JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 819 Performance Characteristics of an Ultrasonic Nebulizer Coupled to a 40.68 MHz Inductively Coupled Plasma Atomic Emission Spectrometer* Isaac B. Brenner Geological Survey of Israel 30 Malkhe Israel Street Jerusalem 9550 1 Israel Phillipe Bremier and Allain Lemarchand Jobin-Yvon 16-18 Rue du Canal Longjumeau Cedex 91 163 France Analytical performance characteristics [limits of detection (LODs) memory effects long and short-time reproducibility effect of high salt content and accuracy] of an air-cooled ultrasonic nebulizer coupled to a 40.68 MHz r.f. inductively coupled plasma (ICP) were evaluated. Approximately 1 0-fold enhancements in the LODs only slightly affected by ICP power were observed with a simultaneous sequential polychromator.Limits of detection were somewhat better than those quoted in the literature for 27 MHz r.f. ICP generators. The present configuration allowed the determination of trace elements at the pg dm-3 and sub-pg dm-3 concentration level in pristine and waste waters. The use of a Trassy-Mermet sheath gas assembly and a 3 mm injector tube allowed the direct analysis of saline solutions containing trace elements at the pg dm-3 level using matrix-matched calibration protocols. With this configuration memory effects were minimum and the long- and short-term variations could be compensated for with the use of internal references. As a result relative standard deviations are improved and the need for frequent system readjustment and recalibration was eliminated.A comparison of the data for standard reference and spiked materials indicated that the accuracy was satisfactory. Keywords Sample introduction; inductively coupled plasma; trace element analysis of water; ultrasonic nebulization The most common devices for sample introduction into an inductively coupled plasma (ICP) are pneumatic nebu- lizers. With the use of these robust devices ICP atomic emission spectrometry (AES) has become the accepted technique for routine multi-element analysis. However the limits of detection (LODs) for several important analytes in particular those that have toxicological significance are inadequate for the analysis of surface and sub-surface waters industrial eMuents and related environmental ma- terials owing to the low efficiency of aerosol transport.Improved LODs can be obtained by using a 40.68 MHz generator1V2 with on-line column preconcentration and flow injection techniques3y4 and hydride generati~n.~ However the desired LODs are still inadequate or the techniques are element-selective and tedious. Thus the growing need for enhanced LODS has resulted in revived interest in the ultrasonic nebulizer (USN). The application of the USN for sample introduction in ICP-AES was described in the late 1 9 7 0 ~ . ~ - ~ A water-cooled USN was used successfully for the analysis of effluents and sludges8 and of sea-water after column precon~entration.~ However the device was not routinely applied for the analysis of solutions containing even moderately high concentrations of acid and salt since droplet size interfer- ence effects related to liquid viscosity and surface tension were identified accompanied by unsatisfactory system reliability.Chemical interferences in the desolvation sys- tem ('desolvation interference effects') and in the plasma were also suggested to explain the decline in element inten~ities.~J~ The latter processes were explored in an in- depth evaluation of an improved version of the original Ames nebulizer.I0 Several unpublished reports described the application of a commercially available air-cooled USN (CETAC Omaha NA USA) for various analytical disciplines. Prelimi- nary data were also published for the analysis of low-salt solution^.