Gas chromatography-mass spectrometry (GC-MS) is a popular analytical technique for identifying and quantifying concentrations of organic substances. GC-MS has a wide variety of applications because of its ability to detect so many types of molecules with high accuracy. Some common uses include quality control in the food industry, screening for impurities in environmental samples, and food contact material migration testing.
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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. GC-MS is used to determine the composition of unknown organic mixtures. This makes it the definitive method for NIAS testing of food contact materials, which involves screening the sample for contaminants without prior information about which substances are present. GC-MS is also commonly used to screen water samples for volatile organic compounds (VOCs) and several sample types for harmful PAH compounds.
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 (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.
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. In this way, it is possible to analyze volatile compounds originating from non-volatile samples, such as soil or building materials.
Suitable sample matrices
- Unknown organic materials
- Pure chemicals and different mixtures
- Food and beverage
- Fuels, oils and lubricants
- Environmental samples
Ideal uses of GC-MS
- Analysis of unknown organic compounds and mixtures
- Food and beverage safety testing to identify harmful compounds and determine their concentrations
- Compositional analysis of oils, fuels and lubricants, e.g. PAH analysis
- Environmental tests such as searching for impurities from air, water and soil samples
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Frequently asked questions
GC-MS is most commonly used for separating and identifying different compounds from a complex mixture of substances as well as determining their quantities. GC-MS can be used, for instance, to search for toxic substances from food products or to determine the amount of different organic sulfur compounds in natural gases. The method can also be used to test petroleum products in the oil refining industry to find PAHs and to perform environmental tests on water, soil, and air samples to find different contaminants.
When the compounds contained in the sample are known and only their concentrations are to be determined, gas chromatography alone is sufficient to obtain the results. However, if the composition of the sample is unknown, mass spectrometry is needed to identify the compounds. In this case, GC-MS should be used.
The compounds that are examined must be volatile or semi-volatile for gas chromatography to work. The sample matrix must be free of non-volatile substances, or a suitable method must be used to prevent the non-volatile matter from entering the gas chromatograph (HS-GC-MS, SPME-GC-MS, or similar).
Liquid chromatography methods (HPLC-MS, HPLC-DAD) can be used to identify non-volatile substances that are not suited for GC-MS analysis.
GC-MS is a suitable method for analyzing gaseous, liquid, and solid samples. Gaseous and solid samples, as well as liquid samples containing non-volatile matter, require pretreatment and/or the use of a special autosampler prior to the analysis. Very complex samples containing many different substances can be analyzed with this method combination, but they must first be separated from possible non-volatiles, such as dirt.
Measurlabs offers a variety of laboratory analyses for product developers and quality managers. We perform some of the analyses in our own lab, but mostly we outsource them to carefully selected partner laboratories. This way we can send each sample to the lab that is best suited for the purpose, and offer high-quality analyses with more than a thousand different methods to our clients.
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