Anal. Chem. 1997, 69, 3183-3187
On-Line Isotope Dilution Analysis with ICPMS Using Reverse Flow Injection Diane Beauchemin* and August A. Specht
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
A simple flow injection manifold is described to perform the addition of isotopic spikes to aqueous samples online with ICPMS. The analysis involves one multielement spike injection in the sample carrier and another injection of the spike solution in a standard carrier. This standard must contain one element which is not present in the spike solution, to allow the determination of the dispersion coefficient. The same standard also allows a reverse isotope dilution (ID) analysis, in addition to corrections for mass discrimination and any spectroscopic interference on one of the two isotopes used for the ID analysis. This flow injection approach, therefore, requires only one isotope free of spectroscopic interference for elements whose isotopic distribution does not vary in nature (two isotopes are still needed for other elements since the “natural” ratio must then also be determined). No preliminary analysis of the sample is required prior to the actual ID analysis. Furthermore, the concentration profile resulting from the flow injection allows the selection of the best isotopic ratio in terms of error propagation. This approach, therefore, makes ID analysis as simple as an external calibration but with added accuracy and precision. It was successfully applied to the analysis of a river water certified reference material and to saline water.
of enriched isotopic spikes to each sample, such that the desired isotopic ratios23 (often close to unity) are obtained. The preliminary estimate of the analyte concentration is, indeed, required to ensure that the spike/analyte molar ratio will be between 0.1 and 10, as the propagated error is then minimum.24 Furthermore, only elements which have two isotopes free of spectroscopic interference can be determined. However, for those elements to which ID can be applied, if a good equilibration of the isotopic spike with the analyte is achieved (ideally, the spike should have the same speciation as the analyte),25 then the isotopic spike acts as the ideal internal standard and will compensate for many sources of error, including sample evaporation losses and any effect of concomitant elements. ID-ICPMS is, therefore, a quasi-absolute technique. It is not quite absolute (i.e., standardless), because a correction for mass discrimination must be carried out on the measured isotopic ratio, and a standard of known isotopic composition is required for this purpose. Nonetheless, the corrected isotopic ratio measured for a given sample can be readily converted into analyte concentration using the following formula:
The stable isotope dilution (ID) analysis technique is the most accurate and precise calibration strategy available with inductively coupled plasma mass spectrometry (ICPMS).1 This explains its increasing popularity, as witnessed by numerous recent publications.2-22 It is, however, time-consuming, since it requires a preliminary analysis of all samples and then involves the addition
where C is the analyte concentration (µg L-1) in the sample solution, m is the mass of isotopic spike (µg) which was added to the sample, V is the volume of sample (L) which was spiked, K is the ratio of the atomic weight of the element over that of the spike, 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, and R is the measured isotopic ratio (reference/spike isotopes), corrected for mass discrimination. Although the sample spiking step can be facilitated by merging the sample with an enriched isotope spike solution on-line with ICPMS,7,17,26,27 a preliminary analysis of the sample is still required
