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Anal. Chem. 1987, 59, 2658-2662
(11) Jensen, N. J; Tomer. K. 9.: Gross, M. L.; Lyon, P. A. I n Desorption Mass Spectrometry; Lyon, P. A.. Ed.: ACS Symposium Series: American Chemical Society: Washington, DC, 1985; p 194. (12) Rosenstock. H. M.;Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data Suppl. 1977, 6(1), 1-752. (13) Page, F. M.;Goode. G. C. Negative Ions and the Magnetron; Wiiey: London, 1969. (14) Janousek. B. K.: Braurnan. J. I. In Gas Phase Ion Chemistry;Bowers, M.,Ed.; Academic; New York 1979; Vol. 2, p 87. (15) Sheppard, W. A.; Sharts. C. M. Organic Fluorine Chemistry: Benjamin: New York. 1969: p 36. (16) Pople. J. A.; Gordon, M. J. Am. Chem. SOC. 1967, 89, 4253. (17) Klabunde, K. J.; Burton, D. J. J. Am. Chem. SOC. 1972, 94, 5985. (18) Andreades, S. J. Am. Chem. SOC. 1964. 8 6 , 2003. (19) Sleigh. J. H.: Stephens, R.; Tatiow, J. C J. Floorine Chem. 1980, 15, 411.
(20) Schieyer, P.; Kos, A. Tetrahedron 1983, 39(7), 1141. (21) Dixon, D. A.; Fukunaga, T.; Smart, 9. J. Am. Chem. SOC.1986, 108, 4027. (22) Streitwieser, A., Jr.; Berke, C. M.; Schriver, G. W.; Grier, D.; Collins, J. 9. Tetrahedron, Suppl. 11981, 3 7 , 345. (23) Streitwieser, A., Jr.: Holtz, D. J. Am. Chem. SOC. 1967, 89, 692. (24) Klabunde, K. J. Burton, D. J. J. Am. Chem. SOC. 1972, 9 4 , 820.
RECEIVED for review January 15,1987. Resubmitted July 22, 1987. Accepted July 22,1987. This work was supported by
3M and the Midwest Center for Mass Spectrometry, a National Science Foundation Regional Instrumentation Facility at the University of Nebraska-Lincoln (Grant CHE8211164).
Inductively Coupled Plasma Mass Spectrometric Determination of Lead Isotopes T h o m a s A. Hinners* a n d E d w a r d M. H e i t h m a r
US.Environmental Protection Agency, Environmental Monitoring Systems Laboratory, Las Vegas, Nevada 89193-3478 Thomas M. Spittler
U.S. Environmental Protection Agency, Lexington, Massachusetts 02173 J o h n M. Henshaw
Lockheed Engineering and Management Services Company, Las Vegas, Nevada 89109
Inductively coupled plasma mass spectrometry (ICP-MS) offers the opportunity to measure stable Isotopes of the elements. When the Isotope proportions dtffer s d k h t i y among source materlals, isotope ratio analysis provides a means to dlstlngulsh the orlgln of pollutants such as lead. Relatlve standard devlatlons of 1.1 %, 0.76%, and 0.80% were obtained for an 8 h perlod using a 5-mln measurement Interval for the m / z 208:206, 207:200, and 204:206 lead ratlos, respectively. By use of 95% tolerance Ilmlts, slgnlflcant dlfferences In lead Isotope ratlos were observed for lead ores from Idaho, Mlssourl, and Yugoslavla, as well as for commerkal lead standards. Lead Isotope ratlos for Mlssourl ore differed as much as 19% from values for Natlonal Bureau of Standards “Common Lead Isotopic Standard” (SRM 981). Lead Isotope data obtalned by ICP-MSare consistent wlth palnt as the source of lead In blood for one case study but not for another. An lsotopk detectlon llml of 2 pptr (parts per Amerlcan trllUon) was both calculated and measured for lead.
