610
Anal. Chem. 1987, 59, 610-613
(31) Moffat, A. C.; Horning, E. C.; Matin, S.5.:Rowland, M. J . Chromatogr. 1071, 66, 255-260. (32) Grimsrud, E. P.; Kim, S. H., Gobby, P. L. Ana/, Chem, 1979, 57, 223-229. (33) Homing, E. C . ; Carroll. D. I.: Dzidic. I.; Lin, S. N.; Stilweii, R. N.. Thenot, J. P. J . Chromatogr. 1077, 142, 481-495. (34) Siegei, M. w.; McKeown, M. c. J . Chromatogr. 1976, 122,397-413.
RECEIVED for review May 5, 1986. Accepted October 10, 1986.
This work was supported by funds provided from the National Institutes of Health (Grants RR07143, NS 15439, NCICA35843), Grant CR 812740 from the RerJroductive Effects Assessment Group ofthe U.S. Environmental protection Agency, and an ACS Analytical Division Graduate Fellowship sponsored by the Analytical Chemists of Pittsburgh (T.M.T.). Contribution no. 294 from the Barnett Institute of Chemical Analysis.
Application of Isotope Dilution Inductively Coupled Plasma Mass Spectrometry to the Analysis of Marine Sediments J. W. McLaren,* Diane Beauchemin, and S. S. Berman Analytical Chemistry Section, Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K 1 A OR9
Isotope dllutlon Inductively coupled plasma mass spectrometry (ICP-MS) has been appiled to the determination of 11 trace elements (Cr, NI, Zn, Sr, Mo, Cd, Sn, Sb, TI, Pb, and U) in the marine sedlment reference materials MESS-1 and BCSS-1. Accuracy and, especially, precision are better than those that can be easily achieved by other ICP-MS callbratlon strategles, as long as Isotopic equiilbration is achieved and the isotopes used for the ratlo measurement are free of isobark interferences by molecular species. The measurement of the Lsotope ratios on unsplked samples provides a sensitive diagnostic of such interferences.
The detection power of inductively coupled plasma mass spectrometry (ICP-MS) makes possible the determination in geological reference materials of many trace elements for which relatively few reliable values have been previously established (1-4). This lack of data in many cases prevents a full assessment of the accuracy of ICP-MS results. A partial solution to this problem is the use of stable isotope dilution techniques ( 5 ) which are immune to many of the sources of error which can adversely affect ICP-MS results obtained by other calibration strategies. This approach would, for example, be an effective means to compensate for the suppression of ion sensitivities by concomitant elements (6) observed in many of the early applications of ICP-MS ( 4 , 7-9). Also, more calibration drift can be tolerated in an isotope dilution analysis because an isotope ratio, rather than an absolute intensity measurement, is used in the calculation of the analyte concentration. This suggests that i t may be easier to obtain accurate and precise ICP-MS results for solutions with appreciable dissolved solids concentrations if isotope dilution techniques are used. Relatively little use has been made of isotope dilution techniques in ICP-MS. Its application to the determination of six trace metals in a coastal seawater sample after a separation by adsorption on silica-immobilized 8-hydroxyquinoline was recently reported (10). Ting and Janghorbani used a 57Fespike for the accurate determination of 54Feand 58Fein human fecal matter after a chemical separation (11). Taylor and Garbarino (12) have applied isotope dilution ICP-MS to the determination of trace elements in natural
Table I. Operating Conditions for Isotopic Analysis by ICP-MS ICP plasma Ar auxiliary Ar nebulizer Ar rf power
sampler skimmer
14 L min-' 2.0 L min-' 0.9 L min-' 1.2 k\V
Mass Spectrometer nickel, 1.2-mm orifice nickel, 0.9-mm orifice
Operating Pressures interface region - 1 torr mass spectrometer chamber -4 x
Lens Voltages
photon stop (S2, Bessel box barrel (B) einzel lenses 1 and 3 ( E l ) Bessel box end lenses cP) einzel lens 2 entrance a.c. rods exit a.c. rods
torr
-7.0 v +2.95 Y -12.0 v -11.3 I' -130 1. 0 1. -3 1.