^^ In this paper performance characteristics are *Presented at the 1992 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 6-1 1 1992.given for the air-cooled system which differed from that described by Olsen et al. and other investigators.6-8 Experimental Ultrasonic Nebulization System The instrumentation (CETAC U-5000) differs from previ- ous USN designs and features an air-cooled transducer assembly for heat dissipation and regulation of the trans- ducer power and the heating and cooling temperatures of the desolvation system. These modifications result in longer transducer life improved analytical repeatability and gen- eral system reliability. The detailed operation procedure is described in the operation manuals and only a few points will be highlighted here. In order to prevent transducer damage and ensure that a minimum layer of about 1 mm for the production of acoustic waves occurs all solutions were continuously pumped at approximately 3 cm3 min-I.Regular pumping was also necessary in order to avoid uneven production of the aerosol cloud. It is now recognized that the limiting factor for enhanced LODs with the USN is the high amounts of water vapour introduced into the plasma resulting in considerable energy consumption.14 In the present system water vapour was removed by employing a desolvation system consisting of a temperature controlled heated cell followed by a water- cooled condenser to obtain dry aerosol particles. The USN spray chamber included a steep drain for rapid gravita- tional removal of solution droplets a tangential argon gas inlet for continuous sweeping of the transducer face plate and an auxiliary rinse for high-salt solutions.The last facility was employed when very high salt concentrations were aspirated. Tube lengths were reduced as much as possible and the system placed in close proximity to the towh housing in order to minimize dead volumes and wash- out time. The time for the desolvation system to attain a tempera- ture of 140 "C was about 5 min. The period of time needed for the refrigeration system to attain a temperature of from - 5 to - 10 "C was approximately 90 min. A further period of about 15 min was needed for stabilization of the USN820 ALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 1 Instrumentation JY 50 Polyscan Gratings Spectral range/nm Dispersiodnm mm-1 Resolu tion/nm Slit dimensions/pm Optical transfer Generator In st rumen t control JY 36 Grating Spectral rangelnm Resolution Slitdpm Photomultiplier tube SIM-SEQ 0.5 m polychromator N flushed 2 dm3 min-l Concave holographic 3600 grooves mm-I 0.55 0.028 Entrance 20 exit 50 Plasma focused on entrance slit via an N flushed extension tube Jobin-Yvon 40.68 MHz maximum power rating 2.5 kW JY (ISA) Spectralink electronics IBM PC Sequential 0.65 m N flushed 2 dm3 min-' Plane holographic 2400 grooves mm-l double order 160-800 ( 1 st order) 160-3 10 (2nd order) Variable depending on slit dimensions and order 0.006 nm mm-I maximum Entrance 30 exit 25 Hamamatsu R300 for the UV R 106 for < 190 nm 160-4 10 Table 2 ICP and USN operating conditions ICP- Power/kW Torch Observation height Coolant gas flow rate/ Aerosol carrier flow rate/ Sheath gas flow rate/ dm3 min-' dm3 min-l cm3 min-' USN- Instrument Sample delivery rate/dm3 mind' Desolva tion Heat ing/"C CoolingPC Integration time/s Washout time/s 0.7-1 Jobin-Yvon Ryton demountable torch with aerosol injector of alumina 3 mm diameter 12 mm above the upper coil 12 0.8 0.2 and 0.6 for alkalis CETAC U-5000 including desolvation system (heater and cooler systems) 2.5 peristaltic 140 -5 to -10 Polychromator 5- 10 and monochromator 1 30-80 itself.It was observed that when an excess of liquid accumulated in the spray chamber condensation occurred in the tube leading to the torch. This effect was minimized by continuous peristaltic pumping. Failure to regulate this level resulted in intensity drifts.The