Gas Chromatography - Tandem Techniques - Introduction > Historical Development of GC Spectroscopy Systems > Page 2


The separation, depicted in figure 1, however, also discloses some further problems that arise, even after having produced a very sophisticated separation. Although each individual component might be separated (and, thus, the total number of components would also be disclosed) and its relative proportion in the original mixture could be estimated, the actual identity of each component would still remain unknown. In the original presentation by James and Martin, they pointed out that the area of a peak could give a quantitative assessment of the amount of solute present in the mixture. In addition, they stated that the retention time of any solute eluted would be unique for a given phase system operated at a constant and known temperature and, thus, could be used for identification purpose (this, of course is confirmed by the Plate Theory). The latter claim, however, although basically true, was an over simplification as, within the precision possible for practical retention measurements in GC, many substances would have very close retention characteristics and, thus, in practice, retention data would have very limited use for solute identification.


Another problem associated with solute identification from retention data is the assumed availability of suitable reference samples to provide reference retention data. For most unknown samples, reference compounds are not available and, thus, retention values for any unknown solute is of little use for identification purposes. Unfortunately, even today, a half a century later, neither the thermodynamic theory of retention nor the interaction theory of retention are sufficiently well developed to be able to calculate the retention of a specific solute on a specific phase system from basic physical chemical data. As a consequence, reference retention data can not be calculated as an alternative to using a reference sample. It follows that the eluted sample must be examined by other means than mere simple detection to help identify (or confirm the identity) of an hitherto unknown substance.



Historical Development of GC Spectroscopy Systems


The concept of connecting the eluent from a gas chromatograph directly to a spectroscopic system was an attractive initial approach because it eliminated any sample handling procedures that might be necessary between the two techniques. However, there would be several necessary requirements before this could be satisfactorily accomplished. Firstly any interface must not denigrate the resolution obtained in the GC column by significant solute dispersion. Secondly the spectrometric system must be sufficiently sensitive to handle the small masses of sample that must be placed on the chromatographic column. In addition, as the solute was diluted during passage through the column, if the ultimate concentration of the eluted sample was insufficient for the spectroscopic system selected, then some form of concentrator would be necessary in the interface.