Anal. Chem. 1988, 6 0 , 2500-2504
2500
with an overall accuracy within &5%. Weighing samples (0.2-1.0 g) directly into polycarbonate containers and dissolving the alloys in aqua regia using a minimum amount of concentrated HF, and employing microwave heating, is a fast, convenient method of sample preparation. Controlling sample size and acid concentration eliminates the need for fluoride complexing agents. Using the 360.07-nm emission line, matrix-matching of yttrium standards with alloy elements is not necessary. Total analysis time for a set of four samples in duplicate typically takes 2 h.
ACKNOWLEDGMENT The authors thank Gerald S. Golden for his helpful suggestions and comments during the course of this investigation and Robert W. Dean for his assistance in preparation of the manuscript.
LITERATURE CITED (1) Whittle, D. P.; Stringer, J. Philos. Trans. 295, 309-329.
R . SOC. London, A
1980,
(2) Tien, J. K.; Pettit, F. S. Metall. Trans. 1972, 3 , 1587-1599. (3) Fornwalt, D. E., Pratt & Whitney Aircraft, East Hartford, CT, unpublished work, 1985. (4) Bolton, A.; Hwang, J.: Voet, A. V. Spectrochim. Acta, Part B 1983. 388, 165-174. (5) Hsu, C. G.; Pan, J. M. Analyst (London) 1985, 110, 1245-1248. (6) Kingston, H. M., National Bureau of Standards, Inorganic Analytical Research Division, Gaithersburg, MD, personal communication, March 1987. (7) Matthes, S. A.; Farrell. R . F.;Mcakie, A. J. Technical Progress Report 120, Bureau of Mines, Analytical Support Services Program, April 1980. (8) Nadkarni, R. A. Anal. Chem. 1984, 56, 2233. (9) Mahan, K. I.; Foderaro, T. A.; Garza, T. L.; Martinez, R. M.; Maroney, G. A.; Trivisonno, M. R.; Willging, E. M. Anal. Chem. 1987, 59, 938. (IO) Wieland, J. R., PCC Airfoils Inc., Minerva, OH, personal communication, March 1987.
RECEIVED for review February 9, 1988. Accepted August 22, 1988. This work was presented in part at the 38th Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 1987 (paper no. 1016).
Evaluation of Inductively Coupled Plasma Mass Spectrometry for the Determination of Trace Elements in Foods R. Duane Satzger Elemental Analysis Research Center, Food a n d Drug Administration, 1141 Central Parkway, Cincinnati, Ohio 45202
Inductively coupled plasma mass spectrometry has been investigated for the determination of trace elements in foods. The effects of the malor elements in foods (Na, Mg, P, K, Ca) on the response of several trace elements found In foods (AI, Cr, Zn, Mo, Cd, Pb) were evaluated. Enhancement or suppresslon of response Is generally less than 10% in the presence of 1000 mg L-’ of each major element added. The effects of mlxed concomitants (Na, Mg, P, K, Ca) at concentratlons of 0.1 % and 0.2% total dlssolved solids on Zn, Mo, Cd, and Pb at 0.01 and 0.10 mg L-’ were investlgated. The greatest suppression observed with the mixed concomitant solutions was on the response of 0.01 mg L-’ Zn. Results are presented for dry ashed reference materials, which compare well with those reported by NBS.
Responsibilities of this laboratory include assessment of the nutritional and/or toxicological significance of trace elements in foods. Elements of interest include those that are naturally incorporated in the food matrix in addition to contributions from agricultural chemicals, processing, environmental contamination, and, in recent years, product tamperings. Several analytical instrumental techniques have traditionally been required to obtain information on major and ultratrace element concentrations. Most nutritional elements are present a t parts-per-million levels in foods and can therefore be determined by using inductively coupled plasma optical emission spectroscopy (ICP-OES) or flame atomic absorption spectrometry (I, 2 ) . However, many elements present a t ultratrace levels, such as Cd and Pb, require the use of more sensitive and time-consuming techniques such as graphite furnace atomic absorption spectrometry (GFAAS) or differential pulse anodic stripping voltammetry (DPASV).
