Advances in atomic spectroscopy, volume 3


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Advances in atomic spectroscopy, volume 3

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  3. 3. ADVANCES IN ATOMIC SPECTROSCOPY Editor: JOSEPH SNEDDON Department of Chemistry McNeese State University Lake Charles, Louisiana VOLUME3 9 1997 Greenwich, Connecticut ~~~~ JAiPRESSINC. London, England
  4. 4. Copyright 91997 by JAI PRESSINC. 55 Old PostRoad, No. 2 Greenwich, Connecticut 06836 JAI PRESSLTD. 38 TavistockStreet Covent Garden London WC2E7PB England All rights reserved. No part of this publication may be reproduced, stored on a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photocopying, filming, recording, or otherwise, without prior permission in writing from the publisher. ISBN: 0-7623-0072-8 155N: 1068-5561 Manufactured in the United Statesof America
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  7. 7. LIST OF CONTRIBUTORS Rudi Avni Isaac B. Brenner Les Ebdon Kimberly S. Farah Andrew S. Fisher Joseph Sneddon Nuclear ResearchCenter-Negev Beer-Sheva, Israel Geochemistry Division Geological Surveyof Israel Jerusalem, Israel Department of Environmental Sciences University of Plymouth Plymouth, England Department of Science Lasell College Newton, Massachusetts Department of Environmental Sciences University of Plymouth Plymouth, England Department of Chemistry McNeese StateUniversity LakeCharles, Louisiana vii
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  9. 9. PREFACE Volume 3 of Advances in Atomic Spectroscopy continues to present cutting edge reviews and articles in atomic spectroscopy as did the previous two volumes in this series. Chapter 1 of this volume is devoted to plasma source mass spectroscopy, in particular inductively coupled plasma mass spectrometry. This was proposed in the early 1980s and has been commercially available since the mid-1980s. It has been suggested that it will be the dominant force for trace and ultatrace metal determina- tion in the coming years. This chapter describes the basic theory, instrumentation, sample introduction techniques, and selected applications. Chapter 2 covers simultaneous multielement atomic absorption spectrometry, mostly with graphite furnace atomization but, where appropriate, with flame atomization. Atomic absorption spectrometry has been around since the early to mid- 1950s and is a well-established and accepted technique for trace and ultratrace determination of elements. However, it is primarily regarded as a single-element technique. The need to perform simultaneous multielement analyses became a need and a desire in the early 1970s (the resurgence of atomic emission spectrometry with the inductively coupled plasma at this time). Atomic absorption spectrometry was slow to respond to the challenge of multielement analyses, with most work from the early 1970s to the late 1980s using laboratory-constructed or modified systems. However, since the late 1980s through to the present time, simultaneous multielement atomic absorption spectrometry has attracted interest. This chapter
  10. 10. x PREFACE describes instrumentation and applications of simultaneous multielement atomic absorption spectrometry. Chapter 3 describes the direct current arc and plasma jets. Direct current arc and plasma spectrometry has been around for a number of years but is still an integral and indispensable method for determining metals in solids and liquids in many laboratories. This chapter describes the arc and plasma, and the physical and chemical interferences of the sample and its trace elemental constituents in the direct current discharge and their correlation with spectral line intensities of each trace element. The authors describe their experiences in the determination of trace elements in refractory-type samples such as uranium, thorium, and plutonium oxides, rare earth oxides, rock phosphates, silicate rocks, aluminum and titanium oxides, and molybdenum and tungsten oxides. Chapter 4 describes basic principles, design, instrumentation, evaluation, char- acterization, and selected applications of the use of a single-stage impactor com- bined with a graphite furnace for the direct collection ofmetals in air and subsequent determination by atomic spectroscopic methods, primarily atomic absorption spec- trometry. The advantage of this type of system is the ability to determine low concentrations of metals, in the ng/m3range, within a few minutes. Joseph Sneddon Editor
  11. 11. PLASMA SOURCE MASS SPECTROSCOPY Andrew S. Fisher and Les Ebdon Abstract ...................................... 2 I. Introduction and Basic Theory .......................... 2 A. Interferences .................................. 4 B. Isotope Ratio Analysis ............................. 7 II. Instrumentation .................................. 8 A. Sample Introduction Systems ........................ 8 B. The Plasma .................................. 11 C. The Interface Region ............................. 12 D. The Ion Lenses ................................ 14 E. Mass Analyzer ................................ 15 F. Electron Multiplier .............................. 15 G. Vacuum Pumps ................................ 16 III. Sample Introduction Techniques ......................... 16 A. Laser Ablation ................................ 17 B. Electrothermal Vaporization ......................... 18 C. Slurry Nebulization .............................. 19 D. Flow Injection ................................ 20 E. Chromatography ............................... 22 F. Hydride Generation .............................. 23 Advances in Atomic Spectroscopy Volume 3, pages 1-31 Copyright 9 1997 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0072-8
  12. 12. 2 ANDREW S. FISHERand LESEBDON IV. Applications ................... ................ 25 V. Conclusions .................................... 26 References .................................... 27 ABSTRACT The use, the instrumentation, and some of the applications of inductively coupled plasma mass spectrometry (ICP-MS) is described. A brief description of some of the basic theory is followed by an outline of the different components and their functions and a more substantial description of the different methods of sample introduction and their inherent advantages and disadvantages. The use of survey analysis and isotope dilution analysis has also been described. A description of numerous appli- cations involving different methods of increasing sensitivity or reducing interference effects has also been included. Conclusions and a prediction of possible future trends have also been made. !. INTRODUCTION AND BASIC THEORY The concept of plasma source mass spectrometry was first proposed by Gray and in collaboration with Fassel's research group the inductively coupled plasma was identified as the most suitable source. Inductively coupled plasma mass spectrome- try (ICP-MS) was first demonstrated in Fassel's laboratory in the mid- 1970s (Houk et al., 1980; Date and Gray, 1981). It is a marriage between two already successful techniques (ICP and MS). The main problem encountered during this coupling was the interface between the two. The ICP part is at atmospheric pressure, whereas the MS is under high vacuum. This interface region has been developed and improved over several years and this will be discussed in a later section (Section II.C). There are several readable summaries of ICP-MS and ICP in general. Books by Boumans (1987) and by Montaser and Golightly (1992) give excellent accounts of the theory behind ICP, and a book by Date and Gray (1989) summarizes the early developments of ICP-MS. This latter reference also contains a very large number of applications. Other publications that are of use include the books by Thompson and Walsh (1989), the handbook of ICP-MS edited by Jarvis, Gray, and Houk (1992) and the Royal Society of Chemistry monograph by Evans et al. (1995). In addition to this, there are journal articles that explain in simple terms the use of such instruments (Ebdon and Evans 1988). There are several types of instruments available commercially. The standard instrument has a quadrupole-based mass spectrometer and costs in the region of $150,000 to $200,000. These are low resolution spectrometers (approximately 0.5 daltons), but are sufficient for the vast majority of applications. For some applica- tions, a more highly resolving spectrometer is required. Such instruments with magnetic sector mass spectrometers are available but at much higher cost (approxi-
  13. 13. Plasma SourceMassSpectroscopy 3 mately $400,000). Much of the most recently introduced instrumentation is either of the high resolution type or reduced size, i.e. benchtop versions of quadrupole instrumentation. An ICP-MS instrument may be used to obtain concentration data for a large number of analytes (approximately 70) in a very short period of time. It has several advantages over other trace element techniques, and may be used in several different ways. For the survey (semi-quantitative) method of analysis, the concentration of approximately 70 analytes may be determined simultaneously to within a factor of 3 of the true concentration. Although this method is not particularly accurate, it does serve to identify contaminants in a previously unknown sample. Once identi- fied, these contaminants may be determined more accurately using fully quantita- tive software with calibration standards in the normal way. In addition, most instruments have single-ion monitoring and time-resolved analysis facilities. These are software packages that monitor the signal at one isotope for single-ion moni- toring or at several isotopes quasi simultaneously for time-resolved analysis. They are most useful with transient signals such as those obtained with laser ablation, flow injection, electrothermal vaporization, or when chromatography is being coupled with ICP-MS, although there are other applications. Another advantage of ICP-MS is ultratrace sensitivity. Modern ICP-MS instruments have limits of detection superior for most analytes than even electrothermal atomic absorption spectrometry. As well as having ultratrace detectability (limits of detection < 0.1 ng m1-1 for many analytes), it is also multielement and has a large linear working range (manufacturers claim 8-10 orders of magnitude). Another advantage is that it can supply isotopic information. This can be especially useful for analytes such as lead whose isotopic ratio varies according to geological origin. The technique also has some disadvantages, the most obvious of these being the high purchasing and operational costs of the instruments. The very basic principles of an ICP-MS instrument are that the sample enters a plasma and is ionized. The ions are then taken from atmospheric pressure through nickel cones into the interface region which is under partial vacuum. From this region, the ions are directed using ion lenses under a stronger vacuum to a mass spectrometer which is under still stronger vacuum. The mass spectrometer sorts the ions into the mass-to-charge ratio and the ions are then detected by an electron multiplier. A more detailed description of the method of work of each of the components will be described later in Section II. For the majority of applications, the sample is introduced to the plasma via the sample introduction system consisting of a peristaltic pump, a nebulizer, a spray chamber, and a torch. In the plasma the sample is rapidly desolvated, atomized, and ionized. The extent of ionization depends on the ionization energy of the respective analytes. The ionization energy for argon (the most commonly used plasma gas) is 15.76 eV. Any analyte with an ionization energy below this value will be at least partially ionized and hence available to be detected. For most metallic analytes the ionization energy lies in the range 5-10 eV. For analytes with a low ionization
