Precision isotope ratio measurements

Figure 15 Isotope-ratio measurement precision as a function of the number of accumulated counts. Each measured value (circles) represents the relative standard deviation of 10 repetitions. The dotted line represents the precision predicted by counting statistics for a given number of accumulated counts. Because the measured values match the predicted values closely, it is confidently expected that longer integration times will yield even better precision (<0.01 %). (From Ref. 47.)...

From a series of isotope ratio measurements, the precision of measurement can be assessed statistically, as shown here. Precision reveals the reproducibility of the measurement method, but it does not provide information on the accuracy of the measurement (see also Figures 48.8 and 48.9). 

Accurate, precise isotope ratio measurements are important in a wide variety of applications, including dating, examination of environmental samples, and studies on drug metabolism. The degree of accuracy and precision required necessitates the use of special isotope mass spectrometers, which mostly use thermal ionization or inductively coupled plasma ionization, often together with multiple ion collectors. 

Accurate, precise isotope ratio measurements are used in a variety of applications including dating of artifacts or rocks, studies on drug metabolism, and investigations of environmental issues. Special mass spectrometers are needed for such accuracy and precision. 

Becker JS, Dietze H-J (2000) Precise and accurate isotope ratio measurements by ICP-MS. Fresenius J Anal Chem 368 23-30... 

Cochran JK, Masque P (2003) Short-lived U/Th-series radionuclides in the ocean tracers for scavenging rates, export fluxes and particle dynamics. Rev Mineral Geochem 52 461-492 Cohen AS, O Nions RK (1991) Precise determination of femtogram quantities of radium by thermal ionization mass spectrometry. Anal Chem 63 2705-2708 Cohen AS, Belshaw NS, O Nions RK (1992) High precision uranium, thorium, and radium isotope ratio measurements by high dynamic range thermal ionization mass spectrometry. Inti J Mass Spectrom Ion Processes 116 71-81... 

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. 

TI is a very precise and accurate method in stable isotope ratio measurements and quantification of inorganic elements, for example, by isotope dilution mass spectrometry [8]. Because TI is a continuous ion source, it could be coupled to any analyzer that is suitable for such sources. However, because the strength of TI lies in the quantitative precision and accuracy, sector analyzers are preferred to ensure maximum quality. 

J. S. Becker and H. J. Dietze. Precise and Accurate Isotope Ratio Measurements by ICP-MS. Fresenius J. Anal. Chem., 368(2000) 23-30. 

Walder, A.J. and Furuta, N. (1993). High-precision lead isotope ratio measurements by inductively coupled plasma multiple collector mass spectrometry. Analytical Sciences 9 675-680. 

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... 

Rigorous correction for instrumental mass bias is required if the precision of an isotope ratio measurement needs to be greater than l%o per mass unit. This concept is well illustrated by the definitive Ca isotope work of Russell et al. (1978), which used a double-spike approach. Prior to the Ca isotope investigation of Russell et al. (1978), natural mass-dependent Ca... 

Hirata T, Hayano Y, Ohno T (2003) Improvements in precision of isotopic ratio measurements using laser ablation-multiple collector-ICP-mass spectrometry reduction of changes in measured isotopic ratios. J Anal At Spectrom 18 1283-1288... 

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-... 

A major attraction is the ability to perform isotope ratio measurements, e.g. in many geological applications to determine the age of rocks, and isotope dilution analysis. The latter in particular is gaining-popularity as a highly accurate, precise and hence traceable, method of analysis, so it is worthwhile describing these techniques in more detail. 

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. 

Q. What are the factors which most affect the accuracy and precision of isotope ratio measurements ... 

The precision of any isotope ratio measurement depends to a great extent on the mass spectrometer operating conditions. Because a quadrupole mass spectrometer is a rapid sequential analyser, the frequency at which it switches between the masses to be ratioed and the amount of time spent collecting data at each mass must be optimized. 

The number of points, or channels, which are chosen for each isotopic peak will also affect the frequency of data acquisition, and hence the final precision of the isotope ratio measurement. Three points per peak is a typical value, but this too could be optimized. 

In addition, one of the main features of mass spectrometry is, and this is the major advantage in comparison to other atomic and molecular non-mass spectrometric techniques, that it offers the possibility of determining isotope ratios and abundances of isotopes with high precision and accuracy in all types of samples (in solid, liquid and gaseous materials as well). Isotope ratio measurements have applied increasingly for stable isotopes in nature, especially for investigating... 

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