Sub-part-per-billion determination of P b by GFAAS requires multiple injections and voltammetric techniques for P b at this level require an extended electrochemical preconcentration step (3). Inductively coupled plasma mass spectrometry (ICP/MS), on the other hand, offers comparable or better multielement sensitivity with direct solution nebulization ( 4 ) . This is a tremendous advantage when investigating an unknown, such as in a product-tampering case. In many situations, samples can be rapidly screened for the type of contaminant. If the adulterant can be isolated, an elemental “fingerprint” may be established in an effort to define its origin ( 5 ) . One drawback is that the sample matrix reportedly has an effect on the analyte response, requiring extensive dilution of the sample in order to reduce sample induced matrix effects. However, these reports have not been consistent in type or degree of matrix induced interference reported (6-10). Matrix effects reported by other investiators appear dependent on the instrument used in the study as well as source operating conditions such as sampling distance, power, and nebulizer flow rate. On the basis of these studies (7-IO), it is apparent that instruments that utilize greater sampling distances (15-20 mm) require higher nebulizer flow rates (0.8-1.2 L min-l) to obtain an optimum analyte count rate. At the above sampling distances and flow rates, matrix-induced effects appear more severe than those effects reported at shorter distances (110 mm) and lower nebulizer flow (0.5-0.7 L min-’) rates ( 4 ) . Source grounding configuration, interface design, and ion optical configuration, including the pressure in the ion lens region, may play a role in elucidating the origin of matrixinduced changes in analyte response. In recent work, Munro et al. (11) investigated the application of ICP-MS to the determination of V, Mo, Cd, and P b in biological standard materials. This work studied molecular
This article not subject to U.S.Copyright. Published 1988 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988 C
a
a
f
Flgure 1. ICP-MS system schematic: (a) sampler, (b) skimmer, (c) first stage rough pumping, (d) ion lenses, (e) viewing port, (f) diffusion pump, (g) quadrupole mass analyzer, (h) turbomolecular pump, (i) analog amplifier, (j) pulse amplifier.
interferences that originated from chemicals used in oxidation of the sample in preparation for analysis. In other work, Dean e t al. (12) experienced little suppression of the P b response in the presence of Na or Ca although the precision of isotope ratios was degraded. In the study to be reported, matrix-induced effects were determined on the analyte response with no modifications in the instrumental optimization scheme. The instrument was tuned t o maximize a plasma background ion count rate and all subsequent measurements were made by using the same tuning parameters. Power, gas flows, sampling distance, and sampling position were based on earlier studies which established the optimum response for singly ionized atomic species relative to doubly ionized and molecular species in the source. Results for Al, Cr, Ni, Zn, Rb, Mo, Cd, and P b determined in biological reference materials are compared with optical emission and DPASV results.
EXPERIMENTAL SECTION The instrument used in this work was designed and constructed in this laboratory and has been described elsewhere as used with the helium (13)and argon (14) microwaveinduced plasmas (MIP). The following modifications illustrated in Figure 1 were incorporated to increase the pumping speed of the system and lower the second and third stage pressures. A 20.3 cm diameter second stage and a 15.2 cm diameter third stage with a 10.2 cm diameter horizontally mounted turbomolecular pump were fabricated in order to improve the system conductance. These improvements resulted in second and third stage pressures during plasma sampling of and lo-' Torr, respectively. First stage pressure with the Ar plasma remained at approximately 1 Torr. A water-cooled brass interface was designed which provides symmetrical cooling of the sampler. This interface replaces the stainless steel interface which did not require cooling when used with the MIP. This interface also incorporates two ports for additional rough pumping during He operation. Sampler and skimmer cones are constructed from nickel. Both cone orifices are 0.07 cm in diameter with a 0.6 cm spacing between cones. Second stage path length, the distance from the tip of the skimmer to the differential pumping lens (DPL), was increased to 15 cm. The diameter of the DPL was reduced to 0.2 cm. The ion lens elements were fabricated from an aluminum rod, which facilitated frequent modifications. The lens stack used in this work is illustrated in Figure 1. A higher current lens supply was constructed in order to improve stability of those lens elements that experience more ion collisions. Each of the lens elements is electrically isolated. There were no modifications in the mass filter. The quadrupole center potential was maintained at 0 V. Resolution was adjusted to 0.5 amu at 50% peak height by using the 100 ppb standards. A +10 V potential applied to the enrance lens reduced ion transmission to a level that was not detectable by using the analog detection mode.
