6 Analytical A tomic Spectroscopy 1.2 PLASMAS The now widespread availability of commercial plasma spectrometers has been reflected by a continued growth in the literature describing their use in analytical chemistry. Interest in microwave plasmas has appeared to decline but that in d.c. plasmas, especially their application, has increased, Most of the published reports deal with the r.f. inductively coupled plasma (ICP), but here there has been a noticeable shift towards a more realistic appraisal of the ICP in genuine analytical situations, as evidenced by the numbers of papers that discuss interference effects, unmentioned until recently.Two books (656, 1830) have contained chapters devoted to accounts of the different forms of plasma excitation used for analytical emission spectroscopy.A tomization and Excitation 7 1.2.1 R.f.Inductively Coupled Plasmas 1.2.1.1 Reviews. An authoritative and comprehensive review of the theoretical aspects of r.f. induced discharges has been published in two parts (655, 1087). Readers who prefer a theoretical discussion with fewer equations are directed to that by Robin (652). State-of- the-art reviews have been presented at conferences by Boumans (126, 1089).An insight into the progress of ICP-AES in various parts of the world was afforded by a report (1086) of activities in seven countries. An attempt has also been made to clarify the role of the ICP in practical analyses in comparison with other techniques (2073). It was concluded that of the available multi-element techniques, the ICP offers the greatest versatility and sensitivity. 1.2.1.2 Fundamental Studies. The present knowledge of the excitation conditions in the ICP was well surveyed by Mermet (842) at the CSIIICAS meeting in Cambridge. The author described the various spectroscopic diagnostic methods that have been used with the ICP for the measurement of temperatures, electron number density and metastable density. A study of excitation temperatures and i o n / a t m line intensities in both a conventional ICP and a mini-ICP (see also Section 1.2.1.3) has been presented (588).This work showed that local thermal equilibrium (LTE) did not exist in the low-power Ar plasma either close to the load coil or higher up in the tail flame. Other workers (150) have confirmed this conclusion and shown that desolvation of the sample moved the plasma even further from LTE (see also ARAAS, 1978, 8, Ref. 755). Lovett (1289) has found very broad lines in the ICP spectrum due to emission from autoionizing states with half-widths of as much as 4 A. These were ascribed to the natural line-width of very short-lived (ca. 10-14 s) excited states lying above the ionization limit of the atom.As the concentration of the element, e.g., Ag, Bi, Cd, Cu and Pb, was increased, the contribution of these emissions to the background became significant, posing problems in metallurgical analysis and demonstrating a unique link between atomic and ionic states in the study of plasma mechanisms. There is a growing appreciation that an understanding of analytically important plasma characteristics will be facilitated by an increased recognition of the inhomogeneity of the ICP.Koirtyohann et d. (201) have attempted to delineate separate zones in the plasma by using both a photodiode array detector and a moveable aperture controlled by a stepper-motor. The profiles obtained help explain the conflicting reports found in the literature concerning ionization interferences and optimum power levels.Other workers (737) have also used a computer-coupled photodiode array spectrometer to study spatial profiles of emitting species and provided evidence of the effects of plasma operating conditions and interferents. The alternative “end-onYy view of the ICP can give information on radial distribution patterns and, using an IDES, some interesting results have been obtained (86 1).As would be expected, under normal conditions most elements exhibited their maximum intensities around the centre of the plasma and background intensity was maximal at the skin zone. This distribution was influenced to some extent by operating conditions. Demers (617) has also studied “end-on” viewing in some detail and concluded that this gave superior detection limits with little or no loss in othcr aspects of analytical performance.Barnes et al. (841, 1092) have continued to refine and discuss their computer simulations of the ICP (see also ARAAS, 1978, 8, 7, and 1977, 7, 12). Computed results confirmed the experimental observation that natural convection exerted a negligible effect on the flow and temperature fields in a confined discharge but was a greater influence for a free plasma discharge.In contrast, the effect of conductive heat losses was found to be greater in the confined plasma.8 Analytical Atomic Spectroscopy The introduction of gases other than Ar into the plasma continues to be an