  14. 14. 4 ANDREW S. FISHERanti LES EBDON energy (e.g., caesium), the ionization is close to 100%; however arsenic which has a much higher ionization energy may only be 30-40% ionized. The extent of ionization obviously has some effect on the sensitivity of the analyte. Other prospective analytes such as the halogens (ionization energies 11.8-17.4 eV) are either very insensitive or completely undetectable. The extent to which the various species within the plasma are ionized may conveniently be described by the Saha equation shown below; although it must be stated that this equation assumes the plasma is in local thermal equilibrium, which most plasmas are not, NONe _ (2xmekT) 3/2 2zij e-eHkr Naj h3 Zaj where: NU= ion concentration of species j; Naj = atom concentration of species j; N e = free electron concentration; m e = electron mass; k = Boltzman's constant; k = Planck's constant; zij = partition function of ions of speciesj; Zaj = partition function of atoms of species j; ej = ionization energy of species j; and T = ionization temperature. A. Interferences Although, when first produced, these instruments were regarded as being inter- ference-free, this has proved not to be the case. A number of interferences and types of interference exist. There are basically four types of interference encountered using ICP-MS. These are termed isobaric, polyatomic, doubly charged, and inter- ferences observed arising from signal drift or decreased/increased nebulization efficiency. There have been several reviews on the different types of interference and methods of overcoming them (Vanhoe et al., 1994a; Evans and Giglio, 1993; Sah, 1995). Examples of each of these types of interference and how they are overcome are given below. IsobaricInterferences These are interferences that occur when more than one analyte has the same nominal mass. Examples of this include 4~ on 4~ and l13In on ll3Cd. Such interferences may be overcome using alternative isotopes, such as 44Ca, but this is a far less abundant isotope and therefore the sensitivity is limited. Another method of overcoming this problem is to use a high resolution spectrometer. As explained earlier, this is an expensive solution. PolyatomicInterferences This is by far the most troublesome type of interference. There are a large number of polyatomic ions that interfere with numerous analytes. The majority of interfer- ences occur below mass 80, although some oxide based ones do exist at higher mass, e.g. for the rare earth elements. Some of the more common interferences are
  15. 15. Plasma SourceMassSpectroscopy 5 shown in Table 1. The interfering ions tend to come as part of the matrix, part of the solvent, or are entrained from the air. There are several possible ways to overcome the interferences. The use of an alternative isotope is the obvious one, but some analytes (e.g. arsenic) are monoisotopic. Judicious tuning of the ion lenses and optimization of torch position, forward powers, and injector (nebulizer) gas flow lead to a decrease in the interferences, but do not always eliminate them completely. The use of alternative gases bled into the nebulizer flow has had great success in removing interferences arising from chloride (Branch et al., 199 la; Hill et al., 1992a), oxides (Hill et al., 1992b; Ebdon et al., 1993a) sulfates and phosphates (Ebdon et al., 1994b). Some authors have also bled nitrogen into the coolant gas flow to overcome interferences (Lam, 1990). Desolvation of the sample will also decrease interference effects derived from the solvent, e.g. oxides, nitrogen-containing interferants, and hydrides. Cryogenic desolvation has been used by Alves et al. (1992) with great effect. By cooling the sample to -80 ~ oxide ratios for many analytes were decreased to 0.02-0.05% while ArO§ ArC1+, and C10§ were also decreased. Desolvation has been achieved in a variety of other ways. The most common method is to use a cooling jacket on the spray chamber, but other methods include the use of Peltier coolers (Hartley et al., 1993), gas permeable membranes (Branch et al., 1991b; Botto and Zhu, 1994; Tao and Miyazaki, 1995) and a mixture of all of these. At least one instrument manufacturer produces a nebulizer that specializes in desolvating organic solvents. Table 1. A List of Some of the More Common Polyatomic Ion Interferences M/Z Element Abundance (%) Interfering Ions 47 Ti 7.72 POt 48 Ti 73.48 32SO§ POH§ 51 V 99.76 35C10+, 34SOH+ 52 Cr 83.76 4~ 35C1OH+ 53 Cr 9.55 37C10§ 54 Fe/Cr 5.92/2.38 4~ +, 37C1OH+ 55 Mn 100 4~ 56 Fe 91.52 4~ + 63 Cu 69.09 4~ § 64 Zn 48.89 HPO~, 32SO~,32S~,63Cull+ 65 Cu 30.91 H32SO~ 69 Ga 60.16 37C10~ 75 As 100 4~ 76 Se 9.12 36Ar4~ + 77 Se 7.58 4~ 78 Se 23.61 4~ + 79 Br 50.54 38Ar4~ + 80 Se 49.96 4~176 +
  16. 16. 6 ANDREW S. FISHERand I.ESEBDON Membrane separators also have the effect of removing interferences. Tao and Miyazald found that the levels of ArO§ and C10§ were decreased by one and two orders of magnitude, respectively, while the CeO+:Ce§ ratio was decreased to 1.1 x 10 -3. The Ba2+:Ba+ratio was also decreased to a larger extent than that found using a cooled spray chamber. Desolvation devices made inhouse have also been used. Tittes et al. (1994) have described such a device and found that interferences arising from chloride (e.g., C10§ C1OH§ ArC1§ CI~, and C10~) were largely overcome. The detection limit for V in 0.4 M HC1 was found to be improved by a factor of 25. DoublyChargedInterferences This occurs only when an element which has a very low second ionization energy is present in the sample. The most problematic element for this is barium. This element produces Ba2+ions which, when in high concentration, can interfere when determinations of gallium are being made at m/z 69. This can be circumvented by determining Ga at its less abundant isotope at rn/z 71. Alternatively, if barium is the analyte, erroneously low results can be obtained if the double ionization is not prevented. This can be achieved by optimization of power etc. SignalDrift The signal may drift over a long-term period if the temperature of the room changes. This is the reason that instruments should always be placed in a tempera- ture-controlled room. Short-term signal drift occurs when the dissolved/suspended solids content of the sample changes. This would lead to a change in the nebuliza- tion efficiency and hence to the amount of sample reaching the plasma. Signal drift can be accounted for by the addition of an internal standard to all samples and standards. The prime requisites for an internal standard are that it should not be present naturally in any measurable amount in the sample, it should preferably have an ionization energy close to the analyte(s) of interest, and should be reasonably close in mass to prevent any mass discrimination effects. Some of the more common internal standards include Co, In, and Rh. The use of an internal standard for normal analyses is now routine, but care must be taken if chromatography is being coupled with the ICP-MS since an internal standard is not always readily available. Simi- larly, for slurry introduction, an internal standard will not necessarily correct for the transport efficiency of the slurry particles to the plasma if these particles are too large. An added disadvantage of samples containing high dissolved solids or an organic solvent is that gradual blocking of the nebulizer, torch, or cones may occur. This will inevitably lead to a decrease in signal. Mass Discrimination Effects In addition to the types of interference listed above, mass discrimination effects may also occur. This happens especially when there is a high concentration of an easily ionized heavy element. The most badly affected analytes are the lighter
  17. 17. Plasma SourceMassSpectroscopy 7 elements that have a high ionization potential. Careful optimization of the plasma operating conditions may overcome these problems to a large extent. It must be noted that the effect is dependent on the absolute amount of the heavy element rather than the molar ratio to the analyte. Therefore, if sensitivity permits, the effects can also be partially overcome by diluting the sample. B. IsotopeRatio Analysis Applications of isotope ratio measurements fall into one of two classes. Unspiked analyses are used primarily in the geological sciences for the determination of the ages of samples, measuring sedimentation rates and determining paleo- and mag- matic-temperatures, or in the nuclear industry where applications include the monitoring of isotopic composition during the production of enriched uranium and monitoring for environmental contamination. Spiked or isotope dilution analysis is used to determine the concentration of an analyte by adding a known concentration to the sample. In ICP-MS, stable isotopes are normally used. In a way it is the ideal form of internal standardization because any matrix-induced interference will affect all the isotopes equally. For this type of analysis it is necessary to know the natural abundance of each of the isotopes in the sample to ensure that the spike is as different from it as practical. The advantage of this technique is that once the sample has been spiked, it is not necessary to analyze it quantitatively. Another advantage is that it can yield results with exceptionally good precision (typically < 1%). This method also compensates for any losses of analyte during the sample preparation procedures. If the amount of spike isotope added is known and by measuring the isotope ratio in the spiked sample, the analyte concentration can be calculated using the formula below (Thompson and Walsh, 1989), C~ MsK(A s - BsR ) W(BR - A) where: C is the analyte concentration (~tgg-l); M~ is the amount of isotope spike (~tg);A is the natural abundance of the reference isotope; B is the natural abundance of the spike isotope; A~is the abundance of the reference isotope in the spike; B~is the abundance of the spike isotope in the spike; K is the ratio of natural and spike atomic weights; Wis the sample weight (g); and R is the measured reference/spike isotope ratio after spike addition. This technique has found applications in several different fields, including geological, environmental and health sciences, and the nuclear industry. One of the problems associated with the technique is that it relies on an analyte having several isotopes that are stable. When isotopic abundance can vary because of radioactive decay, as is the case with lead and uranium, then problems may occur unless a "double spike" technique is used, whereby two accurately mixed spikes are added
