Isotope ratio measurements precision limits

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. 

Both Merritt and Hayes [639,640] and Merritt et al. [641] have investigated the statistical limits to attainable precision for GC-C-IRMS techniques. For carbon isotope ratio measurements with precision not limited by counting... 

A specific variant of El MS is isotope ratio (IR) MS [46]. It is based on electron impact ionization with maximized ionization probability. IR MS is limited to the analysis of gases of high volatility and low reactivity such as CO2, N2 or SO2. The analytes of interest thus have to be transformed into one of these gases before introduction into the IR MS. Information on the position of C labelings in the analyte can be only obtained, if all carbons are isolated position specific and subsequently combusted. In this context Corso and Brenna [47] showed position specific analysis by IR MS for methylpalmitate through pyrolytic fragmentation. IR MS exhibits an extremely high precision of 0.00001 % for the isotope ratio measurement and is optimal to quantify low label enrichments [48]. This is especially important for in vivo studies with ani-... 

The best precision is obtained for isotope ratios near unity (unless the element to be determined is near the detection limit, when the ratio of spike isotope to natural isotope should be between 3 and 10) so that noise contributes only to the uncertainty of natural isotope measurement. Errors also become large when the isotope ratio in the spiked sample approaches the ratio of the isotopes in the spike (overspiking), or the ratio of the isotopes in the sample (underspiking), the two situations being illustrated in Fig. 5.11. The accuracy and precision of the isotope dilution analysis ultimately depend on the accuracy and precision of the isotope ratio measurement, so all the precautions that apply to isotope ratio analysis also apply in this case. 

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. 

The figures of merit of quadrupole-based ICP-MS, such as the precision of isotope ratio measurements and the detection limits, can be improved significantly, especially for elements which are difficult to determine due to the appearance of isobaric interferences (e.g., by the trace, ultratrace and/or isotope ratio measurements of Ca, Fe, S, As, I or Se).16-22 The occurrence of interference problem can be minimized by the insertion of a collision/reaction cell in ICP-MS as the result of defined collision induced reactions using selected collision/reaction gases or gas mixtures (such as H2, He, NH3, 02, CH4 and others). For each analytical problem, which is different, e.g., for U or... 

Precise and accurate isotope analyses by mass spectrometry have attained growing importance in the last few years due to instrumental improvements with respect to sensitivity, detection limits, precision and accuracy.1 As mentioned before, because the isotope abundances of several elements are not constant and vary as a result of nuclear, biological, chemical, geochemical and physical processes, isotope ratio measurements are required for different research and application fields. Isotope ratio measurements are therefore necessary for elements with two or more isotopes for inves-... 

Limits for Precision and Accuracy of Isotope Ratio Measurements and How to Solve the Problems... 

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