active area of research. A He ZCP discharge is particularly interesting as the available excitation energy should be greater than that for an Ar ICP.Using a 54 MHz generator and 6 kW forward power, Robin and co-workers (848) produced a He plasma. Unfortunately, even when 40 1 min-l He was used, only a needle-like discharge was obtained and efforts were being directed towards broadening the plasma sufficiently to permit sample injection. The introduction of small concentrations of He into a conventional Ar plasma did not produce a He-type discharge.Talaat (1094) has maintained an experimental r.f. discharge inside a cylindrical quartz tube using high-velocity He flows at an intermediate pressure. Experi- mental results and thcoretical predictions of the r.f. ionization of He wcre compared. Many European and other spectroscopists continue to use nitrogen as the coolant or outer gas flow and the effects of this have again been discussed (1084).The use of N, in any of the three gas flows has also been studied (1816). When N,/Ar mixtures were used in the aerosol carrier-gas flow, the stability of the plasma was impaired. The use of a N,/Ar mixture in the plasma coolant gas increased both the net analyte and background intcnsities, confirming that a hotter plasma was produced.Little change, however, in detcction limits for 15 elements was obtained. Barnes and Meyer (1299) have investigated both N,/Ar and all N, discharges using a 10 kW, 27 MHz, crystal-controlled r.f. gcnerator. Experimental results suggested that a N, ICP could be operated at lower power than that predicted by a computer model. This model had previously predicted (see ARAAS, 1976, 6, Ref. 314) improved particle decomposition in a N, ICP compared to an Ar ICP. There has been a renewed interest in using air as the coolant gas for the ICP. Ohls and Sommer (654) have used an air/Ar ICP at 27 MHz and 3 kW. Most elements studied showed improved sensitivities in the air/Ar ICP compared to N,/Ar, but detection limits were only improved for about half of the elements.Considerable controversy has in the past been aroused concerning the effects of different plasma parameters, particularly power, on analytical utility. One difficulty in the resolution of such controversies has been that many comparisons have been made with non-optimal conditions. This problem has been compounded because optimization of the ICP is difficult given that the five operating parameters that affect plasma performance (the three gas flows, hcight of observation and power) are closely inter-related.Hopefully this is now a difficulty of the past as two different approaches to optimization have been successfully applied. These wcre the alternating variable search method employed by Greenfield (866) and the variable step-size simplex employed by Ebdon et al.(873). Both groups of workers came to the same general conclusions that low power was optimal for an Ar plasma, but higher power was normally optimal for a N,-cooled plasma. The wide extent of the other para- meters that must be optimized in a particular laboratory situation has also been discussed (2040).The characterization of noise is obviously of importance in this context and a comprehensive study of the noise power spectra of an ICP has been reported (1291). Near the limit of detection the major component was that of 1 /f noise. This noise increased with increasing wavelength. Some very useful lists of prominent lines for ICP analysis have been published this year. One listing of 450 persistent lines for 71 elements (1475) was based on a previously tested empirical relationship between sensitivities in the 4 r ICP and intensities in the NBS copper arc (see also ARAAS, 1978, 8, Ref. 1060). Another listing (1730, 1801) covered the prominent lines of 70 elements in both alphabetical and numerical wavelength order. Additional reference on the preceding topic (W spectrum) - 625.The ICP offers one of the few viable techniques for rare-eurth element analysis. Three different groups have reported on the determination of the lanthanoids by ICP-AES (870, 1275, 1476). In each case various Lines were investigated and limits of detection reported.Atomization and Excitation 9 The line-widths reported and the complexity of the spectra require the use of spectrometers with high resolving powers. The energetic nature of the ICP has led to its use as a source in other spectroscopic techniques.Winefordner and co-workers (1399, 1851) used an ICP as an excitation source for flame AFS. Fourteen elements were studied; promising detection limits and a reduction in spectral interferences were reported. The same system was used to plot three growth curves (the fluorescence, the excitation and the ICP emission growth curves).These showed the absence of self-absorption and self-reversal in the ICP at normal observation heights, even at relatively high concentrations. An ICP has also been used as a source for molecular fluorescence (1096). Interest in the ICP as an ion source for analytical mass spectrometry has been the subject of further study (1288).A small fraction of the ions from the ICP were pumped into a quadruple mass spectrometer. There are few potential interferents above mass number 42 and direct isotopic determinations are possible. We can expect to hear more of this approach in the future. Other references of interest - Ar/C,H, plasma for pyrolysis experiments: 845.Ar/UF Determination of stoicheiometries of heterupolymolybdates and heteropolytungstates by ICP: 2052. lasma for UF, regeneration and product analysis: 1093. 