Measurement isotope amount ratio determination

Using appropriate ions of the natural analyte and the spike, the isotope amount ratios for the spiked sample and the spiked calibration standard are determined. It is suggested that alternating measurements of the isotope amount ratio are made on these two solutions (repeated measurement of the calibration blend allows mass bias correction to be performed for inorganic IDMS (see Section 3 10), repeating each five times. The mean value of the five measurements will minimise the effects of any instrument drift. An improved estimate of the natural analyte concentration in the sample can then be calculated from the data. 

The procedure described in Section 3.8.4 is illustrated using the determination of p,p -DDE in 2,2,4-trimethylpentane. This example shows how the sample and calibration solutions may be prepared so that the natural and isotopically labelled analogue concentrations and their isotope amount ratios are as close to being identical as possible. Additionally, to obtain high accuracy the measured isotopic ion abundance ratios should be as close to unity as possible. For the highest accuracy to be achieved, all solutions should be prepared gravimetrically except where identified below. Conventional volumetric techniques will limit the accuracy attainable by this IDMS method. The symbols used in this example should be read in conjunction with Equation 11 (Annex 3) which was used for the calculation of results. 

Single-collector instruments also prove very usefid for mass content determinations via isotope dilution, as carefiil estimation of all quantities that influence the uncertainty budget demonstrates that the precision on the isotope amount ratio is typically not the dominant factor for high-precision measurements. Often, the accuracy of a mass content measurement will hardly improve through the use of MC-ICP-MS instruments as other influence quantities, such as uncontrolled spectral interferences and sample inhomogeneities, typically deserve more attention. 

Other isotopes can be used to determine the age of samples. The age of rocks, for example, has been determined from the ratio of the number of radioactive atoms to the number of stable gfPb atoms produced by radioactive decay. For rocks that do not contain uranium, dating is accomplished by comparing the ratio of radioactive fgK to the stable fgAr. Another example is the dating of sediments collected from lakes by measuring the amount of g Pb present. 

Isotopic dilution analysis is widely used to determine the amounts of trace elements in a wide range of samples. The technique involves the addition to any sample of a known quantity (a spike) of an isotope of the element to be analyzed. By measuring isotope ratios in the sample before and after addition of the spike, the amount of the trace element can be determined with high accuracy. The method is described more fully in Figure 48.13. 

It is seen that also in the general polyisotopic case the ratio of sample and spike amounts can be determined solely from isotope ratio measurements. Isotope dilution mass spectrometry measures the change in the ratio of two isotopes of the element of interest, induced by the addition of a known amount of the same eiement with an artificially altered isotope ratio of the same isotopes to a weighed aliquot of a sample. Since only isotope ratios in the different materials are measured, it follows that sample treatment or recoveries for chemical separations need not be quantitative once isotopic equilibration after spiking has been achieved. Also variability of chemical separation recovery is generally not important. 

If the isotope abundance (isotope amount fraction) of the sample is not known this must be determined experimentally via an isotopic abundance run. Otherwise, the only information required is the weight of the enriched isotopic analogue added (and hence the elemental concentration of the spike solution), the weight of the sample and the measured ratios. Modem analytical balances are capable of routinely weighing to 1 part in 50 000, so that errors arising from the weights of the sample and spike should be minimal unless very small aliquots are taken. 

The isotopic abundances of the tracee or the tracer can be obtained from measured isotope ratios using Eqs. (16.2) and (16.3), while atomic masses can be taken from the literature (10]. Based on IDMS principles, measured isotope ratios can be converted into tracer to tracee ratios in the sample analyzed. The amount of tracee can then be calculated if the amount of tracer in the sample is known. The latter can be determined either by IDMS using an additional tracer or by non-IDMS techniques for quantitative analysis employing external calibration or standard addition techniques. 

Dombovari, )., Becker, J.S., and Dielze, H.J. (2000) Isotope ratio measurements of magnesiiun and determination of magnesium concentration by reverse isotope dilution technique on small amounts of Mg-spiked nutrient solutions with inductively coupled plasma mass spectrometry. Int. J. Mass Spectrom., 202, 231-240. 

Isotope dilution mass spectrometry (IDMS) can be applied with most of the ionisation methods used in mass spectrometry to determine isotope ratios with greater or lesser accuracy. For calibration by means of isotope dilution, an exactly known amount of a spike solution, enriched in an isotope of the element(s) to be determined, is added to an exactly known amount of sample. After isotopic equilibration, the isotope ratio for the mixture is determined mass spectrometrically. The attraction of IDMS is its potential simplicity it relies only on the measurement of ratios. The... 

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