Correspondence Anal. Chem. 1996, 68, 567-569
Precise Measurement of Isotope Ratios with a Double-Focusing Magnetic Sector ICP Mass Spectrometer Frank Vanhaecke,† Luc Moens, and Richard Dams*
Laboratory of Analytical Chemistry, Ghent University, Institute for Nuclear Sciences, Proeftuinstraat 86, B-9000 Ghent, Belgium Philip Taylor
Institute for Reference Materials and Measurements, European Commission Joint Research Centre, Retieseweg, B-2440 Geel, Belgium
Since its commercial introduction in 1983, quadrupole inductively coupled plasma mass spectrometry (ICPMS) has aroused great interest and has become increasingly popular and widespread in the analytical community. Although the potential of this technique for isotope ratio determination was recognized1-6 from the early days, most ICPMS instruments are currently operated for element determination only. This is mainly due to the
relatively poor precision of isotope ratio determinations via ICPMS in comparison with “classical” isotope techniques such as, e.g., thermal ionization (TIMS) and gas source mass spectrometry (GSMS). With the quadrupole ICPMS instruments presently commercially available, the short-term precision of isotope ratios has typically been limited to 0.2-0.6% (expressed as relative standard deviation (RSD) over a number (often 10) of replicates).7-11 Only in a few exceptional cases have values of ∼0.1% been reported,8,10 and further improvement requires nonstandard instrument modifications. Gray et al.12 obtained experimental RSDs of ∼0.1%, using a bonnet to shield the ICP from the surrounding air and/or using free aspiration of the sample solution to eliminate the peristaltic pump noise. By using a free expansion interface instead of the “normal” interface (using a sampling cone and a skimmer), they were able to further reduce the RSD to 0.02% (value limited by counting statistics under conditions used). It should be emphasized, though, that this interface did not allow routine operation and that the results were obtained for a concentration as high as 100 mg/L of Ag, a level that is quite unusual in normal ICPMS operation. In spite of the limited isotope ratio precision attainable with commercially available instrumentation, for some applications, quadrupole ICPMS remains attractive, mainly as a result of its ease of operation (samples can be introduced in aqueous form, at atmospheric pressure, and no chemical separation is needed), which leads to a much higher sample throughput than is possible with TIMS. Hence, the technique has been used for isotope ratio determination in, e.g., archaeological13 and environmental stud-
† Senior Research Assistant of the Belgian National Fund for Scientific Research. (1) Date, A. R.; Cheung, Y. Y. Analyst 1987, 112, 1531-1540. (2) Hinners, T. A.; Heithmar, E. M.; Spittler, T. M.; Henshaw, J. M. Anal. Chem. 1987, 59, 2658-2662. (3) Longerich, H. P.; Fryer, B. J.; Strong, D. F. Spectrochim. Acta 1987, 42B, 39-48. (4) Price Russ, G., III; Bazan, J. M. Spectrochim. Acta 1987, 42B, 49-62. (5) Ting, B. T. G.; Janghorbani, M. Spectrochim. Acta 1987, 42B, 21-27. (6) Ting, B. T. G.; Janghorbani, M. J. Anal. At. Spectrom. 1988, 3, 325-336.
(7) Ketterer, M. E.; Peters, M. J.; Tisdale, P. J. J. Anal. At. Spectrom. 1991, 6, 439-443. (8) Furuta, N. J. Anal. At. Spectrom. 1991, 6, 199-203. (9) Ketterer, M. E. J. Anal. At. Spectrom. 1992, 7, 1125-1129. (10) Koirtyohann, S. R. Spectrochim. Acta 1994, 49B, 1305-1311. (11) Roehl, R.; Gomez, J.; Woodhouse L. R. J. Anal. At. Spectrom. 1995, 10, 15-23. (12) Gray, A. L.; Williams, J. G.; Ince, A. T.; Liezers, M. J. Anal. At. Spectrom. 1994, 9, 1179-1181. (13) Ghazi, A. M. Appl. Geochem. 1994, 9, 627-636.
