GC-MS Analysis

What is GC-MS and what is it used for?

Gas chromatography-mass spectrometry (GC-MS) is an efficient and accurate combination of two methods: gas chromatography and mass spectrometry. The ability of GC-MS to determine the composition of unknown organic mixtures makes it the definitive method for NIAS testing of food contact materials, which involves screening the sample for contaminants without prior information about which ones are present. Other food contact material testing applications include printing ink set-off testing and phthalates analysis.

In addition, GC-MS is commonly used in environmental analysis to screen water samples for volatile organic compounds (VOCs) and various sample types for harmful PAH compounds. It also has multiple applications in food industry quality control, for example, determining the fatty acid profile of oils and detecting dioxins and PCBs in food, water, and feed.

Gas chromatography

Gas chromatography (GC) is used to separate compounds based on their boiling points and polarities. The sample is vaporized to achieve the gas phase. The gaseous sample is then separated into its components in a capillary column. The column itself is coated with a solid stationary phase and the sample is propelled forward by an inert gas used as a moving phase. In this way, different compounds are separated from each other.

The compounds are then eluted from the column at different times based on their boiling point and polarity. The retention time (RT) of a single compound, that is the time it takes for the compound to go through the column, is affected by the length and temperature of the column and the flow rate of the carrier gas. The concentrations of the compounds in the sample can be determined using internal and external standards (i.e. determining the fatty acid composition of cooking oil).

Mass spectrometry

Mass spectrometry (MS) identifies different compounds separated in a gas chromatograph by creating a mass spectrum that is unique for every compound. Mass spectrometry also detects the quantities of the compounds by producing peaks in the data. These peaks are proportional to the amount of the compound of interest.

In mass spectrometry, either electron or chemical ionization is used to ionize and fragment the compound giving the fragments a positive charge. The fragmented molecules (ions) are then accelerated through a mass analyzer which includes either a quadrupole or an ion trap. In the mass analyzer, the ions are separated based on their mass-to-charge (m/z) ratios with the help of a magnet which bends the stream of the ions toward a detector. Bigger particles take longer to turn than smaller ones, which is why different particles hit the detector in different locations. Thus, the fragmented molecules can be separated and identified by their different masses and they appear as a function of their m/z ratios in the spectrum produced by the mass spectrometer.

The peak areas in the MS spectrum are proportional to the quantity of the corresponding compound. A complex sample produces numerous different peaks in the gas chromatograph and each peak generates a unique mass spectrum. The identification and quantification of unknown compounds are done by comparing the mass spectra obtained to libraries of mass spectra.

Suitable samples for GC-MS

GC-MS is well suited for studying many different compounds from a wide variety of sample matrices. A requirement for GC-MS analysis is that the compounds in the sample are volatile or semi-volatile so that they can be separated from each other with a gas chromatograph and identified by a mass spectrometer. Many gaseous, liquid, and solid samples are suitable for GC-MS analysis without significant pretreatment. However, the analysis of complex samples or very small sample amounts may require more preparation.

Sample preparation

Gases and very easily volatile samples can be analyzed by collecting the substances of interest with the help of solid-phase microextraction (SPME). Then the compounds can be released into the gas chromatograph for analysis. To make the analysis of low-volatility compounds easier, they can be treated to be more volatile with the help of derivatization.

In derivatization, a boiling-point-lowering chemical group such as methyl or silyl is attached to a low-volatility compound. For the examination of samples that contain significant amounts of non-volatile matter, the so-called headspace (HS) autosampler, which dispenses gas into the gas chromatograph from the top of the actual test sample, can be used. This makes it possible to analyze volatile compounds originating from non-volatile samples, such as soil or construction materials.

Another option is starting the analysis with pyrolysis, i.e. breaking the sample through exposure to heat. The resulting py-GC-MS analysis is well-suited for analyzing polymeric materials, lignin, and other complex sample types. A GC-MS setup may also be connected to a thermogravimetric analyzer to analyze the evolved gases that are formed when the sample is heated.

Need an analysis?

At Measurlabs, we offer a comprehensive selection of purity and composition analyses using GC-MS and related methods, such as GC-ECD and GC-FID. Do not hesitate to contact us through the form below to discuss your testing needs. Our experts will get back to you within one business day.