2501
The ICP operating parameters were as follows: 1.2 kW radio frequency (rf) forward power (Jarrell-Ash generator); 0.64 L min-' nebulizer flow rate; 0.8 L min-' auxiliary flow rate; 16 L min-' coolant flow rate. Nebulizer and coolant flows were regulated by using mass flow controllers (tylan). The sampling distance was 1.0 cm above the load coil on center. These ion source parameters were selected after monitoring changes in the response for 1 ppm analyte solutions with changes in nebulizer flow rate at several rf powers. Because this instrument exhibits a visible discharge at the sampler orifice as the source is moved into sampling position, plasma conditions were selected to keep oxide and doubly ionized species below 0.5% and 2%, respectively,while introducing 1 ppm Ba (15). The torch box (VG Isotopes) was grounded to the vacuum chamber and mounted on extra large translation stages (Oriel) to enable three-dimensional plasma profiling. A peristaltic pump (Gilson) was used to regulate the rate of solution delivery to a concentric nebulizer (Meinhard, Type A) at about 1 mL m i d . A double pass spray chamber was water cooled to 6 "C by using a refrigerated recirculator (Neslab), which also cooled the load coil and the interface. Solutions were continuously nebulized into the plasma. Ion optics were tuned to obtain maximum response for 02+ (m/z 32 amu) in the pulse counting mode at an electron multiplier setting of 2000 V. This procedure was used for tuning the instrument throughout this study. Tuning on each analyte mass does provide an improvement in count rates; however, this instrument shows little ion optical discrimination with changes in mass and sufficient sensitivity for all elements is obtained without returning for each analyte. Typical 208Pb+response was 150 kHz per wg/mL-* with a background count rate of 0.085 0.002 kHz. The electron multiplier setting was stepped up to 3000 V for ultratrace (sub-part-per-million) analyte measurements. Analyte element data acquisition was in the single ion, pulse counting mode. Count rates were manually recorded after each 10-s integration period. At least five 10-s integration periods were averaged for each solution studied. Five sets of solutions were prepared in 1% "OB (G. F. Smith) by using aqueous ICP standards (SPEX), with each set containing one of five major elements (Na, Mg P, K, Ca) found in foods and agricultural samples. The concentrations of these major elements (concomitants) were 10,50,200,500, and loo0 mgL-'. All of these solutions contained 0.1 mg L-l Al, Cr, Ni, Zn, Mo, Cd, and Pb. The principal isotope for each element was monitored in the single ion mode, Le., 27A1,52Cr,58Ni,64Zn,98Mo,Il4Cd, and 200Pb. Data for 64Zn were corrected for contribution from 64Ni. Five solutions were prepared with each solution containing 1000 mg L-' of each of the concomitants. These concomitant solutions only defined the contribution of the concomitant to the analyte response, in the form of either an unresolved spectral overlap or contamination of the standard solution with the anal-. The sequence of analysis involved determination of the 0.1 mg L-' analyte response in a 1%HNOBsolution containing no concomitant before and after every concomitant-plus-analyte solution. A 1% HNO, blank was run before and after the sequence of standard and concomitant solutions for each element in order to monitor the background count rate. This sequence of determinations enables correction to be made for instrumental drift due to concomitant deposition on the sampler or skimmer, fluctuation of the gas flows, elevation in the temperature of the system, and changes in detector response. After correction of the analyte ion count rate for contribution from the concomitant-only solutions, the ratio of the analyte ion count rate in the presence of added concomitant to the mean of the analyte ion response without concomitant was calculated for each of the five concentrations of concomitant plus analyte solutions. The assumption here is that the time difference is small between standardizations resulting in drift that is linear with time. The same sequence was used in the analysis of the mixed concomitants and sample digest solutions. Mixed concomitant solutions containing the five major elements (Na, Mg, P, K, Ca) were prepared at 200 mg L-' of each concomitant (0.1%) and at 400 mg L-' of each concomitant (0.2%). These solutions were spiked with 0.01 and 0.10 mg L-' of Zn, Mo, Cd, and Pb. Agricultural samples used in this study were NBS standard reference materials and agricultural crop samples from a previous study (1). Sample weights for digestion and dilution factors were
*
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
2502
Reaponse oonoomitant)/Znl _Ratlo _ [(Zn _ _ _ _ _ ~
__
r-
1.3
-
--1
I
t
1.1
-----
0.7 t 07t
'--
I
1
0.6 0
200
I
400
800
---_I 800
I
I I 05.
