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2.2 Inlets

High quantitative precision benefits from a direct style inlet. Direct injection is typical for packed columns, but capillary columns present difficulties related to column overloading. Thermally labile compounds can often be analyzed by gas chromatography methods utilizing cold-on-column inlet systems, which are a form of direct inlets. Direct injection is also used for analysis of gases where the sample container is plumbed directly to the inlet system.

The split/splitless inlet is perhaps the most common inlet. This system allows both split and splitless operation. Split injection is made with a purge gas flowing at some relatively large multiple of the column flow, so that most of the injected sample is discarded. This reduces the chances of column overloading and helps insure a spatially small band of injected analyte being deposited on the column (which in turn improves chromatographic resolution).

Splitless injection is somewhat of a cross between direct and split injection. In splitless mode, the purge gas flow is zero initially, allowing most of the analyte to be deposited onto the column. However, after a short delay (about one minute), the purge flows to remove any remaining excess solvent or analyte present in the inlet. This technique is more forgiving than direct injection, but is less easily tuned than split injection. However, splitless gives more quantitative precision compared to split injection.

Split/splitless inlets can generally be used with either manual or automatic injection.

Volatile components can be analyzed using headspace or purge and trap. Either can be done manually, though automated samplers can be purchased. The automatic sampler is especially convenient for purge and trap sampling.


next up previous
Next: 2.3 Detectors Up: 2 Instrument Parameters Previous: 2.1 Columns
John S. Riley, DSB Scientific Consulting