Inorganic IDMS

The Approximate signal matching method for Inorganic IDMS 19... 

Additional Organic IDMS Publications Additional Inorganic IDMS Publications Glossary of Terms and Abbreviations Standard IDMS Equations... 

This guide aims to provide a structured approach to the use of IDMS to achieve high accuracy analytical measurements in both organic and inorganic IDMS. The approach outlined in Sections 4 and 5 has the advantage over normal IDMS of ... 

This approach may be inappropriate for inorganic IDMS, where other situations may apply. In the case of silver, its two natural isotopes actually exist in proportions close to 1 1 (51.8% and 48.2%). Thus, the optimum spiking regime requires a final concentration ratio of 1 4 to minimise the propagation of errors. A fuller explanation of this particular issue is given in Section 5. 

This method of calibration has the disadvantage that drift in instrumental response may cause significant errors because the calibration and sample measurements are made some time apart. It has been widely used for organic IDMS but is less attractive for inorganic IDMS where several isotopes may be present at significant levels in both the sample and the spike material. In such instances the calculation of calibration data will be more difficult. This is because both the natural and the spike materials will often contain both isotopes of interest for the IDMS measurement. This in turn leads to a non-linear relationship between the signals observed and the amounts used to create a blend of the natural and spike materials. 

This procedure uses only one calibration standard. To achieve good accuracy the ion abundances of the standard should be as close as possible to those of the sample. This method has been used for both organic and inorganic IDMS, especially using thermal ionisation mass spectrometry (TIMS). 

The analyte isotope in organic IDMS is usually C, H or N. Any of the inorganic isotopes can be used as the analyte isotope in inorganic IDMS. The choice will depend on considerations such as which isotope is available as the enriched isotopic analogue (this is used as the spike isotope) and the detection limit required (which limits the use of lower abundance isotopes). 

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. 

This section includes worked examples of the recommended calibration procedure as applied to organic and inorganic IDMS, together with a graphical overview of the procedure for inorganic IDMS. The equations used for each measurement step are given in a simplified form. A more extensive derivation and explanation of the equations used in IDMS is given in Annex 3. 

The Approximate signal matching Method for Inorganic IDMS... 

Figure 3 Schematic detailing the steps in the definitive inorganic IDMS determination...

In inorganic IDMS, the isotopically enriched analog is not an array of individual labeled molecules, but rather a mixture of individual isotopes that contribute directly to the enrichment of the array, but in different proportion to the sample (Table 3 and Figure 1). Ideally, the sample isotope should be of low abundance in the spike, and the spike isotope should be of low abundance in the sample, but this is very often not the case (Table 3). The isotope ratio is measured by ratioing the signal strengths of the spike and sample isotopes at their corresponding masses. 

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