(1) Jarvis, K. E.; Date, A. L.; Houk, R. S. Handbook of Inductively Coupled Plasma Mass Spectrometry; Blackie and Sons Ltd.: Glasgow, 1992; pp 168-171. (2) Mortlock, R. A.; Froelich, P. N. Anal. Chim. Acta 1996, 332, 277-84. (3) Scholze, H.; Hoffmann, E.; Luedke, C.; Platalla, A. Fresenius’ J. Anal. Chem. 1996, 355, 892-4. (4) Wildner, H.; Wuensch, G. Fresenius’ J. Anal. Chem. 1996, 354, 807-10. (5) Komoda, M.; Chiba, K.; Uchida, H. Anal. Sci. 1996, 12, 21-5. (6) Yi, Y. V.; Masuda, A. Anal. Sci. 1996, 12, 7-12. (7) Gallus, S. M.; Heumann, K. G. J. Anal. At. Spectrom. 1996, 11, 887-92. (8) Hwang, T.-J.; Jiang, S.-J. J. Anal. At. Spectrom. 1996, 11, 353-7. (9) Patriarca, M.; Lyon, T. D. B.; McGaw, B. M.; Fell, G. S. J. Anal. At. Spectrom. 1996, 11, 297-302. (10) Klinkenberg, H.; Van Borm, W.; Souren, F. Spectrochim. Acta 1996, 51B, 139-53. (11) Yi, Y. V.; Masuda, A. Anal. Chem. 1996, 68, 1444-50. (12) Coedo, A. G.; Dorado, T.; Fernandez, B. J.; Alguacil, F. J. Anal. Chem. 1996, 68, 991-6. (13) Katoh, T.; Akiyama, M.; Ohtsuka, H.; Nakamura, S.; Haraguchi, K.; Akatsuka, K. J. Anal. At. Spectrom. 1996, 11, 69-71. (14) Hastings, D. W.; Emerson, S. R.; Nelson, B. K. Anal. Chem. 1996, 68, 3717. (15) Alonso, J. I. G. Anal. Chim. Acta 1995, 312, 57-78. (16) Xie, Q.; Kerrich, R. Chem. Geol. 1995, 123, 17-27. S0003-2700(97)00273-4 CCC: $14.00
© 1997 American Chemical Society
Csample )
mspike (A′ - B′R) K Vsample (BR - A)
(1)
(17) Hill, S. J.; Brown, A.; Rivas, C.; Sparkes, S.; Ebdon, L. Tech. Instrum. Anal. Chem. 1995, 17, 411-34. (18) Campbell, M. J. Tech. Instrum. Anal. Chem. 1995, 17, 27-37. (19) Paschal, D. C.; Caldwell, K. L.; Ting, B. G. J. Anal. At. Spectrom. 1995, 10, 367-70. (20) Alonso, J. I. G.; Sena, F.; Arbore, P.; Betti, M.; Koch, L. J. Anal. At. Spectrom. 1995, 10, 381-93. (21) Enzweiler, J.; Potts, P. J.; Jarvis, K. E. Analyst 1995, 120, 1391-6. (22) Murphy, K. E.; Paulsen, P. J. Fresenius’ J. Anal. Chem. 1995, 352, 203-8. (23) Patterson, K. Y.; Veillon, C.; O’Haver, T. C. Anal. Chem. 1994, 66, 282934. (24) Hall, G. E. M.; Park, C. J.; Pelchat, J. C. J. Anal. At. Spectrom. 1987, 2, 189-96. (25) Rottmann, L.; Heumann, K. G. Fresenius’ J. Anal. Chem. 1994, 350, 2217.
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if maximum accuracy and precision are to be obtained. This paper describes a simple flow injection (FI) manifold which can be used to perform the addition of enriched isotopes to aqueous samples on-line with ICPMS. In contrast to the off-line and abovementioned on-line merging methods, this FI approach requires only one isotope free of spectroscopic interference (for elements whose isotopic distribution does not vary in nature) and does not require knowledge of the sample concentration prior to ID analysis. The proposed approach, therefore, makes ID as simple as an external calibration but with the added accuracy and precision associated with ID analysis. THEORY Flow injection consists in the injection of a discrete volume of sample into an unsegmented continuous flow of solution called carrier.27 As the sample is entrained, it is gradually dispersed into the carrier, and vice versa. The extent of dispersion (D) of the injected solution into the carrier depends on numerous factors (design of the FI manifold, flow conditions, etc.). Because the edges of the FI slug are in direct contact with the carrier, they get dispersed more than its center, resulting in the concentration of the sample being maximum in the center of the sample plug. Furthermore, since the back of the plug has more time to undergo dispersion than the front, an asymmetrical peak usually results (such as that observed for 62Ni in Figure 4). In any case, the dispersion at any point of the sample plug can be expressed as28
