Anal. Chem. 1988. 60, 687-691 (5) Jaklevic, J. M.; Giauque, R. D.. Thompson, A. C. Nucl. Instrum. Methods Fhys. Res., Sect. E 1985, E 1 0 / 1 1 , 303-308. (6) Iida, A.; Yoshinaga, A,; Sakurai, K.; Gohshi, Y. Anal. Chem. 1988, 58,394-397. (7) Aiginger, H.; Wobrauschek, P. I n Advances in X-Ray Analysis; Barrett, C. s., Predecki, P. K., Leyden, D. E., Eds.; Plenum: New York, 1985; Vol. 28, pp 1-10, ( 8 ) Knoth, J.; Schwenke, H. Fresenius’ Z.Anal. Chem. 1980, 301, 7-9. (9) Ihnat, M.; Wolynetz, M. S.;Thomassen, Y.; Verlinden, M. Pure Appl. Chem. 1986, 58, 1063-1076. (10) Currie. L. A. X-Ray Fluorescence Analysis of Environmental Samples; Dzubay, T. G . , Ed.; Ann Arbor Sclence: Ann Arbor, MI, 1977; Chapter 25. (11) Spal, R.; Dobbyn, R. C.; Burdette, H. E.: Long, G. G.; Boettinger, W. J.; Kuriyama, M. Nucl. Instrum. Methods Phys. Res., Sect. A 1984, 222, 189-192. (12) Kuehner, E. C.; Alvarez, R.; Paulsen, P. J.; Murphy, T. J. Anal. Chem. 1972, 44, 2050.
687
(13) Pella, P. A,; Myklebust, R. L.; Darr, M. M.; Heinrich, K. F. J. NBS Special Report No. 77-121 1; US. Government Printing Office: Washington, DC, June 1977. (14) Kingston, H.; Pella, P. A. Anal. Chem. 1981, 53,223-227. (15) Moody, J. R.; Epstein, M. S. “Container Caused Contamination of SRM’s in Serum and Urine”; NBS Report of Analysis, February 1984.
RECEIVED for review July 13, 1987. Accepted September 11, 1987. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Determination of Trace Metals in Marine Biological Reference Materials by Inductively Coupled Plasma Mass Spectrometry Diane Beamhemin,* J. W. McLaren, S. N. Willie, and S. S. Berman
Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9
Inductively coupled plasma mass spectrometry ( ICP-MS) was used for the analysls of two marine biological reference materlals (dogfish liver tissue (DOLT-1) and dogflsh muscle tissue (DORM-I)). The materlais were put Into solution by digestion In a nitric acld/hydrogen peroxide mixture. Thirteen elements (Na, Mg, Cr, Fe, Mn, Co, NI, Cu, Zn, As, Cd, Hg, and Pb) were then determined. Accurate results were obtained by standard addltlons or isotope dllution technlques for ail of these elements in DORM-I and for ail but Cr In DOLT-1.
Inductively coupled plasma mass spectrometry (ICP-MS) is a rapidly expanding technique, the development and many current features of which have been summarized in four recent review articles (1-4). Although the number of applications is increasing in many areas, there are still few of them devoted to the analysis of biological materials. While concentrations of many of the trace metals of interest in these materials are low enough to necessitate a minimum of dilution in the sample dissolution procedure, concentrations of certain concomitant elements (e.g. K, Na, Ca, and Mg) can be high enough to cause significant suppression or enhancement effects (5,6). ICP-MS appears to be more susceptible to interferences by such elements than would have been predicted on the basis of previous experience in inductively coupled plasma atomic emission spectrometry (ICP-AES) (3-5). While the mechanism of these effects is not yet fully understood, processes additional to those occurring in the ICP are clearly involved. The generic term “ion sampling effects” was recently proposed (7) to describe phenomena unique to ICP-MS which may be occurring in the interface region between the two ion sampling orifices (i.e. the “sampler” and the “skimmer”) and in the ion optics between the skimmer and the quadrupole mass filter. Furthermore, ICP-MS is also more susceptible than initially expected to isobaric interferences from polyatomic species arising from the plasma (8),the acids used in sample preparation ( 9 , l O ) or the sample itself (e.g. ref 10). Because of the low concentrations of many of the elements of interest in biological