Inductively coupled plasma mass spectrometry (ICP-MS) entered the commercial instrument market in 1983 after several years of development (1-7). While ICP-MS offers attractive detection limits near 10 pptr for the determination of elemental concentrations, the isotopic proportions of an element may provide information that can distinguish the source of the element. However, the proportions of an element’s isotopes will not have diagnostic value unless source materials have different isotope proportions. Isotopic composition variations on the order of 50% have been reported for boron in nature (8). Differences of 17-36% in isotope
ratios for lead from various geographic areas have been documented (9, 10). Natural materials differ in lead isotope composition because radioactive decay of thorium and uranium yields different lead isotopes and because thorium and uranium were not distributed uniformly in nature. While ICP-MS offers convenience and speed in the determination of isotope ratios, it is not expected to provide the high precision (0.01’70 relative standard deviation) of thermal ionization mass spectrometry (7). The usefulness of ICP-MS for distinguishing the origins of lead (and other polyisotopic elements) by the isotope ratios will depend upon the precision obtainable and upon the variation of the ratios in commercial products and in nature. This study was conducted to assess the performance of an ICP-MS system to distinguish between lead samples by their isotope ratios and to evaluate its potential for determining the source of lead in pollution situations. EXPERIMENTAL SECTION Instrumentation. Isotope determinations were made on an Elan Model 250 ICP-MS instrument purchased from Sciex, Thornhill, Ontario, Canada. Measurements were made with both the original and new ion optics supplied by the manufacturer. The new ion optics differ from the original version by the replacement of the mesh ring lens with an opened-centered front lens, by replacement of the collimating AC rod set with a set of Einzel lenses, and by revised electronics for the plate lens unit and the barrel lens. The new ion optics were designed by Sciex to reduce signal drift and to improve the utility of internal standards. A peristaltic pump (Minipuls 2, Gilson, Middleton, WI) was used for sample uptake. A multichannel mass flow controller (Model FM4575, Linde Division, Union Carbide Corp., Somerset, NJ) was used for the plasma (outer gas) and nebulizer argon flows, unless noted otherwise. Reagents. Deionized water was obtained from a Milli-Q system (Millipore Corp., Bedford, MA) at, or above, 13 Ma cm. Nitric
0003-2700/87/0359-2658$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59,
Table I. Instrumental Conditions parameter rf power argon flows plasma (outer gas) nebulizer auxiliary coil-to-orifice distance nebulizer spray chamber resolution
setting or specification 1.00 kW
12.0 L min-’ 1.07 L min-’ 1.8 L min-’ 21 mm Meinhard “C” Scott-type high (0.6 u full width at 10% maximum peak height) isotopic multichannel
scanning mode measurement mode measurement locations per peak 1 0.500 s or 1.000 s measurement time 10 ms dwell time 0.540 s cycle time 5 integrations 2.5 rnin or 5.0 rnin analysis time 40A, 60B, 40C, 99 Ring original Sciex ion optics 85B, 50E1,30P,2792 new Sciex ion optics 1.0495 X 208Pbcountsls, 1.0202 normalizing factors” X z07Pbcounts/s, 1.0077 X 206Pbcountsls, LOO0 X 2wPb countsls Elan data system version 9.2 software (Sciex) Isotope ratios were normalized by software equations based on the certified values for NBS “Common Lead Isotopic Standard” (SRM 981). acid used was Ultrex grade (J. T. Baker Chemical Co., Phillipsburg, NJ). Certified isotopic standards of lead (SRM 981, 982, 983) as well as a lead standard solution (SRM 2121) were obtained from the National Bureau of Standards (Gaithersburg, MD). A lead standard solution was also obtained from Spex Industries, Edison, NJ (Custom Plasma Standard, lot 485MP). Procedure. Weighed portions of the lead ores (obtained under confidentiality agreements) and of the certified NBS isotopic standards (SRM 981, 982, and 983) were dissolved in 1:l (v/v) Ultrex HN03 with heating before dilution to desired lead concentrations with 0.25 or 0.5 M “0% Portions