D ) C0/C
(2)
where C0 is the original analyte concentration and C is the concentration at any point of the FI profile. Because of the wide linear dynamic range of ICPMS, it can be readily measured by comparing any point of the transient FI signal to that resulting from continuous aspiration of the sample:
D)
steady-state signal height of FI peak
(3)
Obviously, the concentration profile of the carrier is the complement of that of the sample (i.e., the concentration of the carrier is greater around the edges of the sample slug than in its center). Therefore, the dispersion of the carrier (Dcarrier) can be expressed in terms of the sample dispersion using29
Dcarrier ) D/(D - 1)
(4)
In reverse flow injection (r-FI), the sample is used as the carrier, and a solution is injected into it. This approach is preferable in this instance in order to minimize isotopic carryover, which may limit the sample throughput. Indeed, if a solution containing a low concentration of a certain enriched stable isotope is aspirated immediately following one with a high concentration of the same isotope, an erroneous ratio may result if the washout time is not long enough. In r-FI, this problem is readily alleviated by injecting only a discrete volume of the enriched isotope solution (26) Heumann, K. G.; Rottmann, L.; Vogl, J. J. Anal. At. Spectrom. 1994, 9, 13515. (27) McLeod, C. W. J. Anal. At. Spectrom. 1987, 2, 549-52. (28) Ru˚zˇicˇka, J.; Hansen, E. H. Anal. Chim. Acta 1978, 99, 37-76. (29) Tyson, J. F. Analyst 1985, 110, 419-29.
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Figure 1. Schematic of the flow injection manifold. P, peristaltic pump; S, spike solution injection valve; C, carrier solution (sample or standard).
into the sample carrier. Under these conditions, the sample effectively acts as a washout solution, which drastically minimizes isotopic memory effects. Furthermore, the continuous monitoring of the FI profile also provides an instantaneous check-up of the expected ratio (based on natural abundances for many elements). When performing ID in the r-FI mode (see Figure 1), the analyte concentration at any point of the transient profile will be given by eq 1, modified to take into account the dispersion of the sample into the spike (Dsample) and of the isotopic spike into the sample (Dspike) as follows:
Csample Cspike (A′ - B′R) ) K Dsample Dspike (BR - A)
(5)
where Cspike corresponds to mspike/Vsample of eq 1. Because of the complementarity of the two dispersions,
Dspike ) Dsample/(Dsample -1)
(6)
and, in the FI mode, the analyte concentration can simply be calculated as
(A′ - B′R) (BR - A)
Csample ) (Dsample - 1)CspikeK
(7)
where Dsample is the dispersion of the sample in the spike plug, Cspike is the concentration of the isotopic spike solution which was injected into the sample carrier, and the rest are the same as for eq 1. EXPERIMENTAL SECTION Apparatus. The instrumentation consisted of a Perkin-Elmer/ SCIEX ELAN 500 inductively coupled plasma mass spectrometer (Concord, ON, Canada) coupled to a simple single-line FI manifold, which is illustrated in Figure 1. The system is composed of a peristaltic pump (Minipuls II, Gilson Medical Electronics, Middleton, WI) and a sample injection valve (Model 5020, Rheodyne, Cotati, CA) which was electronically actuated by a switching module (Universal, Anachem, Luton, U.K.). Approximately 150 cm of 0.3-mm-i.d. Teflon tubing connected the valve to the Meinhard (C-3) nebulizer of the ICPMS instrument. Several modifications were made to the latter, which included the addition of a mass flow controller on the aerosol carrier gas line and an x-y-z translation stage under the torch box. The standard ICPMS torch was also replaced by a short ICP emission one. The operating conditions which were used throughout this work are listed in Table 1. The plasma position with respect to the interface and the aerosol carrier gas flow rate were optimized daily to maximize sensitivity while continuously aspirating a 100 µg L-1 solution of Li, Rh, and Pb. The lens settings were adjusted