drain peristaltic pump rate only slightly exceeded that of the sample. Spectrometer and ICP Operating Conditions The main body of data presented here was obtained with a Jobin-Yvon (Longjumeau France) JY 50 Polyscan Sim-Seq multichannel spectrometer. This instrument has a facility which allows the utilization of spectral lines k2.2 nm on either side of the fixed spectral ~hanne1.l~ Using the 20 channel instrument additional spectral lines were mea- sured with adequate LODs. Additional data were obtained using a JY 36 high-resolution sequential system.16 This spectrometer has a variable resolution facility consisting of computer controlled variable exit and entrance slits. The main details of this configuration are listed in Table 1 and the operating conditions in Table 2.It was observed that when high-salt solutions (> 1%) were injected continuously into a USN the narrow torch injectors were clogged within a short period of time. Consequently a JY Ryton demountable torch with a wide injector (3 mm) was employed. A Trassy-Mermet sheath gas tube was also installed between the spray chamber and torch base. This facility permitted optimization of the plasma for maximum LODs and reduction of the memory effects due to salt deposition on the inner wall of the inje~t0r.l~ Conditions for multi-element simultaneous analysis were determined by adjusting the so-called normal analytical zone using a 100 mg dm-3 Y solution. The LODs of the alkali elements were enhanced by increasing the sheath gas flow to 0.6 dm3 min-I.I8 Calibration Pristine water Multi-element calibrations were performed using aqueous solutions.A 5% HN03 (Suprapur Merck Darmstadt Germany) solution was used as the low standard. A multi- element composite Spex QC-19 (Spex Industries Edison NJ USA) was diluted to form a graded series of standards varying from 10-100 pg dm-3. The concomitant elements were also present in these standards and facilitated multi- element analysis for the major and minor elements. High-salt solution In order to compensate for interference effects in the nebulizer and in the ICP due to high concentrations of concomitants in high-salt solutions matrix-matched cali- brations were performed. For the analysis of water contain- ing 0.1-1 Yo NaCl calibration standards were prepared using Suprapur NaCl spiked with Spex QC-19 standard to produce standards varying from 10 to 500 pg dm-'.All operations were performed in a class 100 work hood with ultrapure (Millipore Bedford MA USA) de-ionized water. For the analysis of LiBOz fusion solutions portions of aJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 82 1 5 0.5% LiB02 (Johnson Matthey London UK) solution were spiked to produce analyte concentrations in the pg dm-3- mg dm-3 range. a' 0 Results General Obervations The USN produced a large amount of aerosol over an extended period of time of at least 12 h. Plasma extinction and blockage did not occur in the presence of 1% NaC1. ! Limits of Detection Limits of detection were determined at several ICP power levels. The data in Figs. 1-3 indicate that the majority of lines used were not significantly affected by changes in power in the range 1-0.6 kW.However several LODs improved slightly with decreasing power and some of the 'hard lines' e.g. P S and Se were somewhat degraded with decreasing power (Figs. 1 and 2). A multi-element solution containing 100 pg dm-3 of analytes was then employed to determine LODs ( 2 4 using both simultaneous and sequen- tial multi-element detection at a power of 0.7 kW. These are compared with those obtained with a conventional Mein- hard nebulizer (denoted CONV in Table 3) and the JY 40.68 MHz generator. In comparison with LODs obtained by pneumatic nebulization those obtained by USN are about ten times better. (Enhan F is the LOD enhancement factor defined as the ratio between the LODs obtained by conventional nebulization and those obtained by USN.) Thus many types of solutions containing low salt contents can be analysed without the need of preconcentration.In Fig. 4 the USN LODs obtained in the present study are compared with the values reported by other investiga- tors.