Gas Chromatography - Tandem Techniques - The Combination of the Gas Chromatograph with Other Spectroscopic Systems > Page 8

The Combination of the Gas Chromatograph with Other Spectroscopic Systems

 

Unfortunately the resolution of the MS 12 was little better than 1 mass unit and so the actual molecular formula of the eluted peak could not be determined by calculation but only by conventional analytical procedures. However, unit mass resolution was often inadequate for accurate spectra interpretation, and further information, such as the presence of functional groups, was found necessary for reliable structure elucidation.

 

The limited mass resolution of the early fast scanning mass spectrometers motivated the development of an IR spectroscopic system that could be linked to a gas chromatograph in much the same way as the mass spectrometer. Such a combination would provide IR spectra and, consequently, indicate the presence of specific chemical groups to aid in structure elucidation. This was a far more difficult development project because the sensitivity of the IR spectrometer was many orders of magnitude less than that of the mass spectrometer and, consequently, significant solute concentration was often necessary. Initially, the specific peaks from which further structural information was required were trapped and the IR spectra obtained off-line by conventional procedures. Subsequently, the 'light pipe' (which will be described in due course) was developed to improve the IR sensitivity and spectra were again obtained on-line with the column eluent passing directly through the light pipe.

 

An alternative procedure for increasing the sensitivity of the IR spectroscopic system was to trap each peak as it was eluted, stop the chromatographic development and regenerate the solute from the trap into an IR gas cell. The IR spectrum was then taken and the chromatographic development restarted until the next peak was trapped. In one GC/spectroscopic system, after the IR spectrum had been taken a sample of the vapor was also drawn from the IR gas cell into a mass spectrometer and a mass spectrum taken. This produced, virtually simultaneously, mass and IR spectra of the sample, thus, simplifying the interpretation considerably. As the resolution of the mass spectrometer was improved the need for associated IR spectra became redundant and the empirical formula of the eluted solute could be obtained from the molecular weight by calculation.

 

An example of the chromatogram and also some spectra obtained from a combined GC/IR/Mass Spec. combination is shown in figure 6.