  18. 18. 8 ANDREW S. FISHERand LES EBDON to a portion of the sample and then the spiked portion and the unspiked portion are analyzed separately and the data is normalized to the ratio of the spikes. There have been numerous applications of isotope dilution published. A few of them are outlined below. A study of the systematic and random errors for the determination of fission products and actinides has been made by Garcia'Alonso (1995). Systematic errors arising from mass discrimination effects, detector non- linearity, and isobaric interferences were corrected. Blood lead has been determined by Paschal et al. (1995). These authors spiked blood with NIST 983 radiogenic lead isotopic standard enriched with 2~ to 92.15%. After acid digestion of the blood in a microwave oven, the isotope ratio of 2~176 was measured. The authors claimed that exceptionally accurate results were obtained and up to ten whole blood samples could be analyzed in a day. Another application of isotope dilution has been demonstrated by Enzweiler et al. (1995). These authors determined Ir, Pd, Pt, and Ru in sodium peroxide fusions of geological materials. Analysis of several certified reference materials (WGB-1, TDB-1, UMT-1, WPR-1, WMG-1, and SARM-7) yielded results in excellent agreement with certified values. Elemental speciation with liquid chromatography ID-ICP-MS has been ad- dressed by Heumann et al. (1994). The isotope spike was applied in one of two ways. When the chromatographic system was well-characterized and the species were well-defined, a spike was added to the sample before the separation stage. If the species were not well-defined, a continuous on-line introduction of a spiked solution was used. For the species specific spikes, the method was applied to the determination of iodide and iodate in mineral water. The range found was 0.5-20 ng ml-l, and the precision of the analysis was 2%. !1. INSTRUMENTATION The instrumentation may be conveniently split into several sections, starting from the sample introduction systems and ending with the electron multiplier. A. SampleIntroductionSystems There are a number of different ways that samples can be introduced to an ICP-MS instrument. The most common method is by aspiration of aqueous-based samples, but other methods such as laser ablation (LA), electrothermal vaporization (ETV), flow injection (FI), hydride generation (HG), chromatography, and slurry nebulization also exist. These sample introduction techniques will be discussed in more detail in Section III. The sample introduction system for simple aqueous samples consists of a peristaltic pump, a nebulizer, a spray chamber, and a torch. In general the nebulizer produces a fine mist of droplets that enter the spray chamber which acts as a
  19. 19. Plasma Source Mass Spectroscopy 9 dampener for pump noise and droplet size filter. The smallest droplets (typically 1-2% of the total solution) reach the torch and enter the plasma. Nebulizers There are several different types of nebulizers available commercially. These include: concentric glass pneumatic nebulizers such as the Meinhard; V-Groove nebulizers such as the De Galan, Babington, and Ebdon (Ebdon and Cave, 1982); direct injection devices; and ultrasonic nebulizers (Bear and Fassel, 1986). Various frit (Layman and Lichte, 1982) and grid (Brotherton et al., 1987) style nebulizers have also been developed. The type of nebulizer used will depend on the application. The Meinhard nebulizer is effective for aqueous applications, but may become blockecl if the solution has a high dissolved/suspended solids content. The Meinhard operates with low noise levels since the solution need not necessarily be pumped to it by a peristaltic pump. The frit and grid style nebulizers have greater nebulization efficiency (and hence a higher percentage of the sample reaches the plasma leading to reduced wastage of sample), but are also prone to blocking if samples with high solids are aspirated. The V-Groove nebulizers are far more robust. They are often made of a polymer, thus are less fragile than the glass Meinhard. For example, the Ebdon nebulizer is made of corrosion-resistant Kel-F and has proved to be virtually unblockable, therefore it is used when the sample is corrosive or if it contains a high dissolved/suspended solids. The disadvantage with V-Groove nebulizers is that they rely on a peristaltic pump to transport the sample. This means that they are more liable to pump noise, but less prone to variations in uptake rate due to sample viscosity. Direct insertion (or direct injection) devices lead to far higher sensitivity because close to 100% of the sample reaches the torch (i.e. there is no spray chamber). Since there is no spray chamber, the dead volume is very much decreased and the time of analysis is decreased because there does not have to be a wash-out period between samples. This has advantages for chromatographic applications, but the sample flow rate has to be low (typically < 100 ~tl min-1) to prevent plasma extinction. In addition, the DIN can be easily blocked if a sample or a chromatographic mobile phase has a high dissolved solids content. There have been several papers published using this type of nebulizer. A DIN has been evaluated in a paper published by Zoorob et al. (1995). Other authors to have used DIN's include Shum et al. (1992), Shum and Houk (1993), and Powell et al. (1995). It should be remembered that one of the roles of a spray chamber is to dampen noise and fluctuations in sample uptake, therefore nebulizers with no, or reduced, volume spray chambers may result in noisier signals. Other types of nebulizers such as the thermospray (Vanhoe et al., 1994a) have also been developed. The advantages of this device are that it improves the sensitivity by an order of magnitude and improves the M+:MO+and M+:M2§ratios by a factor of 2.5 when compared with a pneumatic nebulizer used in conjunction
  20. 20. 10 ANDREW S. FISHERand LES EBDON with a spray chamber. Vanhoe et al. have also produced other papers on this topic (Vanhoe et al., 1994b and Vanhoe et al., 1995). Other authors have also reported the use of a thermospray device (Arpino, 1992; Koropchak and Veber, 1992). The hydraulic high pressure nebulizer (HHPN) has been described (Jakubowski et al., 1992). The same research group has also used this type of nebulizer for speciation of chromium (Jakubowski et al., 1994). Once combined with an effective method of desolvation, the HHPN increases the sensitivity for most analytes when compared with a conventional pneumatic nebulizer. The results for the speciation yielded detection limits of down to 1 ng ml-~ for the different chromium species. Although ultrasonic nebulizers were developed in the 1920s, they have been used by several authors to improve the detection limits in atomic spectrometry (Woller et al., 1995). Detection limits for many analytes are improved by factors of 5 to 50 when compared with a nondesolvated pneumatic nebulizer, or 3 to 8 times when desolvation is used. Desolvation is normally used for ultrasonic nebulization, otherwise the increased solvent loading of the plasma leads to plasma cooling and possibly extinction. The increased solvent loading of the plasma would arise because of the smaller droplet size generated by this type of nebulizer (Tarr et al., 1992). Some commercial USN's consist of a temperature-controlled heated cell followed by a water-cooled condenser to obtain dry aerosol particles. Ultrasonic nebulizers are also prone to blockage by samples with high dissolved solids and may suffer from troublesome memory effects. Yang and Jiang (1995b) have also reported on the use of a USN recently. High efficiency nebulizers have also been developed (Sang-Ho Nam et al., 1994). This nebulizer operates at a very low solution uptake rate (10-100 ~tl min-1) and its analytical performance in terms of detection limits (ng I-l), precision (0.7-4%), and the M+:MO§ and doubly charged species ratios compared favorably with a conventional nebulizer. An excellent review containing 209 references on the theory, mechanism of operation, and operating characteristics of pneumatic nebulizers was produced by Sharp (1988a). In a more recent review, the noise characteristics produced by the aerosols of different ICP nebulizers has been published (Luan et al., 1992). SprayChambers The basic function of the spray chamber is to ensure that only the smallest droplets reach the plasma. A review (140 references) of the fundamental processes occurring within spray chambers has been produced by Sharp (1988b). This review also covers references that compare nebulizer and spray chamber types. Droplets in excess of 5-8 ~tm are effectively removed from the system in the spray chamber and drawn or are pumped to waste. There are several types of spray chambers that may be used, but the large majority of applications use a double-pass Scott-style spray chamber. These are frequently made of glass but variants exist for when corrosive materials such as hydrofluoric acid are being analyzed. These variants are usually made from PTFE. The drawback with a double-pass spray chamber is that