Table 1. ICPMS Operating Conditions ICPMS torch sampler and skimmer forward rf power plasma gas auxiliary gas
ELAN 500 aerosol carrier gas PlasmaTherm carrier flow rate Ni (standard) dwell time sweep/reading 1.2 kW reading/replicate 12 L min-1 point/spectral peak 2.0 L min-1
0.9 L min-1 1.0 mL min-1 40 ms 1 1 1
(if needed) to equalize the Li and Pb signals with little sacrifice in the Rh signal. Reagents. Multielemental solutions in 1% (or 0.15 M) HNO3 were prepared using high-purity HNO3 (Ultrex II from J.T. Baker Inc., Phillipsburg, NJ), 1000 mg L-1 monoelemental standard solutions (Spex Industries, Edison, NJ), and deionized distilled water (Milli-Q Plus System, Millipore, Mississauga, ON, Canada). Some NaNO3 (Analar grade, BDH, Toronto, Ontario, ON, Canada) was used to prepare the 0.036 M Na solution containing 3 µg L-1 Mo. The riverine water reference material for trace metals SLRS-2 was obtained from the National Research Council of Canada (Ottawa, ON, Canada). Procedure. To perform an ID analysis, a 1% HNO3 blank was first aspirated continuously (i.e., used as carrier), followed by a standard solution into which the enriched isotope solution was injected (using a 100-µL loop). The sample was then used as the carrier, and another injection of the enriched isotope solution was then made into it. Data Processing. From the steady-state signal measured for the blank, an average blank signal was computed and subtracted from all other measurements. An average natural ratio was then computed from the steady-state signals of both the standard solution carrier and the sample carrier. Any discrepancy between the two ratios indicated a spectroscopic interference. The magnitude of the interfering signal was found as follows:
S2sample -
S1sample ) Si Rstd
(8)
where S1 and S2 are the individual averaged count rates obtained from the steady-state signals of the reference and spike isotopes, respectively, in the sample, Rstd is the S1/S2 ratio obtained for the standard solution, and Si is the signal contributed by the interferent on S2. A similar approach could be used if the interference was on the reference isotope instead of the spike isotope. A point-by-point correction was then made to the interfered isotope by subtracting from it Si/Dsample, since the extent of the interference indeed decreased upon dispersion with the injected solution. A point-by-point computation of Dsample was accomplished using the injection performed in the standard carrier, with an element which was present in the standard but not the spike solution injected. Under these conditions, a negative peak resulted, as shown in Figure 2, and the dispersion (shown in Figure 3) computed using eq 3 was then that of the carrier. It should be noted that Dsample was not computed when the sample was used as carrier because any effect of concomitant element would be maximum in the steady-state region but would decrease during the injection, thereby resulting in erroneously high dispersion values during the FI peak. The determination of Dsample was,
Figure 2. Signal for 59Co+ resulting from the flow injection of 100 µL of Co-free isotopic spike solution into a 1 µg L-1 multielement standard. It was monitored simultaneously with those reported in Figure 4.
Figure 3. Dispersion profile resulting from a 100-µL injection, which was computed from Figure 2, by ratioing each point to the average of the steady-state signal between 0 and 240 s.