waters. Longerich et al. ( 4 ) briefly described the determination of samarium using a '*'Sm isotopic spike in three geological reference materials. The purpose of the present work was to examine in detail the application of isotope dilution ICP-MS to the determination of trace elements in solutions of the marine sediment reference materials MESS-1 and BCSS-1. These materials were chosen with two objectives in mind: verification of the methodology by comparison of the results with published reliable values, and assessment of the potential of isotope dilution ICP-MS to contribute accurate data toward establishment of reliable values for additional elements. EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer used for this work was an ELAN 250 from SCIEX Division of MDS Health Group Ltd. (Thornhill,Ontario, Canada) that had recently undergone a modification of the original ion optics by the manufacturer to improve stability and reduce suppression of analyte ion sensitivity by concomitant elements. This involved replacement of a set of ac rods between the skimmer and Bessel box lenses with a three-cylinder einzel lens and re-
0003-2700/87/0359-0610$01.50/0@ 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987
placement of a screen (termed the “ring” electrode), positioned immediately behind the skimmer and held at -10 to -50 V, with a 5-mm-diameter grounded stop. The lens voltage settings used for this work are listed in Table I. As described in ref 10 a mass flow controller was used on the aerosol carrier gas line and solution uptake was controlled by a peristaltic pump. As described in ref 9 the extended torch provided with the instrument was replaced with a conventional ICP-AES torch and the torch box was positioned as close as possible to the sampler. Other details of the operating conditions are given in Table I. Reagents. All acids were purified by subboiling distillation of reagent grade feedstocks in quartz or polypropylene stills prior to use. Enriched isotopes purchased from the Oak Ridge National Laboratory included %r, 61Ni,67Zn,%Sr,‘%lo, ‘llCd, l17Sn,“%b, 203Tl,and 207Pb. These stable isotopes were received as metals or oxides (lead as the nitrate) with an isotopic enrichment greater than 95%. Stock solutions of approximately 100 mg L-’ of each were prepared by dissolution of an accurately weighed quantity of the material in nitric acid and dilution to volume. Cr2O3 was dissolved by prolonged digestion with several milliliters of perchloric acid. Sn02was dissolved by a lithium tetraborate fusion. The source of 235Uwas the U S . National Bureau of Standards SRM U-930. The concentrations of the spike solutions were verified by reverse spike isotope dilution ICP-MS. Dissolution of Marine Sediments. A series of solutions of the marine sediment reference materials MESS-1 and BCSS-1 was prepared by a mixed acid digestion procedure which results in a total dissolution of a 0.5-g sample in 50 mL of 0.5 M nitric acid. The initial digestion was performed by heating each sample with a mixture of 3 mL of hydrofluoric acid, 3 mL of nitric acid, and 2 mL of perchloric acid in a sealed pressure decomposition vessel made of Teflon in a boiling water bath for 3 h. The mixture was then transferred to a beaker made of Teflon and evaporated to fumes of perchloric acid. Two further digestions were performed by adding 1-mL portions of hydrofluoric acid and perchloric acid and evaporating to dryness. Concentrated nitric acid (2 mL) was then added to the residue and the mixture gently boiled. The mixture was transferred to a 50-mL flask and diluted to volume with deionized distilled water. A small amount of a light-coloredundissolved residue that remained at this point slowly dissolved if the solution was left to stand for 48 h. For the isotope dilution analyses, the isotopic spikes were added to the sediment samples prior to the first digestion. The aliquots of the isotope spike solutions were calculated to result in a ratio near 1 for each of the isotope pairs. The reference spike isotope pairs were as follows: 52Cr/53Cr;GONi/61Ni;68Zn/6Zn; 8sSr/ssSr; 98Mo/1WMO; 114Cd/lllCd; l16Sn/117Sn; 121Sb/123Sb; 205T1/203T1; 208pb/ 207pb. 238u/ 235u. Isotope Ratio Measurements. Intensity data for