6 P 1.2.1.3 Torches. Of the various developments in torch design reported, by far the greatest interest has been in the so-called “mini-torches”, or reduced gas-flow torches (see also ARAAS, 1978, 8, Ref. 1412). The miniaturized torch of Savage and Hieftje has been further described (210, 733, 860, 1194, 1701).This compact torch was sustained by less than 1 kW of power and consumed less than 8 1 min-1 Ar. Two new torch configurations, with reduced diameter and smoothed contours have been reported (1985). Both power and gas consumption for these 9- and 13-mm diameter torches were lower than for the conventional 18-mm torch and the detection limits for 19 elements were encouraging. Another interesting development was the emergence of a water-cooled torch (844, 1298, 2012, 2042).As the water replaced the cooling function of the outermost argon flow, a tenfold reduction of Ar consumption was achieved with excitation temperaturee comparable to a conventional system. This ICP could be sustained at powers as low as 0.2kW, but as the carrier gas flow-rate available was only about 0.1 1 min-1, there were problems with sample introduction and hence detection limits were not as low as for conventional torches.Torches designed for improved performance with samples in orgaizic solvents have been reported. Boom and Browner (225, 559, 853) have inserted quartz capillaries in the two outer annular channels to produce a laminar flow of the gases.This reduced turbulence and hence improved stability while spraying organics. A new streamlined torch, for the analysis of liquids with organic constituents (1090), has been described. It has been pointed out that at higher power (5 kw) the N,/Ar ICP gives a stable discharge with organic solvents and that no carbon deposits occur (852).A modified Greenfield torch was reported, which was shown to have particular advantage when using Ar as coolant (873). The intermediate tube was flared and the torch was demountable. The three tubes were inserted into a brass support, which also featured replaceable threaded brass jets for control of the tangential gas velocity of the intermediate and outer gas flows; another demountable torch featured threaded and slotted spacers between the coolant / plasma tubes and plasma / injector tubes (1 725).This novel arrangement gave a greater tangential component to the outer gas flow. 1.2.1.4 Sample Introduction. Perhaps no other aspect of ICP-AES would more repay study than sample introduction, Fortunately it is increasingly being recognized that this is often10 Analytical Atomic Spectroscopy the limiting factor controlling the applicability of the ICP.Methods of sample introduction for the ICP have been briefly surveyed (590). Pneumatic nebulization is still the most popular form of sample introduction and the studies of Browner and co-workers (199, 226, 559, 593, 843, 853, 1320) on the chemistry of aerosol transport and the characterization of droplet sizes promises to be of great practical significance.Droplet-size distributions for a variety of pneumatic and ultrasonic nebulizers were characterized in the range 0.01 to 10pm for both aqueous and organic solvents. Information was obtained by laser scattering, electrostatic precipitation and cascade impac- tion.They have found in these studies a new form of interference, aerosol ionic redistribufion, arising from the observation that the relative concentrations of metals in droplets formed from mixed salt solutions are a function of droplet size. Doubtless we will hear more of this effect in future work. Two new cross-flow pneumatic nebulizers have been described (1 320), which are robust and of fixed geometry.Both, it was claimed, ran continually with solutions containing 10% NaCl. One of these nebulizers operated at a net sample uptake rate of 0.2 ml min-1, without loss of analytical signal, by recycling the untransported sample. More detailed descriptions of methods for wetting the injector gas stream to avoid the salt blockages that can m r with conventional concentric nebulizers (see also ARAAS, 1978, 8, Ref. 1424) have been published (51 5, 1541). The complex relationship between the analytical signal and stability, and the characteristics of