materials, the dissolution procedure is an especially critical step. Three reports on the analysis of reference biological materials (11-13) by ICP-MS have appeared. Pickford and Brown (11) described the analysis of three NBS standard reference materials (orchard leaves, bovine liver, and oyster tissue) and two other bovine liver materials by ICP-MS and ICP-AES. They had problems of poor precision with ICP-MS compared to ICP-AES using external calibration (i.e. the use of reference solutions) only and recommended the use of isotope dilution. They also reported matrix suppression not seen in ICP-AES. Ward and co-workers (12) described the analysis of three reference materials to assess errors associated with the matrix, using holmium as an internal standard. They noted some problems of polyatomic isobaric interferences a t low anal@ levels. Finally, Munro and co-workers (13)looked a t two NBS reference materials and one other proposed reference material by ICP-MS (using Bi and Y as internal standards) and electrothermal atomization atomic absorption spectrometry (ETA-AAS). They noted that ICP-MS was less precise than ETA-AAS, especially a t the lower levels. However, none of these studies used either the method of standard additions or the isotope dilution technique which have been reported to be less susceptible to ion sampling effects than external calibrations (e.g. ref 14 and 15). The recent development of two marine biological reference materials (16) in this laboratory (a dogfish liver tissue with the acronym DOLT-1 and a dogfish muscle with the acronym DORM-1) provided an opportunity to assess the performance of ICP-MS in the analysis of two biological tissues which were characterized at the same time by a number of other methods, including atomic absorption spectrometry and neutron activation analysis. Trace elements were determined by isotope dilution ICP-MS whenever possible and additional analyses were performed by the method of standard additions.
EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer used for this work was the ELAN 250 SCIEX Division of MDS Health Group, Ltd. (Thornhill, Ontario, Canada).
0003-2700/88/0360-0687$01.50/00 1988 American Chemical Society
688
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
Table I. ICP-MS Operating Conditions torch rf power reflected power plasma gas flow auxiliary gas flow nebulizer gas flow
Plasma Conditions conventional ICP-AES 1.2 kW -15 U' 14 L/min 2.0 L/min 0.9 L/min
Mass Spectrometer Settings Bessel box stop -5.9 to -7.3 V Bessel box barrel 2.9-5.0 V -8.0 to -12.0 V Einzel lenses 1 & 3 Einzel lens 2 -130 V -10.9 to -11.4 V Bessel box end lenses sampler orifice diameter 1.14 mm skimmer orifice diameter 0.89 mm interface running pressure 0.8-2 Torr mass spectrometer running pressure (1.3-8.0) X Torr Three modifications were made to the originally supplied instrument. A mass flow controller (Model 5850, Brooks Instrument Division, Emerson Electric, Hatfield, PA) was added to the nebulizer gas line and a peristaltic pump (Minipuls 11, Gilson Medical Electronics, Inc., Middleton, WI) was used to maintain the sample delivery to the nebulizer at 1.1 mL/min. Also, a conventional ICP-AES torch was used instead of the approximately 15 mm longer one that was provided with the instrument. The operating conditions used throughout this work are summarized in Table I. Under these conditions, the sampling height (or sampling depth), defined as the distance between the tip of the sampler and the initial radiation zone (17) observed while aspirating a 1000 mg/L Y solution (18) is about 10 mm. The sampler and skimmer were both made of nickel, although some brass samplers and skimmers plated with either Ni or Cr were also used for short periods of time. The measurement parameters used were the following. The low resolution setting was used (which corresponds to a peak width of 1.1 u a t 10% of peak height). The sequential measurement mode (in which the measurements are made by spending the total specified measurement time a t one mass before going to the next) was used for all the elements. Either 0.1 or 0.2 s was used as the measurement time with three measurements taken per peak, one measurement being done at the central mass while the two others were done at f O . l u from the assumed peak