Organic IDMS

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

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 involves making measurements on each sample between measurements on two calibration standards prepared such that their ion abundances fall just above and below the ion abundances of the sample. Analyte concentration is calculated by linear interpolation between bracketing standards and good precision and accuracy can be achieved using this procedure. This is a specialised version of the graphical method and, again, is mostly used for organic IDMS. 

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). 

The level of the target analyte should be significantly above the limit of quantification to ensure a good signal-to-noise ratio. With ICP-MS measurements the instrumental response should be 10 -10 counts s (for each ion). For organic IDMS the signal-to-noise ratio should be at least 10 1. 

The mass-to-charge ratio of the specific ions monitored should be high, where a choice exists (specifically for organic IDMS). 

Several ions should be monitored where this is possible (specifically organic IDMS). 

The Exact signal matching Method for Organic IDMS... 

Vogl, J. and Heumann, K.G. (1998) Development of an ICP-IDMS method for dissolved organic carbon determinations and its application to chromatographic fractions of heavy metal complexes with humic substances. Anal. Chem., 70, 2038-2043. 

A calibration chain B sample treatment, C associated quantities. The numbers indicate the link between steps. TS transfer standard (masses, wavelength calibration devices, etc), RM laboratory or any non certified reference material, CRM certified RM, PCRM primary CRM e.g, pure substances or matrix CRMs certified with a primary method (balance, IDMS etc). RMs and CRMs can be used at various stages of the measurement process. For chain A PCRMs or CRMs do not exist for all substances in particular for organic and organo-metallic determinations. For step 10, 11 and 12, PCRMs or even CRMs rarely exist. For step 6 laboratories may prepare raw extracts to follow the purification stage. 

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 ... 

With IDMS it is particularly important that full equilibration between the analyte and the isotopic analogue is achieved. This will ensure identical behaviour during the analytical procedure. Sufficient time for equilibration must be allowed but, even so, it is often difficult to ensure that full equilibration has taken place. Particular attention must also be paid, for example, to the chemical forms of the analyte and the isotopic analogue, e.g. oxidation state (inorganic MS), analyte form present (organic MS) etc. 

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). 

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