  21. 21. PlasmaSourceMassSpectroscopy 11 it has a high internal surface area and a large volume. For chromatographic applications this can lead to peak broadening. A single pass spray chamber has less volume and a smaller internal surface area, so is often used for coupling high performance liquid chromatography (HPLC) or flow injection (FI) with ICP-MS. A large number of custom-made spray chambers have also been produced. Wu and Hieftje have developed a cyclone style spray chamber that has a substantially higher transport efficiency than many other spray chambers. A water jacket is incorporated into most commercial spray chambers. This is to cool the aerosol and hence lower the solvent loading of the plasma. This can be important when the solvent is water since large amounts of oxygen entering the plasma can lead to deleterious effects and interferences. Decreasing the solvent loading can be even more important when organic solvents are being used, because many solvents cause quenching of the plasma and increased interferences. A detailed study on the effects of organic solvents on plasmas has been made by Boorn and Browner (1982). Although this work was performed on an emission spectrome- ter, the overall conclusions hold true for MS instruments. Torches There are many types of torches available, but most are basically similar in design. The Fassel-based torch (18 mm in diameter) is used far more than the larger Greenfield type. The internal diameter of the injector (typically 2 mm) may differ depending on the sample type. For high solids, a wider bore may be necessary (3 mm), whereas for organic solvents a narrow bore (1 mm) will reduce solvent loading in the plasma. Demountable torches have the advantage that a variety of injector bore sizes may be used in an attempt to optimize the system. Low-flow torches in which the gas flow may only be half that of conventional torches have also found some use (Evans and Ebdon, 1991). Other Methods of SampleIntroduction Many manufacturers produce "bolt-on" devices that may be bought as accesso- ries and are easily interchangeable. Such devices exist for LA, ETV, FI, and HG, although many laboratories use equipment that they already possess. Certainly for FI and HG applications it is common for simple manifolds to be prepared in-house rather than opting for the more expensive manufacturers products. For LA and ETV applications, modification to already existing devices is common, but it is more usual to invest in the manufacturers product for software compatibility reasons. B. The Plasma Torch boxes and RF generators differ between manufacturers. Some manufac- turers use a standard 27.12-MHz generator, whereas others make use of the extra stability and tolerance of 40.68-MHz generators. Virtually all generators are solid state, but older instrumentation made use of the Henry generator. Most generators
  22. 22. 12 ANDREW S. FISHERand LES EBDON supply power up to 2000 W, but for most applications a power of 1200-1500 W is sufficient. Exceptions include the low power plasma work developed by Evans and Caruso (1993) where powers of less than 100 W may be used. This will be discussed in more detail later. The tolerance of generators to organic solvents depends on the design of the instrumentation. Most modem ICP-MS instruments can withstand very high per- centages of solvents such as methanol, ethanol, and acetonitrile without plasma extinction, although for many, a bleed of oxygen into the nebulizer gas flow is necessary to prevent the buildup of carbon on the cones, which will lead to excessive signal drift and ultimately to complete blockage. The basic principles of the ICP have been given in several texts (e.g., Boumans, 1987). Basically, a flow of argon gas is seeded with free electrons via a Tesla coil. This produces a potential that overcomes the dielectric resistance of the gas. The load coil produces a fluctuating magnetic and electric field which sustains the plasma. These fields couple energy into the plasma by accelerating free electrons into a region within the load coil. These electrons then transfer energy to other plasma species by collision. This produces further breakdown and an avalanche effect is produced. The argon then continues collisional energy exchange and a fireball (plasma) is produced. The temperature reached in the plasma ranges between 10,000 K at the hottest part to 6000 K in the sampling part. C. The Interface Region The plasma acts as an ion source, i.e. the constituents of the sample are dried, atomized, and then ionized. The ionized analytes then pass from the atmospheric pressure plasma to the interface region (also called the expansion chamber). A complete diagram of the plasma and interface region is shown in Figure 1. A comprehensive description of ion sampling in plasma mass spectrometry has been given by Douglas and French (1988). Douglas has also given an account in a book Figure 1. A diagram of the plasma and interface region.
  23. 23. Plasma Source Mass Spectroscopy 13 edited by Montaser and Golightly (1992). The sampling and skimmer cones are usually made of nickel because it is inexpensive, relatively easy to machine, and is durable, but any material with high conductivity will suffice. Other materials that have been used include aluminum, copper, and platinum. Platinum tipped cones are still used if the sample is likely to be corrosive to the nickel ones, e.g. if it contains large amounts of phosphate or sulfate. Behind the sampling cone a rotary pump produces a partial vacuum. The ionized sample in the form of a gas passes through the aperture in the sampling cone and on entering this region of lower pressure it accelerates until it exceeds the speed of sound. The temperature also drops dramatically. Under these conditions, the kinetic energy of the sample is converted into a directed flow along this axis. In effect, a free jet is formed that is bounded by a shock wave known as "barrel shock". Barrel shock helps prevent the gas jet from mixing with any surrounding gas and hence helps prevent the formation of molecular species. A second shock wave exists across this axis. This is formed when the expansion is halted by the background gas pressure. This second shock wave is called the Mach disc. TheMach disc's position is dependent on the diameter of the aperture in the sampling cone and on the pressure. Typically, the Mach disc is approximately 10 mm behind the aperture. Behind the Mach disc the ion beam becomes subsonic again and may mix with any surrounding gas. To prevent this as much as possible a second nickel cone (the skimmer) is placed at a distance of just over 6 mm away from the aperture on the sampling cone. This then allows the gas jet to pass through to the next stage of the spectrometer. The condition of the aperture on the skimmer cone is also vital. If it is misshaped then further shock waves will be caused and this will attenuate the transport of the gas jet through the orifice. Once passed the skimmer, the gas jet becomes random and requires focusing onto the detector by a set of electrostatic ion lenses. Cleaning of the cones has been shown to assist in the prevention of the formation of some polyatomic interferences. Secondary discharge has also been shown to cause interference effects that have a severely detrimental effect on the performance of the instrumentation. Secondary discharge is caused by an excessively high plasma potential causing discharge to occur between the plasma and the sampling orifice. A bad discharge causes crackling of the plasma and is characterized by bright white emission from the gas flowing into the orifice. The overall result is that the sampling orifice becomes ablated (i.e. it becomes much larger), leading to decreased vacuum in the expansion region which therefore leads to increased interference effects such as more doubly charged ions and more polyatomic ions. There have been several methods used to overcome this problem. Among these are careful optimization of the sampling depth (i.e. from what part of the plasma the ions are sampled) using a low nebulizer gas flow (<11 min-l) and reducing the solvent loading of the plasma. The account given above is a very simplified version of the events going on in the interface region. Intense research into the positioning and dimensions of the
  24. 24. 14 ANDREW S. FISHERand LESEBDON cones is continuing. Since the inception of the technique the sensitivity of the analysis has been improved over 10-fold by refining the engineering of the interface region. Much fuller accounts of molecular beams and the sampling of them and of the way in which polyatomic interferences may be formed in this region are given in the literature (Campargue, 1984; Olivares and Houk, 1985; Douglas and French, 1988, Vaughan and Horlick, 1990). Another readable account is given in a mono- graph written by Evans et al. (1995). D. The Ion Lenses The function of the ion lenses is to focus as many ions as possible from the cloud formed behind the skimmer cone through a differential pumping aperture into an axial beam of circular cross section at the entrance to the quadrupole mass analyzer. The ion lenses are basically metal rings with electric potentials applied to them. They are housed in the intermediate region of the instrument. There are several different systems in use, but all have a photon stop present on the axis to prevent photons from the plasma reaching the detector and adding to the background signal. The diameter of the photon stop is another factor that determines the geometry of the ion lenses. The geometry of the lens system used by one manufacturer is shown in Figure 2. The negative voltage on the extraction lens attracts the positive ions in the sample cloud and accelerates them towards the lens stack. Negative ions are repelled and the neutral atoms pass to waste in the vacuum pump. The ion beam is then collected on the "collector electrode" prior to being focused through the differential pumping aperture by lens 1 and 2. Lens 3 and 4 then refocus the ion beam into the entrance aperture of the quadrupole mass analyzer. It must be noted that the region between the skimmer cone and the collector has a very high "space charge" and a high background pressure of neutral species. This makes the calculation of ion trajectories difficult. The optimization of the ion trajectories obviously enables greater sensitivity to be achieved, therefore instru- slidevalve , 'L4"~L3"~ ~~~~'~~ -r". "~"skimmercone L1 extiactibriI Figure2. A diagram of the interface region and the ion lensarrangement.