center. Finally, from 20 to 50 repeats were made, with a counting precision of 0.2% and a threshold of 1 ion/s. Reagents. All acids were purified by subboiling distillation in a quartz still. The enriched isotopes used for the stable isotope dilution analysis were purchased from the Oak Ridge National Laboratory. They included 53Cr,"Cu, 67Zn,"'Cd, and '07Pb. All these stable isotopes were dissolved as described previously (9). The marine biological reference materials DOLT-1 and DORM-1 are partially defatted protein powders produced by acetone extraction and spray drying of homogenized dogfish tissues. (Complete information on the procurement of the marine biological reference materials DOLT-1 and DORM-1 and other marine reference materials can be obtained from S. Berman, Marine Analytical Chemistry Standards Program, Chemistry Division, M-12, National Research Council of Canada, Ottawa, Ontario, Canada K1A OR9.) Digestion Procedure. All sample preparations were carried out in a clean laboratory providing a class 10 working environment. Either 0.5 or 1 g of material was weighed and transferred to a 125-mL quartz Erlenmeyer flask. For isotope dilution analyses, the stable isotope spikes shown in Table I1 were added. Then 25 mL of concentrated HN03 was added and the flasks were covered and heated on a hot plate (60-70 "C) for about 2 h. The temperature was then raised to about 155 "C and the mixtures were left to reflux for several hours. The covers were then taken off to let the solutions evaporate down to dryness. The flasks were left to cool for about 30 min, and then 5 mL of concentrated HNO, was added to each flask. Then, handling one flask at a time (because a vigorous reaction is possible), 1 mL of 30% H,Oz was added, the mixture was swirled and put back on the hot plate (70 "C). When all the solutions stopped foaming, the plate's
Table 11. Spikes Added to 0.5 g of Biological Material, Prior to Its Digestion, for Its Isotope Dilution Analysis
isotope
DOLT-1 spike, pg
Wr 65CU 67Zn "'Cd ZOIHgU 207Pb
3.96 8.29 0.461 0.050 0.250
DORM-1 spike, pg
digestion blank spike, Pg
1.51 1.03 1.64 7.02 0.200
0.0304 0.0411 0.0650 0.0084
0.105
0.0128
0.0321
This spike was made to 1 g of biological material. temperature was raised to take them down to dryness (for the last milliliter, the flasks were tilted by putting a glass tube under one side, so as to maximize the evaporation),taking care to avoid charring of the residue. They were left to cool before repeating the addition of 5 mL of concentrated HNOBand 1 mL of HzOz and evaporation to dryness as described above. Finally, 1 mL of concentrated HN03 and 0.5 mL of HzOzwere added to each flask. The solutions were heated on the hot plate until they stopped foaming (without boiling), a t which point they were removed and left to cool before transferring them to 50-mL volumetric flasks and diluting to volume with deionized distilled water. A different digestion procedure was used for the determination of Hg. In this case, 1 g of material was weighed and transferred to a 125-mL quartz Erlenmeyer flask along with about 5 mL of deionized distilled water (to make a slurry). The stable isotope spikes were then added, followed by 10 mL of doubly distilled HNO,. This mixture was left to stand for 10 min before adding 10 mL of subboiling distilled HC104. The flasks were covered and heated on a hot plate (120 "C) for about 1 h. The caps were then removed and the temperature raised (170 "C) to take the solutions down to about 2 mL. After cooling, these solutions were transferred to 25-mL volumetric flasks and diluted to volume with deionized distilled water. Whether for the determination of Hg or that of the other elements, blanks were prepared by following exactly the same procedure as for the analyte but without any biological material. Analysis Procedure. Initialization. The ion lens voltages of the instrument were set as described previously (15). Since this was done each time the sampler and skimmer were replaced and/or cleaned, a range of operating voltages (listed earlier) was obtained during