concentric nebulizers, e.g., pressure, uptake rate, annulus area and flow velocity, has been explored (850). Babington-type nebulizers are becoming increasingly popular. A design fabricated in Teflon and glass was shown to be tolerant to a wide-range of sample flow-rates, 0.2-4 ml min-1, and was operable even with samples containing suspended solids (592).A factorial design-optimization of another promising prototype has been reported (591). A new development was the fritted-disc nebulizer. Sample was allowed to flow across a fine frit 15-ml Buchner funnel while Ar was introduced from the other side.Sample uptake rates as low as 0.02mlmin-1, with a 60% efficiency, were used but problems with drift and memory effects were encountered. The use of an injection cup technique has again (see also ARAAS, 1978, 8, Ref. 1442) been recommended for the rapid analysis of solutions with up to 20 mg ml-1 solids content (1 572). As more workers discover that the practical power of detection of the ICP is somewhat poorer than the lists of “distilled water detection limits” had led them to believe, there is increased interest in pre-concentration techniques and the use of organic solvents.Such solvents not only cause problems at the torch when using the conventional low-power all Ar systems, but also pose problems of sample introduction.A new cross-flow pneumatic nebulizer with adjustable Teflon capillaries and a novel spray chamber has been described (857, 1090). This nebulizer appeared to be well suited to samples dissolved in HF, those with high salt contents, or those dissolved in MIBK. The analysis of wear metals in oils may present an additional problem, namely the presence of metal particulates in the sample.It was reported that, using a conventional nebulizer-spray chamber system, the analyses of samples containing particles greater than 10 pm were not quantitative (1 321). The critical particle size that can be tolerated has been calculated using a theoretical model (600). In other studies of the same problem, attention has been given to appropriate dilution of the sample before aspiration (882, 1698). The formation of vohtile hydrides (see also Section 1.5) is well recognized as an attractive method of circumventing the relatively poor sensitivity for certain important elements.A continuous on-line NaBH, reduction cell for the routine simultaneous deter- mination of As, Ge, Hg, Sb, Se and Te has been described (1306), as well as a more detailed survey of the conditions for Sn and Ge hydride generation (1722).The influence of excess H, on the discharge stability can be important. It has been reported that when using anAtomization and Excitation 11 Ar ICP at a power of more than 2 kW, this factor can be neglected (852). Alternatively, the hydride, e.g., ASH,, may be collected in a liquid Ar cold-trap and the H, allowed to pass to waste (1739).Further studies of electrothermal vaporization of samples into the ICP have been reported (3, 851, 1312, 1697). A carbon rod is generally used as the vaporizer and some interesting transport effects in the tube connecting the vaporizer to the torch have been described. Enhancement of Cd and As signals in the presence of Se appeared to be due to the formation of condensed aggregates.The use of a halocarbon/Ar atmosphere was found to aid the vaporization of refractory compounds, e.g., in the determination of trace metals in uranium or in the determination of B, Cr, Mo, W and Zr using 0.1% trifluoromethane/Ar as the transport gas. Spark sampZing of solids is a technique with considerable attractions for metallurgical analysis. The use of an electronically controlled waveform spark source to sample a metal for ICP excitation has been further described (74, 192, 525, 874).It was reported that the linearity of response of the ICP was maintained and that matrix effects were reduced and precision improved compared to conventional spark excitation. An interrupted arc has also been used for this type of sample introduction, using either conventional metallic samples or briquetted mixtures of copper powder and oxides (877, 2028).Other methods of soZid sampling that have been reported include laser vaporization and direct solid injection. A system using a pulsed C0,-TEA laser to vaporize samples into the injector Ar stream has been shown to give good accuracy when used with reference materials (222).A wide variety of samples, e.g., metals, powders as pellets or on tape, have been analysed using a pulsed ruby laser as the vaporizer (594). A method was developed by Horlick and Salin (211, 595, 736, 1857) for the direct insertion of a graphite cup containing the sample (powders, solids or desolvated liquids) into the central core of the ICP. The sample holder occupied the position normally held by the aerosol tube.The graphite holder became white hot approximately 2 s after insertion, but the inert Ar ensured