therefore, carried out using the injection performed in the standard solution rather than in the sample because the dispersion is dependent only on the FI manifold and carrier flow rate, which both remained the same for the whole study (i.e., D should be independent of the solution used as carrier). A point-by-point computation of the ratio R (corrected, if needed, for spectroscopic interferences as described above) was then carried out. This ratio was also corrected for mass discrimination, if needed. For elements whose isotopic distribution does not vary in nature, this was accomplished by multiplying R by a correction factor equal to the ratio of the isotopic ratio computed from the natural abundances and that measured for the standard solution (in the steady-state region). A point-by-point computation of Csample was then done using eq 7. Finally, an average Csample was computed in the region where the ratio was around 1. RESULTS AND DISCUSSION Choice of Injection Volume. Figure 4 shows the transient signal resulting from the injection of 100 µL of a 5 µg L-1 62Ni spike solution into a 1 µg L-1 Ni (in 1% HNO3) solution. Because the spike solution is enriched in 62Ni, a positive peak results for 62Ni+, whereas a dip is observed for 58Ni+. Since the natural abundance of 58Ni is greater than that of 62Ni, the two signals also cross one another twice, resulting in two regions where the isotopic ratio is around unity (a value which is frequently desired in ID analysis). Because mixing is more intensive at the back than at the front of the injection plug, the second region, where the ratio is around unity, is longer than the first one and is, therefore, preferred for quantitation purposes. Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
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Figure 4. Signals for 58Ni+ (solid line) and 62Ni+ (dashed line) resulting from the flow injection of 100 µL of 5 µg L-1 62Ni isotopic spike solution into a 1 µg L-1 multielement standard. These were monitored simultaneously to the signal in Figure 1.
Figure 5. Isotopic ratio profile of 58Ni+/62Ni+, corrected for mass discrimination, obtained by point-by-point ratioing of the individual signals shown in Figure 4.
Figure 6. Result of the ID analysis of a 1 µg L-1 standard using a 100 µL injection of 5 µg L-1 62Ni isotopic spike solution. The Ni concentration was calculated by the point-by-point application of eq 7 to the ratio shown in Figure 5, using the dispersion values from Figure 3. The result is obtained by taking the average of the steadystate concentration (i.e., between 275 and 287 s).
In fact, Figure 4 readily demonstrates one major advantage of the FI approach: as long as the concentration of the spike element is greater in the spike solution than in the sample carrier, the signals observed for the spike and reference isotopes will always cross each other at two different points. Therefore, the FI approach readily provides a range of reference/spike isotopic ratios, from much greater than unity to much smaller than unity. The best ratio for a particular analysis can, therefore, always be selected. Thus, a preliminary analysis of the sample (as must be 3186 Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
Figure 7. Ni concentration obtained from the ID analysis of a 1 µg L-1 standard using a 250-µL injection of 5 µg L-1 62Ni isotopic spike solution. The result is obtained by taking the average of the steadystate concentration (i.e., between 180 and 200 s).
carried out when the ID method is carried out off-line) is no longer required. Figure 5 shows the isotopic ratio profile (corrected for mass discrimination) corresponding to the injection of Figure 4. A fairly steady-state region is observed between 260 and 290 s. The corresponding concentration profile, resulting from the point-bypoint application of eq 7 (along with the dispersion profile shown in Figure 3) to the isotopic ratio from Figure 5, is shown in Figure 6. A region of constant concentration can be seen between 275 and 287 s. An average over this period (n ) 26) yields a Ni concentration of 0.97 ( 0.05 µg L-1 (where the error is 1 standard deviation). This corresponds to the second half of the FI slug, as can be seen in Figure 4. (As predicted, the front of the slug results in a much narrower region of constant concentration.) The position and width of the constant concentration regions depend on several factors, including the injection volume, the FI manifold, and the concentrations of the sample and spike solutions. For example, Figure 7 shows the concentration profile obtained with 250-µL injections under otherwise identical conditions. Again, two regions of constant concentration are observed, the second one being, as for the 100-µL injections, wider and steadier than that resulting from the front of the FI slug. The biggest difference with Figure 6 is that the steady concentration regions are both broader than those obtained with 100-µL injections. An average over the 180-200-s region (n ) 44) yields a Ni concentration of 1.06 ( 0.16 µg L-1. Compared to “off-line” ID-ICPMS, the relative errors are greater because the precision on individual ratios depends on the measurement time, which was kept short to allow the acquisition of a relatively large number of data points across the FI peak. However, the precision would improve if the results from several injections of a given sample were pooled. In any case, the exact “sample” and “spike” volumes are unimportant since the dispersion coefficient is determined each time. The only requirement is that they are selected so that the concentration gradient results in a region where the ideal ratio is encountered. Since comparable results were obtained with different injection volumes but any memory effect from the enriched isotope level will be lower with a smaller spike solution volume, injections of 100 µL were performed for the remainder of this study. Although it was used here to show the effect of a change in injection volume, the spike injection performed in the standard carrier was actually used to perform a reverse-ID analysis of the spike solution. This step is required for any accurate ID analysis whether carried out off- or on-line with ICPMS.
