Investigation of gas phase absorption spectral similarity for stable-isotopically labeled compounds in the 125-240nm wavelength range.

Stable-isotopically-labeled internal standards are commonly incorporated in methods for trace analysis that utilize mass spectrometric detection. They closely mimic the physicochemical properties of the analyte, but their signal is easily differentiable based on a change in molecular mass. To investigate the potential to transfer methods incorporating such internal standards for analysis by vacuum ultraviolet detection, a study was conducted to compare the spectral shape (from 125 to 240nm) of stable-isotopically-labeled compounds with their non-labeled counterparts. Gas chromatography - vacuum ultraviolet spectroscopic analysis was performed for a series of benzene isotopologues, as well as for clinically- and environmentally-relevant standard compounds and their deuterated counterparts. The absorption characteristics of the benzene isotopologues were evaluated based on similarity (normalized spectra), as well as on their magnitude of absorption. In general, very minor differences in absorption spectra were observed; however, increasing degree of deuteration did generally increase the spectral difference between labeled and non-labeled analytes. Sum of squared residuals were used as quantitative measures to assess spectral similarity (and dissimilarity). Theoretical computation of absorption spectra for benzene using time-dependent density functional theory was also examined; though, further work is needed in this area to extend the analysis to isotopologue analysis.