of paint (25 mg), dust (100 mg), and soil (100 mg) samples from the Boston area were digested a t 95 OC for 30 min with 10 mL of concentrated Ultrex HNO, before dilution to 250 mL with 0.5 M “OB. These digests of paint, dust, and soil were centrifuged for 5 min at 2000 rpm to remove suspended solids. Lead was leached from 3-in. X 4-in. portions of glass-fiber filters containing air particulates (collected in the Boston area) by soaking for 48 h in 50 mL of 0.5 M Ultrex HNO,. Previous experience indicated that lead in air particulates is extracted quantitatively in 16 h with dilute nitric acid at ambient temperature. Since the majority of lead in blood is associated with red blood cells, whole blood samples (2 g, when available) were digested by heating a t 95 “C with 3 mL of concentrated Ultrex HN03before dilution with 0.5 M HNO, to 10 times their original weights (to keep dissolved solids below 850 ppm). The instrument was operated in the high-resolution mode. Since doubly charged ions were not a concern and oxide ions were considered unlikely in the m/z 204-208 region, the plasma conditions and lens settings (Table I) that maximized the signalto-noise ratio for 208Pbwere selected. The lead isotope ratios generated by the instrument software were normalized by using data obtained for the NBS “Common Lead Isotopic Standard” (SRM 981) and entering factors in the software Elemental Equations. These normalizing factors (Table I) were derived from the measured ion count rates by determining the increases needed to yield the NBS ratios when the 204Pbintensity value was unaltered. When isotopes of an element are not equally abundant, it i3 desirable to measure the less-abundant isotopes for longer times than the most-abundant isotope in order to obtain similar precisions for all the isotopes. The isotope composition data for the NBS “Common Lead Isotopic Standard” indicate that ‘04Pb is the least-abundant stable isotope of lead with a value of 1.4255%
NO. 22, NOVEMBER 15, 1987 2659
Table 11. Normalized Lead Isotope Ratios for NBS Isotopic Standards” Dercent of certified values standard SRM 981” SRM 982 SRM 983
mlz 208206 mlz 207:206 mlz 204:206 97.6
100.4
100.5
101.7 98.4
99.2
100.9 99.8 b
certified isotope ratios standard SRM 981 SRM 982 SRM 983
mlz 208:206 mlz 207:206 mlz 204:206 2.1681 1.00016 0.013619
0.91464 0.46707 0.071201
0.059042 0.027219 0.000371
” These isotope ratios were obtained (for 400 gg L-I total lead in 0.5 M “Os) using normalizing factors in the software equations derived from previous, unnormalized measurements on SRM 981. bZ04Pb content in SRM 983 (at 0.0342 atom %) yields an isotope concentration below the LOQ value when the total lead level is 400 gg L-1. while 2osPbis the most-abundant isotope at 52.347%. To obtain similar precision for the four lead isotopes in the NBS “Common Lead Isotopic Standard”,the 204Pbsignal should be measured 36 times as long as the 208Pbsignal while the 206Pband %‘Pb signals should be measured twice as long as the 208pb signal. Since nearly all lead materials have isotope abundances (10) simiiar to the NBS “common lead-, this measurement time scheme is suitable for general use. The exception to the latter are certain isotopic standards with atypical abundances. When a dwell time of 10 ms and one measurement location per isotope are selected in the multichannel mode, a cycle time of 540 ms is needed to allow for 36 dwells on 204Pb,2 dwells each on %Pb and 207Pb,and 1 dwell on mPb and for software overhead time. To permit 50 scans per determination, the software “measurement time” needs to be set at 500 ms when the dwell time is 10 ms. When five integrations are included for a single analysis (where each integration of 50 scans is considered a “determination”), the data-collection time for one analysis is 2.5 min. If the “measurement time” setting is doubled, the number of scans per determination is doubled as is the analysis time. Other measurement settings were used and evaluated as reported below. Solutions were nebulized for at least 1 min before data collection was initiated, and each solution was followed by the introduction of deionized water (to rinse the tip of the sampling tubing in order to avoid contaminating other solutions).