Determination of Ni in River Water. The method was applied to the analysis of a certified riverine water (SLRS-2) using 58Ni as the reference isotope and 62Ni as the spike one. In this case, the isotopic ratio (which was 31.40 instead of 18.535 from natural abundances) had to be corrected for the spectroscopic interference of 58Fe on 58Ni, as well as for mass discrimination. The Ni concentration found in SLRS-2 using an average over the whole 40 s of steady concentration obtained (n ) 88) was 1.03 ( 0.14 µg L-1 (where the error is expressed as the standard deviation). A narrower concentration window (10 s, n ) 22) could have also been used to yield 1.07 ( 0.17 µg L-1, with no significant difference in the result. Either result was in excellent agreement with the certified value of 1.03 ( 0.10 µg L-1 (where the error is a 95% confidence limit). Determination of Mo in Saline Water. One of the strengths of ID analysis is that the spike isotope acts as the ideal internal standard for the determination of that element, effectively compensating for most matrix effects (i.e., as long as the signal of either the reference or the spike isotope, or both, is not suppressed down to background level). To check how efficient the method was at compensating for nonspectroscopic interferences, 3 µg L-1 solutions of Mo in each of 0.036 M NaNO3 and 1% HNO3 were analyzed using a 5 µg L-1 97Mo spike solution, with 96Mo as the reference isotope. Despite the substantial suppression experienced in 0.036 M Na (where all the Mo signals were 46% of those observed in the Na-free solution), a Mo concentration of 3.01 ( 0.26 µg L-1 was obtained, in good agreement with 3.06 ( 0.22 µg L-1 found in the Na-free solution. CONCLUSION The FI-ID approach, therefore, presents clear advantages over both the off-line method and the merging zones method. No preliminary analysis of the sample is needed (the only requirement is that the concentration of the spike element in the spike solution be greater than that in the sample, a prerequisite which is readily
achieved if trace analysis is carried out and a 5 µg L-1 spike solution is injected). The time-consuming disadvantage of ID1 is, therefore, eliminated. Furthermore, reduced sample consumption results from the fact that the sample must be aspirated only long enough to appropriately surround the spike injection. This, in turn, leads to a higher sample throughput. Because the whole analysis is carried out in a closed system, contaminations are reduced. In addition, isotopic carryover is minimized since only discrete injections of the spike solution are made, the sample itself acting as a washing solution. Finally, a broader range of applications is possible since, except for elements whose isotopic distribution varies in nature, only one isotope free of spectroscopic interference is required. It should be noted that manual injections were performed in the present study which were mainly aimed at verifying the theory. Future work will focus on automating the procedure so that matching the dispersion curve (for instance, Figure 3) to the ratio profile (such as Figure 5) is no longer dependent on the reproducibility of the injection. Automation will also facilitate replicate analyses of a given sample, which will, in turn, improve the precision (through pooling of data). ACKNOWLEDGMENT The principal author gratefully acknowledges funding from the Natural Sciences and Engineering Research Council of Canada (through Grant No. OGP0039487). She is also thankful to PerkinElmer (Norwalk, CT) for the donation of ELAN-5000 software, and to Ellyn Beary of the National Institute of Standards and Technology (NIST) for providing the solutions of enriched stable isotopes used in this work. Received for review March 11, 1997. Accepted June 2, 1997.X AC970273L X
Abstract published in Advance ACS Abstracts, July 15, 1997.
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