In addition to the ability of a capillary gas chromatographic-chemical reaction interface/mass spectrometric technique (CRIMS) to detect the presence of 13C, 15N and 2H (D) it can also quantify the level of the enriched substance. The microwave-powered chemical reaction interface converts materials from their original forms into small molecules whose mass spectra serve to identify and quantify the nuclides which make up each analyte. The presence of enrichment of each element is followed by monitoring the isotopic variants of CO2, NO or H2 which are produced in the chemical reaction interface. Chromatograms showing only enriched 13C and 15N are produced by subtracting the abundance of naturally occurring isotopes from the observed M + 1 signal. A selective chromatogram of 2H (D) is obtained by measuring HD at m/z 3.0219 with a resolution of 2000. Enrichment of 13C and 15N is quantified by measuring the ratio of excess 13CO2 to total 12CO2 or excess 15NO to total 14NO. To evaluate linearity and detection limits, we have used phenytoin as an example of an unlabeled substance and added various labeled phenytoin analogs. Atom enrichments of 0.3% were detectable for (2,4,5−13C3) phenytoin and 0.06% for (1,3−15N2)labeled phenytoin, each in the presence of 500 ng of unlabeled phenytoin, respectively. For deuterium, enrichment could not be directly determined. However, 1 ng of (ring D10) phenytoin was determined in the presence of 500 ng of unlabeled diethylated phenytoin. We have found CRIMS capable of quantifying 13C-, 15N- and D-enriched substances.