virtually no graphite consumption. Selective volatilization and thermochemical reactions are possible problems with this new form of sample introduction. Zil'bershtein et al. (224, 875) have developed a similar system that was used successfully for the direct multi-element analysis of thin sticks of oxide, silicate glass, and water evaporated onto carbon.Descriptions of coupled chromatography-ZCP systems have been published. For example, a coupled GC-ICP was employed for the determination of tetraethyl lead in petrol (1610). It has been suggested that GC-ICP is sufficiently accurate and precise to permit the determination of elemental compositions and hence empirical formulae for hydrocarbons and halogens (2003).The utility of the ICP as an HPLC detector has been evaluated for 25 elements (227, 1855). Its ability to detect HPLC peaks composed of EDTA and NTA chelatcs of Cu was compared with that of AAS. Detection limits were comparable, but the ICP possessed the advantage of multi-element capability. 1.2.1.5 Instrumentation. Complete ICP instruments are described in Section 2.4.1, where, it will be seen that there has been a considerable increase in the interest in rapid-scanning sequential instruments. A commercial instrument, based on a microcomputer-controlled rapid-scanning double monochromator, has been announced (535, 55 1, 864, 865, 1 30 1, 1594). Other workers have published accounts of sequential ICP-AES systems using computer- controlled monochromators (1 78, 188, 56 1, 862, 1 179, 1 300, 2039).The importance of high resolution in geological analysis has been demonstrated using gratings of different dispersion in a Paschen-Runge sequential spectrometer (872). It was concluded in this work that a holographic grating was superior, but another study reported12 Analytical Atomic Spectroscopy few advantages of holographic over ruled gratings, especially regarding stray light inter- ferences (6 14).Echelle spectrometers offer very high resolution and reports are beginning to appear concerning their application with ICPs (731, 739, 1290). It is possible that the geometry and light throughput of commercially available echelle spectrometers are unsuited to the ICP source.The use of specific detectors, either AAS or AFS cells, offers excellent “resolution” and a preliminary examination of a Hg detector has been made (213). Specifications for ICP systems have been reviewed (355) with special attention being given to data reporting. An ICP-spectrograph instrument for general survey analysis, using a computerized microphotometer, has been further developed (858).A interactive computer language for a direct-reading ICP spectrometer has been evaluated (10 19). Further details concerning optics, detector systems and data processing are to be found in Chapter 2, Other references of interest - Optical coupling of an ICP to an AA spectrometer: 343. Spectrometer modifications for K determination: 562. 1.2.1.6 Interferences. Assurances that there are no interferences in the ICP are much rarer than in the recent past and largely confined to those who do not operate an ICP for practical analysis. The present climate favours a frank discussion of the few problems that do exist, as one comprehensive discussion evidenced (5%). Sample trartport interferences are still being reported.These, because of the limitcd gas flow available for nebulization, can oftcn be more acute than in FAAS. For example, different acids have been shown to affect the emission from Cu when using both pneumatic and ultrasonic nebulization (1527, 2016). These effects were correlated with the density of the solution. Most workers now use a peristaltic pump routinely to minimize sample up-take rate interferences.The effects, at present unexplained and apparently varying, of easily ionizable elements seem to present a more acute problem. Perhaps some of these phenomena will be explained in future by the aerosol ionic redistribution mechanism (843) discussed in Section 1.2.1.4. Some workers report negligible effects from, for example, Na on K (1050).Others found matching of the Na level necessary when analysing samples following sodium peroxide fusions (650). Similarly, systematic errors caused by easily ionizable elements have been reported (1091) as have methods for dealing with matrix interferences (1281). Robin (652) has commented on the complexity of these phenomena, and has shown that generally they were less severe as the height of observation increased and the carrier gas flow-rate decreased, There was general agreement with these observations (1654, 201 5).Often the observed effects dcvelop similarly for atom and ion lines, contrary to expectation. The effect of K on atom lines was reported to be more severe than that on ion lines (1698). It has been suggested that high concentrations of Na quench metastable Ar and that this was therefore responsible for the