the period of the certification process (i.e. several months). The plasma operating conditions (which were chosen according to a previous work (14)) allow a compromise between high sensitivity and low oxide levels. Standard Additions Analysis. Standard additions analyses of tissue digests and digestion blanks were done by using three aliquots of each of the following: one unspiked aliquot, one aliquot spiked with approximately the same analyte concentrations as in the digest, and a third aliquot with twice these quantities. These thiee aliquots were then diluted with deionized distilled water to the same final volume (by a factor of 2,10, or 1000, depending on the analyte concentrations). The net intensities, obtained by subtracting the background intensity (measured while running a nitric acid solution of the same concentration as the diluted digests) from that of the three solutions, were used in the calculation of the analyte concentrations. No correction was made for drift. The results for the digestion blanks were subtracted from those of the corresponding tissue digests. The major isotope of each analyte was used in all cases except Ca and Fe for which 'Ca and 54Fewere used instead because of major isobaric interferences (from 40Arand 40Ar160,respectively) on their most abundant isotopes. The method of standard additions was used especially for the monoisotopic elements since isotopic dilution cannot be used in these cases. Three to five separate standard additions analyses of a t least two different bottles of reference biological material were made for each one of the two materials. Isotopic Ratio Measurements. A series of isotopic ratio measurements was made on some unspiked undiluted digests to determine which isotopic pairs were free from isobaric interferences from polyatomic species, and could be used for isotope
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
689
Table 111. Isotopic Ratios Measured in the Digests of Marine Biological Materials DOLT-1 and DORM-1 element
isotopic ratio
expected ratio”
DOLT-1 ratio
DORM-1 ratio
Cr Ni
53/52 61/60 62/60 65/63 67/68 1111114 204/208 2061208 2071208
0.117 f 0.001 0.0448 f 0.0003 0.147 f 0.002 0.465 f 0.001 0.217 f 0.002 0.444 f 0.002 0.0279 f 0.0002 0.471 f 0.006 0.426 f 0.004
0.128 f 0.019* 0.107 f 0.044 0.216 f 0.061 0.464 f 0.025 0.219 0.010 0.441 f 0.032 0.028 f 0.006 0.488 f 0.057 0.411 f 0.048
0.110 f 0.006 0.084 i 0.030 0.139 f 0.039 0.470 f 0.026 0.214 f 0.024 0.43 f 0.16 0.054 f 0.038 0.48 f 0.11 0.414 f 0.075
cu
Zn Cd Pb‘
*
“Measured while aspirating a 100 pg/L standard solution of each element. bPrecisionexpressed as the standard deviation (n = 2). ‘The expected ratios were that of the Pb standard SRM 981 from NBS. dilution analysis. It was noted previously (9)that any difference between the measured ratios and the expected ones (Le. that of standard solutions) indicated the presence of isobaric interference(s). Isotope Dilution Analysis. For each element, three to five separate measurements were made on at least three different bottles of each biological material. In all cases, a blank consisting of 1.5 M HN03 (which is the approximate final acid concentration of the digests) was subtracted to eliminate contribution from the background. The analyte concentrations were calculated by using the following formula: M,K(A, - B$E) C= W(BR - A ) where C is the analyte concentration in the sample (micrograms per gram), M, is the mass of the stable isotope spike (microgrania), W is the weight of sample (grams), A is the natural abundance of the reference isotope, B is the natural abundance of the spike isotope, A, is the abundance of the reference isotope in the spike, B, is the abundance of the spike isotope in the spike, K is the ratio of the natural and spike atomic weights, and R is the measured ratio (reference isotope/spike isotope), corrected for mass discrimination where needed (as in ref 9), after the spike addition. The results of the isotope dilution analyses of the digestion blanks were subtracted from those of the corresponding samples.
