How to Read a Gas Chromatograph

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Gas chromatography is used to analyze compounds that can be vaporized without breaking down into simpler compounds. It's a common technique in analytical chemistry and is typically used to determine the purity of a substance. A gas chromatograph (GC) essentially tells you the time that a component separates (elutes) from the sample, assuming the GC is able to detect it. The GC then produces a graph, which an analyst must interpret.

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    • 1

      Examine the form of the chromatographic data. The output of a GC is called a chromatogram and is generally a two-dimensional graph. The amount of detector response is measured on the y-axis, and the retention time is measured on the x-axis.

    • 2

      Study the chromatogram. It will show a spectrum of peaks, where each peak represents an analyte in the sample. The retention time can help identify an analyte under the proper test conditions.

    • 3

      Learn as much as you can about the sample. The analyst generally must have some idea of what components are in the sample in order for the chromatogram to be meaningful. Even with this information, a component may not be detected if the sample contains another component in a higher concentration with the same elution time.

    • 4

      Connect the GC to a mass spectrometer. While the pattern of peaks can be used to identify analytes directly, a GC normally provides its results to a mass spectrometer or other detection device. This detector then identifies the analyte that's represented by each peak.

    • 5

      Determine the amount of each analyte shown on the chromatogram. GC analysis uses integral calculus to calculate the area under each peak curve. This value is proportional to the original concentration of each analyte in the sample.

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