isotope ratio determinations were acquired by “peak-hopping” through the 22 isotopes listed above a total of 50 times. The low resolution setting of the ELAN was found to be adequate in all cases. At each peak of interest, three measurements of 0.1-s duration were made, one at the assumed peak center, and the other two at f O . l u. Each isotope ratio reported was thus the mean of 50 determinations. The total analysis time was approximately 7-8 min. Corrections for known isobaric interferences were made automatically by the ELAN software where necessary. Mass Discrimination Corrections. Before the results of isotope ratio measurements are applied to isotope dilution calculations, it is important to check for mass discrimination effects. If such effects are significant, a correction must of course be made to the ratios (11). In this work, checks for mass discrimination were made with 100 pg L-l natural abundance solutions of each of the elements of interest, except lead, for which a solution of the NBS SRM 981 lead isotopic standard was used, and uranium, for which a solution of the U-930 reference material was used. It was found that isotope ratios were reproducible to better than fl% from day to day provided that none of the lens voltages or plasma operating conditions were changed. With the ion lens voltages set as described in Table I, the sensitivity of the ELAN (on a molar basis) is highest, and rather uniform, in the middle of the usable mass range (i.e., 100-150 u); it decreases quite gradually toward higher mass, but somewhat more quickly in the other direction. Because of this, mass discrimination effects are
.c
611
largest for the lighter elements. For example, the mean value of the 61Ni/60Niratio observed over a 1week period was 0.0448 & 0.0002, about 3% higher than the expected value of 0.0434. The measured 235U/238U ratio for the uranium standard was 18.68 f 0.16, about 7.5% higher than the certified value of 17.35, corresponding to a mass discrimination of about 2.5%/u. In this work, correction factors of 2-3%/u were applied to isotope ratios for chromium, nickel, and uranium, but for the other eight elements, mass discrimination effects were found to be insignificant. No evidence that the isotope ratios are dependent on concentration nor upon the presence of concomitant elements has been observed in measurements on unspiked solutions of the marine sediments. Even at solution concentrations as low as 10-100 pg L-’, it was possible to measure the ratios with a precision of 1% or better. In every case where isotope ratios differed significantly from the values obtained for pure solutions of the elements, the discrepancy was attributable to an isobaric interference. Isotope Dilution Analyses. Each set of isotope dilution analyses involved duplicate determinations for 11 isotope pairs in four solutions. As each determination required approximately 8 min, the total analysis time was about 1 h. A continuous decrease in sensitivity for all elements was always observed, with sensitivities at the end of the run ranging from half of the original values for the heavy elements (e.g., U, Pb) to one-quarter of these values for the lighter elements (e.g., Ni, Zn). There is no doubt that most, if not all, of this sensitivity loss was caused by deposition of material on the sampler and skimmer. It was always possible to recover the original sensitivity simply by removing and cleaning these two parts. Although the rate of change of sensitivity observed in these experiments would probably be intolerable if calibration by external standardization or the method of additions were attempted, it represented only a minor inconvenience in isotope dilution analyses, in which each element has the ideal internal standard-another of its isotopes. Analyk concentrations in the sediment samples were calculated by means of the formula