RESULTS AND DISCUSSION The accuracies of the isotope ratio values determined with the ICP-MS for three NBS isotopic standards are provided in Table 11. The system was normalized by using the NBS “Common Lead Isotope Standard” (SRM 981). Consequently, the percentage values provided in Table I1 for SRM 981 show how well the system was normalized (when SRM 981 was reanalyzed using the normalizing factors in the software) while the percentages for SRM 982 and 983 show that the normalization is also applicable to different isotope ratios. Interference Evaluation. Scans of the m / z 198-209 region for samples studied to date have not shown bismuth, mercury, or thallium a t levels sufficient to affect the P b isotope values. The potential interference effect found in this work for thallium on *04Pbis less than 0.02% while the effect on 2osPbis near 0.0001%. Similarly, scans of the m / z 169-192 region have not revealed the presence of any elements whose potential chloride or oxide ions could contribute to the signals measured a t the m / z values for the lead isotopes. Limits of Detection and Quantitation. When the 2.5rnin analysis program was appplied to a blank solution of 0.25 M HNO,, the limit of detection (LOD) and limit of quantitation (LOQ) concentration values shown in Table 111 were obtained by calculations. Table IV provides data verifying
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
Table 111. Calculated Lead Limits of Detection and Limits of Quantitation"
parameter
concn (in ng L-' or pptr) 204Pb total Pb
ion rate (in counts/s) at m / z 204
average (n = 11) std dev (n = 11) LOD (3 std dev) LOQ (10 std dev)
9.35 0.75 2.3 7.8
2.2
7.5
i60 550
nLimit of detection (LOD) and limit of quantitation (LOQ) values from 11 2.5-min analyses of a 0.25 M Ultrex HN03 blank solution during a 2-h period. bConcentrationsfor the LOD and LOQ values were calculated using the net 54.7 counts/s ion rate (n = 3) observed at mlz 204 for the 57 ng L-' 2MPb(4.0 pg L-' total Pb) in a 0.25 M Ultrex HNO, solution of NBS "Common Lead Isotopic Standard" (SRM 981). Since SRM 981 contains 1.4255% 204Pb, the corresponding LOD and LOQ values in terms of total Pb concentrations could be calculated to one significant figure (indicated by an overlined digit). Table IV. Detection Limit Verification analysis no.
solution 0.25 M Ultrex HNO, 0.25 M Ultrex HNO, 0.25 M Ultrex HNO, 4.0 ng L-' total Pbb*c 4.0 ng L-' total Pb 4.0 ng L-' total Pb 4.0 ng L-' total Pb 0.25 M Ultrex HNOB 0.25 M Ultrex HNO,
total ion rates (in countsfs) at mfz 208" 16.86 f 0.56 15.50 f 0.46 14.57 f 0.59
Table V. Isotope Ratio Precision Related to Lead Concentration" % re1 std dev
total lead concn, L-l
mlz 208:206
6 (n = 3) 20 (n = 4) 40 (n = 4) F test sig levelb
mlz
207:206
mlz 204:206
0.57
1.3
1.2
0.91
3.0 0.99 0.62 0.047, 0.016d
0.56 a11>0.10
0.47 0.074c
Short-term precision for consecutive determinations (n)using National Bureau of Standards "Common Lead Isotopic Standard" (SRM 981) in 0.25 M Ultrex nitric acid. bThe F test significance level (when less than 0.10) for statistical differences between variances. Significance level for statistical difference between variances for the solutions with 6 and 40 pg L-' lead. dSignificance levels for statistical differences between variances for solutions with 6 and 20 pg L-' lead and 6 and 40 pg L-l lead, respectively. Table VI. Precision Relat.ed to the Number of Measurement Locations Per Peak" no. of measurmt locations per peak
mlz 208
net ion rates (in counts/s)b mlz 207 mlt 206
mlz 204
15012 f 266 6239 f 103 6824 f 78 408.1 f 4.1 14 132 f 496 5873 f 204 6435 f 249 388.6 f 16.8 t test sig 0.0082 0.0073 0.010 0.036 levelC 1
21.63 f 0.22 21.94 f 0.52 21.98 f 0.25 21.30 f 0.46 16.07 f 0.21 17.10 f 0.42