RESULTS AND DISCUSSION Isotopic Ratio Measurements. A series of isotopic ratios was measured in some undiluted digests and the values were compared with those of standard solutions. The results are shown in Table 111. The precision of the ratio measurements for DOLT-1 and DORM-1 is poorer than for the standards because most of the concentrations are considerably lower than 100 pg/L. I t can be seen that, except for Ni, all the ratios agreed with the expected values within the error of the measurements. The Ni 61/60 ratios for both DOLT-1 and DORM-1 were however considerably higher than expected, presumably because of isobaric interference of 44Ca160Hon 61Ni, a problem encountered previously in the analysis of marine sediments (9). Surprisingly though, the Ni 62/60 ratio for DOLT-1 was also higher than expected. The source of the interferences was strongly suspected to be the overlap of the adjacent, and much larger, ”Cu peak on 62Ni. This was verified by measuring the Ni 62/60 ratio of two solutions (labeled “DOLT” and “DORM”) containing only Ni and Cu in the proportions found in DOLT-1 and DORM-1 digests, respectively. The Ni 62/60 ratio was measured in low resolution and in high resolution for both these solutions and for a 100 pg/L Ni standard solution. The results are summarized in Table IV, along with the ratios measured in low resolution for DOLT-1 and DORM-1 digests (from Table 111). The contribution of 63Cuon 62Ni,in low resolution, can readily be seen; the Ni 62/60 ratio is higher than expected for both “DOLT” and ”DORM”. The overlap is further illustrated in Figure 1. When the ratios were remeasured in high resolution, the expected value was obtained in both cases. The copper
Table IV. Ni 62/60 Ratio Measured in Test Solutions, in Low and High Resolutions solution
low resolution
high resolution
“DOLT”” DOLT-1 digest “DORM”* DORM-1 digest 100 pg/L Ni
1.54 f 0.03 0.216 f 0.061 0.222 f 0.007 0.139 f 0.039 0.146 f 0.001
0.147 f 0.010 0.143 f 0.007 0.145 f 0.002
Standard solution containing 2.6 pg/L Ni and 208 pg/L Cu (as in a DOLT-1 digest). bStandard solution containing 12 rg/L Ni and 52.4 ug/L Cu (as in a DORM-1 digest). 10000 T--
, .
,
:: .: ,.
I .
‘9
0 59.0
60.0
61.0
63.0
62.0
‘i
1 ./
11
_PI
64.0
66.0
65.0
m/z
Figure 1. Low-resolution ICP mass spectra in the range 60-65 u, of test solutions “DOLT” and “DORM” which contain Ni and Cu in the 0.1 proportions of DOLT-1 and DORM-1 digests, respectively: (-) M “0,; “DOLT”; (---) “DORM”. (.e.)
8000
I
h
.t:
e
6000-
I
3
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I 2000
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,
,
,
,
630
620
,
7
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.-,. 650
,
660
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Figure 2. High-resolution ICP mass spectra in the range 60-65 u, of test solutions “DOLT” and “DORM” which contain Ni and Cu in the proportions of DOLT-1 and DORM-1 digests, respectively: (-) 0.1 M “03; “DOLT”;(---) “DORM”. (.as)
interference could therefore be removed by operating at higher resolution, as is clearly illustrated in Figure 2. However, the fact that the low-resolution Ni 62/60 ratios in the two digests
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
690
Table V. Estimation of the Extent of Mass Discrimination measdl
isotopic
expected
element
ratio
ratio“
measd ratiob
expected
Cr Ni
53/52 61/60 62/60 65/63 67/68 1111114 2041208 2061208 2071208
0.1140 0.0434 0.1399 0.4474 0.2214 0.4464 0.0272 0.4612 0.4218
0.117 f 0.001 0.0448 f 0.0003 0.147 f 0.002 0.465 f 0.001 0.217 f 0.002 0.444 f 0.002 0.0279 f 0.0002 0.471 f 0.006 0.426 f 0.004
1.026 1.032 1.051 1.039 0.980 0.995 1.026 1.021 1.010
Cu Zn Cd Pb‘
