Enhancement of process mass spectrometric analysis of mixtures utilizing parameterization and a zeolite membrane inlet

Process mass spectrometry was used for simultaneous quantitation of mixtures of hydrocarbons that included up to four butene isomers and a total ofup to six components. Remarkably, even for mixtures contaimng multiple isomeric compounds, differences in relative intensities ofelectron ionization mass spectra provided a basis for quantitation. Quantitation accuracy and precision were found to decrease as the number of components in a mixture and the spectral similarity of those components increased. In general, when cis-2-butene and trans-2-butene were both present in mixtures, quantitation performance was compromised. Selection of which ions to monitor (parameterization) was critical to optimum analysis accuracy, precision, and speed for all mixtures tested. An empirical parameterization algorithm based on comparison of reference spectra of mixture components was developed for ion selection. Empirical algorithm parameterizations gave analysis accuracy and precision values statistically equal to, or better than all-mass parameterizations (using all masses above 1% relative intensity in the spectra of the mixture components). In addition, empirical algorithm parameterizations had better accuracy and precision than square matrix parameterizations (one massfor each componentin the mixture). A zeolite membrane inlet to the mass spectrometer (MIMS) was tested for its ability to differentiate components. Even when there was little or no difference in the steady-state permeation rates of compounds through the membrane, distinctive time dependence of component approaches to steady state provided a basis for differentiation. "Normal" permeation behavior for a binary mixture of gases was characterized by enrichment of one component in the permeate during initiation of a sample pulse and enrichment of the other componentin the trailing edge of a pulse. For example, in binary ethane/propane mixtures ethane was enriched at sample pulse initiation and propane was enriched in the trailing edge of the pulse. In contrast, when a pulse of a mixture of cis- and trans-2-butene was passed by the membrane, the permeate was significantly enriched in cis-2-butene both during pulse initiation mdin the trailing edge ofthe pulse, even though steady state permanences ofthe two gases were equal. This was due to cis-2-butene both diffusing faster and absorbing more strongly than trans-2-butene. Time dependent permeation ennchments were exploited to quantitate binary mixtures of cis-2-butene and 1-butene using dynamic (pulsed sample) MIMS. In this experiment each pure component had a characteristic ion signal phase shift relative to the timing of the pulsing valve, with the more quickly permeating compound (I-butene) having the smaller phase shift. Mixtures of cis-2-butene and 1-butene were found to have ion signal phase shifts between those ofthe two pure components that correlated with relative concentration. Taken together, these studies of zeolite membrane inlet mass spectrometry and simultaneous analysis will expand the applicability of quantitative process mass spectrometry to a wider range ofprocess mixtures.