1000
Concomitant concentration, mg/L Figure 2. Effect of concomitant concentration on the ratio of the 0.1 mg L-' Zn response in the presence of a concomitant element [B, Na;
+, Ca; *,
' 1
K; 0, Mg; X, P] to the response with no concomitant.
0
L -
L L -
6
10
16
20
.A 26
Time, minutes Figure 4. Drift of normalized response for a 0.10 mg L-' Cr standard
after introduction of each concomitant solution [B, Na; +, Ca; *, K; 0, Mg; X, P; at 10, 50, 200, 500, and 1000 mg L-'1 as a function of time.
Response Retlo [(Pb * concomltant)/Pbl
1 i
1
0.7
I
I
1.1 t
1
0.71
Y.8
0
Response Ratio (t/t-0)
200
400
800
800
1000
Concomitant concentration, mg/L Figure 3. Effect of concomitant concentration on the ratio of the 0.1 mg L-' Pb response in the presence of a Concomitant element [B, Na; +, Ca; *, K; 0, Mg; X, P] to the response with no concomitant. selected to maintain less than 0.1% total dissolved solids after complete oxidation of the organic matrix. Dissolved solid content was based on data obtained from NBS and ICP-OESanalyses. Samples were dry ashed with 3 mL of 40% H2S04(Ultrex). After oxidation at 500 "C,samples were reashed with 1 mL of HN03 at 500 "C, acidified, dissolved, and diluted with distilled deionized water ( 3 ) .
RESULTS AND DISCUSSION Figures 2 and 3 are characteristic of the effects of concomitant matrix elements on the response obtained for 0.10 mg L-' solutions of Al, Cr, Zn, Mo, Cd, and Pb. Plotted are ratios of analyte responses in the presence of Na, Mg, P, K, or Ca to analyte elements without concomitant elements versus the concentration of the concomitant element. For most elements investigated, enhancement or suppression of the analyte response was less than 10% in the presence of 1,000 mg L-' added concomitant. Aluminum has the lowest atomic mass and first ionization potential of those analyte elements investigated. In general, because of problems with spectral overlap due to inadequate resolution of major background ions at adjacent masses, the data exhibited considerable scatter. For all six analyte elements investigated, no trends were observed with small changes in atomic mass of the concomitant elements. The first ionization potential of the concomitant elements, which ranged from 4.34 eV for K to 10.48 eV for P, showed few differences in their effects on the analyte response. The signal enhancements for A1 in the presence of Mg may be the result of an inability to completely resolve 27Al+from 2sMg+. The response of a lo00 mg L-' Mg blank, with no Al added, at mlz 27 was approximately twice the magnitude of the 0.10 mg L-' A1 response. Therefore, determination of A1 in samples containing Mg will require accurate blank or background correction. The background count rate at mlz 27 without the
05L..-0
L-I
6
10
A_-/
16
20
26
Time, minutes Figure 5. Drift of normalized response for a 0.10 mg L-' Mo standard after introduction of each concomitant solution [B, Na; +, Ca; *, K; 0, Mg; X, P; at 10, 50, 200, 500, and 1000 mg L-'1 as a function of time. presence of concomitant elements was higher (several hundred counts) than that observed at higher masses, where no overlaps or spectral interferences occur. Nz also makes a contribution a t mlz 27 on this instrument. A notable effect on Cr, Zn, and Mo responses was a 15-20% suppression in the presence of P. This is illustrated in Figure 2 with the Zn response. Since the first ionization potential of P is 10.484 eV, P is only partially ionized and a shift of the analyte ion population in the direction of neutral atom formation would not be the major mechanism of signal suppression. Cr, Zn, and Mo response suppression may be a result of a chemical interference. The P solutions were made from NH4H2P0,in HzO. Analyte vaporization interferences due to the presence of phosphate during optical emission measurements are well documented (16). However, based strictly on thermal properties, Al, Cd, and