Calculated using the natural abundances recommended by IUPAC (19). *Measured while aspirating a 100 Fg/L standard solution of each element. ‘The expected ratios were that of the P b standard SRM 981 from NBS. Table VI. Trace Metal Concentrations (in p g / g ) Determined in the Dogfish Liver Tissue DOLT-] element standard additions isotope dilution Nan Mtf
Cr Mn Fe Ni
co cu Zn As Cd
Hg Pb
7.97 f 0.30b 1.06 f 0.03 0.68 f 0.04 9.12 f 0.13 677 f 7 0.251 f 0.001 0.157 f 0.034 20.4 f 1.4 94.4 f 2.9 8.83 f 0.32 4.06 f 0.03 1.39 f 0.03
19.6 f 0.2 92.3 f 0.8 4.41 f 0.08 0.225 f 0.011 1.39 f 0.24
accepted value 7.26 f 0.73c 1.10 f 0.15 0.40 f 0.07 8.72 f 0.53 712 f 48 0.26 f 0.06 0.157 f 0.037 20.8 f 1.2 92.5 f 2.3 10.1 f 1.4 4.18 f 0.28 0.225 f 0.037 1.36 f 0.29
Concentrations in mg/g. bPrecision expressed as the standard deviation (n = 3-5). cThe uncertainties for the accepted values are 95% tolerance limits-not standard deviations. are lower than those measured in the corresponding test solutions clearly shows that there must also be a significant interference on 60Ni,probably from 44Ca160,with the result that neither the 61Ni/60Ninor the “Ni/’?Ni isotope pairs can be used for the determination of Ni in either DOLT-1 or DORM-1. The fact that the low-resolution Ni 62/60 ratio for the DORM-1 digest agrees with the expected value is apparently fortuitous. Another apparently fortuitous result is the agreement of the Cr 53/52 ratio in DOLT-1 with the expected value; it was subsequently discovered (see next section) that there is a significant isobaric interference by a polyatornic species on 52Cr. T o summarize, Cu, Zn, Cd, and P b can be determined by isotope dilution in DOLT-1 using the following pairs of isotopes: 63Cu/65Cu, “Zn/”Zn, 114Cd/111Cd,and zosPb/207Pb.The same four elements, plus chromium, can be determined in DORM-1 using: 52Cr/53Cr, 63Cu/65Cu,@Zn/”Zn, ll4Cd/ll1Cd, and 208Pb/2’YlPb.It should be noted that since the P b isotopic abundances vary in nature, the ratios of Table I11 were used as the natural abundances in the isotope dilution calculations. Finally, the expected ratios were compared to the values calculated using the natural abundances recommended by IUPAC (19) (see Table V) to check for mass discrimination. Correction for mass discrimination in the subsequent isotope dilution analyses was made only if the measured ratio differed from the expected value by more than *3%. Thus, only the Cu 65/63 ratio was corrected. Analysis of the Marine Biological Materials DOLT-1 and DORM-1. The results obtained are summarized in Tables VI and VII, together with the accepted values, for analyses of DOLT-1 and DORM-1, respectively. The accepted values for these elements were derived from statistical
Table VII. Trace Metal Concentrations (in p g / g ) Determined in the Dogfish Muscle Tissue DORM-1 element standard additions isotope dilution Nan Mgd Cr Mn Fe
co Ni cu Zn As
Cd
Hg Pb
8.50 f 0.81* 1.16 f 0.06 3.85 f 0.09 1.29 f 0.06 59.2 f 4.4 0.057 f 0.004 0.97 f 0.08 5.24 f 0.18 20.6 f 0.4 16.4 f 0.5 0.093 f 0.016 0.487 f 0.028
3.49 f 0.14
4.84 f 0.19 21.4 f 0.9 0.075 f 0.003 0.793 f 0.024 0.374 f 0.036
accepted value 8.00 f 0.60‘ 1.21 f 0.13 3.60 f 0.40 1.32 f 0.26 63.4 f 5.3 0.049 f 0.014 1.20 f 0.30 5.22 f 0.33 21.3 f 1.0 17.7 f 2.1 0.086 f 0.012 0.798 0.074 0.40 f 0.12
*
