7.1.1.a Temperature Settings: There are four main temperature controlled regions on a GC-MS. The first region is the injector, which is set at least 20 degrees higher than the final oven temperature. The column oven is set to run either an isothermal mode or in as a temperature program, with the latter being the most common. The oven temperature is usually initially set between 10 and 15 degrees below the boiling point of the solvent, held at this point during the split-less mode of the injection, followed by one or more temperature ramps, and typically held at a high final temperature to remove late eluting analytes that may or may not be of interest. Modern GC-MS systems automatically return to the starting temperature after a given time. The oven is initially held at a relatively low temperature (compared to the boiling point of the solvent) to concentrate the analytes at the head of the column. If a higher temperature is used, the solvent will rapidly volatize and spread the analytes over a broad area and decrease peak resolution. The detector is always set at a constant temperature 15 to 20 degrees above the maximum temperature of the oven. The injector and detector are held at higher temperatures to prevent recondensation of analytes onto this surfaces which would interfere with quantification due to peak tailing and potential cross contamination between samples. The final temperature region is the MS vacuum chamber. It can be set above or at a lower temperature than the GC components (~150 to 250 degrees C in the quadrupole mass analyzer) due to the more volatile nature of analytes at low pressure.
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