*J 1*12 The data indicate that a considerable improve- ment has been achieved with the present design. It is also evident that some of the LODs obtained in the present study are significantly enhanced. This could be attributed to the use of the high frequency 40.68 MHz r.f. generator as reported by Mermet and co-workers.1*2 An important observation is that the low LOD of 4 pg dm-3 for the Na I 330.2 nm spectral line using these conditions eliminated the need for additional alkali detec- tors in the near-infrared range.In Table 3 the results obtained when several spectral lines were accessed using the polyscan device i.e. lines located within the 2.2 nm interval of the fixed slits are shown. In these cases the LODs were not degraded. The sub-pg dm-3 limit for A1 permits the analysis of biological fluids for this important element and the enhanced LODs for the hydride- and vapour- forming elements As Se Sb T1 and Hg eliminated the 0 ; 1000 800 600 Power/W Fig. 1 B; D Se; E Pb; and F Zn Influence of ICP power on the USN LODs for A P; B S; C L E PowertW Fig. 2 Influence of ICP power on the USN LODs for A Al; B Ca; C Cr; D Mg; and E Cu 1.6 I p 0.8 \ n B \ - 1000 800 600 PowertW Fig. 3 Influence of ICP power on the USN LODs for A Ni; B Co; C Cd; D Ba; E Fe; and F Mn I This work Refs.11 and 12 [7 Ref. 8 I n 1 E 2 7J cn 0 0 -J 0.1 0.01 I 1 I I I I I 1 1 I 1 1 I 1 1 1 1 1 1 I I 1 I Ag As Ba Be Cd Co Cu Mg Mo Pb Se Zn Al B Be Ca Cd Cr Fe Mn Ni Sb TI Element Fig. 4 Comparison of the air-cooled USN LODs with data from the literature refs. 8 1 1 and 12; all values + 2 a need to apply hydride generation which requires the installation of a different sample introduction system and utilization of time-consuming chemical procedures to con- dition the samples and the standards (reduction of high to lower valency states). An additional advantage of the USN procedure is the ability to perform a multi-element analysis of these elements whereas with hydride generation this is not feasible owing to the individual chemical treatments required to optimize analyte responses.A reasonable explanation for the amount of the enhance- ment is not clear from the present data. A relationship with excitation and ionization potentials was not observed. It was however noticed that the LODs for Hg and B were822 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 3 LODs in aqueous media by USN (20) measured in mg dm-3 Element Wavelengthhm Ag A1 Al As As B Ba Ba Be Ca Cd Cd Cd c o c o Cr Cr c u Fe Fe Hg Mg Mg Mn Na Ni Ni P Pb S Sb Se Se Se Si Sr TI V Zn Ag I 328.0 A1 I 308.21 5 Al I 396.15 As I 189.0 As I 193.69 B I 249.77 Ba I1 233.527 Ba I1 493.44 Be I1 31 3.042 Ca I1 317.933 Cd I1 214.438 Cd I1 226.502 Cd I1 228.802 Co I1 228.616 Co I1 237.862 Cr I1 205.55 Cr I1 267.716 Cu 1324.754 Fe I1 238.204 Fe I1 259.940 Hg I 194.22 Mg I1 279.553 Mg I 285.213 Mn I1 257.610 Na I 330.2 Ni I1 216.556 Ni I1 231.604 P I 178.287 Pb 11 220.353 S I 180.734 Sb 1206.833 Se I 196.026 Se 1203.985 Se 1206.83 Si 1288.158 Sr I1 216.59 TI I 190.864 V I1 310.230 Zn I1 213.856 LOD/pg dm-3 This work 0.3 0.2 0.1 0.7 0.4 0.1$ 0.1 0.18 0.02 0.2 0.2 0.15 0.1 0.2 0.2 0.2 0.1 0.05 0.1 0.05 1.5$ 0.04 0.06 0.02 4 0.3 0.2 3.5 1 0.5 1 2.3 5 1 0.4 0.4 0.5 0.5 0.09 CONV* 1 10 I 15 8 1 0.1 2.39 0.1 2 0.6 0.65 0.4 1 5 1 0.8 0.9 1 0.8 4 0.07 0.7 0.15 100 2.7 3 10 5 20 7.4 10 1159 3009 10 10 8.3§ 2.5 0.5 Enhan F t 50 10 21.4 20 10 1 12.8 5 10 3 4.3 4 5 25 5 8 18 10 16 3.3 3.7 1.7 11.6 7.5 25 9 15 2.9 5 40 7.4 4.3 23 300 25 20.8 20 5 5.6 *LODs (20) using a