P b form more stable metal orthophosphates and these elements exhibit a slight enhancement in the presence of P as illustrated in Figure 3 with the P b response. If a chemical interference caused by recombination with phosphorus occurs, the probability of recombination is greatest before entering the skimmer due to a reduction in the temperature and in the collision frequency of the ions as the pressure is reduced. The effects of concomitant elements on P b illustrated in Figure 3 demonstrate less scatter a t high concentrations of all concomitants than was encountered with the lower mass elements. This supports reports that high mass elements are less susceptible to matrix-induced effects (8-10). Matrix-induced enhancement or suppression effects that are sustained beyond the time where their introduction to the system is discontinued as described by other investigators were not observed in these studies ( 4 9 ) . Figures 4 and 5 illustrate drift in instrument response during the time required to collect
ANALYTICAL CHEMISTRY, VOL. 60,
concn (ppb) Mo Pb Cd Zn
7.10 7.42 8.99 9.39
98 208 114 64
10"
100"
IOb
1OOb
NCC NC
NC NC
8 13
6 9
6 8 17 30
9 7 18 18
"200 ppm of Na, Mg, P, K, Ca added (0.1%). b400ppm of Na, Mg, P, K, Ca added (0.2%). cNo change in response. dCRC Handbook of Chemistry and Physics, 53rd ed.: CRC Press: Cleveland OH, 1972.
Table 11. Comparison of the Results Obtained by ICP-MS and ICP-OES for the Determination of Trace Elements in NBS (1570) Spinach
concentration, lg/g NBS ICP-OES
element
ICP-MS
Pb Cd Zn
1.21 1.52 50.4 (52.7)" 5.93 12.9 4.74 0.305 529 (654)"
Ni Rb Cr Mo A1
1.2 f 0.2 (1.5)b 50 f 2
51.8
(6Ib
12.1 f 0.2 4.6 h 0.3 870 f 50
Value based on standard addition. ported by NBS.
645
Noncertified values re-
Table 111. Comparison of the Results Obtained by ICP-MS and ICP-OES for the Determination of Trace Elements in NBS (1567) Wheat
element
ICP-MS
Pb Cd Zn Ni Rb Cr Mo A1
0.028 0.034 10.6 0.226 0.964 0.270 0.436 3.09
concentration. UE/P NBS
ICP-OES
(0.022f 0.01)" 0.032 f 0.007 10.6 f 0.1 (0.18)" (1)"
2503
sample introduction system due to the introduction of high concentrations of matrix elements, instrumental drift is a function of system temperature equilibration and rate of salt deposition in the sampler orifice. The data in Figure 4 were obtained after an equilibration period of 15 min and illustrate a worst case situation. If the instrument has not reached an equilibrium temperature, the rate of change in the analyte response is rapid as illustrated during observation of the net Cr response during the Cr plus Na matrix study. These data illustrate a decrease in the response by 31% over the first 25 min of operation. The change in response is not as rapid during acquisition of the remainder of the Cr data; however, instrumental drift was in the direction of a reduction in response for all elements investigated. Figure 5 illustrates the drift in the Mo response after a 1.5-h equilibration period. In this case, the rate of drift was less rapid; however, it is typical of the amount of drift experienced with this instrument. The equilibration period used in this work was 1 h. A major contribution to drift could be deposition of salts in the sampler orifice. This is supported in Figure 5, which shows that Ca and Mg, which form stable oxides, exhibit greater drift than Na, K, and P between T = 20 and 25 min, which is when the lo00 mg L-'concomitant element solutions are introduced. The effects of a mixture of concomitant elements on the analyte response are illustrated in Table I. A minimum of five 10-s count integration periods for the 1% HNOBblank, mixed concomitant background, and mixed concomitant plus analyte were used in determination of percent suppression. The sequence of analyses was identical with that described earlier. The precision of the count integration measurements was within 3% relative standard deviation