Isotope ratio precision

As discussed before, quadrupole based ICP-MS allows multi-element determination at the trace and ultratrace level and/or isotope ratios in aqueous solutions in a few minutes as a routine method with detection limits of elements in the sub pgml-1 range and a precision for determined trace element concentration in the low % range (RSD - relative standard deviation). The precision for isotope ratio measurements varies between 0.1% and 0.5% RSD. This isotope ratio precision is sufficient for a multitude of applications, e.g., for evidence of contamination of sample with depleted or enriched uranium in urine (this technique is used in the author s laboratory in a routine mode14) or the isotope dilution technique for the quantitative determination of trace element and species concentration after doping the sample with enriched isotope spikes. 

Data acquisition parameters. Precision and accuracy in the measurement of isotope ratios can be improved if the number of measurements is increased (e.g. if the measurement time is increased). Various measurement protocols can be applied and those whereby the time actually spent on measuring the isotope ratios of interest is maximised are preferable. The data acquisition parameters of an ICP-MS device that can be changed to improve the isotope ratio precision... 

The ideal internal standard is the same element as the analyte because it has similar mass, ionization energy, and chemical properties. Therefore, isotope dilution based calibration provides high accuracy as long as isotope equilibration is attained and the measured isotopes are free of spectral overlaps [192,193]. Standards do not need to be matrix-matched. Quadrupole-based ICP-MS instruments can typically provide isotope ratio precision of 0.1% to 0.5%. Much better isotope ratio precision can be obtained by using simultaneous MS detection, such as a multicollector-based instrument or perhaps time-of-flight MS. In comparison to thermal ionization mass spectrometry, ICP-MS provides much higher sample throughput and simpler, faster sample preparation. 

Propagation of errors using isotope dilution ICP-MS has been considered to determine how to optimize the measurements [201]. Comparison of analysis results from external calibration versus isotope dilution can be used to assess the quality of external calibration results and the effectiveness of internal standards with external calibration [202,203]. Because isotope ratio precision depends on the total ion count rate, the use of high-efficiency sample introduction to generate larger signals can improve isotope ratio precision and, therefore, analysis precision [204]. 

ICP-MS with a hexapole collision cell is advantageous for isotope ratio measurements of difficult to analyze elements." Due to possible interferences with Ar+, ArO+ and dimer argon molecular ions Ar2 " the measurements of the isotope ratios °Ca/ "Ca, Fe/ Fe and Se/ Se by ICP-MS are extremely difficult or completely impossible. Whereas for these isotope ratios precisions between 0.2 and 0.3 % were observed using ICP-MS with hexapole collision cell, a precision of 0.07% was measured for a lOp-gF uranium isotope reference solution 1) in the... 

Collaborative efforts with experts in nutrition and bioavailability have been carried out at NBS and have resulted in the use of TIMS for the measurement of stable Isotope tracers in adults and newborns(, J 0). Typical data for these efforts are summarized in Tables II and III for Ca and Zn isotopes For each of these elements optimum Isotope ratio precisions of about 0 1% (R S D )were achieved which permitted the resolution of extended term kinetic effects that would be impossible to achieve with less accurate and precise mass spectrometry. 

For the determination of isotope ratios, the precision of TOF-ICP-MS has been studied in a preliminary comparison with other mass spectrometer systems [521]. Typical isotope ratio precisions of 0.05% were obtained, thus overtaking sector field mass spectrometry with sequential detection, for which values of 0.1-0.3% for 63Cu/65Cu in Antarctic snow samples have been reported [522], Similar results were obtained by Becker et al. [523] for Mg and Ca in biological samples (0.4-0.5%). In principle, the features of TOF-ICP-MS may be superior to those of sequential sector field or quadmpole mass spectrometry, however, true parallel detection of the signals as is possible with multicollector systems may be the defini-... 

Finally, the improved isotope ratio precision that can be obtained with MC-ICPMS opens the doors for the isotopic analysis of new elements (e.g., Cu) that might offer further fingerprinting potential [115]. An increase in the number of LA-ICPMS applications that deal with isotopic analysis of archaeological samples might also be predicted. 

It should be taken into account, however, that the isotope ratio precision is typically worse than that with pneumatic nebulization as a means of sample introduction. Recently, Aramendia et al. provided a description of strategies to improve the isotope ratio precision attainable with the combination of LA and single-collector ICP-MS [62]. As the use of LA also affects the degree of mass discrimination, matrix-matched standards are required for adequate mass bias correction [63],... 

This is the ultimate precision attainable and the isotope ratio precision observed in practice should be compared with that predicted by Poisson counting statistics to evaluate whether or not further progress can be made. 

Isotope Ratio Precision with Single-Collector ICP-MS... 

With a standard quadrupole-based ICP-MS instrument, the optimum isotope ratio precision attainable is 0.1% RSD. This is the within-run or internal precision, expressed as the RSD (%) estimated on the basis of N (typically N = 10) replicate measurements. Some authors rather express the internal precision as the relative standard error, which corresponds to RSD (%)/a/N- As a result, care has to be taken when comparing isotope ratio precisions reported in the literature. 

Figure 2.23 ° Ag/ Ag isotope ratio precision (RSD for 10 replicate measurements) as a function of the acquisition time per nuclide (a) predicted on the basis of Poisson counting statistics (b) observed experimentally. Reproduced with permission of the Royal Society of...

Collisional damping in a collision/reaction cell provides a significant improvement in the isotope ratio precision [98, 99]. This effect is created by pressurizing the cell with a nonreactive collision gas, typically Ne. As a result, ions extracted from the ICP at slightly different moments in time are admixed in the cell, thereby damping the short-term variations in the ion beam to some extent. The effect of the use of Ne as a non-reactive collision gas on the isotope ratio precision observed in practice is illustrated in Figure 2.24. 

By using collisional damping, the isotope ratio precision obtained can be improved to values of approximately 0.05% RSD (under optimum conditions). This gain in isotope precision, however, comes at the cost of a more pronounced instrumental mass discrimination [100-103], caused by preferential collisional losses of the lighter nuclide (Figure 2.25). 

Figure 2.24 Internal isotope ratio precision (RSD) for ° Ag/ ° Ag as obtained using a quadrupole-based instrument equipped with a collision/reaction cell. Filled squares, vented cell open squares, with Ne as an inert collision gas, introduced into the cell at a flow rate of 2 ml min The dotted line represents the precision as predicted by Poisson counting statistics. Reproduced with permission of the Royal Society of Chemistry from [99].

With a sector field ICP-MS instrument operated at low mass resolution, a slightly better isotope ratio precision (better than 0.05% RSD) is obtained than with a quadrupole-based instrument [104, 105], at least when pure electrical scanning (E-scanning) is opted for (the magnetic field strength is not adapted during the measurement). This is a consequence of the flat-topped spectral peak shape observed at low mass resolution with sector field instrumentation, a result... 

Table 2.3 Isotope ratio precision attainable with commercially available ICP-MS instrumentation under optimum conditions.

Type of mstrumentation Isotope ratio precision [%) (RSDforn = 10)... 

One manufacturer of sector field ICP-MS instrumentation has recently introduced the use of a slit that allows combination of a higher mass resolution (R sr 2000) with fiat-topped peaks, thus maintaining the isotope ratio precision as typical for low mass resolution [15]. 

Willie, S., Mester, Z., and Sturgeon, R.E. (2005) Isotope ratio precision with transient sample introduction using ICP orthogonal acceleration time-of-flight mass spectrometry. J. Anal. At. Spectrom., 20, 1358-1364. 

---