The potential of a commercially available double-focusing magnetic sector ICP mass spectrometer (Element, Finnigan MAT, Bremen, Germany) for precise isotope ratio measurement at the low-resolution setting (R ) 300) was evaluated. Optimization of scanning conditions led to a relative standard deviation for a set of 10 consecutive 2 min measurements of ∼0.1% (206Pb+/207Pb+) at signal intensities of ∼200 000 counts/s (peak height). This compares favorably with the best values ever reported for quadrupole ICPMS and barely exceeds the theoretical value (counting statistics). Increasing the signal intensity to values g500 000 counts/s (peak height) resulted in a further reduction of the RSDs obtained (for both 25Mg+/ 26Mg+ and 206Pb+/207Pb+) to typically 0.04%. These figures are remarkably better than those reported for commercially available quadrupole ICPMS systems. This improvement significantly reduces the difference between isotope ratio precision of ICPMS on one hand and those of thermal ionization mass spectrometry and plasma source multiple collector mass spectrometry on the other.
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ies,14 nutritional studies using tracers11 and also of course, for elemental assay using isotope dilution. To combine the advantages of a plasma source with the high precision of TIMS, a mass spectrometric device, exclusively intended for precise isotope ratio determination, was developed by coupling an ICP ion source to a double-focusing magnetic sector mass spectrometer equipped with seven Faraday detectors.15 This instrument (Plasma 54, Fisons Instruments, Winsford, U.K.) permits simultaneous measurement of the ion currents of up to seven isotopes, which minimizes the effect of source instability on the precision of isotope ratios and results in RSDs (typically 0.01-0.05% when not corrected for variations in mass bias by internal normalization) comparable with those obtained using TIMS.15,16 Since 1989, ICP mass spectrometers equipped with a doublefocusing magnetic sector mass spectrometer, instead of a quadrupole filter, have also become commercially available.17 These instruments are capable of resolving isotopes from molecular ions resulting from the plasma, the matrix, and the surrounding air and hence are most often referred to as high-resolution (HR) ICP mass spectrometers. The recent introduction of a “second generation” of HR-ICPMS instrumentation by several manufacturers18,19 was accompanied by a significant reduction in price and led to a growing interest in the technique from both routine and research laboratories. So far, double-focusing magnetic sector mass spectrometers have almost exclusively been used as even more sophisticated elemental detectors, while very little has been reported on their performance at isotope ratio measurement. The present paper reports on the precision of isotope ratio determination using such a commercially available instrument, the Element (Finnigan MAT, Bremen, Germany), in its standard configuration and at the low-resolution setting (R ) 300). EXPERIMENTAL SECTION Instrumentation. All measurements were carried out on a Finnigan MAT Element high-resolution ICP mass spectrometer. This recently commercially introduced instrument is equipped with a compact, double-focusing sector field mass spectrometer of reversed Nier-Johnson geometry. Predefined resolution settings of 300, 3000, and 7500 allow the resolution to be adjusted, depending on the analytical problem at hand. In contrast to previous double-focusing magnetic sector ICP mass spectrometers, the plasma torch, sampling cone, skimmer, and other mechanical parts of this instrument are kept at ground potential. The acceleration of the extracted ions over 8 kV takes place only in the vacuum region of the instrument.19 As a result, plasma operation and plasma extraction are to a large extent similar to the corresponding processes in quadrupole-based instruments, and owing to the absence of capacitive coupling of the acceleration voltage to the high-frequency plasma, fast electric scanning is possible.19 In its standard configuration, the Element is equipped with a Spetec multichannel peristaltic pump, a Meinhard Tr-30A3 concentric nebulizer, a Scott-type spray chamber with sur(14) Campbell, M. J.; Delves, H. T. J. Anal. At. Spectrom. 1989, 4, 235-236. (15) Walder, A. J.; Freedman, P. A. J. Anal. At. Spectrom. 1992, 7, 571-575. (16) Walder, A. J.; Koller, D.; Reed, N. M.; Hutton, R. C.; Freedman, A. P. J. Anal. At. Spectrom. 1993, 8, 1037-1041. (17) Bradshaw, N.; Hall, E. F.; Sanderson, N. E. J. Anal. At. Spectrom. 1989, 4, 801-803. (18) Hertens, R. C.; Morita, T.; Kubota, M. Second Regensburg Symposium on Massenspektrometrische Verfahren der Elementspurenanalyse; 1993; p 2. (19) Giessmann, U.; Greb U. Fresenius J. Anal. Chem. 1994, 350, 186-193.
