs) are used to change theanalyzing volatile and semivolatile environment of the sample for analysis byhydrocarbons. GC. 6.1.2 Headspace In order to measure VOC’s with good precision and accuracy, the sample has to be in a dilute (ppm level or below) solution. Henry’s law applies as long as solute molecules never interact significantly, because then the escaping tendency is strictly proportional to the number of solute molecules in the in the fixed amount of solvent. The measurement of low concentrations of organics in water can be accomplished through the application of Henry’s Law which states that, at Fig. 9 Photo of HNU GC311 equilibrium, the solubility of a gas in a liquid Fig. 10 Photo of HNU GC321 is proportional to the partial pressure of a gas in contact with a liquid as given below: VOC (aq) ⇔ K PVOC where VOC (aq) is the concentration of benzene in the liquid phase, K is the Henry’s Law constant which governs the solubility of gases in water, and PVOC is the partial pressure of benzene in the gas phase. As a result of the above equation, it can be seen that if the concentration of benzene in the gas phase and at equilibrium is6.1 Sampling measured, this is related to the concentration of benzene in the dilute 6.1.1 Air, water, soil aqueous solution by a proportionality constant (K) that can be determined by Volatile hydrocarbons can present in a variety of matrices in field Simple headspace measurementssamples including: air, water, soil, soil gas, can be made by equilibrating the liquid orsludge, etc. Of course, the air samples can soil sample in a sealed container (jar, VOAbe analyzed directly by manually injecting a vial, or plastic bag) with a small headspace.1 or 5 cc of air into the GC. Many of the Stewart and collogues have developed theportable GCs have an automatic mode Static Headspace Method so that it provideswhere the air is injected into the GC at a a useful and reproducible methodology forfixed interval. Water or soil samples can not field measurements. This is described in abe measured by directly injecting into the following section.GC since the former would quickly overloadthe column and the possible the detector.Instead, methods such as headspace, purge 6.1.3 Soil gasand trap (volatiles), or solvent extraction (forDriscoll, John N. 11Environmental InstrumentationWiley (2004) Ch 8
  12. 12. Gas Chromatography Chapter 8 Although headspace analyses (13) In Europe and other parts of theare common for volatile hydrocarbons, world, the purge and trap method has notone of the most commonly used field been accepted for the analysis of volatiles.analysis technique for site characterization Instead, solvent extraction is the method ofis soil gas analysis (13) where the sample is choice. The sample can be water or soil andcollected by in-situ pumping of a well. an organic solvent is used the extract theThese wells are relatively inexpensive to drill trace organic compounds from the sample.and can be surveyed rapidly (as many as Then the solvent can be injected into the35-50 per day). This is a useful procedure to heated injection port of the GC.quickly evaluate the extent and source Field methods for the extraction and(since specific pollutants can be identified analysis of volatiles and semivolatileswith the GC) of contamination for a site. (pesticides, PCBs, and PAHs) have beenWith an HNU GC311, that has a built in described in detail previously (10). Providedsampling system, the sample stream can be that the GC has sufficient versatility, all ofsampled and analyzed directly. these samples can be analyzed with the 6.1.4 Carbon Bed same instrument. GC oven temperature The GC is an ideal device for control at temperatures between 150-200 °Cmonitoring the output of a carbon bed. and a heated injection port are required forThese devices are used to remove the analysis of the semivolatiles.residual hydrocarbons from the air pumped In Table X, we compare theinto the soil and pulled our (pump and treat detection limits for soil and water samplesmethod). EPA requires a monitor on the for extraction and headspace methods (14).output of these devices. To obtain a fasterresponse, the GC column can be replaced Table Xwith a piece of 1/8" or 1/16" tubing. Now the Detection Limits for Soil and Waterinstrument will be a monitor for total VOCs. Samples by GC6.2 Extraction Methods Method H C HC GC GC detection limit PID conc. conc. detecti In Inj. on 6.2.1 Purge & trap- soil/wa Into limit This method is for VOCs which are ter GC FIDnot very soluble in water. This technique extract Headspac 10 ppm 10 ng2 0.1 ng 0.005 ngwas adopted by EPA () for water analysis e1and is the basis for most of the water Solvent 1 1 ng 0.1 ng 0.005 ngmethodology. A 15 mL sample is purged extraction µg/ml.