Meinhard concentric nebulizer.?Enhancement factor; LOD conventiona1:LOD USN. #In several cases higher values were obtained. §Values quoted from ref. 19. significantly reduced in several cases. This intensity incline for B and Hg was attributed to analyte loss in the desolvation system as a result of volatilization during aerosol dehydration.1° In acid media B forms volatile species that can be redeposited. This interesting phenome- non requires further investigation. In the presence of high concentrations of NaCl (1% m/m) LODs were degraded by factors varying approximately from 2 to 10 (Table 4). Despite this decrease numerous trace elements can be determined in brines and similar high-salt solutions (polluted sea-water and mineralized brines) by performing a preliminary dilution.Background Correction For the analysis of pristine water it was observed that background differences between standards and samples were usually insignificant and that compensation was only rarely required However for saline solutions where Ca and Mg concentrations were high background compensa- tion was required. In the case of matrix-matched calibration systems background correction was applied when neces- Table 4 LODs of USN for solutions containing high NaCl concentrations. Blank solution prepared from ultrapure NaCl (Suprapur Merck). Spectral lines listed in Table 3 LOD/pg dm-3 Element 1 O/o NaCl B 0.5 Cd 0.8 Ba 0.6 c o 0.5 Mn 0.2 Fe 0.15 Cr 0.8 A1 1.4 c u 0.6 Aqueous 0.07 0.2 0.1 0.2 0.02 0.1 0.2 0.2 0.05 sary. Regions of background compensation were selected by scrutinizing spectral profiles of the solutions.The positions selected were similar to those listed by Brenner and Eldad.20,21 Wash-out Times The wash-out time was determined by nebulizing an acid blank (2% HN03) followed by a 0.5% NaCl solution containing several trace elements at the mg dm-3 level. The wash-out time was defined as the time required for the system to evacuate the saline aerosols and produce 1% of the original element concentration. The data illustrated in Fig. 5 indicated that this level was attained after about 80 s. It should be noted that this period included nebulization of dead volume for approximately 10 s. This is considered to be adequate for the USN. For pure aqueous solutions the wash-out period amounted to about 40 s.Small contamina- tions of the elements of interest in the blank solutions were taken into account. The wash-out times reported here are similar to those observed by Chan and co-workers.11J2 The influence of tailing-off effects was significant only when trace concentrations were determined consecutively with solutions containing very high concentrations of the ele- ments to be determined. Spurious emission spikes were not observed. In the case of trace concentrations of Hg and B in aqueous solution the wash-out times were about 60 s. In order to take full advantage of the linear range of 4 orders of magnitude an additional wash-out period of 15-30 s was needed. However this situation is uncommon when trace metals are to be determined. 10000 0.1 ' ' I I I 0 20 40 60 80 100 Time/s Fig.5 Wash-out times of the USN for a 0.5% NaCl solution for A Na; B Ba; C Ni; and D MnJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 823 Table 5 Comparison of long-term precision of the USN with and without an internal reference (Co); 0.1% NaCl containing a multi- element spike of 100 pg dm-j Without internal reference With internal reference Mean/ SD/ Mean/ SD/ Element pg dm-3 pg dm-) RSD (Yo) pg dm-3 pg dm-j RSD (O/O) Zn Pb Cd Ni Ba co Mn Fe Cr 101.7 100.8 99.1 98.9 99.5 100.3 101.5 97.9 99.4 0.9 0.8 0.9 1.1 0.8 0.7 0.8 1.2 0.7 0.9 0.8 0.9 1.1 0.8 0.7 0.8 1.2 0.7 100.9 101.4 100.2 98.6 99.2 101.2 99.9 99.2 - 0.5 0.6 0.5 0.6 0.4 0.7 0.7 0.3 - 0.5 0.6 0.5 0.7 0.4 0.7 0.7 0.3 - Table 6 Data for SLRS-1 and NIST SRM 