where C is the analyte concentration in micrograms per gram, M , is the mass of the stable isotope spike in micrograms, 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, BEis the abundance of the spike isotope in the spike, K is the ratio of the natural and spike atomic weights, W is the sample weight, and R is the measured isotope ratio after spike addition. A separate set of isotope dilution analyses was performed to determine the blanks for the dissolution procedure. The isotopic spike masses were somewhat arbitrarily chosen to be 4% of those made for the 0.5-g sediment samples. Two blanks were carried with each set of four samples. Tin was the only element for which a significant blank was observed. RESULTS AND DISCUSSION Isotope Ratio Measurements for Unspiked Solutions. An essential requirement for an accurate stable isotope dilution analysis is that neither of the pair of isotopes chosen is subject to any significant isobaric interference for which correction cannot be easily performed. The measurement of isotope ratios for an unspiked sample provides a very sensitive diagnostic of such interferences. Corrections for known isobaric interferences (e.g., Il4Sn on l14Cd)are made automatically by the ELAN software. Any significant deviation of the measured ratio from the value calculated from the natural abundances of the two isotopes is then diagnostic of an interference by a molecular species. In Table 11, results of isotope ratio measurements on unspiked solutions of MESS-1 and BCSS-1 are compared with the expected values. Values for the mPb/208Pb and 235U/238U ratios are not included in the table. Because lead isotopic abundances vary in nature, ratio measurements cannot be compared with a single expected value. No measurements
612
ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987
Table 11. Results of Isotope Ratio Measurements for Unspiked MESS-1and BCSS-1 Solutions ratio
expected
MESS-1
BCSS-1
0.114 0.0434 0.221 0.119 0.403 0.446 0.315 0.747 0.418
0.161 f 0.018 0.091 f 0.006 0.382 f 0.064 0.119 f 0.001 0.416 f 0.003 0.841 f 0.032 0.314 f 0.004 0.835 f 0.041 0.428 f 0.004
0.155 f 0.020 0.077 f 0.002 0.736 f 0.125 0.118 f 0.001 0.416 f 0.007 1.16 f 0.08 0.314 f 0.004 0.897 f 0.032 0.421 f 0.008
were made for uranium because of the very low natural abundance of 235U. Values for strontium, molybdenum, thallium, and tin agree well with the expected values, but there are clearly problems for the other elements. The 53Cr/52Crratios for both MESS-1 and BCSS-1 are much higher than expected, indicating an interference on the spike isotope, 53Cr. The interfering species is almost certainly 3iCl'60; perchloric acid used in the dissolution procedure may not be completely removed in the evaporations to dryness. Chromium was successfully determined in more dilute BCSS-1 solutions by the method of standard additions (9);apparently the potential interference of 35C1160Hon 52Crwas insignificant in that case. The interference at m / z 53 is much more severe not only because of the lower natural abundance of 53Crbut also because the 37C1160peak is considerably more intense than that of 35Cl'60H. Overlap of 67Znby 35C11602 probably accounts for the much higher than expected 61Zn/68Znratios observed for the sediment solutions. Because of its low natural abundance (4.11%), "Zn is very susceptible to spectroscopic interferences. Zinc was also successfully determined in more dilute BCSS-1 solutions by the method of standard additions, using '%n (9). Other zinc isotopes are subject to oxide interferences by either titanium oxides (for 64Znand "Zn) or iron oxide (for 70Zn). Thus, a successful isotope dilution method for zinc in marine sediments will require either a significant reduction in the chloride concentrations in the final solutions (perhaps by reducing or eliminating the use of perchloric acid) or a reduction of oxide levels, or a t least an accurate correction procedure for oxide overlaps. Overlap of 61Niby 44Ca160His believed to be responsible for the higher than expected values of the 61Ni/60Niratio. The fact that the value is higher than expected implies that the interference by the corresponding oxide on 60Ni is less significant. This situation arises not because the hydroxide species is more abundant than the oxide but rather because 61Ni,with a natural abundance of only 1.13%, is much more