(10-15 minutes) with clean nitrogen or Static < 0.5 < 10 ppbhelium to sweep the VOCs out of the water headspace ppmsample. The VOCs in the nitrogen are - soil Static < 50 < 1 ppbcollected on a tenax trap which absorbs the headspace ppbhydrocarbons. Once the purging is -watercomplete, the tenax trap is rapidly heated 1. 1 g. of soil or water in 100 cc container, heatedand the sample is injected onto the GC mildly and cooled- assume aromatic HCcolumn for analysis. This method can be 2. assume 1 cc gas sample injected 3. 4 g. in 25 cc DI water , 50 µL headspace injectedused for water and soil samples and willdetect low ppb concentrations of manyhydrocarbons. 6.2.2 Solvent extraction- 6.2.3 Static headspaceDriscoll, John N. 12Environmental InstrumentationWiley (2004) Ch 8
  13. 13. Gas Chromatography Chapter 8 This field method (static headspace)will allow the analyst in the field to rapidlyscreen the soil or ground water samples andif a no detect is found, another sample canbe taken and analyzed. One obviousadvantage of this technique is that theequipment needed is minimal compared tothe purge and trap technique yet Robbinsand Stuart (14) have shown that comparableresults can be obtained with detection limitsof the order of 1 ppb. This method was developed byRobbins and Stuart (15) at the University of Connecticut for the extraction of low levels of volatile organics from water. A 4 g. soil 6.3 VOCssample is added to 25 ml of water in a 40ml VOA vial and 100 µL of mercuric Typically, at a site, the GC can chloride (2.4 g/L) was added as a initially be used for industrial hygienepreservative. Each vial was shaken for 10 surveys to evaluate the level of toxic VOCsseconds, inverted and placed in a water and implement a plan to protect the workers.bath for thirty minutes at 25 +/- 0.3 C to Then it can be used for soil gas surveys,reach thermal equilbrium. A 50 uL gas and checking contaminated soil and water.sample is injected into the GC. An example .An example of a sample containing BTX byof the comparison between static PID is shown in Fig.12.headspace and purge and trap for benzeneand toluene is shown in Table XI. The 2correlation coefficients (r ) for the static 35000headspace and purge and trap data in 7Table XI was 0.999 for benzene and 0.89 for 30000 8toluene. A chromatogram of VOCs in water 25000 11is shown in Fig. 11. 20000 13 4 Table XI 6 15000Comparison of Static Headspace & Purge 16& Trap Techniques 10000 15 9 14 1 17 5000 2 10 Method Benzene Toluene 3 12 Static HS 17.3 61.8 0 5 min Purge & Trap 12.7 40.6 0:00 2:36 5:12 7:48 10:24 13:00 Static HS 1144 4320 Purge & Trap 709 2170 Static HS 496 3180 Fig. 12 Purge & Trap 330 2800 Air Sample of BTX via PID Static HS 21.2 ND Purge & Trap 18.0 ND Several years ago, some Static HS 10.5 ND underground gasoline tanks ruptured in Fal- Purge & Trap 8.1 ND mouth, MA. Since the soil is sandy, the contents of the tanks spread quickly over a considerable area. Initially, the site was investigated using a portable PID (HNU Model 101) by measurements in a number of soil gas wells to determine the extent of the plume. The plume had migrated more Fig. 11 than 300 yards from the original source. Chromatogram of VOC s in Water This type of Level I screening could be used to determine the extent of contamination ofDriscoll, John N. 13Environmental InstrumentationWiley (2004) Ch 8