  25. 25. Plasma SourceMassSpectroscopy 15 ment manufacturers have performed intense research into this area of the design. The effects of space charges etc. have been described in a series of papers by Tanner (Tanner, 1992; Tanner et al., 1994). E. Mass Analyzer The function of the quadrupole is to produce an electric field that selectively allows a stable trajectory for ions that have a narrow mass-to-charge ratio. Although it does not have as good a resolution as a magnetic sector instrument, it does have an acceptable sensitivity/resolution trade-off. Typically, it is comprised of four electrically conducting rods (12 mm diameter x 230 mm long) made of molybde- num that are arranged to produce an oscillating electric field between them. As the rods are arranged in a square, the electric field they produce between them is a good approximation of the ideal hyperbolic quadrupole field. Before the main mass analyzer there is a series of pre-rods that are only about 20-25 mm long. These rods are used to improve the transmission of the lighter ions and to prevent contamination of the main analyzer. Similarly at the end of the main analyzer, a set of rods are present to improve the extraction of the ions. The entire assembly is under high vacuum to ensure that there is no residual gas that can disrupt the ion trajectories by scattering, and hence causing decreased sensitivity. The basic principles of a quadrupole mass analyzer are beyond the scope of this text, but accounts of how they work are given elsewhere (Dawson, 1986). It is sufficient to say that the ions of the selected mass units "corkscrew" their way through the filter while the ions of unrequired mass units are deflected off this trajectory and are lost. Quadrupole analyzers have the advantages of being capable of scanning mass units very rapidly (up to 3000 mass units per second), and they are substantially cheaper and can withstand higher operating pressures when compared with magnetic sector spec- trometers. As discussed earlier, high resolution spectrometers that utilize a magnetic sector instead of a quadrupole mass selective filter also exist. These spectrometers are more highly resolving than the quadrupole-based instruments (they have a resolving power of about 5000). It must be noted though that sensitivity and resolution are not always compatible. Although the magnetic sector instruments are far more resolving, they are frequently unable to compete with the detection limits obtainable using quarupole-based instrumentation. There are however some applications that require high resolution to obtain accurate results, including the semiconductor industry, some geological applications, the nuclear industry, and laboratories that analyze certified reference materials. F. Electron Multiplier The ions transmitted from the quadrupole mass analyzer are detected by an electron multiplier. Again, the detailed description of how electron multipliers work is not appropriate in this text, but basically when a positive ion strikes the funnel
  26. 26. 16 ANDREW S. FISHERand LES EBDON of the multiplier one or more secondary electrons are ejected from the surface and are accelerated down the tube. During the passage down the tube they collide with the walls dislodging further electrons and thus an avalanche effect quickly builds up. The curved nature of the tube prevents ionic feedback, i.e. the traveling of electrons to the beginning of the funnel to restart another avalanche effect. At the bottom of the tube the cloud of electrons leaves the base of the channel and is attracted by a collector electrode. The signal is therefore measured as an electrical pulse. Recently, another type of detector has been developed. The use of this "active film multiplier" detector has become fairly widespread in the U.S. and it is envisaged that its use will spread further. It is a new type of discrete dynode multiplier that is coated with a material that is reputedly both resistant to chemical attack and which is stable in air. Other advantages this detector possesses include an increased total surface area (1100 mm2compared with 160 mm2in conventional electron multipliers), which should enhance its lifetime substantially and offer a large linear dynamic range. The Faraday cup is also used by some manufacturers. This device is especially useful when high concentrations of analytes are being determined. G. VacuumPumps As mentioned earlier, the mass spectrometer from the interface region to the detector is under vacuum. The interface region is evacuated to typically 2 mbar by a single-stage rotary pump. Often two rotary pumps in series are used i.e. the output from one is the input for the second. This improves the vacuum further. The intermediate (the ion lenses) and analyzer (quadrupole and detector)regions are under much stronger vacuum. In older style instruments, diffusion pumps were used but more recently the large majority of ICP-MS instruments use turbomolecular pumps. This has several advantages. After cleaning the lens stack the turbo pumps may evacuate the inner chambers within only a few minutes, whereas the diffusion pumps would take several hours. This has the obvious advantage of decreasing the downtime of the instrument. Although cleaning the lens stack is normally a 3 to 6 monthly task, if samples containing organic solvents are in regular use. the proce- dure may have to be repeated every 2 to 3 weeks. !!!. SAMPLEINTRODUCTION TECHNIQUES As well as the conventional aspiration of aqueous samples, there is an array of other methods of introducing samples to the instrument. These include the direct analysis of solids by laser ablation, ETV or slurry nebulization; or the use of chromatogra- phy, hydride generation, and flow injection. The relative merits of each of these will be discussed in this section. An annual review of the advances of methods of sample introduction to ICP-MS instruments is provided in theAtomicSpectrometryUpdate
  27. 27. Plasma Source Mass Spectroscopy 17 produced in each October issue of the Journal of Analytical Atomic Spectrometry. This review also describes the fundamental studies, theory, and, to some extent, the applications of ICP-MS. A. LaserAblation Most manufacturers produce a laser accessory that simply bolts on to the front end of their instrument. A laser (often a Nd:YAG laser) is focused onto (or close to) the surface of a solid sample causing it to vaporize. The sample vapor is then swept by a flow of carder gas to the plasma for ionization and detection. The sample does not have to be conducting, so it has advantages over arc/spark technology. Laser ablation (LA) has enjoyed great popularity with geologists since it enables them to analyze rocks without resorting to traditional methods of sample destruction such as fusion, or acid digestion using hydrofluoric acid. Other advantages it has include the ability to analyze small samples and depth profiling (i.e. the ability to determine elemental constituents at different distances from the surface of the sample). The production of a dry vapor of the sample also leads to decreased interference effects that normally arise from the solvent (i.e. the production of oxides, hydrides etc.). There are, however, problems associated with the technique. As the laser only vaporizes very small areas of the sample, spurious results will be obtained if the sample is not homogeneous. Another very important disadvantage is that of standardization. The calibration standards have to be very closely matrix-matched with the samples and this can be very problematic. The use of certified reference materials as standards is common for LA-ICP-MS. Because of the difficulty of standardization, LA is used most frequently for qualitative analysis of samples. As described earlier, the technique has had many applications in the geological field. A review of the applications for geological exploration has been published by Hall (1992). In a very comprehensive review (1019 references) the interaction of laser radiation with solid materials and its significance to analytical spectrometry has been discussed (Darke and Tyson, 1993). This review gives a simple explanation of the ablation process, an introduction to laser microprobes, and a large number of applications. Because this is such a comprehensive review, only a few of the more recent applications will be discussed here so that a feel of what is possible is obtained. Laser ablation has been used to determine numerous analytes in several different geological samples. These include the rare earth elements in geological materials (Jarvis and Williams, 1993); chalcophile elements in rocks, soils, and sediments (Guo and Lichte, 1995); sulfide minerals (Watling et al., 1995); rocks (Lichte, 1995); garnet (Fedorowich et al., 1995); alkaline and rare earth elements in marine ferromanganese deposits (DeCarlo and Pruszkowski, 1995): and weathered marble (Ulens et al., 1994). As well as geological applications, LA has been applied to numerous other samples. These include the analysis of steel (Yasuhara et al., 1992), biological tissue (Wang et al., 1994), tree rings (Hoffmann et al., 1994), glass (Stix
  28. 28. 18 ANDREW S. FISHERand LES EBDON et al., 1995), and teeth (Outfidge and Evans, 1995). Other applications that have been performed include the analysis of individual fluid inclusions by Shepherd and Chenery (1995), the analysis of powders (Raith and Hutton, 1994), and the laser vaporization of small volumes of solutions (Prabhu et al., 1993). Laser ablation has been used to obtain semiquantitative results in a paper by Cromwell and Arrows- mith (1995). A paper that reports the direct determination of metals in silver and gold without matrix-matched standards has been published by Kogan et al. (1994). This is a novel application, because as described earlier, the standards used for calibration normally have to be matrix-matched. Some papers have also been published on hardware developments. Cousin et al. (1995) have developed an autofocus system that enables reproducible focusing of the laser. Laser ablation using a microprobe has been used in several applications (Chenery and Cook, 1993; Fryer et al., 1995; Gunther et al., 1995). A microprobe has the advantage of permitting ablation from a much smaller site (10% of the size required by conventional laser ablation). This greatly facilitates the analysis of small individual grains, crystals, and powders. In another application a laser microprobe was used to find the three-dimensional distribution of precious metals in sulfide minerals. Isotope ratio measurements of copper have been made by Allen et al. (1995). Precision was found to be 0.85%. This paper also utilized a novel twin quadrupole mass spectrometer. Having two spectrometers enabled the authors to analyze two masses simultaneously. B. ElectrothermalVaporization Electrothermal vaporization can be used to analyze liquid or solid samples. Some manufacturers produce an accessory, but many laboratories simply choose to modify existing hardware (Lamoureux et al., 1994; Wang et al., 1994). A review of ETV into ICP-MS has been produced by Carey and Caruso (1992). The overall benefits offered by this technique have been summarized in a paper produced by Beres et al. (1994). This paper also discussed the importance of vaporizer design and illustrated its conclusions with several applications. For the majority of ETV applications the sample is dispensed on a rod or platform made of graphite or other suitable material (tungsten or some other refractory metal), dried, ashed, and then vaporized. The vapor is then swept to the plasma by a flow of carrier gas. This technique has the advantage of separating the analytes from the matrix, which is removed during the ash phase. This obviously decreases the prospective interfer- ences arising from phosphates, halides, sulfates etc. Since the sample is dry, the determinations are also free from interferences arising from solvents (e.g., oxides and hydrides). A study of the interferences in ETV-ICP-MS has been made by Shibata et al. (1993). These authors concentrated on the formation of oxides. The attenuation of oxide formation in ETV-ICP-MS has also been reported by Clemons