susceptible to interference than 60Ni,with an abundance of 26.1 70. The fact that nickel was successfully determined in BCSS-1 by the method of standard additions using the latter isotope (9) suggests that interference by 44Ca160on 60Niis small in this case. Nickel has also been determined by isotope dilution ICP-MS in a river water reference material, but only after a separation and preconcentration by adsorption on silica-immobilized 8-hydroxyquinoline (13). It was found that even after the separation, 61Ni was not usable because of interference by 44Ca160Harising from the residual calcium (-30 mg L-l) in the concentrate. This problem was circumvented by using 62Nias the spike isotope and 60Ni as the reference. This solution is not so attractive in the case of the marine sediments because of overlap of 62Ni by 46Ti160. Despite the large discrepancies of the 61Ni/60Niratios from the expected value, results of isotope dilution analyses obtained by using this isotope pair are only slightly lower than the accepted values. The interference by 44Ca160His less severe in the spiked sample, in which the abundance of 6'Ni has been adjusted to be approximately equal to that of 60Ni. It might be considered in this case that the isotope ratio measurement for an unspiked sample overestimates the severity of the interference problem; on the other hand, it provides a warning that there is a problem which will become increasingly severe as the calcium/nickel concentration ratio of the sample increases. Another situation in which overlap by a hydroxide species posed a serious problem arose in the determination of cadmium. The only stable isotope available was l W d , and 111Cd/lL4Cdratios were much higher than expected for solutions of both sediments. As the molybdenum concentrations in these materials are not much higher than the cadmium concentrations, it seemed very unlikely that the discrepancy was due to an overlap of W d by 95M0160,a problem encountered by McLeod et al. (8) in the analysis of nickel-base alloys. A more likely possibility is an overlap by 94Zr160H. The mass spectrum of a 3 mg L-' zirconium solution in the mass range from 90-96 and 106-112 u, plotted on a logarithmic intensity scale, is shown in Figure 1. While the peaks at 106, 107, 108,110, and 112 u can be assigned as oxides of the five isotopes of zirconium, two additional peaks a t 109 and 111 u are almost certainly due to 92Zr160Hand 94Zr160H. (A remarkably similar set of spectra for a 10 mg L-' Mo solution showing oxide and hydroxide peaks has been published by McLeod et al. (8).) Zirconium/cadmium concentration ratios for MESS-1 and BCSS-1 are approximately 850 and 1400, respectively; as would be expected from these data, the isotope ratio discrepancy is larger for BCSS-1 than for MESS-1. A possible solution to this problem would be the use of l13Cd as the spike isotope; while this would necessitate a correction
Table 111. Isotope Dilution Analysis of Marine Sediment Reference Materials MESS-I
BCSS-1
element
found"
accepted
found"
accepted
Cr Ni Zn Sr Mo Cd Sn Sb T1 Pb U
55.2 f 0.8 26.3 f 0.3 171 f 2 94 f 1 2.2 f 0.1 0.40 f 0.03 3.8 f 0.2 0.67 f 0.04 0.74 f 0.02 33.1 f 0.7 4.1 f 0.4
71 f 11 29.5 f 2.7 191 f 17 (89)b (2.2 f 0.3)c 0.59 f 0.10 (3.4 f 0 . 2 ) d 0.73 f 0.08 (0.70 f 0.03)' 34.0 f 6.1 (4.2 f 0.3)e
95.0 f 1.2 53.5 f 0.6 85 f 4 108 f 2 2.03 f 0.04
123 f 14 55.3 f 3.6 119 f 12 (96)' (1.9 f 0.2)C 0.25 f 0.04 (1.8 f 0.2)d 0.59 f 0.06 (0.60 f 0.05)' 22.7 f 3.4 (2.7 f 0.2)e
1.75 & 0.06 0.47 f 0.05 0.65 f 0.03 22.9 f 0.5 2.7 f 0.2
4Results in pg/g; precision expressed as the standard deviation (n = 16). *Flame atomic absorption spectrometry. Isotope dilution spark source mass spectrography with preconcentration by electrodeposition. Hydride generation graphite furnace atomic absorption spectrometry (14). e Isotope dilution spark Rource mass spectrography.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987
613
CONCLUSIONS The use of stable isotope dilution techniques has been shown to be a very useful complement to alternative calibration strategies in ICP-MS. More concentrated solutions can be analyzed without any sacrifice of precision; in fact, precision is greatly improved because the ratioing of intensities for a pair of isotopes for each element is the ideal form of internal standardization.
Flgure 1. I C P mass spectrum illustrating the relative abundance of zirconium and zirconium oxide and hydroxide species at a nebulizer gas flow rate of 0.9 L min-'.
