Internal isotope ratio precision

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].

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]. 

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. 

For precise measurement of isotopic composition by mass spectrometry, it is also common to use either a natural, known isotopic ratio to correct for instrumental mass fractionation (e g., internal normalization) or to add a tracer for this purpose. For example for natural uranium samples, one can use the natural U/ U of 137.88 to correct for fractionation. Alternatively, one can use an added double spike of ratio -unity... 

Accuracy for all thorium measurements by TIMS is limited by the absence of an appropriate normalization isotope ratio for internal correction of instrumental mass fractionation. However, external mass fractionation correction factors may be obtained via analysis of suitable thorium standards, such as the UC-Santa Cruz and IRMM standards (Raptis et al. 1998) for °Th/ Th, and these corrections are usually small but significant (< few %o/amu). For very high precision analysis, the inability to perform an internal mass fractionation correction is probably the major limitation of all of the methods for thorium isotope analysis discussed above. For this reason, MC-ICPMS techniques where various methods for external mass fractionation correction are available, provide improved accuracy and precision for Th isotope determinations (Luo et al. 1997 Pietruszka et al. 2002). 

Flegal and Stukas [406] described the special sampling and processing techniques necessary for the prevention of lead contamination of seawater samples, prior to stable lead isotopic ratio measurements by thermal ionisation mass spectrometry. Techniques are also required to compensate for the absence of an internal standard and the presence of refractory organic compounds. The precision of the analyses is 0.1 -0.4% and a detection limit of 0.02 ng/kg allows the tracing of lead inputs and biogeochemical cycles. 

The accuracy with which absolute isotope abundances can be measured is substantially poorer than the precision with which relative differences in isotope abundances between two samples can be determined. Nevertheless, the determination of absolute isotope ratios is very important, because these numbers form the basis for the calculation of the relative differences, the 5-values. Table 1.6 sununarizes absolute isotope ratios of primary standards used by the international stable isotope community. 

Isotope Ratios and Internal Standardization The ability to extract all ions from an incoming ion beam simultaneously allows the TOF-MS to provide better precision than sequentially scanned ICP-MS instruments. Provided that the dominant source of noise is multiplicative in nature, all elements and isotopes should experience the same perturbations. Therefore, ratioing techniques such as isotopic dilution should allow compensation for drift and source noise, and isotope-ratio measurements should improve in precision. 

Figure 6, Projection onto two dimensions of lead isotope ratios for the objects listed in Table II. The ratios chosen are different than those shown in Figures 4 and 5, and the error bars, which represent all the points, are for the relative internal precision at our points, the proper error measure for judging clustering within our own data set. The clear clustering of the Benin + group is visible, as well as the separation from the Igloo-Ukwu and Ife samples.

Fig. 16. Matrix effect on Li isotope ratios measured by MC-ICP-MS. Pure solution contains 100 ppb Li and 0 ppb Na. With increasing addition of Na, Li/ i decreases. Note also that the internal error of the analysis (2a) increases with increasing Na. For accurate and precise measurement of Li isotope ratios, Li must be properly separated from Na. This can be achieved by cation exchange chromatography (e.g. James Palmer 2000).

The stable carbon isotope ratios of dissolved inorganic carbon (DIC) and benthic foraminiferal calcite generally are determined with isotope ratio gas mass spectrometers calibrated via NBS 19 international standard to the VPDB (Vienna Pee Dee Belemnite) scale. All values are given in 8-notation versus VPDB with an overall precision of measurements including sample preparation usually better than +0.06 and +0.1%o for calcite and DIC carbon isotopes, respectively. Except one single-specimen based dataset (Hill et al. 2004), all stable isotope data from papers referred to in this overview are from species-specific multi-specimens analyses. The number of specimens used for a single analysis depended on size and weight of species but usually varied between 2 and 25. 

Isotope ratios of inorganic trace metals which are the topic of this chapter are not modified when incorporated into the sediment (note that some minor isotope fractionation might occur on incorporation into the sediment, but usually such shifts are either smaller than analytical precision or they are removed by the internal correction procedures of the techniques used). The variation in isotope ratios only varies... 

Pu5/92138 issued by AERE, Harwell (Hamilton et al. 1989). The isotope fractionation at each step is corrected by reference to the certified isotope ratios. The idea was further developed by Dubois et al. into a dynamic multidetection measurement mode (Dubois et al. 1989), which practically eliminates mass fractionation effects and possible mismatches of cup efficiencies with a 2-isotope internal standard and MIC/TIMS. According to the results presented, precisions and accuracies of 0.01 % are achievable with this procedure. An ultimate refinement has been introduced by performing total evaporation measurements with peak tailing correction in dynamic multicollection mode, using a MIC/TIMS with magnetic sector equipped with a dispersion quadrupole (Goldberg et al. 2002 Richter and Goldberg 2003). 

Truong QS, Keeffe R, Ellacott T, Desson K, Herber N (2001) In IAEA symposium on international safeguards, IAEA-SM-367/8/08/P Tuttas D, Schwieters JB, Quaas N, Bouman C (1998) Improvements in TIMS high precision isotope ratio measurements for small sample sizes. Application note 30136, Thermo Eisher Scientific, Bremen Tuttas D, Schwieters JB, Bouman C, Deerberg M (2005) New compact discrete dynode multipliers integrated into the thermo scientific TRITON variable multicollector array. Application note 30192, Thermo Fisher Scientific, Bremen United Nations Security Council (1991) Text of UNSC Resolution 687. Available at http //www.fris.org/ news/un/iraq/sres/sres0687.htm... 

Because three of the four isotopes of the Pb isotope system are radiogenic, there is no stable reference isotopic ratio. Thus, there is no stable isotope pair for internal checking of the instrumental bias, which results in a precision of about one order of magnitude less than that obtained for other isotope systems used in geochemistry (Sr, Nd, etc.). As a consequence, it is very difficult to refer to absolute reference values even for a standard material after careful double-spike (DS) TIMS measurements. 

Preliminary measurements were made with a quadrupole-based system and reported (Gunther-Leopold et al. 2003), but after a multicollector ICPMS was installed in the hot laboratory, more precise isotope ratio measurements were made. The results were adjusted by use of internal correction (known isotope ratio of the same element) or by a bracketing procedure with certified reference materials (external correction). Additional measurements were made with a specially designed laser-ablation system that was coupled to the ICPMS instrument. The nuclear fuel burn-up assessment was based on measurement of the ratio between Nd isotope and the four main fissionable nuclides ( U, Pu, and " Pu) for UOj or MOX fuel. " Nd is formed... 

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