What is gas chromatography and describe it. In connection with this discuss the sampling, detection limit, precision, and error sources. Regarding to the analysis method explain, for example, application, principle, equipment and assay performance.

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Gas chromatography (also commonly called GC) is a method of analyzing chemical samples for purity.  Different chemical components within a sample injected onto a GC will separate and read as separate compounds by a detector.  A sophisticated system can not only separate different chemical components but it can quantify them as well.  So one very useful application of GC is detecting for the presence of illegal chemicals in samples in crime and drug labs.  Assay performance on a sophisticated system is very high.

The principle and equipment used are as follows.  The chemical sample is dissolved and diluted in a volatile chemical solvent like methanol or acetone.  The sample is injected using a syringe into an injector assembly.  The injector assembly is very hot (>200 degrees C) and vaporizes the solvent and atomizes the chemical sample into gaseous form.  The gaseous sample is picked up by the carrier gas (usually helium or hydrogen) and then carried through the GC column.  The column is a long (~25 meters) and thin (<1 millimeter) flexible tube coiled inside an oven.  The sample travels through the stationary phase in the column tube where the individual chemical entities are separated according to their affinity for the column.  The temperature in the oven can be varied to help larger compounds travel faster.  The gas then exits the column and enters a detector, which is usually a flame ionization detector (FID).  The output of the detector is plotted over time to give a graph of the sample being analyzed.

The sampling for GC is very easy.  The chemical sample needs to be dissolved in any number of basic chemical solvents.  Only microgram levels of material are required for an effective GC trace so the detection limit is down to parts per billion levels (ppb).  Precision of analysis can be very high when using an autosampler to inject the samples since it can measure and replicate injection volumes with a high degree of precision.  Error sources include sample preparation and potential gas leaks with the system (it should be gas tight).  Probably the biggest drawback to GC is that the chemicals injected need to be a low enough molecular weight to effectively atomize and exist in the gaseous state.  Compounds that are too large will not travel through the column effectively and can damage the column with regard to future analysis.

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