5.2 Sample Introduction from GC and Analyte Ionization
The purpose of coupling GC with MS is to provide confirmatory identification with minimal effort. Prior to the common availability of mass spectrometers, confirmatory identification was possible but required twice the effort. GC analysis alone can provide confirmatory analysis, but it is usually necessary to analyze a sample using two different columns. With capillary systems, it is possible to perform two independent analyses by installing two different capillary columns into one injector system and monitoring each column effluent with a separate detector. If the same retention time and concentration are obtained, the identity of a compound is determined and the results are considered confirmatory.
Capillary column systems are more easily interfaced with a mass spectrometer than packed columns. The high flow rate of packed columns (30 to 60 mL/min) created problems in maintaining the necessary low pressure of a mass spectrometer. On the other hand, capillary columns typically have a flow rate between 1 and 5 mL/min which has a minimal effect on the low pressure MS requirements. The GC and MS are interfaced by inserting the effluent end of the capillary column into the MS with a standard nut and ferrule system near the ionization source (Section 5.1.2a). Since GC analytes are volatile, the interface and MS must be maintained at temperatures and pressures that keep the analyte (or ionized form) in a volatile form.
As implied in the previous paragraph, mass spectrometer systems require a low operating pressure, typically 10-5 to 10-6 Torr through out the system (ionization source, mass analyzer, and detector). This is necessary to avoid collisions between ionized molecules. If collisions are prominent, the mass resolving capabilities will be effected which decreases the detection limit and the resolution. Collisions also affect the interpretative value of the mass spectrum preventing identification.
The MS works by (1) ionizing each analyte as it exits the GC column, (2) accelerating and focusing the ionized compound and its fragments into the mass analyzer, (3) separating the fragments in the mass analyzer based on mass to charge (m/z) ratios, and (4) detecting the fragments as they exit the mass analyzer. There are a variety of ionization systems and mass analyzers that achieve these results. The following sections are dedicated to a simple description of most common ones.
| Frank's Homepage |
©Dunnivant & Ginsbach, 2008