and this precision was not degraded during measurement of 10 ppb standard in the presence of 0.2% total concomitants. As expected, the extent of suppression is greater in the presence of a higher level of total concomitant elements. Reproducibility of the degree of suppression was within 2% for a replicate set of P b measurements. The data indicate at both 0.1% and 0.2% total concomitant element levels that the ionization potential of the analyte shows a correlation with the degree of suppression. The data also indicate that an increase in the molar ratio of concomitant elements to analyte element by varying the analyte concentration while holding the concomitant level constant was not responsible for increased suppression in most cases, with Zn being the exception. After establishment of the level of dissolved solids that could be present in solution while permitting a single standard calibration, NBS standard reference materials were H2S04 dry ashed, diluted in 1% "OB, and analyzed. A single digestion was performed for each sample. The H2S04dry ash is used in this laboratory for the determination of Cd and P b by using stripping voltammetry (3). The same sequence of determinations was used, whereby standards were run before and after all samples. An average of seven integration periods of 10-s duration for each element were acquired for samples and standards. After method blank correction, the sample
% suppression at analyte
isotope m / z 1st IP,d eV
NOVEMBER 15, 1988
L-'concomitant, respectively. Assuming no changes in the
Table I. Effect of Mixed Concomitant Elements on Analyte Element Response
element
NO. 22,
10.8
(0.4)" 2.79
" Noncertified values reported by NBS. data for Cr and Mo in the presence of the five concomitant elements investigated. Plotted are ratios of net analyte response at t = 5,10, 15,20, and 25 min to net analyte response at t = 0 versus time. At t = 2.5, 7.5, 12.5, 17.5, and 22.5 min, 0.10 mg L-' analyte in the presence of concomitant elements was introduced in the sequence 10,50,200,500, and 1000 mg
Table IV. Comparison of the Results Obtained by ICP-MS with DPASV and ICP-OES for the Determination of Pb, Cd, and Zn in Agricultural Crop Samples
concentration, l g / g Pb ~.
a
Cd __
Zn
sample
ICP-MS
DPASVO
ICP-MS
DPASV"
ICP-MS
ICP-OES
corn lettuce potato soybeans
0.10 0.37 0.13 0.067
0.12 0.37 0.17 0.065
0.059 0.92 0.56 0.16
0.066 0.88 0.45 0.13
46 28 13 36
48 30 13 39
Values based on single scan.
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
concentration based on the mean of standards in 170H N 0 3 analyzed before and after each sample was determined. Tables I1 and I11 report data obtained for eight elements in NBS spinach and wheat. Approximately 1g of spinach was diluted to 100 mL in order to reduce the level of dissolved solids in solution to less than 0.1% . Three grams of wheat was diluted to 50 mL. With these dilution factors, determination of most trace elements, which was routine by ICP-MS, was not possible with ICP-OES. However, even with a smaller dilution, several elements that are of interest such as Cr, Cd, Mo, and P b often may not be determined directly by ICP-OES, requiring utilization of an alternative technique. ICP-MS values are generally within the concentration range reported by NBS with A1 in spinach being an exception. Since the ICP-MS value obtained by standard additions agrees with the optical value, there appears to be a problem in the dry ash for Al. A white precipitate in the spinach dilution was not analyzed for the presence of Al. In Table IV, results obtained by ICP-MS are compared with DPASV and ICP-OES data on Pb, Cd, and Zn in raw agricultural crop samples ( 3 ) . Data are based on a single digest of each sample type. With most P b and Cd solution concentrations in the range of 0.001-0.003 mg L-I, there is good agreement between the ICP-MS values and the voltammetric data.