Concentrations in mg/g. *Precision expressed as the standard deviation (n = 3-5). cThe uncertainties for the accepted values are 95% tolerance limits-not standard deviations. treatment of a large number of data obtained by two or more of the following methods: cold vapor atomic absorption spectrometry (for Hg); graphite furnace atomic absorption spectrometry (for As, Cd, Co, Cr, Cu, Pb, Mn, Ni, Se, and Zn); flame atomic absorption spectrometry (for Fe, Zn, Mg, K, and Na); instrumental neutron activation analysis (for As, Co, Cr, Cu, Fe, Mn, Hg, Ni, Se, Zn, C1, Na, K, and Mg); ICP-AES (for Cu, Fe, Ni, Zn, Mg, K, and Na); ICP-MS (see Tables VI and VII); isotope dilution ICP-MS (see Tables VI and VII);hydride generation atomic absorption spectrometry (for As and Se); vapor phase chromatography (for As and Se); and titrimetry (for Cl). Dogfish Liver Tissue DOLT-1. Results for 13 elements determined by ICP-MS in DOLT-1 are compared with the accepted values in Table VI. The only result that does not lie within the 95% confidence interval of the accepted value is the value for Cr obtained by standard additions, which is high, suggesting an isobaric interference. Two polyatomic species were considered as possible interferents: 40Ar12Cand 35C1160H.At first the former was considered the more likely candidate primarily because neither hydrochloric nor perchloric acid had been used in the digestion procedure. In addition, it was felt that complete destruction of the organic matter in DOLT-1, with a relatively high fat content of 2470, might not have been achieved by the H N 0 3 / H 2 0 2digestion. Ward and co-workers (12) suggested that 40Ar12Ccould arise from residual organic matter in solutions of biological materials. An attempt to achieve a complete breakdown of the organic matter in DOLT-1 was made by preparing some solutions by a nitric acid microwave digestion procedure (in pressure decomposition vessels) which was recently developed in our laboratory (20), but results of the analysis of these solutions were not significantly different. While the possibility of an interference by 40Ar12Cstill cannot be ruled out, it is also possible that the chlorine concentration of DOLT-1 (0.69%) is high enough to result in the presence of enough 35C1160Hto seriously interfere with the determination of chromium. Whatever the source of the difficulty in the accurate determination of Cr in DOLT-l, good results were obtained for Cr in DORM-1 by both standard additions and isotope dilution (see Table VII). While the chlorine content of DORM-1 (1.13%) is about twice as high as that of DOLT-1, the chromium content is almost an order of magnitude higher. It is also possible to speculate, however, that the lower fat content of DORM-1 ( 5 % ) results in a more complete breakdown of the organic matter. Thus, further experimentation would be necessary to identify the source of error in the determination of Cr in DOLT-1.
Anal. Chem. 1988, 60,691-696
Dogfish Muscle Tissue DORM-1. The concentrations of the same 13 elements were determined in DORM-1 digests and the results are summarized in Table VII. The results obtained by standard additions and isotope dilution analysis were in good agreement with the certified values for all 13 elements (Na, Mg, Cr, Fe, Mn, Co, Ni, Cu, Zn, As, Cd, Hg, and Pb). ACKNOWLEDGMENT The authors thank A P. Mykytiuk for the preparation of biological digests for the determination of mercury. Registry No. Na, 7440-23-5;Mg, 7439-95-4;Cr, 1440-47-3;Mn, 7439-96-5; Fe, 7439-89-6; Ni, 7440-02-0; Co, 7440-48-4; Cu, 7440-50-8; Zn, 7440-66-6; As, 7440-38-2; Cd, 7440-43-9; Hg, 7439-97-6; Pb, 7439-92-1. LITERATURE CITED (1) Douglas, Donald, J.; Houk, Robert S . Prog. Anal. At. Spectrosc. 1985, 8, 1-18. (2) Gray, Alan L. Spectrochlm. Acta, Part6 1965, 408, 1525-1537. (3) Houk, Robert S. Anal. Chem. 1986, 58, 97A-105A. (4) Gray, A. L. Fresenius’ 2.Anal. Chem. 1988, 324, 561-570. ( 5 ) Olivares, J. A.; Houk, R. S. Anal. Chem. 1986, 58, 20-25. (6) Beauchemin, Diane; McLaren, J. W.; Berman S . S . Spectrochim. Acta, Part 6 1987, 428, 467-490.
