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Science

Analysing substances

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GC-MS

Gas chromatography (GC) and mass spectrometry (MS) are two instrumental methods of analysis. They can be linked together as GC-MS to allow substances in a sample to be separated, then identified. You need to have a basic understanding of how gas chromatography works.

Gas chromatography

Gas chromatography allows a mixture of compounds [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ] to be separated. The GC machine consists of a long glass tube packed with a powdered solid material, which is fitted into an oven. The tube is called the column, even though it is usually wound into a coil so that it fits into the oven.

  1. The sample is dissolved in a solventsolvent: A solvent is the liquid in which the solute dissolves to form a solution., then injected into one end of the column.
  2. An unreactive gas - usually nitrogen - carries the sample through the column.
  3. Different substances in the sample travel through the column at different speeds and so become separated from each other.
  4. The separated substances leave the column one after the other. As they leave, they are detected by a detector.

The time taken for a substance to travel through the column is called its retention time. A detector produces a graph where each substance is represented by a peak:

  • The number of peaks shows the number of compounds present in the sample
  • The position of each peak shows the retention time for each compound

Mass spectrometry

A mass spectrometer is able to identify substances very quickly and accurately. It can detect very small amounts. If a GC machine is linked to a mass spectrometer, this combined GC-MS method allows the separated substances leaving the column of the GC machine to be detected.

Mass spectrometry - Higher tier

The mass spectrometer can be used to identify substances quickly and accurately, and in very small amounts. It can also provide the relative formula mass of the substances separated by gas chromatography. The peak furthest to the right in a mass spectrum is called the 'molecular ion peak'. Its relative massrelative mass: The relative mass is the number of times heavier a particle is, compared to another. is the relative formula mass of the substance being analysed.

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