" Each ion rate value shown (using single-ion monitoring) is the average of a single (2.5-min) analysis of a solution f the standard error (for the five integrations involved in a single analysis). bThe Pb solution used was NBS "Common Lead Isotopic Standard" cSince 208Pbconstitutes (SRM 981) in 0.25 M Ultrex "OB. 52.3470% of the total Pb in NBS SRM 981, the analyte concentration present at mlz 208 (i.e., the 208Pbisotope) is 2.69 ng L-' in a solution of NBS SRM 981 with a total Pb concentration of 4.0 ng L-1. the capacity to detect a P b isotope when present a t 2.1 ng L-l (2.1 pptr). Since the least abundant isotope of P b (204Pb)can be measured at its LOQ concentration (Table 111)when the total P b concentration exceeds 600 ng L-l(0.6 ppb), the m / z 204:206 ratio can be considered reliable above this same concentration for the total P b in a solution. Similarly, the m / z 207:206 ratio reaches an LOQ value a t 100 ng L-l (0.1 ppb) total P b and the m/z 208206 ratio reaches an LOQ value a t 80 ng L-I (0.08 ppb) total Pb. Precision and Linearity. Relative standard deviations of 1.1%, 0.76%, and 0.83% (n = 5) were obtained for 400 Mg L-' total lead (SRM 981) during an 8-h period using a 5-min analysis period (with the original ion lens set and before use of the mass flow controller) for the 208:206, 207:206, and 2M206 lead ratios, respectively. Table V provides information on short-term precision for the P b isotope ratios as a function of the lead concentration. Since ion counting was observed to become nonlinear between 180000 and 364000 ionsls, isotope ratios determined a t signal levels above 180000 ions/s can be distorted. As the major isotope, the 2oaPbsignal becomes nonlinear before the other isotope signals as ion rates increase. Consequently, the 208206 lead ratio is altered before the other ratios are affected as ion rates increase. When the number of measurement locations per isotope peak were changed from 1 to 3 (0.05 u apart) for a 2.5-min analysis, the ion-rate precision decreased significantly for 20BPband especially for 204Pb,as indicated in Table VI. The highly significant precision difference for 2oJPb(Table VI) is asso-
3
no. of measurmt locations per peak 1
3 F test sig leveld
% re1 std dev for ion rates
mlz 208 mlz 207 mlz 206 mlz 204 1.77 3.51 0.13
1.65 3.48 0.11
1.14 3.86 0.022
1.02 4.28 0.0095
"From five analyses (for each condition) of a 40 pg L-' solution of NBS "Common Lead Isotopic Standard" (SRM 981) in 0.25 M " 0 % bIon rate values are the averages f the standard deviations (not the standard errors because the variances are being subjected to the F test) for the blank-subracted results from five 2.5-min analyses for each condition. cThe t test significance level for the null hypothesis on the average ion rates. dThe F test significance level for statistical difference between variances.
ciated with a decrease in the proportion of the total measurement time spent on 204Pbwhen three locations per peak are used because then the minimum time on the other peaks is three dwells and not just one dwell. A different reason must account for the statistical difference for the 206Pbprecision shown in Table VI. Since the rate of change for a signal is zero a t the peak maximum, measuring a t three locations over a peak could be subject to more variation than measuring a t one location (at the maximum). The statistically significant decreases in the ion rates shown in Table VI for the average of three measurement locations per peak (0.05 u apart a t the nominal peak maximum) compared to a single location (at the peak maximum) are expected and do not affect the ratios because all four isotope intensities are altered to the same extent. The data in Table VI1 show that the isotope ratio precision values are statistically different when the proportion of time spent measuring each isotope is changed. When the amount of time spent measuring each of the four lead isotopes is the same, the relative standard deviation is several times larger for the ratio involving the least-abundant isotope in comparison to the other ratios. Analysis of Lead Samples. The isotopic ratios for lead ores from Idaho, Missouri, and Yugoslavia, as well as for commercial standards and foundry ash, are contrasted (where statistically significant a t the 95% level) with the NBS
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
2661
~~
Table VII. Lead Isotope Ratio Precision Related to Measurement Time per Isotopea measurmt time per isotope
no. of analyses
equalc
4
proportionald
5
time per isotope
equalc proportionald F test sig levele
