Application Note #2

Transfer-Line GC-MS for Process Analysis

Description of Technique:

Transfer-line GC-MS (TLGCMS) uses a short, narrow-bore capillary GC column as the sampling interface between a mass spectrometer detector system and a process that is to be analyzed on-line. The GC column acts both to reduce the sample pressure for introduction into the mass spectrometer, and to provide a high-speed GC separation of the sample mixture. A block diagram of a TLGCMS system is shown in Figure 1. Combining a high-speed GC separation with a mass spectrometry-based process analysis system increases the potential applicability of the technique since more complex and challenging processes can be analyzed. In addition, the valved-injection sampling system improves quantitative analyses by controlling the sampling loop volume and pressure. The data system is also able to integrate a peak rather than monitoring an absolute signal level and consequently, background signals are more easily eliminated. The valved injection system also improves analyzer reliability in corrosive processes applications since the MS and much of the sampling system is not continuously exposed to the sample.

Figure 1: Sampling and Instrumental Block Diagram:

Example Applications

Characterization of corrosive, high-temperature and high-pressure chemical processes on-line and in real time
Real-time monitoring of compressed gas distribution systems for cross contamination and leaks.
Support process startups with sensitive, component-specific, real-time analytical data
Support environmental compliance through characterization, optimization, and qualification of process vent streams

Description of Instrumentation and Sampling System:

The process is sampled by taking a sidestream off the main process stream, passing through the sampling loop of an automated injection valve. The sidestream either returns to a lower pressure point in the process, or passes to an appropriate scrubber system. Upon receiving a signal from the control computer, the injection valve switches to the inject position and a plug of the process sample is injected onto the head of the GC column/ transfer line. Components present in the process sample are separated on the GC column and are introduced into the quadrupole mass spectrometer.

The data acquisition and control system collects mass spectra in either continuous-scan or selected-ion modes during elution of the sample components. At the completion of the fast separation the data system generates extracted ion chromatograms (EIC) for each of the masses of interest. Each EIC is integrated, and the peak area for each of the target masses is passed to the post run data analysis and trend-plotting subsystem. The system is designed to run continuously, re-injecting and analyzing the process at intervals that are determined by the fast GC separation: Typically, less than 1 minute. Data can be automatically archived, control-charted, and configured to generate high/low alarms. A variety of sampling systems can be designed to meet the needs of each application. In addition, the transfer-line GC separation can be controlled by proper selection of the column stationary phase, adjusting the isothermal transfer-line temperature, or through pressure/flow programming of the carrier gas.

References

"Pressure Programming for Transfer-Line GC/MS", S.J. Doherty, W.L. Winniford, and S.J. Hein, 42nd ASMS Conference on Mass Spectrometry, 1994.
"Transfer-Line GC/MS for Process Monitoring", S.J. Doherty, 42nd ASMS Conference on Mass Spectrometry, 1994.
"Vapor Sampling Device for Direct Short Column Gas Chromatography/Mass Spectrometry Analyses of Atmospheric Vapors", N.S. Arnold, W.H. McClennen, and H.L.C. Meuzelaar, Anal. Chem., 1991, 63, 299-304
"Compact Gas Chromatograph Probe for Gas Chromatography/Mass Spectrometry Utilizing Resistively Heated Aluminum-Clad Capillary Columns", M.E. Hail and R.A. Yost, Anal. Chem., 1989, 61, 2410-2416.

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