1643b by USN; all data in pg dm-j except where indicated SLRS- 1 NIST SRM 1643b Element Determined Recommended* Determined Recommended* RSD (Yo) A1 As B Ba Be Cd c o Cr c u Fe Mn Mo Ni Pb Se srs V Zn CaS 9 MgS § NaS 9 K4 9 18 180 - 20.7 - - - 0.3 3.5 1.6 0.9 1.8 32 - - 135 0.87 2.1 26.4 5.8 9.9 0.97 23 NDt 22.2 - - - - 0.4 3.6 31.5 1.8 0.8 1.I - - 136 0.66 1.34 25.1 5.99 10.4 1.3 15 58 47 42.4 18.5 23 29.5 22.4 20.3 87 28 89 49 19 12.5 224 43 60 29.9 6.9 - - *From refs. 22 and 23. TND = not detected. $Value obtained by extrapolation using a 500 pg dm-3 calibration standard. §Values in mg dm-3. ND 49 ND 44 19 20 26 19 21.9 99 28 85 49 24 10 227 45 66 1.2 1.4 - - 2 5 5 1.8 2 1.4 2.2 2.6 1.5 1.8 1.8 2.2 1.6 10 8 1.2 1.4 1.9 30 8 - - Precision The short-term precision in a pure aqueous solution was approximately 1% at the p g dm-3 level and < 1% at the mg dme3 level.The long-term precision of the USN for a solution containing 0.1 O/o NaCl (m/v) and a 100 pg dm-3 multi-element spike was determined for an analysis period of 90 min. The data listed in Table 5 indicate that the relative standard deviations (RSDs) for the ten measure- ments were approximately 1 %. These values were improved by a factor of about 2 with the use of the internal reference method. Under these conditions it can be concluded that both the short- and long-term variations of the USN are similar to those obtained with a conventional pneumatic nebulizer. Accuracy The accuracy of the USN was evaluated by replicate analysis of a National Institute of Standards and Techno- logy (NIST) Standard Reference Material (SRM) 1643b Trace Elements in Water (n= 10) and National Research Council of Canada (NRCC) SLRS- 1 Riverine Water ( n = 4).These samples represent pristine waters of low-salt content. The data provided in Table 6 indicate that bias between the USN and the recommended value^^^^^^ is absent. With few exceptions the percentage deviation [(USN value - recommended value)/( recommended value)] x 100 varied from < 1 to 15%. The precisions for the SLRS-1 data are similar to those for SRM 1643b. Noteworthy is the capability of determining As and Se directly. In order to determine the accuracy of analysis of high-salt solutions 0.5% m/v NaCl and LiB02 solutions containing 5-500 pg dm-3 multi-element spikes were analysed using a matrix-matched procedure.The results are listed in Table 7. At the 10 pg dm-3 concentration level the trace element recoveries varied from about 80 to 120%. At higher analyte concentrations recoveries were in the 95- 105% range and even better indicating that Mo Cr Zn Cd Ni Co Mn Fe V Cu and Pb can be determined with satisfactory accuracy and precision in the presence of relatively high-salt concen- trations. In the case of Pb a background correction was824 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 Table 7 Spiked data for 0.5% NaCI and 0.5% LiBOl solutions; calibration with matrix-matched procedure 0.5% NaCl 0.5% LiBOz Addition of spike/Pg dm-3 Addition of spike/pg dm-3 Element Cd c u Cr Fe Mn Mo Pb 10 11.3 9 .6 12.4 9.75 9 . 2 11.8 5 10 20 7.7 11.7 19.8 50 52 47 51.7 50.5 48.5 49 30 31.1 100 98 103 102 101 99 102 40 50 36.6 50.9 Element Cd c u Cr Fe Mn Mo Sr Ti ~ 10 11.4 13 8.5 12 9.6 9.2 10.3 1 1 100 98 102 102 101 108 110 99 97.5 500 496 506 492 490 49 5 5 10 503 500 applied. Recent data obtained in our laboratories indicate that the use of Sc as an internal reference results in a considerable improvement in the precision and accuracy when solutions containing 1000-5000 mg dm-3 of total salt are aspirated. In geoanalysis samples are frequently decomposed using an LiBOz fusion (0.5 g of sample and 1-2 g of fusion reagent) and dissolved in 200-500 dm3 of dilute acid usually HN03. However the final volume of 500 cm3 precludes trace element determination employing conven- tional ICP-AES