effects observed in the analysis of rare-earth minerals (1654).In the miniature ICP, the enhancement by Na of a Ca atom line was greatest (30%) at higher powers and nebulizer gas flows (1285). In contrast, a Ca ion line was only slightly enhanced, and this only at low power. The use of Ce as internal standard to overcome suppressions by K has been recommended (587) as has the use of Li to enhance Pb signals (148).Spectral interferences due to the superposition of lines are enhanced by line-broadening in the plasma, as Robin has noted (652). An excellent tutorial rcview of these phenomena has been presented (847). There have been many reports this year of such interferences (e.g., 883). The greater number of spectral interferences in ICP-AES compared with AAS was commented upon by several authors (548, 879, 1524, 1996).Spectral interferences haveA tomization and Excitation 13 been predicted from wavelength tables and then evaluated in a practical study orientated towards environmental and clinical samples (1 282). Stray light effects continue to cause problems (221, 1541).The use of glass filters was found to be an effective way of reducing the interference of Cu and Ni on 0 s determina- tions (319). Band rejection filters for Ca and Mg have been used to reduce stray light effects on shorter wavelength channels (ca. 200 nm) in a commercial direct-reading plasma spectro- meter (218). A comparison of transmission filters placed before the detectors and a single rejection filter placed at the entrance slit (opaque to 390-4OOnm) for the removal of stray light caused by emission from Ca has been made (1153). The former was found more effective.Various devices for background correction are now in use. A novel triple entrance slit prefaced by an oscillating chopper so that radiation entered alternatively through the central slit and the two adjacent slits was proposed (878).A SIT has been used to provide back- ground correction (862). A spectral window of 5 nm with resolution of 0.01 nm per channel was used. Boumans (863) presented a progress report concerning the use of three silicon photodiodes for automatic background correction. Each was provided with a current t o voltage converter and connected to the measuring circuitry.The line peak was centred on the second photodiode and the other two viewed the background on either side of the line. The nature of micro-wavelength scan background correction systems and their performance on two different instruments has been discussed (616, 1284), as have microcomputer- controlled background-correction systems (705, 1050, 1283, 1308).Other references of interest - Concomitant effects in AAS using a plasma atomizer: 11 13. Influence of concomitants on analytical results: 231. 1.2.2 Microwave-excited Plasmas Research work in the field of microwave-excited plasmas seems to have entered a quiescent period, certainly fewer papers are being published. 1.2.2.1 Fundamental Studies. A comparison has been made of different cavities for micro- wave induced plasmas (MIP) (1696).Of the Beenakker (see also ARAAS, 1977, 7, 16), 3- and +-wave Evenson and +-wave Broida cavities, the former was preferred as being easier to tune, more reproducible and capable of sustaining a He plasma at atmospheric pressure. A plasma at atmospheric pressure, obtained by surface-wave propagation using a surfatron, was reported (1634).Produced within a silica tube, the plasma had a typical electron density of 3X 1014 cm-3 but the gas temperatures were lower than 4000 K. Measure- ments of temperatures and electron densities in the Beenakker cavity have been reported (607, 849). These were shown to vary with gas flow rates and applied power, The combina- tion of this He MIP with a micro-arc sampler, for analysis, was also briefly reported (733, 1194).The characteristics of the MIP, such as the atomization efficiency, have been studied (1 359). Preliminary results suggested good correlation between atomization efficiency and electron temperature, electron density and an energy parameter (power/pressure). The radiative ionization-recombination model has been shown to offer reasonable predictions of the experimentally measured intensity ratios of the H a- and P-lines in a 0.2 Torr He and Ar MIP, but so did a hypothetical thermal model (1888).The absorption intensities, how- ever, were very poorly predicted by the thermal model. A kinetic scheme based on a radiative charge-recombination process was shown to explain the scavenging and other effects observed when 0, is added to a conventional GC-MIP detector (1549).A reduced-pressure plasma operating at 5 kW and a frequency of 2300 MHz was14 A n d y tical A t omic Spectroscopy reported (846). A more conventional MIP has been employed to measure hydrogen isotope ratios in a GC eluant using an oscillating slit mechanism to view alternately the H emission at 656.28 nm and the D emission at 656.10 nm (2045).Agterdenbos et al. (966) have con- tinued their work with sealed MIP-AES and have determined Pb in human serum. The serum was ashed by microwave excited 0,, the Pb extracted with dithiocarbamate and the freeze dried extract sealed in a quartz tube before excitation. 