Gas Chromatography (GC)























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GC is used to analyze the content of a chemical product, for example in assuring the quality of products in the chemical industry; or measuring toxic substances in soil, air or water. GC is very accurate if used properly and can measure picomoles of a substance in a 1 ml liquid sample, or parts-per-billion concentrations in gaseous samples.


GC and specifically gas-liquid chromatography, involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of an inert, gaseous mobile phase.


The column itself contains a liquid stationary phase, which is adsorbed onto the surface of an inert solid. The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector, which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities. For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. Capillary columns, on the other hand, need much less sample, typically around 10-3 mL. For capillary GC, split/splitless injection is used. Have a look at this diagram of a split/splitless injector;


For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapor; slow injection of large samples causes band broadening and resolution loss. The most common injection method is where a micro-syringe is used to inject sample through a rubber septum into a flash vaporizer port at the head of the column. The temperature of the sample port is usually about 50C higher than the boiling point of the least volatile component of the sample. In general, substances that vaporize below ca. 300 C (and therefore are stable up to that temperature) can be measured quantitatively. The samples are also required to be salt-free; they should not contain ions. Very minute amounts of a substance can be measured, but it is often required that the sample must be measured in comparison to a sample containing the pure, suspected substance.


Various temperature programs can be used to make the readings more meaningful; for example to differentiate between substances that behave similarly during the GC process. GC analysis needs quite of a time; sometimes a single sample must be run more than an hour according to the chosen program; and even more time is needed to "heat out" the column so it is free from the first sample and can be used for the next. Equally, several runs are needed to confirm the results of a study - a GC analysis of a single sample may simply yield a result per chance (see statistical significance). Also, GC does not positively identify most samples; and not all substances in a sample will necessarily be detected. All a GC truly tells you is at which relative time a component eluted from the column and that the detector was sensitive to it. To make results meaningful, analysts need to know which components at which concentrations are to be expected; and even then a small amount of a substance can hide itself behind a substance having both a higher concentration and the same relative elution time.


Last but not least it is often needed to check the results of the sample against a GC analysis of a reference sample containing only the suspected substance. A GC-MS can remove much of this ambiguity.