Gas Chromatography - The Modern Gas Chromatograph > Page 4
The third unit is the column unit which contains the column, the essential device that actually achieves the necessary separation, and an oven to control the column temperature. It is interesting to note that despite the complexity of the apparatus, and its impressive appearance, the actual separation is achieved either in a relatively short length of packed tube or a simple wall-coated open tube. The rest of the apparatus is merely there to support this relatively trivial, but critical device. The oven also will contain a temperature sensor and if necessary an appropriate temperature programmer. As the mobile phase is a gas, there are virtually no interactions between the sample components and the mobile phase and thus the elution time can not be controlled by techniques such as solvent programming or gradient elution. The counterpart to gradient elution in gas chromatography is temperature programming. The column temperature is raised continuously during development to elute the more retained peaks in a reasonable time. It is a similar technique to flow programming but decreases the retention exponentially with temperature as opposed to linearly with flow rate. The temperature was originally programmed in a linear manner using electro-mechanical devices but modern temperature programmers contain a dedicated micro processor for the purpose. Sometimes all controls are initiated from a central computer that is also employed for acquiring and processing the chromatographic data.
The fourth unit contains the detector which is situated in its own oven. There is a wide range of detectors available each having unique operating parameters and its own performance characteristics. The detector, and the conduit connecting the column to the detector, must be maintained at a temperature at least 15˚C above that of the maximum temperature the oven will reach during analysis to ensure no sample condenses in the conduits or detector, consequently, separate conduit heaters are necessary. Any condensation introduces serious detector noise into the system and also reduces the detector response thus effecting both the detector sensitivity and the accuracy and precision of the results. The detector oven is set at a user defined temperature and is operated isothermally, controlled by its own detector-oven temperature controller. The output from the detector is usually electronically modified and then acquired by the data processing computer which processes the data and prints out an appropriate report.