  29. 29. Plasma Source Mass Spectroscopy 19 et al. (1995). These authors used a graphite torch injector to decrease the amount of oxide formation. Many applications have been produced recently. Vaporization from a metal platform has been achieved by Marawi et al. (1995); the vaporization of boron has been assisted by the addition of mannitol as a matrix modifier (Wei et al., 1995); rare earth elements, uranium, and thorium have been determined (Gre- goire et al., 1995); and As, Cd, Pb, and Zn have been determined in soils and sediments (Zaray and Kantor, 1995). The analysis of solids in this way leads to greatly enhanced sensitivity as no sample preparation or dilution is required. An alternative approach is to place the sample on the end of a graphite rod and then insert the rod into the base of the plasma. This technique is not so common, although there has been a move recently towards the "electrically heated wire loop in torch vaporization" (Karanassios et al., 1995). Similarly, an assessment of direct solid sample analysis by graphite pellet ETV-ICP-MS has been made by Ren et al. (1995). As well as being an analytical tool, ETV-ICP-MS has been used to elucidate vaporization mechanisms in electrothermal atomic absorption spectrometry. The mechanism of chloride interference has been discussed by Byme et al. (1993) and the mechanism of vaporization of uranium in a graphite tube has also been reported (Goltz et al., 1995). In the former paper, the use of ICP-MS allowed direct observation of the signals for manganese along with the matrix components during the ash (pyrolysis) and vaporization steps. The loss of Mn was reported to be due to vaporization of manganese chloride. Exotic combinations of techniques have also been published recently. An ETV method of introducing slurries to an ICP-MS instrument has been reported by Gregoire et al. (1994). This technique also used a patented ultrasonic slurry agitator to ensure homogeneity of the sample. Electrothermal vaporization has also been used for isotope dilution analysis (Bowins and McNutt, 1994). This procedure enabled the determination of lead in blood to below the ng m1-1 level. Although the technique of ETV-ICP-MS has proved very successful and sensi- tive, there are a few drawbacks. These include the very slow sample throughput, and the possibility of some refractory analytes not being vaporized quantitatively from the graphite rod. This has led to the use of matrix modifiers as in electrothermal atomic absorption. C. Slurry Nebulization Slurry nebulization has had wide acceptance as a means of sample introduction into plasma emission spectrometry, and its use has spread into plasma mass spectrometry. Slurry nebulization requires no extra instrumentation, although the use of a high solids nebulizer is obligatory. Slurry nebulization has several advan- tages. It eliminates lengthy sample preparation procedures, decreases the number of sample preparation steps, decreases the use of hazardous acids, and may decrease the amount of contamination. A major advantage is that slurries may be analyzed
  30. 30. 20 ANDREW S. FISHERand LES EBDON using aqueous solutions as calibrants. A comparative study of aerosol sample introduction for solutions and slurries in atomic spectrometry has been made by Ebdon et al. (1989). There are several methods of making slurries, but basically, sample is powdered and then ground in the presence of an aqueous dispersant using either a ball mill or the "bottle and bead" method. The dispersant used will depend on the sample properties, but Aerosol OT and Triton X-100 are frequently used for carboniferous samples, whereas sodium pyrophosphate or sodium hexametaphos- phate are used for samples that are more inorganic in nature. The most important parameter for slurry nebulization is the particle size. Particles larger than 2 ktm are not transported to the plasma as efficiently as water droplets, hence low recoveries will be obtained if calibration is against aqueous solutions. The particle size may be decreased by lengthening the grinding time. The use of an internal standard may not compensate for the poor transport efficiency since the standard is normally in the aqueous phase. There have been several applications of slurry nebulization ICP-MS. These include geological samples (Jarvis, 1992; Halicz et al., 1993; Totland et al., 1993) and environmental samples, e.g. sediments (Zaray and Kantor, 1995). A wide range of analytes may be determined in this way. Zaray and Kantor determined As, Cd, Pb, and Zn, whereas Totland et al. determined platinum group metals and gold. For some analyses, slurries have been analyzed using ETV-ICP-MS (Gregoire et al., 1994; Fonseca and Miller-Ihli, 1995; Zaray and Kantor, 1995). Some studies have concentrated on the theoretical aspects of slurry introduction. A paper by Goodall et al. (1993) gave a good theoretical basis for slurry nebulization into plasmas. Fonseca and Miller-Ihli (1995) concentrated on transport studies of slurries in ETV-ICP-MS and Hartley et al. (1993) found that desolvation improved the transport and atomization efficiencies of slurries. D. Flow Injection Flow injection has been used by numerous workers to obtain a variety of effects. It has proved to be a very popular and successful method of sample introduction because it is so easily coupled with ICP-MS and is so versatile. The first authors to couple flow injection with ICP-MS were Dean et al. (1988). Basic flow injection has been used to introduce samples with a very high solids content (Stroh et al., 1992; Richner, 1993), or to introduce organic solvents (Hill et al., 1992c,d). The use of flow injection for these samples prevents a continual loading of the plasma with high solids, thereby preventing torch/nebulizer/cone blockage; or, in the case of the organic solvents, prevents continually high-reflected powers. Richner com- pared the limits of detection for samples with increasing solids loading. He found that LOD's at the ng g-1 level were obtained when samples containing 3% m/m Ni were analyzed. This was attributed to the very low dilution factor used. Precision was 2% on 13 replicate 200-~d injections. Organic solvents have been introduced into ICP-MS by flow injection to maintain plasma stability (Hill et al., 1992c).
  31. 31. Plasma Source Mass Spectroscopy 21 Trimethylgallium and methyllithium were dissolved in diethyl ether and aliquots of this solution (10-25 ~tl) were introduced to a 2% nitric acid stream. Limits of detection in the ng ml-I range were obtained for several analytes including A1, Cu, In, Pb, and Zn. Detection limits were improved further by the use of a desolvation membrane. The authors concluded that this approach improved detection limits, increased sample throughput, decreased memory time, avoided sample pretreat- ment, and therefore improved the safety aspects. Flow injection is also useful when only a limited amount of sample is available. If microcolumns of exchange media are used, analytes may be preconcentrated and the matrix can be eliminated and hence interferences removed. There have been a large number of applications papers that have used this approach. Examples include the determination of trace analytes in seawater (Orians and Boyle, 1993; Bloxham et al., 1994), concentrated brines (Ebdon et al., 1993a), biological samples (Ebdon et al., 1993b; Ebdon et al., 1994; Huang et al., 1995), steels (Coedo and Dorado, 1994; Coedo and Lopez, 1994), iron (Coedo et al., 1995), waters (Dadfar- mia and McLeod, 1994; Fairman and Sanz-Medel, 1995; Gomez and McLeod, 1995), high purity zinc (Sayama et al., 1995), soils (Hollenbach et al., 1994), and geological materials (Eaton et al., 1992). Many of these papers use ion exchange resins to retain the analytes while the bulk matrix is eluted to waste. This obviously has the advantage that when the analytes are eluted to detection, the large majority of potentially interfering species have already been removed. Large preconcentra- tion factors are also obtainable using this methodology. If 5 ml of sample is passed through the column and then the analytes eluted to detection in 250 ~tl of eluent, a theoretical preconcentration factor of 20 is obtained. Often the preconcentration factor will be substantially higher than this; e.g. Gomez and McLeod achieved a factor of 160 when they determined gold in natural waters. Many exchange or adsorption media have been used. Examples include chelating resins (especially for transition metals, Cd, Pb, and Zn), acidic alumina (for As, Se, V, and Cr), ion exchange resins and sulfydryl cotton (for Hg and Au). Work has also been per- formed in which flow injection of samples into a gaseous carrier (air) was achieved (Beauchemin, 1993). Some workers have again been coupling several well known techniques together to improve the overall detection limit or precision. Lu et al. (1993) have coupled flow injection with isotope dilution ICP-MS to determine Cd, Cu, and Pb in biological and environmental samples; Colodner et al. (1993) have used the same technique to determine Re and Pt in waters and Ir in sediments. Stroh and Vollkopf (1993) have described a method of flow injection-hydride generation ICP-MS for the determination of As, Hg, and Sb in water and seawater. Their results were validated by the analysis of several certified reference materials. Quijano et al. (1995) have used a similar approach to determine Se in water and serum. Using this method, the authors claimed that the sensitivity was improved by two orders of magnitude when compared with pneumatic nebulization (35 ng 1-l compared with 3 1-1).
