Gas Chromatography - Tandem Techniques - The Combination of the Gas Chromatograph with Other Spectroscopic Systems > Spectroscopy Techniques > Page 11


an automatic sampling system so that the instrument can be operated continuously over 24 hour periods for as many days as necessary. It also has full data acquisition and data processing facilities that will allow identification of eluted solutes by comparison with standard reference spectra. The data processing software supplied with many combined instruments may also be capable of structure elucidation by either fragment analysis or, if the mass spectrometer has sufficiently high resolution, will provide a calculated empirical formula of the eluted solute. Most mass spectrometers used in modern GC/MS combined instruments are quadrapole, double quadrapole of octapole-quadrapole devices, which can provide very high resolution in a relatively compact form. GC/IR instruments are also fairly compact but are not nearly so popular due to the inherent limited information that can be obtained from the infra red spectrum. They are, however very useful in forensic studies where solutes can be accurately identified with a high degree of confidence from reference spectra. Other gas chromatography spectroscopy combinations are much less common but, nevertheless, have areas of application which makes them uniquely extremely useful.


Spectroscopy Techniques


Identification techniques that are used in conjunction with separation systems are almost exclusively spectroscopic in nature, particularly when associated with GC. Originally the term spectroscopy was given to the measurement of adsorption, transmission, or reflection of electromagnetic waves but today the term has been extended to mass spectroscopy (which measures the individual mass of charged ions) and to nuclear magnetic spectroscopy (which measures the precession frequency of asymmetric spinning atomic nuclei when situated in a magnetic field). The only common factor that remains between the different spectroscopy's is that, in their traditional manner of presentation, the different types of spectra bear some slight resemblance to each other.


The three major spectroscopic techniques that are associated with GC are infra red spectroscopy, atomic spectroscopy and mass spectroscopy, the first two, as already stated, involves the absorption of electromagnetic waves whereas the last the measurement of ion masses. There are rare examples of the use of the UV spectrometer and the fluorescence spectrometer in conjunction with a GC but, for the reasons discussed below, these combinations have very limited advantages., nevertheless, they will be discussed.


Electromagnetic radiation consists of a sinusoidal electric field in phase with sinusoidal magnetic field at right angles to it propagated at a constant speed (c) of 3 x 108 m/sec.