CONCLUSIONS The analyte elements chose for study represent a spectrum of analyte masses, ionization potentials, and mass spectral background problems. Data obtained for these analyte elements should be applicable to other analytes in the same matrix. On the basis of the results obtained in the matrix investigations and in the sample analyses, it is apparent that ICP-MS is a valuable tool for those laboratories investigating food samples. Matrix-induced suppression of analyte response in the presence of a mixture of the five major elements found in foods appeared dependent on analyte ionization potential. Accurate results were obtained during the analysis of standard reference materials in the presence of 0.05-0.1 % dissolved solids with a single point calibration. Drift correction
was used in all determinations. Although drift correction using replicate standardization is time-consuming, it is more rapid than standard additions. Interferences due to insufficient resolution in this laboratory-built instrument generally were not a problem, although a higher resolution quadrupole or a narrower ion energy distribution would improve the accuracy of analyte elements whose m / z is adjacent to major background or matrix ions.
ACKNOWLEDGMENT The loan of the torch box by J. A. Caruso of the University of Cincinnati is gratefully acknowledged. Registry No. Na, 7440-23-5; Mg, 7439-95-4; P, 7723-14-0; K, 7440-09-7; Ca, 7440-70-2; Mo, 7439-98-7; Pb, 7439-92-1; Cd, 7440-43-9; Zn, 7440-66-6; Ni, 7440-02-0; Rb, 7440-17-7; Cr, 7440-47-3; Al, 7429-90-5.
LITERATURE CITED (1) Wolnik, K. A.; Fricke, F. L.; Capar, S. G.; Braude, G. L.; Meyer, M. W.; Satzger, R. D.; Kuennen, R. W. J. Agrlc. Food Chem. 1983, 31, 1244-1 249. (2) Fricke, F. L.; Robbins. W. 6.; Caruso, J. A. Prog. Anal. At. Spectrosc. 1979. 2. 185-286. (3) Satzger, R . D.;'Bonnin, E.; Fricke, F. L. J. Assoc. Off. Anal. Chem. 1984, 67, 1138-1 140. (4) Gray, A. L. Spectrochim. Acta, Part B 1986, 418, 151-167. (5) Wolnik, K. A.; Fricke, F. L.; Bonnin, E.; Gaston, C. M.; Satzger, R . D. Anal. Chem. 1984, 56, 466A-474A. (6) Olivares, J. A.; Houk, R. S. Anal. Chern. 1986, 58, 20-25. (7) Vaughan, M. A.; Horlick, G.; Tan, S. H. J. Anal. At. Spectrorn. 1987, 2, 745-763. ( 8 ) Gregoire, D. C. Spectrochim. Acta, Part B 1987, 428, 895-907. (9) Beauchemin, D.; McLaren, J. W.; Berman, S. S. Spectrochim. Acta, Part B 1987, 428,467-490. (10) Tan, S . H.; Horlick, G. J. Anal. A t . Spectrorn. 1987, 2 , 745-763. (11) Munro, S.; Ebdon, L.; McWeeny, D. J. J. Anal. A t . Spectrom. 1986, 1, 211-219. (12) Dean, J. R.; Ebdon. L.; Massey, R. J. Anal. A t . Spectrorn. 1987, 2. 369-374. (13) Satzger, R. D.; Fricke. F. L.; Brown, P. G.; Caruso, J. A. Spectrochirn. Acta, Part B 1987, 428, 705-712. (14) Satzger, R. D.; Fricke, F. L.; Caruso, J. A. J. Anal. A t . Spectrorn. 1988, 3 , 319-323. (15) Satzger, R. D.; Fricke, F. L.; Caruso, J. A. 29th Rocky Mountain Conference. 1987. (16) Kornblum, G. R.; de Galan, L. Spectrochirn. Acta, Pari B 1977, 328, 455-478.
RECEIVEDfor review April 4, 1988. Accepted August 1, 1988.