because of high dilution factors.The USN data for the LiBOZ spike demonstrate that trace elements can be determined in this type of solution. Dynamic Range In environmental analysis it is essential that the dynamic range exceeds the expected range of concentrations which can vary unexpectedly. In the present application Ca Na K and other elements in the mg dm-3 concentration range were determined in NIST SRM 1643b and SLRS-1 using a two-point calibration consisting of an acid blank and a multi-element standard of 500 pg dm-3. Thus satisfactory values were obtained even when the concentration ex- ceeded the upper calibrator. As a result of the 4-fold linear range of calibration the number of standards was reduced. Conclusions The data demonstrate that the USN coupled to an ICP-AES can potentially be used for the determination of pg dm-3 and mg dm-3 concentration levels allowing the direct analysis of low-salt pristine water biological fluids and environmental geological and related materials.In the case of high LOD enhancement the use of the USN overcomes the need to analyse the samples by electrothermal atomic absorption spectrometry and hydride generation. More- over the alkali elements can be determined using less sensitive spectral lines in the ultraviolet and the ultravio- let-visible regions of the spectrum. The large dynamic range of calibration allows the determination of minor elements including the alkalis in the mg dm-3 range. High-salt solutions can be analysed on a routine basis using the sheath gas attachment and the wide injector tube which prevent salt accumulation and eventual blockage.However a matrix-matched calibration technique was required to compensate for interference effects. References 1 Capelle B. Mermet J.-M. and Robin J. Appl. Spectrosc. 1982 36 102. 2 Marichy M. Mermet M. Murillo M. Poussel E. and Mermet J.-M. J. Anal. At. Spectrom. 1989 4 209. 3 Hartenstein S. T. Ruzicka J. and Christian G. D. Anal. Chem. 1985 57 21. 4 Ruzicka J. Anal. Chem. 1983 55 1041A. 5 Huang B. Zeng X. Zhang Z. and Liu J. Spectrochim. Acta Part B 1988 43 38 1 . 6 Olsen K. W. Haas W. J. and Fassel V. A. Anal. Chem. 1977 49 632. 7 Boumans P. W. J. M. and De Boer F. J. Spectrochim. Acta Part B 1975 30 309. 8 Taylor C. E. and Floyd T. L. Appl. Spectrosc.198 1,35,408. 9 Berman S. S. McLaren J. W. and Willie S. N. Anal. Chem. 1980 52 488. 10 Fassel V. A. and Bear B. R. Spectrochim. Acta Part B 1986 41 1089. 1 1 Chan S. K. and Zajac J. P. paper presented at the 21st Annual Conference Canadian Mineral Analysts Ontario Canada September 26-29 1989. 12 Chan S. K. Zechmann C. A. and Yanak M. M. Analysis of Water and Wastes Using ICP-AES With Ultrasonic Nebuliza- lion CETAC Report 1990. 13 Freeden K. J. Am. Lab. (Fairfield Conn.) 1990 December 24 23. 14 Tang Y. Q. and Trassy C. Spectrochim. Acta Part B 1986 41 143. 15 Myers R. Spectroscopy 1989 4 16.. 16 Brenner I. B. and Lemarchand A. paper presented at the XXVII Colloquium Spectroscopicum Internationale 199 1 Bergen Norway June 9-14 1991. 17 Murillo M. and Mermet J.-M. Spectrochim. Acta Part B 1987 42 1151. 18 Brenner I. B. and Myers R. paper presented at the 1989 Pittsburgh Conference Atlanta GA USA March 6-10 1989 paper 301. 19 Winge R. K. Fassel V. A. Peterson V. J. and Floyd M. An Atlas of Spectral Information. Physical Sciences Data 20 Elsevier 1985. 20 Brenner I. B. and Eldad H. ICP Inf Newsl. 1984 10 451. 21 Brenner I. B. and Eldad H. ICP In$ Newsl. 1986 12 243. 22 U.S. Dept. of Commerce National Institute of Standards and Technology Certificate of Analysis for I643b Trace Elements in Water Gaithersburg MD USA. 23 National Research Council Canada Marine Analytical Chemistry Standards Program Certificate of Analysis for SLRS-1 Riverine Water Halifax Nova Scotia Canada. Paper 2/002 I IF Received January 9 1992 Accepted May 14 I992