  14. 14. Gas Chromatography Chapterthe soil ("total" but not individualhydrocarbons) and groundwater whichoccurred. Following this, a portable GC(GC311) was used to characterize the composition of the fuel detected and theultimate source of the contamination. Whenmultiple sources are present, thisfingerprinting data can be used to identifythe source of a leak.6.4 SVOCs Semivolatiles including pesticides,herbicides, polychloro biphenyls (PCBs),and polyaromatic hydrocarbons (PAHs) doaccount for nearly 30% of the field samples.Field methods for sample preparation andanalysis have been described (10). Somechromatograms of PCBs and PAHs aregiven in Fig. 13 and 14. One field method (10) involvestaking an 800 mg soil sample or 10 ml watersample, add 1 cc of a 1:4 water methanolmixture, the add 1 ml of hexane, shake for30 seconds then let stand for 30 seconds (ifthe mixture emulsifies, then centrifuge thesample) and inject the top layer (hexane)into the GC. This technique is useful forextraction of PAHs, polychlorbiphenyls(PCBs) and other non volatilehydrocarbons. This method was modified (16) andused for the determination of DDT in soil. AGC311 with a PID (10.2 eV) was employedfor the field analysis. This site was one if thebest examples of the need for a field methodology and detectors were bothmethod of analysis. The site had been different.visited two times previously and samples A useful detector combination forhad been sent back to a laboratory for sample confirmation is the PID and ECD.analysis. Although this was the third time in Detector response ratios are used to providethe field, new areas of contamination were additional confirmation of the presence ordiscovered that had not been encountered the structure of a particular compound in apreviously (16). Forty four samples were peak. For example, trichlorobenzene wouldcollected and analyzed in a three day time be expected to have a strong response onperiod and enough information was both the PID and ECD while anothergathered to finally cleanup the site. Nearly a compound with same retention time wouldyear had passed since the first visit to the produce a very different response ratio. Assite and if this doesnt demonstrate the need observed above, the need of field screeningfor good field methodology for volatiles, is obvious even for semivolatiles. Driscollnothing does. Excellent agreement was and Atwood (17) evaluated the 8000 series of EPA methods and found that essentially Fig. 13 Chromatograms of PCB s all of the methods including phenols, pesticides, herbicides, nitrosamines, PAHs, nitroaromatics, PCBs, and phthalate estersobserved between the field (PID) and can be analyzed by GC using a PID andlaboratory (ECD) methods even though theDriscoll, John N. 14Environmental InstrumentationWiley (2004) Ch 8
  15. 15. Gas Chromatography Chapter 8ECD individually or in common. A typical magnitude lower if an aliphatic hydrocarbonchromatogram for some of the semivolatiles is present in place of the aromaticis shown in Fig. 11. hydrocarbon making the identification process relatively easy. Dual detectors have been used in6.5 Dual detectors the laboratory for many years to analyze Dual Detectors are an important difficult "unknown" environmental samples.consideration for field analyses because The PID has interchangeable lamps and thethey are the minimum needed for 11.7 eV lamp can detect the low molecularconfirmation of a particular compound. With weight chlorinated hydrocarbons , which areGC, one has to run a sample on two so prevalent in wells and groundwater. Thecolumns of different polarity (e.g. polar and PID with a 10.2 eV lamp is used fornon polar) to confirm the identity of a hydrocarbons aromatic, olefinic, andparticular compound. This is not necessarily alkanes >butane.something that should be done in the field. The FUV detector has a moreInstead, it is more useful to identify general response and is very useful forcompounds by comparing both retention landfills since it responds to CH4, CO & CO2.times and detector response factors with None of these compounds respond with theknown standards. This is the basis of a PID. The detector is also useful for thenumber of environmental methods. detection of low molecular weight Gasoline hydrocarbons are one of chloroalkanes which are not detected by thethe most common contaminants found in the PID (10.2 eV). These latter species are quitefield. PID and FID response ratios (18) can common on hazardous waste used to identify alkanes (PID/FID ratiosof 8-10), alkenes (PID/FID ratios of 18-24) 6.6 Site or Fenceline Monitoringand aromatics (PID/FID ratios of 40-50) incomplex mixtures (18). When working with air samples at For groundwater applications in the the ppb levels, severe