Time- and concentration-dependent relative peak intensities observed in electron impact membrane inlet mass spectra

This paper demonstrates that membrane inlet mass spectrometry (MIMS) experiments in some cases can result in electron impact mass spectra where the relative peak intensities change both with time and concentration of the sample. The phenomenon was observed both for phenoxyacetic acid (POAA), a highly polar compound with low volatility, and for the highly volatile chloroethylenes vinyl chloride (VC), 1,1- and 1,2-dichloroethylene (DCE) and trichlororethylene (TCE), exemplified here by VC. Following step changes in concentration some of the fragments were observed to have transient patterns deviating from the molecular ion and from most of the fragment ions. Moreover, the relative peak intensities of these fragment ions at steady state were observed to depend on sample concentration. The unusual behaviour of the ions could be explained by a model for the MIMS system, which takes into account the possibility of a surface-catalysed decomposition of the primary molecules (POAA and VC) inside the vacuum system. In the POAA experiments the response patterns for m/z 94 deviated from the other major ions and it was assumed that POAA dissociated into phenol by interaction with the surfaces. In the VC experiments the masses and the isotope patterns for the deviating ions indicate that HCl and Cl-2 were produced. On the basis of the model of molecule/surface interactions in vacuum systems, we predict that surface-catalysed dissociation of labile molecules can distort both MIMS transients and spectra, whenever the analyte flow into the vacuum system is comparable to the rate of dissociation of analytes. This condition will typically be fulfilled with signals close to the detection limit.