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Table 1. Instrument Settings rf power (W) gas flow rates (L/min) plasma auxiliary nebulizera sample uptake rate (mL/min) ion sampling depth ion lens settings sampling cone skimmer a
1250 14 0.700 0.675-0.750 1 a a nickel, 1.1 mm aperture diameter nickel, 0.8 mm aperture diameter
Adjusted in order to obtain maximum signal intensity.
rounding liquid jacket, maintained at 5 °C using a recirculating refrigeration-heating system (and drained with the same peristaltic pump), a Fassel torch, and a 27 MHz generator (ICP20, RF Power Products Voorhees, NJ). Instrument settings are summarized in Table 1. The Element allows (i) magnetic scanning (by varying the magnetic field (B-field) at a fixed acceleration voltage), (ii) electric scanning at a fixed magnetic field, and (iii) synchroscanning19 (during which both the magnetic field and the electric field are varied). Isotope ratio measurements were carried out only using electric scanning (E-scanning) and at the lowresolution setting (R ) 300). Scan conditions were adjusted in order to obtain optimum repeatability and are discussed under Results and Discussion. Standard Solutions. Mg and Pb standard solutions were prepared using commercially available stock solutions (1000 mg/ L). Scan conditions were optimized using Pb standard solutions. The concentration of the latter solutions was adjusted such that the signal intensity for the nuclides monitored (206Pb and 207Pb) exceeded 200 000 counts/s (counts per second peak height). This led to Pb concentrations ranging from 20 to 40 µg/L, depending on the actual sensitivity of the instrument. In our attempts to evaluate the best achievable repeatability, Pb concentrations of 100 µg/L and Mg concentrations of 5 mg/L were used to keep the contribution of the counting statistics to the total uncertainty as low as possible. Measurements. The scanning conditions were optimized for the best isotope ratio precision (short-term, 2 min) by monitoring the 206Pb+/207Pb+ isotope ratio obtained for 20-40 µg/L Pb standard solutions. All measurements were limited to 2 min acquisition time per replicate. The latter (i) permitted the acquisition of a relatively large number of counts for the isotopes involved, such that the contribution of the counting statistics to the total uncertainty did not exceed 0.1%, and (ii) is completely acceptable in view of routine use. Experimental RSDs were calculated on the basis of 10 successive 2 min measurements and were compared with the theoretical values (calculated on the basis of Poisson statistics). RESULTS AND DISCUSSION It is general knowledge that for precise isotope ratio determination, the scanning (or peak-hopping) rate should be sufficiently high to smooth out signal fluctuations due to plasma flicker and variations in, e.g., the sample uptake rate and the nebulization, ionization, and extraction efficiencies. E-scanning is operated at a fixed B-field and allows rapid scanning of one or more discrete mass intervals. Hence, this is the appropriate scanning mode for isotope ratio determinations. With the B-field fixed at a given
Table 2. Optimum Scanning Conditions for Isotope Ratio Measurement scan type magnet settling time (s) magnet mass (u) Pb Mg mass range (u) Pb Mg dwell time/measurement point (ms) scan duration/sweep (s) no. of sweeps measurement time/ replicate(min)
electric scanning 0 205.974 24.986 205.700-206.249, 206.700-207.252 24.952-25.019, 25.948-26.017 1 2 × 0.05 1200 2
Figure 1. Normalized 25Mg+signal intensity (O) and 25Mg+/26Mg+ signal ratio (+) as a function of the measurement number (data taken from Table 4, set 2: RSD 25Mg+signal intensity, 3.7%; RSD 25Mg+/ 26Mg+ signal ratio, 0.033%).