  32. 32. 22 ANDREW S. FISHERand LES EBDON Some manufacturers produce accessories for achieving flow injection/matrix elimination/preconcentration, but basically all that is required is an injection valve, a peristaltic pump, a glass tube, and some fittings to make the column. The vast majority of laboratories make their own manifolds since it is substantially cheaper and is often more robust. E. Chromatography While ICP-MS gives information on total elemental concentration, it is often the form of the element, so-called "speciation", which is important. To obtain specia- tion information it is necessary to couple a separation technique such as HPLC or GC with ICP-MS. A review of coupling chromatography with plasma spectrometric detection has been made by Hill et al. (1993), while a review concentrating on coupling chromatography with ICP-MS has been produced by Seubert (1994). There have been numerous papers appearing in the literature describing the cou- pling of high performance liquid chromatography (HPLC) with ICP-MS. Chroma- tography is frequently used for speciation studies and there have been numerous papers published speciating several different analytes in many different sample types. Examples include gold from anti-arthritis drugs (Zhao et al., 1992), alumi- num in tea infusions by size exclusion chromatography (Owen et al., 1992); platinum from anticancer drugs (Zhao et al., 1993); arsenic in water (Thomas and Sniatecki, 1995), in urine (Larsen et al., 1993a), in fish (Branch et al., 1994), and in chicken (Dean et al., 1994); organotin (Kumar et al., 1993 and Rivas et al., 1995); and cadmium in pig kidney (Dean et al., 1987; Crews et al., 1989). Coupling HPLC with ICP-MS is in theory very simple. Normally a PTFE tube from the end of the column to the nebulizer is sufficient, but occasionally, when the mobile phase contains organic solvents, it may be necessary to desolvate the nebular in some way. There have been several methods published in the literature on this subject. A summary of methods available for desolvation is given in Section I. Some authors are again becoming more adventurous and in attempts to improve the sensitivity or the precision of their analyses they have coupled together several techniques. An example of this is the use of chromatography to separate Sb III and Sbv, followed by hydride generation into ICP-MS detection (Smichowski et al., 1995). In this paper the detection limits for a 100 ~tl sample were 0.04 and 0.008 ng of SblII and Sbv, respectively, which was an improvement of over an order of magnitude compared with HPLC-ICP-MS. In a similar approach, lead has been speciated by HPLC-HG-ICP-MS (Yang and Jiang, 1995a). Again, the limits of detection for the HPLC-HG-ICP-MS were comparable to or better than conven- tional pneumatic nebulization. The limits of detection were reported to be 0.6-6 ng 1-1 depending on the species. Lead has also been speciated using HPLC - isotope dilution -ICP-MS (Brown et al., 1994). Gas chromatography has been coupled with ICP-MS on far fewer occasions. This is partially because of the difficulty in coupling the two together. A heated transfer
  33. 33. Plasma SourceMassSpectroscopy 23 line from the end of the chromatograph to the torch is required, but ensuring that there are no cool points where analytes can condense often proves to be very problematic. Another problem is ensuring that the transfer line reaches as far into the torch as possible while ensuring that it does not act as an aerial for RF power which, if directed back to the chromatograph, could lead to severe instrumental failure and possible personal danger to the operator. That said, a few workers have succeeded in making the coupling (Kim et al., 1992a,b; Peters and Beauchemin, 1993; Pretorius et al., 1993; Ebdon et al., 1994b). All of these papers describe the transfer line or the interface between the chromatograph and the ICP torch. The majority of papers in this field analyze organometallic compounds such as al- kylleads in fuel (Kim et al., 1992b) and metalloporphyrins (Pretorius et al., 1993; Ebdon et al., 1994b). A novel interface that can double for both GC and normal sample nebulization has been described by Peters and Beauchemin. This interface reportedly yielded detection limits that were favorable compared with those ob- tained from a conventional nebulizer and spray chamber. Other workers that have attempted to couple GC with ICP-MS include Prange and Jantzen (1995) and Hintelmann et al. (1995). This latter paper described the determination of mercury and organomercury species in sediments. Supercritical fluid chromatography is one of the latest types of chromatography to be developed, and this too has been coupled successfully with ICP-MS. Appli- cations of this have been produced by Vela and Caruso (tin compounds)(1992) and Blake et al. (various organometallic compounds)(1995). An overview of the method has also been published (Carey and Caruso, 1992). The basic instrumentation including the transfer line is similar to that for gas chromatography. A few papers coupling capillary electrophoresis with ICP-MS have also been published. One of the drawbacks with coupling these two techniques is the different flow rates of the sample. Capillary electrophoresis uses a flow rate at the ~tl min-1 range or below, while ICP-MS typically uses 1 ml min-l. Lu et al. (1995) have developed a special interface to overcome this problem. A speciation application has also been produced by Liu et al. (1995). F. Hydride Generation Hydride generation (HG) is a technique that is used for several of the metalloid elements. Arsenic and selenium are particularly prone to interference from chloride ions. The use of hydride generation is an extremely useful way of separating the analyte from the matrix. This means that even for difficult samples such as seawater, As and Se may be determined. The other advantage of HG is that the transport efficiency of the analyte to the ion source is far higher than for aqueous nebulization (close to 100%). This inevitably leads to a substantial increase in sensitivity. Hydride generation has a particular advantage for arsenic determinations in that by separating As from the matrix the troublesome interference from ArC1§ at mass 75 may be avoided. At very low levels of As, chloride in the aerosol carried over from
  34. 34. 24 ANDREW S. FISHERand LES EBDON a conventional U-tube gas-liquid separator may be problematic. The use of a membrane separator eliminates this very fine aerosol and has therefore particular advantage in ICP-MS (Branch et al., 1991). Other analytes have also been determined using this technique. Besides As and Se, Ge, Pb, Sn, and Te may be determined by their hydrides. Other analytes such as Hg may be determined by reducing any mercury present in a sample to the elemental form using stannous chloride and then sweeping the Hg to the plasma in a flow of argon. Cadmium has also been determined by ethylating it into a volatile vapor. The only difficulty with this procedure is that the different species of an analyte may form a hydride with different efficiency. Examples include SeTM which is hydride forming and SevI which is not, and inorganic arsenic (AsnIand Asv) which form hydrides (albeit at different rates) and arsenobetaine (the main arsenic com- pound in many marine samples) which does not form a hydride (Dean et al., 1990). There are procedures to overcome this problem, e.g. reducing the selenium and arsenic species to SeTMand AsIu, respectively, using a reagent such as hydrochloric acid or L-cysteine. This however does not work for some species, e.g. arsenobetaine and selenomethionine, which are extremely stable. To destroy these compounds, either very harsh acidic conditions such as perchloric acid are required, which can on occasions be hazardous for the analyst; or photolysis may be used. There have been numerous papers that have been published recently that describe the use of HG-ICP-MS. Antimony, arsenic, and selenium have been determined in waters (Haraldsson et al., 1992); As, Hg, and Sb have been determined in seawater (Stroh and Vollkopf, 1993); and total and leachable As and Se have been determined in soils (Anderson et al., 1994). Other solid materials have been analyzed. Arsenic and selenium have been determined in marine CRM's with a detection limit of 0.3 and 2.5 pg ml-l respectively (Tao et al., 1993). Other researchers have coupled different types of chromatography with HG-ICP- MS to obtain speciation data. However, one of the major problems with this technique is obtaining full recovery of all the species present without altering the speciation. This obviously prohibits the use of conventional acid digestion of solid samples. Arsenic speciation has been achieved as reported in papers by Le et al. (1994) and Rubio et al. (1993). This latter paper is interesting because it involved on-line photooxidation to convert the nonreducible species into ones that can form hydrides. Arsenic has also been determined in seawater by HPLC-HG-ICP-MS (Hwang and Jiang, 1994) and water (Thomas and Sniatecki, 1995). This work utilized an interesting in-situ nebulizer hydride generator. Other speciation studies that have been performed include the separation of SbIu and Sbv (Smichowski et al., 1995), lead compounds (Yang and Jiang, 1995a), mercury compounds in biota (Brown et al., 1995), and organotin compounds in mussels (Rivaro et al., 1995). Other types of chromatography that have been used include micellar liquid chro- matography for the speciation of tin (Inoue et al., 1995) and vesicle-mediated HPLC for the speciation of Hg (Aizpun et al., 1995) and for As (Liu et al., 1993). The
