Gas chromatography–mass spectrometry

Gas chromatography–mass spectrometry (GC-MS) is an effective analytical technique for identifying and determining concentrations of organic substances.

GC-MS has a wide variety of different applications because of its ability to detect so many types of molecules with high accuracy. Gases, liquids and solid samples can be analysed and almost all kinds of sample matrices are suitable for the analysis of their volatile and semi-volatile components.

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Gas chromatography–mass spectrometry (GC-MS) is an efficient and accurate combination of two methods, gas chromatography and mass spectrometry. GC-MS is applied to determining the composition of unknown organic mixtures. 

Gas chromatography

Gas chromatography is used to separate compounds based on their boiling points and polarities. The sample is vaporized in order 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 a cooking oil).

Mass spectrometry

Mass spectrometry 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 towards 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 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. Identification and quantification of unknown compounds is done by comparing the mass spectra obtained to libraries of mass spectra.

Samples

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 have to be volatile or semi-volatile, so that they can be separated from each other with a gas chromatograph and identified by a mass spectrometer. Some gaseous, liquid and solid samples are suitable for GC-MS analysis without pretreatment of the sample. However, the analysis of gaseous and very poorly volatile samples requires pretreatment of the samples or an appropriate autosampler. 

Sample preparation

Gases and very easily volatile samples can be analyzed by collecting the substances of interest that the sample contains with the help of solid-phase microextraction (SPME). Then the compounds can be released to the gas chromatograph for the 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 lots of non-volatile matter, the so-called Headspace (HS) autosampler that dispenses gas into the gas chromatograph from the top of the actual test sample, can be used. In this way, it is possible to study volatile compounds 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 amounts in foodstuff
  • Compositional analysis of oils, fuels and lubricants, such as determining the amount and kind of hydrocarbons (e.g. PAH) of an oil sample
  • Environmental tests such as searching for impurities from air, water and soil samples

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Frequently asked questions

What is GC-MS commonly used for?

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 for searching for toxic substances from food products or for determining the amount of different organic sulfur compounds from natural gases. This method can also be used for testing petroleum products in the oil refining industry for example to find PAHs and performing environmental tests to water, soil and air samples to find different contaminants. 

If it is known what compounds the sample contains and it is only wanted to find out their concentrations, the gas chromatography part of the method alone is sufficient for getting the results. However, if there is no idea of the composition of the sample, mass spectrometry is also needed to identify the compounds in the sample. In this case, GC-MS could be a suitable method.

What are the limitations of GC-MS?

The compounds that are examined must be volatile or semi-volatile so that gas chromatography can work. The sample matrix must be free of non-volatile substances or a suitable method must be used to avoid the non-volatile matter entering the gas chromatograph (HS-GC-MS, SPME-GC-MS or similar). Non-volatile matter, e.g. dirt, can also be removed from the sample prior to the gas chromatographic analysis.

What kinds of samples can be analysed using GC-MS?

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 but first they have to be separated from possible non-volatiles, such as dirt.

What is Measurlabs?

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.

How does the service work?

When you contact us through our contact form or by email, one of our specialists will take ownership of your case and answer your query. You get an offer with all the necessary details about the analysis, and can send your samples to the indicated address. We will then take care of sending your samples to the correct laboratories and write a clear report on the results for you.

How do I send my samples?

Samples are usually delivered to our laboratory via courier. Contact us for further details before sending samples.

GC-MS is a good method for identifying different compounds present in complex sample matrices. Measur offers commercial gas chromatography–mass spectrometry testing in accredited laboratories. Our laboratory network consists of the world's best laboratories, carefully selected by our team of experts. With GC-MS and all other services by Measur, you can expect the highest possible reliability and fastest delivery of results.