1.2.2.2 Sample Introduction. A variety of approaches continues to be investigated with the MIP.Conventional nebulization is difficult because of the low tolerance of the plasma to water; however, plasmas operated in the Beenakker cavity are more robust in this respect. A new demountable torch has been described, which allowed a conventional cross-flow nebulizer to be used at a flow-rate of 1-2 1 min-1 (735). The plasmas was stabilized by the generation of an annular toroid and operated without desolvation with both pncumatic and ultrasonic nebulizers.Work has continued on laser vaporization of solids for excitation in an MIP (see also ARAAS, 1978, 8, Ref. 1061) (1657). The procedure was applied to the analysis of trace elements in aluminium and zinc samples using both photographic and photoelectric detection. The precision of the method was limited by the small number of photons arriving at the detector.Electrothermal vaporization continues to be a popular way of introducing samples into the MIP. A graphite cord was used to vaporize 1 pl samples and a temperature programme enabled the distortion of the linearity of the peak height caused by 1% KC1 solutions to be removed (855). Peak-height responses linear over 2 to 3 orders of magnitude were thus obtained.Van Dalen et al. (856) reported that use of a carbon rod vaporizer overloaded the plasma with C. Therefore, a home-made Ta furnace was used for the determination of C1, Br, I and S over the mass range 1-4000 ng. Metal oxides have been electroplated onto a wire, which was then used either as a wire loop or moving wire Vaporizer for MIP emission (90). The extraction into CHC1, of Mg from aluminium as the TFA complex, with subsequent vaporization of the chelate into an MIP was reported (641).The low tolerance of the MIP to solvents has encouraged investigation of the hydvide generation technique. Various means of hydride formation and their corresponding sensi- tivities have been reported (89).The H, p!oduced during the hydride generation has been vented and the hydrides condensed in a hquid-N, trap (91, 146). The hydrides were then passed to the MIP via a simple chromatographic column that removed condensed con- taminants. The linear dynamic range of the method extended over 3 orders of magnitude from the ngml-1 level. The most popular application of the MIP is as a GC detector.The advantages of the Beenakker cavity for GC detection of halogens and other elements has been stressed, reported detection limits were in the pgs-1 range (75, 460, 608, 945). The element specificity of the MIP was used to eliminate interferences, found with an electron capture detector, in the determination of polybrominated biphenyls in food (1 385). 1.2.2.3 Capacitively Coupled Microwave Plasmas.The need for ionization buffcrs and hence matrix matching has limited the application of the CMP. This has been stressed in a comparison of the CMP and the ICP for geological analysis (1691). Provided that the appropriate ionic and atomic partition functions are known, the degree of ionization may be calculated by comparing the intensity of atomic and ionic emission.A four-line method (2 atom and 2 ion) has been developed on this basis to calculate the excitation temperatures of atoms and ions and the dcgree of ionization, and hence correct for cationic interference (180, 1721). The method was successfully applied to the determination of Mn in the presence of Sr. By gradual electrical heating of a coal sample, S species were vaporized andAtomization and Excitation 15 introduced discretely into a CMP (1331).Four S species were found using an atmospheric pressure Ar plasma and monitoring at 190.027 nm. The air plasma torch of Hanamura reported last year (see also ARAAS, 1978, 8, Ref. 176) has now been used to detect Pb above an open container of gasoline, and Hg in air (537). A novel background corrector for use with this plasma, consisting of a quartz cell containing vapour of the metal being determined, is being evaluated. 