errors can be intro-vicinity of gasoline stations, it is necessary duced by carryover from the teflon in theto measure levels of aromatic hydrocarbons syringe. With an unskilled analyst precisionin the presence of gasoline or fuel oil as poor as 20-30% would not be unusual.contaminated samples. EPA method 602 or The latter technique does not depend on the8020, does not have adequate selectivity for operator since it is automatic. This samplethis particular analysis since high molecular introduction mode can be used for air,weight alkanes can coelute with the headspace (soil, water, sludge), and soilaromatic hydrocarbons resulting in an gas. The precision at low ppm levels is +/- 1-interference. The approach we took (18) 2 %; at ppb levels +/- 5-10 %. Thefollowing purge and trap involved the use of instrument can be run in a continuous modea highly polar capillary column (carbowax or one sample at a time. Automatic calibra-initially but then DB5 because of the tion at a specified time interval can beimproved long term stability) which would programmed if an area is to be monitoredelute the non polar alkanes quickly (and in over a period of broad peak) while providing adequate During the remediation process,resolution for the aromatic hydrocarbons, pockets of pollution can be stirred up andparticularly the xylenes.. An added feature VOCs and semivolatile hydrocarbons canof this method is that alkanes and aromatics be released to the atmosphere. Since manycan be quantitated, if desired. One of these sites are in urban areas, it isadvantage of this technique is that we can important to continuously monitor theidentify interferences from aliphatic fenceline to minimize the exposure ofhydrocarbons in the determination of of surrounding neighbors to these pollutants.aromatic hydrocarbons from the differences A concentrator was describedin their relative responses on the PID and previously (20) which is available as anFID (19). For example, it is possible for C14 option for the GC311. This system allowedor C15 (from fuel oil) hydrocarbons to the detection of ppt levels of aromaticcoelute with the aromatic hydrocarbons. The hydrocarbons in the atmosphere on anPID/FID response ratio will be an order of automatic basis. A typical chromatogram ofDriscoll, John N. 15Environmental InstrumentationWiley (2004) Ch 8
  16. 16. Gas Chromatography Chapter 8an ambient air sample is shown in Fig. 15. Detector for Gas chromatography," Res.This system improves chromatography by & Dev. , 27, 50 (1976)eliminating any 6. Langhorst, M.L., "Photoionization detector Sensitivity for Organic Fig. 15 Compounds," J. Chrom. Sci., 19,98Ambient Air Analysis with a Concentrator (1980) 7. Driscoll, J. N. & A. W. Berger, " A New air or water peaks which would interfere FPD with Rare Earth Filters," J.with the early eluting peaks at low ppb Chromatogr. (1989)levels. The concentrator also improves the 8. Driscoll, J.N., M. Duffy, S. Pappas, andperformance of the FUVAD as shown in Fig. M. Sanford, "Analysis of ppb Levels of6. This detector is useful down to 0.1 ppm Organics in Water by Means of Purgewithout preconcentration but the and Trap Capillary GC and Selectiveinterference from both water and oxygen is Detectors", J. Chromatog., 441, 73 (Mayvery significant since both these species 1988)absorb uv and thus produce a detector 9. Anon., "Data Quality Objectives forresponse. The material in the concentrator Remedial Response Activities," USis hydrophillic and the water can be swept EPA, Development Process, PB88-through without any loss of volatiles. 131370. March 1987Applications for this accessory include 10. Anon., "Field Screening Methodsfenceline monitoring, background soil Catalogue (Users Guide,checks, following the emissions from or to a EPA/540/2/88/005)" Hazardous siteparticular source, checking carrier gases for Evaluation Div., USEPS Office ofcontamination, and any applications where Emergency and remedial Response,additional sensitivity is required. Washington, DC, 1988 In this chapter we have described a 11. Fribush, H., and J. Fisk, "Survey of U.S.number of basic aspects of chromatography EPA Regional Field Analytical Needs,"in order to provide the reader with a American Laboratory, Oct. 1990, p 29reasonable understanding of both field andlaboratory