isotope ratios ( m / z ) 208206 207:206 204206
* *
* *
0.9260 0.0024 0.9197 0.0082
2.242 0.006 2.229 0.019
no. of analyses 4 5
Table VIII. Lead Isotope Ratios for Bloods and Potential Lead Sources
*
0.0598 f 0.0004
208206
207:206
204:206
0.27 0.87 0.042
0.26
2.6 0.67 0.016
0.035
isotope ratio at m/z 208206 blood potential source
0.0621 0.0016
% re1 std dev for ratios ( m l z )
0.89
%
aFrom 2.5-min analyses of a 40 fig L-’ solution of NBS ‘Common Lead Isotopic Standard” (SRM 981) in 0.25 M Ultrex “OB. bRatio values are the averages and standard deviations (not the standard errors because the variances are being subjected to the F test) from blank-subtracted ion rates for the four or five analyses conducted (as indicated in the second column). eMeasurement time per Pb isotope was essentially equal with 26% for mlz 208,24% for mlz 207,26% for mlz 206, and 24% for m/z 204. dMeasurementtime per isotope was inversely proportional to abundance, so most measurement time was applied to the least abundant isotope: 88% for mlz 204,5% for mlz 206,5% for m/z 207, and 2% for m / z 208. ‘The F test significance level for statistical difference between variances.
2.028 (2B1)
2.050 (1B1)
2.212 (paint 2P1) 2.021 (air particulate) 2.052 (paint 1P1) 2.101 (soil 1Sl) 2.111 (dust 1D4) 2.021 (air particulate)
difference for ratio pair“ +9.1 -0.3 +0.1
+2.5 +3.0 -1.4
statistical differenceb Yes
no no no no no
a Difference between ratios as percent of blood ratio. Statistical difference between isotope ratios using 95% tolerance limits for sinele analvses. ~~
lead”, statistical differences were detected for the other isotope ratios (t test significance level 0.016 for the m / z 208206 ratio and 0,0010 for the m / z 207:206 ratio). Not only could digests of the Missouri and Idaho ores be distinguished by the isotope ratios but also a mixture of these digests (with 25% of the lead from Missouri ore and 75% from Idaho ore) could be distinguished from the unadulterated Idaho ore, even though the predicted ratio differences were only about 5 % . The isotope ratios for the lead standards from Spex Industries and from NBS (SRM 2121) could originate from a 1 1 mixture of Missouri and Idaho ores. Table VI11 shows lead isotope data for two blood lead case studies in the Boston area. For one case the lead isotope ratio in the blood sample is statistically different from that for the paint suspeded as the main source of the blood lead. Although the suspected paint for the other blood case could be the source for the lead in that blood (since there is no statistical difference between the isotope ratios), the same statement can be made about the dust, soil, and air particulate. While the air particulate sample shown in Table VI11 was not collected as part of these blood case studies, it could reflect the lead isotope composition for air particulate throughout the Boston area. This air particulate value of 2.021 does agree closely with the value of 2.024 obtained by thermal ionization mass spectrometry for air particulate collected in California (11). Whiie the digestion blank lead contribution is not a significant concern for the high lead concentrations in air particulate, dust, paint, and soil samples, there is more uncertainty for the low lead levels in blood digests. However, the m/z 208206 blood lead isotope ratios of 2.028 and 2.050 found in this study are near the averages of 2.040 and 2.062 obtained by thermal ionization mass spectrometry in another study (11). Isotope ratio determinations by ICP-MS can be used to exclude a suspected material as the source of lead in pollution situations.
+
m
0
-4 -8
12
--
-20 I
208/206
Y
207/206
204/206
Flgure 1. Statistical differences for Pb isotope ratios from NBS “Common Lead Isotopic Standard” (SRM 981). Only those differences exceeding the 95% tolerance limits for single analyses are included in the figure where numbers identify the following: (2) Pb Custom Plasma Standard (Spex Industries, lot no. 485MP); (3) NBS Spectrometric Standard Solution (SRM 2121); (4) Idaho ore sample; (5) Missouri ore sample; (6) Yugoslavia ore sample; (7) foundry ash sample no. 1023; (8) foundry ash sample no. 1076; (9) foundry ash sample no. 1130.
