Variations in GC–MS Response Between Analytes and Deuterated Analogs

Abstract

Isotopic analogs are commonly used as appropriate internal standards. It was observed that analytes usually have higher mass responses than their equimolar deuterated analogs leading to quantification discrepancy. The standard addition method was adopted on dimethyl azelate (DMA) and d6-dimethyl azelate (d6-DMA) to investigate possible reasons for this behavior. Cross contribution of mass responses, intermolecular deuterium–hydrogen exchange during chromatographic separation, and deviation in mass ionization response of C–H against C–D bonds were studied. GC–MS analysis revealed that neither cross contribution of ions nor H2/H exchange was responsible for the difference in responses between DMA and d6-DMA. On the other hand, a study of carbon nucleus relaxation conducted by 13C-NMR showed that relaxation rate of carbonyl carbon in d6-DMA is faster than DMA (9 and 3 s−1, respectively). This indicates rapid interaction between spin of deuterium nucleus with spin of unpaired electrons in the structure. Accordingly, DMA has more electrons in triplet/singlet state (i.e., unpaired electrons) in each time moment promoting propagation of radicalization reactions inside the electron impact chamber. In conclusion, this should increase the response of total ion current (TIC) and generate overestimated results for equimolar ratios of analytes such as dimethyl azelate, dimethyl adipate and dimethyl phthalate against their deuterated counterparts.