methods. Should the reader be 12. Driscoll, J. N., J. Hanby, and J. Pennaro, "Review of Field Screeninginterested in learning more about Methodology for Analysis of Soil," ppchromatography or detectors, the books in 153-172 in Hydrocarbon Contaminatedreferences 3 or 19 would be recommended. Soils ,P. Kostecki, E. Calabrese, and M. Bonazountas edited, Lewis Publishing, Ann Arbor, MI (1992) 13. Anon, "Field Measurements, Dependable Data When You Need It," US EPA, EPA/530/UST-90-003, Sept. 1990 14. Driscoll, J. N., "Review of Field Screening Methods for the Analysis of References Hydrocarbons in Soils and Groundwater", Int. Labmate., pp 27-32,1. Symposium on Air Quality Oct. 1992 Measurement, MIT (Oct. 1993) 15. Roe, V., M.J. Lucy, and J.D. Stuart2. Dandeneau, R., and E.H. Zerenner, J. "Manual Headspace Method to analyze HRC & CC, 2, 351, 1979 for the Volatile Aromatics in Gasoline in3. Giddings, J. C., Theory of Groundwater and Soil Samples," Anal. Chromatography in "Chromatography: A Chem., 61, 2584 (1989) Laboratory Handbook" Heftmann Ed., 16. Driscoll, J. N., M. Whelan, C. Woods, M. Van Nostrand Reinhold, NY (1975) Duffy, and M. Cihak, "Looking for4. Daniels, F., & R.A. Alberty, "Physcial Pesticides," Soils, Jan./Feb. 1992, pp Chemistry", Wiley, NY, NY (1965) 12-155. Driscoll, J.N. , & F.F. Spaziani, " 17. Driscoll, J. N., and E.S. Atwood, "Use of Development of a New Photoionization Selective Detectors for Field Analysis ofDriscoll, John N. 16Handbook of Environmental InstrumentsWiley (to be published 2004) Copyright ©1999
  17. 17. Gas Chromatography Chapter 8 Semivolatiles in Soil and Water", J. Table V- PID Response with Various Chromatog. (July 1993) Solvents18. Driscoll, J. N. , J. Ford, and E.T. Gruber, Table VI- FID Characteristics " Gas Chromatographic Detection and Table VII- Thermal Conductivities for Identification of Aromatic and Aliphatic Selected Compounds Hydrocarbons in complex Mixtures,", J. Table VIII- Detection Limits for the FUV Chromatog., 158, p 171, 1978 Detector19. Driscoll, J.N., "Far UV Inoization Table IX- Summary of Detector (Photoionization ) and Absorbance Characteristics Detectors," in Detectors for Capillary Table X- Detection Limits for Soil and Water Chromatography, ed by H. Hill & D.G. Samples by GC Mc Minn, Wiley & Sons., NY (1992) Table XI- Comparison of Static Headspace20. Whalen, M., J. N. Driscoll, J. N., and C. & Purge & Trap Techniques Wood, ”Detection of Aromatic Hydrocarbons in the Atmosphere at PPT Keywords Levels,” Atmospheric Environment, 28, Gas chromatography, photoionization, PID, 567 (1993) flame ionization, FID, thermal conductivity, TCD, electron capture, ECD, far UV absorbance, FUV, flame photometric, FPD, theoretical plates, stationary phase, solid support, packed columns, capillary columns, detector, air,soil, water, solvents, detector characteristics, accuracy, precision, environmental, sampling, sample prep, detection limit, resolution, fenceline, soil List of Figures gas, groundwater, EPA, contaminated, CLP,Fig. 1 Retention Times concentrator, Level II, purge & trap, staticFig. 2 Polarity of Compounds and Phases headspace, headsapce, extraction, carrierFig. 3 Schematic Representation of Packed gas, chlorinated hydrocarbons, organic,and Capillary Columns inorganic, sulfur, IP, packings, liquid phase,Fig. 4 Chromatogram of Packed and peaks, GSC, GLC, polarity, plates, capacity,Capillary Columns partitioning, diffusion, lab and field methods,Fig. 5 Comparison of column types temperature, dual detector, VOC, SVOC,Fig. 6 Effect of Film thickness and Other phases, mobile phase, elution, retentionParameters on column Performance time, peak height, peak area, resolved,Fig. 7 Schematic of a Gas Chromatograph confirmationFig. 8 Schematic Representation of the PIDFig. 9 Photo of HNU GC311Fig. 10 Photo of HNU GC321Fig. 11 Chromatogram of VOC s in WaterFig. 12 Air Sample of BTX via PIDFig. 13 Chromatograms of PCB sFig. 14 Chromatogram of PAHsFig. 15 Ambient Air Analysis with aConcentrator List of TablesTable I- Comparison of Packed andCapillary columnsTable II- List of GC Applications and ColumnPhasesTable III- PID CharacteristicsTable IV- PID Sensitivity for OrganicCompoundsDriscoll, John N. 17Environmental Instrumentation & Analysis HandbookWiley, Interscience, NY (2004) Ch 8