“Common Lead Isotopic Standard” ratios in Figure 1. By use of 95% tolerance limits, significant differences from NBS “common lead” were observed for the materials identified in Figure 1. The lead isotope ratios for the Missouri ore differed the most from the NBS “common lead” values with percent differences of -14% for 208206, -19% for 207:206, and -18% for 204:206. In comparison to the NBS “common lead” (containing24% %Pb), the Missouri ore is richer in 206pb with 27%. The increased proportion of 206Pbin the Missouri ore is associated with more decrease in mPb than in m8Pb. Since 2MPbconstitutes only 1.4% of the lead in the NBS “common lead” standard, a 10% change in the 204:206 ratio could be associated with an indistinguishable change of 0.3% in the 208206 ratio and no change in the 207:206 ratio. This pattern of a significant ratio change only for the 204206 ratio was observed (Figure 1) for Idaho ore in contrast with NBS “common lead” using the 95% tolerance limits for a single analysis. When a t test was conducted using the averages for five analyses each of the Idaho ore and of the NBS “common
ACKNOWLEDGMENT The authors thank Gerald L. McKinney, EPA Region 7, and Wayne Grotheer, EPA region 10, for assistance in obtaining the lead ore samples. Registry No. 2MPb,13966-26-2; 206Pb,13966-27-3; z07Pb, 14119-29-0;‘OSPb, 13966-28-4.
LITERATURE CITED Houk, R. S.;Fassel, V. A.; Flesch, 0. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. Anal. Chem. 1880, 52, 2283-2289. Date, A. R.; Gray, A. L. Analyst (London) 19S1, 106, 1255-1267. Date, A. R.; Gray, A. L. Analyst (London) 1983, 108, 1033-1050. Date, A. R.; Gray, A. L. I n t . J . Mass Spectrom. Ion Phys. 1883, 48, 357-360. Doualas, D. J.: Quan, E. S. K.; Smith, R. G. Spectrochim. Acta, Part 8 lg83, 388, 39-48. Gray, A. L. Spectrochim. Acta, Part 8 1985, 408, 1525-1537. Houk. R. S. AnalChem. 198s. 58. 97A-105A. Splvack, A. J.; Edmond, J. M. h i . Chem. 1886, 58, 31-35.
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(9) Auk, W. U.; Senechai, R. G.; Erlebach. W. E. Environ. Sci. Techno/. 1970. 4 , 305-317. (10) Brown, J. S. €con. Geol. 1962, 57, 673-720. (11) Yaffe, Y. Arch. Environ. Health 1983, 38.237-245.
RECEIVED for review November 24, 1986. Accepted July 24,
1987. The use of trade names in this report d w not constitute endorsement by the US.Environmental Protection Agency. This paper was presented in part a t the Environmental Protection Agency Seminar on Atomic Spectroscopy, Kansas City, MO, in April 1986.
Lithium Isotope Analysis by Thermal Ionization Mass Spectrometry of Lithium Tetraborate Lui-Heung C h a n Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803
AmaosspecttomeMcmedhodbasedonthethermallonization of iithtum tetraborate has been developed for the isotopic analysis of iitMum. The measurement of Li2B02+reduces lsotoplc fractfonatkn during analysis compared to the normal use of Li+ ion. The technique is capable of achieving a reiative precision of 1.3%0( I u ) in the determination of ilthium isotoplc ratios. A chemical procedure for quantltative and clean extraction of lithium from natural waters is described. This method has been applied to seawater wlth reproductMty withln the analytical uncertainty. Preltminary study indicates this technique is also applicable to silicate rocks.