Table 3. Experimental and Theoretical RSDs (%) for 206Pb+/207Pb+ Obtained Using the Scanning Conditions Presented in Table 2a measurement set
exptl RSD (n ) 10)
theor RSD
1 2 3 4
0.11 0.044 0.12 0.063
0.062 0.062 0.065 0.053
Table 4. Experimental and Theoretical RSDs (%) for 25Mg+/26Mg+ and 206Pb+/207Pb+ (Peak Heights g 500 000 counts/s) Obtained Using the Scanning Conditions Presented in Table 2 ratio
exptl RSD (n ) 10)
theor RSD
25Mg+/26Mg+
0.047 0.033 0.041
0.022 0.022 0.038
25Mg+/26Mg+ 206Pb+/207Pb+
a
The Pb concentration was varied in order to correct for day-today variation in instrument sensitivity (peak height g 200 000 counts/s).
magnet mass (mB), the mass region that can be scanned electrically is limited to the range from mB to (1 + 0.3)mB. Optimization of the scanning conditions indicated that the scan duration per sweep is of great importance: a 2-3-fold improvement in the RSDs (n ) 10) was obtained upon a 10-fold increase in the scanning rate (reduction of the scan duration per sweep from 1 to 0.1 s). Further reduction of this scan duration to 0.02 s did not significantly improve the precision any further. Table 2 shows the scanning conditions that were found to give the best performance. Table 3 compares the experimental RSDs obtained under the conditions shown in Table 2 with the theoretical values (Poisson counting statistics). It is clear that these results, obtained under optimized scanning conditions on an instrument in its standard configuration, are comparable with the best values ever reported for commercially available quadrupole-based ICPMS instruments. Moreover, the precision obtained can be reproduced easily. It can also be calculated that the experimental RSD exceeds the theoretical value by only 30% on average. Enhancement of the count rate by increasing the concentration of the standard solutions to 100 µg/L Pb and 5 mg/L Mg produced a further reduction of the RSDs (for both 25Mg+/26Mg+ and 206Pb+/207Pb+) to e0.05% (Table 4), or ∼0.01% standard error on the mean (s/ n1/2). Except for the results obtained with a highly experimental setup (free expansion interface) by Gray and co-workers,12 such values have never been reported for quadrupole ICPMS and represent a significant reduction in the difference between the
isotope ratio precision of ICPMS on one hand and those of thermal ionization mass spectrometry and plasma source multiple collector mass spectrometry on the other. Finally, for those not acquainted with the power of isotopic measurements, Figure 1 (data taken from Table 4, set 2) is an elegant illustration. This figure shows that excellent stability of the ion signal intensities is not a prerequisite for precise isotope ratio determinations. Even for a change in ion currents of >10% over a 20 min period (significantly worse than usual), the isotope ratios stay constant to within 0.03%, a value barely exceeding the theoretical value (0.022%). CONCLUSIONS It has been shown that double-focusing magnetic sector ICP mass spectrometry, operated under low-resolution conditions, yields a remarkable improvement in isotope ratio precision when compared to quadrupole-based ICPMS. Further research is presently being carried out to evaluate the isotope ratio precision attainable at higher resolution settings. Whatever the outcome of that study, the results presented in this paper suggest that this double-focusing magnetic sector mass spectrometer opens the way to certain isotope ratio measurements which previously have been beyond the reach of quadrupole ICPMS instruments. Received for review July 19, 1995. Accepted October 12, 1995.X AC9507247 X
Abstract published in Advance ACS Abstracts, December 15, 1995.
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