1.2.3 D.c. Arc Plasmas Clearly, the availability of commercial d.c. plasma spectrometers is encouraging a growth in the literature on their applications and interference effects. Several evaluation papers have appeared. It was suggested that the commercial plasma has a sensitivity advantage over a rotating d.c.arc (see also ARAAS, 1975, 5, Ref. 1246), but a disadvantage in respect of the high rate of anode consumption (54). A comparison of the d.c. plasma and the ICP for forensic analysis concluded that the ICP offered better precision and lower detection limits, although this was not based on first hand experience (876). From a study of stray light and spectral overlap problems in the d.c.plasma/echelle spectrometer, it was concluded that these were less severe than those reported for ICP/direct-reading spectrometer systems (1741). Schmidt and Sacks (53) have compared the two-electrode plasma jet, the three-electrode stabilized plasma jet, and the gas-stablized bent-arc and noted the problems in each with sample introduction into the high-temperature regions, and plasma stability.It has been claimed that significant advances have been mad2 in the analytical capabilities of the commercial system by alteration of the electrode con- figuration (to 30" from 75"), the use of a small, 2.4mm, diameter chimney for aerosol introduction and doping the electrode streams with 25% V / V He (55, 603) (see also ARAAS, 1978, 8, Ref. 182). These modifications all seem to favour more entrainment of the aerosol in the plasma. This new system has been characterized using Hg vapour in order to minimize vaporization and desolvation effects (1 322). Using a computer-controlled stepper-motor, the emission of elements in the d.c. plasma has been mapped (604). This improved the selection of optimum conditions for multi-element analysis.The effects of easily ionizable concomitants in the d.c. plasma, the so called "salt- effects", have generated considerable interest. High concentrations of Na, Ca and Mg were found to increase line intensities and background emission, and alter the shape of the plasma (1539). It was therefore concluded that the d.c. plasma could not be used for the direct determination of metals in sea-water.Other workers were able to perform such analyses in the three-electrode plasma by adding appropriate amounts of alkali-metal salts to standards and samples (514, 960), or by buffering with high-purity Li,CO, (952), or other easily ionizable elements (1952). Barium was found to exert similar effects at the centi-molar level and self-enhancement was observed that produced non-linear calibration curves (62).A correction procedure, in which apparent concentration values were fitted to a series of simple regression equations, has been employed to correct for the effects of Ca, K, Na and Si on 19 elements in the analysis of geological waters (1595). By using a LiBO, fusion and 0.4% m/V CsCl buffer, Bankston (58) was able uniformly to minimize matrix effects in geological analyses.Experimental studies of possible solute vaporization mutual interference effects of P on Ba, Ca and Sr and of Ca on A1 have been reported (1986). Ionization buffers were used throughout. The only systematic trends observed were the suppression of Ca emission at 393.3nm by excess of Al. The extent of the suppression was approxin~ately 10% at molar ratios in excess of 20.A simple flow-injection device has been connected in line with the nebulizer flow system of a d.c. plasma spectrometer (61 8). This permitted appropriately16 Analytical Atomic Spectroscopy buffered samples of 50-1OOpl to be injected for the determination of Al, Ca, Mg and P in dental plaque and Zn in skin. As with other forms of plasma, samples may be introduced in a gaseous form. Hydride generation methods have been described for As (65) and As, Sb and Se (1 360). In the latter study, detection limits of 1, 2 and 1 ppb, respectively, were reported. Sediment samples have been heated in an induction furnace in an HCl/Ar atmosphere. The AsC1, formed was trapped and then swept into the plasma (1471). Using CaSO, to enhance the signals and suppress interference from organics, the detection limit quoted was 15 ng. Trace levels of S in various oxidation states have been determined by reduction to H,S by HI (564, 1780). The resultant gas was swept into a d.c. Ar plasma and the emission monitored in the V.U.V. at 180.7 nm. The detection limit was 10 ng S, with less than 2 min required per sample. The factors that governed the rate of H,S evolution and possible interferences were investigated. An interface for coupling the plasma to a GC has been described (454). The heated interface enabled quantitative transfer of relatively involatile organometallics. Detection limits in the range 10-8 to 10-12 gs-1 were reported. Selective detcction of Si with this system has been demonstrated for silylated derivatives (59). Examples of the analysis of complex mixtures by GC-d.c. plasma emission have also been presented (60). A portable GC-d.c. plasma was reported (538) that was capable of separating and detecting Hg species present in air and determining total Hg in air. The sample was either passed through the GC column or collected on Ag and Au before being flushed into the plasma. Other references of interest - Analysis of silver scrap by d.c. plasma: 64. Automation of a multi-channel d.c. plasma emission spectrometer: 81. Wavelength selection for a multi-element d.c. plasma emission spectrometer: 63.