ACKNOWLEDGMENT Spark source mass spectrographic analyses were performed by A.P. Mykytiuk of this laboratory, whom we also thank for many helpful discussions on isotope dilution techniques. R. E. Sturgeon, also of this laboratory, performed the graphite furnace atomic absorption analyses. Registry No. Cr, 7440-47-3; Ni, 7440-02-0; Zn, 7440-66-6; Sr, 7440-24-6; Mo, 7439-98-7; Cd, 7440-43-9; Sn, 7440-31-5; Sb, 7440-36-0;T1, 7440-28-0;Pb, 7439-92-1;U, 7440-61-1.
for the '131n isobaric interference, the interference from
LITERATURE CITED
48 9- l 92-
1
96
'
1
100
'
1
104
'
1
108
'
1
ml2
96Zr160Hwould be about six times less severe. The r23Sb/121Sbratios for MESS-1 and BCSS-1 are slightly higher than expected. This may be due to an overlap of lZ3Sb by 91Zr1602.The signal for a 3 mg L-' Zr solution a t m / z 123 appeared to be very slightly above the background level. Results of Isotope Dilution Analyses. Results of isotope dilution analyses of eight solutions each of MESS-1 and BCSS-1 are presented in Table 111. These values are compared with the published accepted values for the elements for which these have been established; for the other elements, results obtained by other methods available in this laboratory are used for comparison. Results for chromium and zinc are consistently low because of interferences by chlorine-containing molecular ions on the 53Crand 67Znspike isotopes. Values for nickel and antimony are only slightly low, indicating that the interferences discussed for these elements in the previous section are not severe. Results for strontium, molybdenum, tin, thallium, and uranium are in very good agreement with the values obtained by other methods, and the lead results are in excellent agreement with the accepted values. A striking feature of all of the results is the very high precision obtained, even for elements such as antimony and thallium a t the sub-microgram-per-gram level in these materials.
(1) Date, A. R.; Gray, A. L. Spectrochim. Acta, Part 8 1985, 4 0 6 , 115-122. (2) Date, A. R.; Hutchison, Dawn Spectrochim. Acta, Part 8 1986, 4 7 8 , 175-1 81. (3) Doherty, W.; Vander Voet, A. Can. J . Spectrosc. 1985, 30, 135-141. (4) Longerlch, ti. P.; Fryer, 8. J.; Strong, D. F.; Kantipuly, C. J., submitted for publication in Specfrochim. Acta, Part 8 . (5) Heumann, K. G. PAC Trends Anal. Chem. (Pers. E d . ) 1982, 7 , 357-361. (6) Olivares, J. A.; Houk, R. S. Anal. Chem. 1986, 58, 20-25. (7) Pickford, C. J.; Brown, R. M. Spectrochim. Acta, Part 8 1986, 4 7 8 , 163-187. (8) McLeod, C. W.; Date, A. R.; Cheung, Y. Y. Spectrochim. Acta. Part 6 1986, 4 7 8 , 169-174. (9) McLaren, J. W.; Beauchemin, Diane; Berman, S. S. J . Anal. A t . Spectrom ., in press. (10) McLaren, J. W.; Mykytiuk, A. P.; Willie, S. N.; Berman, S. S. Anal. Chem. 1985, 57, 2907-2911. (11) Ting, 8. T. G.; Janghorbanl, Morteza Anal. Chem. 1986, 58, 1334. .. 1340. .- . ..
(12) Taylor, H. E.; Garbarino, J. R. Colloquium Spectroscopicum International XXIV, Garmlsch-Partenkirchen, West Germany, Sept. 15-20, 1985, Abstract NO. 52. (13) Beauchemin, Diane; McLaren, J. W.; Mykytiuk, A. P.; Berman. S . S . Anal. Chem., in press. (14) Sturgeon, R. E.; McLaren, J. W.; Willie, S . N.; Beauchemin, D.; Berman, S. S . Can. J . Chem., in press.
RECEIVEDfor review May 15,1986. Accepted October 9,1986. This is NRCC Publication No. 26681.
