Gas chromatography/mass spectrometry (GC/MS) combines two analytical tools to identify and measure the concentration of chemicals found in foods, consumer products, pharmaceuticals, fuels, the environment, and more.
Read on to learn more about the typical components of a GC/MS instrument and its basic principle of operation.
The gas chromatograph
Gas chromatography (GC) is used to separate volatile components in a mixture. It works by heating a liquid sample until it converts into a vapor that can be carried by a gas like helium or hydrogen. The gas (called a carrier gas or mobile phase) transports the sample through a long, thin glass or metal tube (column) that is coated with a chemical (stationary phase).
As the vaporized compounds are pushed through the column, they slow down when they interact with the stationary phase. Different chemicals will take longer or shorter times to reach the end of the column based on their chemical properties. As the chemicals are separated, they are transferred to the mass spectrometer.
The mass spectrometer
A mass spectrometer (MS) is a detector that identifies and measures the vaporized compounds separated in the GC. While GC provides retention time and peak intensity information, mass spectrometry adds a third dimension: mass information. Mass information can be used to identity, quantify, and determine the structural and chemical properties of molecules.
The first component the chemicals encounter in the mass spectrometer is called the ion source, where neutral molecules that elute from the GC column are ionized. A common ion source is an electron ionization (EI) source that usually contains a metal filament, similar to the filament in a light bulb. When an electrical charge is applied to the filament, it emits a stream of electrons at the incoming compounds, breaking them into fragments, and many of them with a positive charge. The pattern of resulting fragments acts as a highly specific “fingerprint” that can be used to identify the chemical.
Within the ion source, a series of electrodes known as lenses direct the charged molecules away from the source and into a quadrupole mass analyzer (or mass filter). A quadrupole consists of four rods to which a direct current voltage and radio frequency are applied. Various combinations of these forces ensure that only fragments of a specific mass (called a mass-to-charge ratio or m/z) will travel down the electric field of the quadrupole toward the detector at a given time.
The mass spectrometer rapidly cycles through different voltages, measuring many m/z ratios. The ions that pass through the instrument are measured by a detector called an electron multiplier, which provides a signal intensity for each ion species present. The recorded data at each point in time of an experiment is called a mass spectrum. The pattern of this mass spectrum can be used for identification purposes, much like a fingerprint. The response recorded for the different ion species can be calibrated for quantitative purposes.