trometry, the lithium isotopic ratios are normally measured on 6Li+and 7Li+ions. As the relative mass difference of the two isotopes is large, isotopic fractionation during ionization of the ion source may be appreciable. To maintain reproducibility strict adherence to specific instrumental conditions and filament heating time is required (8). The use of a molecular ion of higher mass, Li2B02+was investigated in this study with the view of reducing the isotopic effect during ion emission. The procedure, which involves the thermal ionization of Li2B407is adapted from the CszB4O7method developed for high-precision boron isotopic analysis (9). EXPERIMENTAL SECTION
Lithium isotopes are of interest from the viewpoint of geochemistry and cosmochemistry. Lithium participates in a wide variety of geological processes ( I ) . Its two stable isotopes (6Li and 7Li) are susceptible to fractionation during natural processes, owing to their relatively large mass difference. In addition, the isotopic abundance in extraterrestrial matter holds clues to the nuclear synthesis and cosmic abundance of light elements. Yet the development of the lithium isotopes as a diagnostic tracer has been hampered by the difficulty in accurate isotopic measurement of this light element. The isotopic composition of lithium has been studied by many investigators. Earlier data have been summarized by Heier and Billings, which show a spread of several percent in the isotopic abundance ratio in reagenb and natural lithium bearing minerals ( I ) . More recently a wide range of 6Li/7Li ratios (0.07257-0.1022) was obtained in material of different geological origins (2-4) by the method of thermionic emission from a silicate source (5). Meier (6) summarized values determined by atomic absorption, which show that the 6Li/7Li ratios of hydrothermally altered rocks vary between 0.0787 and 0.3774 and those of natural waters can be as high as 0.1545. The most precise assays to date involve standardization of reference material and determination of the atomic weight of lithium (7, 8). Two commercial natural lithium samples analyzed by conventional mass spectrometry were found to have absolute 6Li/7Li ratios of 0.0832 0.0002 (7) and 0.08137 f O.OO0 34 (8). The large variations in the isotopic composition of lithium reported in the literature have been ascribed to isotopic effects during chemical treatment, mass discrimination in the ion source, and instrument factors, as well as natural variation (7, 8). The purpose of this work is to develop a mass spectrometric technique suitable for precise determination of the lithium isotopic composition in geological material. In mass spec-
Instrument and Apparatus. The isotopic measurements were made on a solid-source mass spectrometer at MIT which has a 60" magnet sector with a 12-in. radius of curvature. It is equipped with a HP computer-controlled peak selection and data acquisition system. The detector assembly consists of a Faraday cup collector with a 9.2 X 1O'O R input resistor. The samples were loaded on single tantalum filaments (0.025 X 0.5 mm) mounted on glass beads (Cathodeon, Cambridge, England). Cation exchange columns were made of Vycor glass with a diameter of 2.5 cm. They were filled with Bio-Rad AG50X8 resin (200-400 mesh) to a height of 20 cm. The resin was initially washed with 6 N HCl. Prepacked Bio-Rad AGlX8 columns in chloride form (bed volume 2 mL) were used for anion exchange. The ultraviolet photooxidizing apparatus, evaporation apparatus, subboiling still, and Teflon beakers were the same as those described by Spivack and Edmond (9). Reagents. The isotopic standard used for reference in this study is a secondary Li2C03standard obtained from the National Bureau of Standards. This purified Li2C03was prepared from virgin ores (Lithium Corp. of America) and assayed by Flesch et al. (7). Boric acid with 95% enriched llB (Eagle-Picher Industries) was used for the synthesis of Li2B407.All acids used were purified by distillation in Vycor. Water was distilled at subboiling temperatures (-65 "C)by using the method of Spivack and Edmond (9)following deionization and distillation in a Corning still. HCl (0.5 N) was prepared from Vycor-distilled 6 N HCl and subboiling water. Reagent grade NaOH was used to convert anion exchange resin from the chloride form to the hydroxide form. Procedure. Chemical Separation. Lithium was separated from natural waters and rock samples by cation exchange chromatography. Rock samples were first ground in an agate mortar and decomposed by low temperature digestion in a mixture of HF, "Os, and HCIOl and evaporated to dryness. The residue was dissolved in subboiling water for chemical separation. The lithium concentrations in samples were determined by flame or furnace atomic absorption. Typically, samples containing 0.5 wmol of lithium were processed for isotopic analysis. Sample solution was passed through the cation exchange column and the resin was washed with 6 bed volumes of subboiling
0003-2700/87/0359-2662$01.50/00 1987 American Chemical Society
