A critical review of on-line mass spectrometry is presented, illustrating instrumentation, data
treatment, and applications drawn from both control and research environments. Both direct
and membrane inlets are included. The discussion of instrumentation focuses on commercially
available options, although some research options that offer promise for future process
applications are also mentioned briefly. Special considerations attendant upon the high-precision
measurements needed for simultaneous analysis and continuous process control are emphasized.
Applications illustrate a range of problems benefitting from rapid, simultaneous analysis of
volatiles.
The feasibility of simultaneous analysis of mixtures containing two to four butene isomers and up to six total components using process mass spectrometry is assessed. As for typical (nonisomeric) applications of process mass spectrometry, simultaneous analysis is based on the assumption that the electron ionization mass spectra of mixtures are linear combinations of the spectra of the individual constituents. Limits of detection for binary isomer mixtures are on the order of 0.1% to 10%, limited by the ability to distinguish small differences between similar spectra. As spectral and mixture complexity increase, both accuracy and precision decrease. Not surprisingly the similarity of the spectra of stereoisomers cis- and trans-2-butene is greater than that of the other (nonstereoisomeric) isomer pairs, and mixtures containing both cis- and trans-2-butene are the most difficult to quantitate. However, even for mixtures of all four butenes, accuracy (root-mean-square error = 2.43%), precision (average coefficient of variation = 6.72%), and linearity (correlation coefficient of a plot of measured versus actual concentration r2 = 0.985 +/- 0.002) are reasonably good.
The time-dependent permeation behavior of binary gas mixtures through a ZSM-5 zeolite membrane was studied. Although steady-state permeation rates were indistinguishable for CO(2) and N(2) or for cis- and trans-2-butene in binary mixtures, differences in the rate of approach to steady state allowed component distinction. In "normal" systems, one component is initially enriched in the permeate following application of a pulse of analyte gas to the membrane, and then disappears more quickly upon termination of the pulse. Mixtures of cis- and trans-2-butene exhibit qualitatively different behavior; the permeate is enriched in cis-2-butene during both the leading and trailing edges of a sample pulse (though not at steady state). These differences in permeation behavior reflect different balances among multiple transport mechanisms through the zeolite membrane, thought to reflect a combination of selective component sorption and intracrystalline diffusion; in the case of cis- and trans-2-butene, these two factors oppose one another. It is known that this mechanistic complexity can engender synergistic effects, wherein the presence of one component can affect the permeation of another. These may limit applicability to true "unknowns", but resulting complications should be less problematic in well-defined process applications.
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