  35. 35. Plasma SourceMassSpectroscopy 25 vesicle-mediated HPLC methods have used a Cl8-reversed phase column and a mobile phase consisting of didodecyldimethylammonium bromide vesicles in a phosphate buffer containing 0.5% methanol. Under these conditions, the four toxic forms of As were separated in 10 minutes with sub ng ml-l detection limits. As an application, As was speciated in urine and tap water. Several papers have reported on the coupling of flow injection with HG-ICP-MS (Dean et al., 1990). In one study Se was determined in water and serum (Quijano et al., 1995) and in another the effects of organic solvents on the hydride generation of selenium was evaluated (Olivas et al., 1995). In this latter paper, it was found that the presence of the organic solvents enhanced the Se signal. Under optimum conditions (sodium borohydride 0.2%, pH 1, methanol load 6%) an LOD of 1 pg for a 200 ~t1 sample was obtained. Isotope measurements of antimony have been made in seawater by Kumamaru et al. (1994): A method was developed to overcome the well-known chloride interference on As determinations (Story et al., 1992). Other more exotic method- ologies include the preconcentration of As, Bi, and Te hydrides in a palladium- coated graphite tube, followed by ETV into the ICP-MS (Marawi et al., 1994). This technique has the advantage of separating the excess hydrogen produced during reduction, enabling a more stable plasma to be obtained. The linearity was limited to the sub ng ml-~ range, but the procedure was validated by the analysis of CRM's. The LOD for As was 0.002 ng m1-1. IV. APPLICATIONS There have been a very large number of applications involving determination by ICP-MS over the last 10 years. Many of these applications have already been detailed in the individual sections describing the different types of sample intro- duction. Extremely useful sources of information include the AtomicSpectrometry Updates (ASU) published in the Journal of Analytical Atomic Spectrometry. Fundamental papers involving the theory, hardware, modifications, etc. of ICP-MS are published annually in the October issue of the journal. Environmental applica- tions including water, geological materials, air/airborne particulates, soils, etc. appear in the February issue; clinical, biological materials, food, and beverages appear in the April issue, and industrial analysis incorporating metals, chemicals, and advanced materials appear in the December issue. These updates give informa- tion on papers published in all majorjournals during the previous year. The majority of the ASU's tabulate the data so that the samples being analyzed, the sample preparation procedures, the techniques used, the analytes determined, etc. may be seen at a glance. Several reviews have also been published in recent years. Gray, (1994) has discussed the current status of ICP-MS and considered the major problems. Other reviews concerning clinical applications have also been published. These include ones by Barnes (1993)(138 references) who summarized the advances in the
  36. 36. 26 ANDREW S. FISHERand LES EBDON application of ICP-MS to human nutrition and toxicology; Vanhoe (1993) who reviewed the capabilities of ICP-MS for trace element analysis in body fluids and tissues; and McKay (1993) who reviewed (with 17 references) the use of ICP-MS for the study of pharmacokinetics of platinum drugs. Carey (1993) has discussed the relative merits of conventional sample nebulization, ETV, HG, GC, LC, and SFC (120 references). Uden (1995) has discussed the relative merits of HPLC- and GC-ICP-MS. In a similar paper, Hill et al. (1993) also reviewed the coupling of chromatography with ICP-MS. In another invaluable review, McLaren (1993) provided an ICP-MS applications bibliography update. This review was designed to include all of the most important ICP-MS applications papers. A review of speciation in aquatic and biological environments has been produced by Lespes et al. (1992). Reviews have also been produced for solid sample analysis by ICP-MS (Baumann, 1992; Voellkopf et al., 1992; Darke and Tyson, 1994). Darke and Tyson concentrated on slurry introduction, ETV, and laser ablation. Recently, the use of mixed gas or alternative gas plasmas has increased substan- tially. Sheppard and Caruso (1994) have produced a review concerning the use of mixed gas plasmas to overcome polyatomic ion interferences (64 references). The use of helium as an alternative gas has been used successfully to overcome the interferences arising from argon, e.g. ArO§ ArC1§ and ArAr§ Similarly, mixed gas plasmas that use ethene (Ebdon et al., 1994c), nitrogen (Hill et al., 1992a), hydrogen (Ebdon et al., 1993a), and methane (Hill et al., 1992b) to reduce polyatomic ion interferences have been reported. The use of mixed gas plasmasmcertainly nitrogen to remove chloride interferencesmis now routine in many laboratories. The use of the low-powered plasma developed by Evans and Caruso will probably gain popularity as it is a halfway house between conventional ICP-MS and other types of mass spectrometry (Evans and Caruso, 1993; Castillano et al., 1994). Depending on the operating conditions, it may be used to break down molecules completely in a similar manner to ICP-MS, but it may also be used to obtain a fragmentation pattern in a similar manner to typical mass spectrometry techniques (Evans et al., 1994). This will therefore enable a more unequivocal characterization of a sample. Although some modification to the hardware of the interface region of the instrument is required, this can be machined and may easily be interchangeable with normal hardware. V. CONCLUSIONS Inductively coupled plasma mass spectrometry has now come of age. A review by Horlick (1994) containing 36 references compared the current status of ICP-MS in the context of Shakespeare's seven ages of man, and there has been an overview by Gray (1994). Both concluded that the method had reached an encouraging degree of maturity. Many of the problems associated with ICP-MS have been overcome, such as the removal of interferences by either matrix separation using flow injec- tion, hydride generation, or ETV; or by the use of mixed gases.
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  43. 43. MULTIELEMENT GRAPHITE FURNACE AND FLAME ATOMIC ABSORPTION SPECTROMETRY JosephSneddon and KimberlyS. Farah Io II. III. IV. Abstract ...................................... 34 Introduction .................................... 34 Instrumentation .................................. 35 A. Multielement Line Sources .......................... 35 B. Continuum Source Multielement AAS .................... 36 C. Multielement Systems ............................ 38 D. Multielement Systems Using Lasers ..................... 46 Flame Atomization Applications ......................... 47 A. Multichannel Systems ............................ 47 B. SIMACC ................................... 48 C. Hitachi .................................... 49 D. Thermo Jarrell Ash-Baird .......................... 49 Graphite Furnace Applications .......................... 49 A. FREMS .................................... 51 B. Hitachi .................................... 51 C. Dual-Channel ................................. 52 Advances in Atomic Spectroscopy Volume 3, pages 33-61 Copyright 9 1997 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0072-8 33
  44. 44. 34 JOSEPH SNEDDON and KIMBERLYS. FARAH D. SIMAAC .................................. 53 E. Time-Divided, Single-Channel ....................... 54 F. Thermo Jarrell Ash-Baird .......................... 54 G. Fast Fourier Transform (FFT) ........................ 55 H. Perkin-Elmer SIMAA ............................ 57 V. Multielement System Summary ......................... 58 Acknowledgments ................................ 59 References .................................... 59 ABSTRACT In recent years there has been a renewed interest in the use and application of simultaneous multielement atomic absorption spectrometry (AAS). This chapter describes the historical development of simultaneous multielement AAS, tracing its use in laboratory-constructed or modified systems through to commercially available systems. Several approaches have been proposed and used and are described, includ- ing multielement line sources, continuum source, multichannel systems, and laser- based systems. Selected results of the applications of simultaneous AAS are presented, primarily on graphite furnace atomization, but where appropriate flame atomization. I. INTRODUCTION Atomic absorption spectrometry (AAS) is widely used for the determination of trace elements in numerous and complex matrices. It offers excellent and low detection limits, high precision, high sensitivity and selectivity, and acceptable accuracy. One drawback of AAS has been its lack of ability to simultaneously determine several elements (multielement determinations). If knowledge of several elements in a sample is desired, an analysis must be run for each sample separately. This leads to an increase in the time required to complete the analysis. It also requires that a separate sample be used for every analysis. This can prove to be difficult when the available sample is small in quantity or is difficult to obtain. Since the late 1960s, researchers have used a variety of techniques in performing multielement AAS analysis. Both continuous and line sources have been used. Early problems with these systems included a decrease in sensitivity, lack of linearity, and spectral interferences. Not until the early 1980s did systems become available that eliminated some of these problems. In the past few years several commercial systems have become available. Newer systems, some using a laser light source and solid state electronics for detection, are being studied. Commercial multielement AAS systems are available from Hitachi (Danbury, Connecticut) since 1988, Thermo Jarrell Ash-Baird (Franklin, Massachusetts) since 1990, Leeman Labs (Lowell, Massachusetts) since 1993, and