Gas Chromatography - Tandem Techniques - Modern GC/IR Systems > The Cryostatic Interface > Page 55

As the sample is contained in a frozen argon matrix, the samples can be held for a very long time on the drum without loss or change. Consequently, the spectrum of any particular sample can be taken repeatedly, any number of times, to improve the signal-to-noise ratio and, thus, the overall sensitivity. The interferometer's modulated IR beam is focused on the narrow frozen argon 'stripe' that contains the sample. Employing this device, the IR sensitivity is commensurate with that of the mass spectrometer.

Courtesy of Mattson Instruments Inc.

Courtesy of Mattson Instruments Inc.

Figure 43. Absorption Peaks for Liquid, Solid and Matrix Isolated Samples

The improved sensitivity is partly due to the detector element being approximately the same size as the sample 'stripe' and partly due to the sample being in an inert argon matrix, which causes the absorption bands to be much sharper and thus much higher. An example of the relative absorption peak heights for a liquid sample, solid sample and a matrix-isolated sample according to Mattson is shown in figure 43. All peaks represent the same mass of solute and it is seen that matrix isolation sharpens the peak considerably and provides a very significant increase in sensitivity. The interferometer itself is situated in a vacuum chamber to eliminate any contaminants and thus background effects. This arrangement produces a remarkably stable output. The system can be cleaned very rapidly by warming the drum, pumping out the argon and sample vapor, leaving the disk with no trace of residue.

Courtesy of Mattson Instruments Inc.

Figure 44. Spectra of 1,4 and 1,5,-Dimethyl Naphthalenes Obtained Using the Cryostat Interface.