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(7) McLaren, J.; Beauchemin, D.; Vander Voet, T. Can. J . Spectrosc. 1985, 3 0 , 29A-32A. ( 8 ) Tan, Samantha H.; Horiick, Gary Appl. Spectrosc. 1988, 4 0 , 445-460. (9) McLaren, J. W.; Beauchemin, Diane; Berman, S. S. Anal. Chem. 1987, 59,610-613. (IO) Vaughan, M. A.; Horlick, Gary Appl. Spectrosc. 1986, 4 0 , 434-445. (11) Pickford, C. J.; Brown, R. M. Spectrochim. Acta, Part 8 1966, 4 1 8 , 183-187. (12) Ward, N. I.; Gray, A. L.; Williams, J. G. Poster presented at the Royal Society of Chemistry Annual Chemical Congress, 1986. (13) Munro, Seumas; Ebdon, Les; McWeeny, David, J. J . Anal. At. Spectrom. 1888, 1 , 211-219. (14) McLaren, J. W.; Beauchemin, Diane; Berman, S . S . J . Anal. At. Spectrom. 1987, 2 , 277-281. 15) Beauchemin, Diane; McLaren, J. W.; Mykytiuk, A. P.; Berman, S . S. Anal. Chem. 1987, 5 9 , 778-783. 16) Berman, S. S. Rep.-Natl. Res. Counc. Can., Mar. Anal. Chem. Stand. Program 1988. 17) Koirtyohann, S. R.; Jones, J. S.;Jester, C. P.; Yates, D. A. Spectrochlm. Acta, Part 6 1961, 3 6 8 , 49-59. 18) Beauchemin, Diane; McLaren, James I C f Inf. News/. 1985, 1 1 , 44 1-446. (19) De BiBvre, Paul; Galiet, Marc; Holden, Norman E.; Barnes, I . Lynus J . Phys. Chem. Ref. Data 1984, 13, 809-891. (20) Nakashima, Susumu; Sturgeon, Ralph E.; Willie, Scott N.; Berman, Shier S . Analyst (London), in press.
RECENEDfor review June 4,1987. Accepted December 5,1987. This is NRCC Publication No. 28571.
The Role of Metastable Atoms in Glow Discharge Ionization Processes K. R. Hess’ and W. W. Harrison* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
Ionization processes have been studied In a glow discharge using neon and argon as discharge gases. A tunable laser served to depopulate metastable atom populations, and the effects were followed by optical galvanic spectroscopy and mass spectrometry. I n the neon discharge, depopulation of the metastables caused a general reduction in all measured lon slgnals, while similar studies with argon showed that many species with ionization potentlais greater than the metastable energies of argon were not affected by laser depopulation. These results provide evidence for the role of Penning lonizatlon in the glow discharge.
The glow discharge has served effectively as an atomization/ionization source for solids elemental mass spectrometry (1, 2). The intensities of ion signals obtained from the sputtered sample constituents generally correspond to their concentrations in the bulk material, indicating that the sputter atomization and ionization efficiencies are generally uniform, with most relative sensitivity factors falling within a factor of 2-3 of each other. Sputter yields are known to vary in approximately this range (3,4 ) , so the ionization processes generating glow discharge ions of sputtered species appear to do so rather nonselectively. The concentration of discharge gas species in a 1.0 Torr glow discharge is several orders of magnitude greater than that of the sputtered species, 10l6 Present address: Department of Chemistry, F r a n k l i n & Marshall College, Lancaster, PA 17604.
atoms/cm3 of argon at 1.0 Torr vs typically lo9 atoms/cm3 as measured for copper (5), yet their ion signals are roughly the same intensity. The ionization processes appear, therefore, to show a preferential ionization of the sputtered species. The investigation of primary ionization mechanisms for ions sampled in a glow discharge, including ions arising from background gases such as Nz+, HzO+, etc., and other molecular species such as CuAr+ and CuNz+,is of interest from both a fundamental and practical standpoint. The practical advantages include the ability to adjust conditions to maximize analytical signals while reducing ion signals from interfering species. The two mechanisms believed to be primarily responsible for ionization in the glow discharge are electron impact ionization, termed a collision of the “first kind” where there is a kinetic energy transfer from the energetic electron to the atom, and Penning ionization, a collision of the “second kind” in which a potential energy transfer creates the ion, if the transferred potential energy is greater than the ionization potential of the species to be ionized. In the glow discharge, this potential energy is stored in an excited metastable state of the discharge gas. For argon these metastable states are 3Pzat 11.55 eV and 3P0at 11.72 eV, while for neon the metastable levels are a t 16.62 eV, 3P2,and 16.71 eV, 3P,. Due to this difference in metastable energies, the behavior of species with ionization potentials between 11.72 and 16.62 eV may be discharge gas dependent. Several other collisional ionization mechanisms exist in the glow discharge, although their relative importance is thought to be less than the electron impact and Penning processes. Of some importance is an associative ionization mechanism that generates species such
0003-2700/88/0360-0691$01.50/00 1988 American Chemical Society
