Combining isotope ratio monitoring with isotope dilution techniques for high accuracy quantification in organic chemical analysis

The combination of gas chromatography, combustion, and isotope ratio monitoring (IRM-GC/MS) is usually used for measuring small variations of the isotope ratios of carbon and nitrogen in organic chemical compounds. This technique is very well developed in terms of both instrumentation and methodology. High precision is, therefore, attained in isotope ratio mesurements. It has been the aim of our investigation to check the applicability of the IRM-GC/MS technique for the quantitative analysis of organic chemical compounds using the isotope dilution technique. The influence of the different steps of the instrumental setup on the uncertainty of the isotope ratio measurement was investigated. It follows from the results that the chromatographic separation in connection with the combustion of the analyte exerts the strongest influence on the uncertainty of the measurement results. The suitability of the method for amount-of-substance measurements has been checked by investigating model mixtures of carboxylic acids. The results exhibit low values of the relative type A standard uncertainty of 2 × 10−3. It is the main feature of the new method that, on the one hand, the conversion of the analyte to carbon dioxide makes it possible to measure the n(13C)/n(12C) isotope ratio with a high precision. On the other hand, the high selectivity of mass spectrometry is lost due to the combustion step. © 1998 John Wiley & Sons, Ltd.