What Is a Gas Chromatograph Mass Spectrometer?

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  • Written By: Maggie J. Hall
  • Edited By: Susan Barwick
  • Last Modified Date: 15 January 2020
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A gas chromatograph mass spectrometer (GCMS) combines gas chromatography and mass spectrometry to identify and quantify substances. The process begins with the substance under study being diluted with either a gas or liquid and introduced into the machine. The GCMS separates the substance into components to be identified by a computer. Gas chromatograph mass spectrometers have a wide range of uses including illicit drug investigations, environmental detection, and medical diagnosis.

The gas chromatography or GC phase of the analysis typically involves placing a sample into the injector portal. The substance might be dissolved in an aqueous solution and injected into the chamber with a hypodermic needle. Once in the glass or stainless steel chamber, the compound travels into an oven where it is transformed into a gas.

Gradually increasing the temperature causes the compound to break into its component parts, leave the chamber, and pass at variable rates into the detector. Electrically charged, the particles trigger an electronic signal. The greater the concentration of particles the stronger the signal created.

These signals are then analyzed by computer software. The computer produces a chromatogram with peaks that indicate the retention times and signal intensities of the compounds. Knowing the retention times of various chemical compounds allows investigators to identity them.


Further analysis occurs in the mass spectrometer. Once compounds enter the spectrometer, they encounter an array of electrons or gas. Ionization occurs, breaking the compound into fragments, each having a specific atomic mass. These fragments undergo analysis by means of compiling a mass fragment to charge ratio, or M/Z.

This information passes to the gas chromatograph mass spectrometer computer which formulates a graph with peaks based on M/Z versus signal intensity. The computer software typically includes a database of known M/Z ratios and signal intensities for different chemical molecules. The computer identifies substances, comparing the acquired information to the library of known substances. The gas chromatograph mass spectrometer can complete each phase of analysis individually or simultaneously. Depending on the level of detail desired, completing the process can take 20 minutes or over 20 hours.

Law enforcement officials use a gas chromatograph mass spectrometer when they suspect the presence of illegal drugs or poisonous substances. Environmental agencies use the technology for analyzing air, soil, or water samples to detect possible pollutants. Health care professionals use a GCMS on urine samples to detect chemical compounds found in individuals with certain metabolic disorders.


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