Characteristics of the sample: All the components of the mixture must be volatile at the temperature of the injector (300°C, approx.),unless the sample is to be analysed using the headspace technique.For this reason it cannot contain metals, salts, inorganic acids or bases and other non-volatile components such as long-chain polymers, with the exception of the samples analysed by head space.The solvent of the sample and, whenever possible, the possible components of the sample must be reported.The usual concentration range of the analytes in the sample is 1 µg/ml.

Samples dissolved in water, DMSO, DMF and other less volatile solvents should be analysed using the headspace technique.

You will need to give the toxicity and any storage precautions. Once the report has been delivered, if you do not collect the sample within one week, any remaining sample will be destroyed.

On the one hand, the chromatography of gases allows for the separation of the components of a mixture according to its boiling point and the different degree of interaction of the components in the mixture with the stationary phase of the column.The coupling with spectrometry of masses allows for the individual detection of the components and to obtain information about the mass and the structure of the molecule.

The GC-MS technique is made up of several parts: a system introducing the sample, a separation system (column), a source to ionise compounds (EI or CI), an analyser of masses to sort out the ions (quadrupole), a detector and a system for processing data.

Ionisation sources

UEM has two different sources of ionisation:

Electronic ionisation (EI): This is the most usual ionisation in GC-MS.The impact of a bundle of electrons with a relatively high energy (70 eV) produces the ionisation of the sample.The primary process consists of the abstraction of an electron to give a cation-radical (molecular ion).According to the stability of this molecular ion, a major or minor fragmentation will occur.The very stable molecular ions will have little tendency to fragment and will be very abundant.For the specificity and robustness of the fragmentation in the registered spectres, their study provides relevant structural information.Besides, through a computer comparison of the registered spectre with a library of spectres, the identification of the substance can be achieved.The UEM has a commercial NIST library of more than 190,000 spectres.

Chemical ionisation (CI): In this type of ionisation the forming of ions of the sample involves a lot of less energy and is much softer than in EI.Because of this, the CI produces a lot less fragmentation and the CI spectre shows a greater abundance of the molecular ion.This is why it is often used to determine the molecular weights of the components in the sample.In chemical ionisation, besides the sample and the carrying gas, great amounts of reaction gas are introduced into the ionisation chamber.As there is much more reaction gas than the sample, the majority of electrons issued collide with the reaction gas molecules, forming reaction gas ions.These ions react between them causing several processes until a balance is reached.These ions react in a different way with the molecules of the sample and form ions of the sample.

In the UEM we have methane and ammonia 5% in methane as a reaction gas.

The most common applications for this technique entail the analysis, quantification and identification of volatile or semivolatile compound mixtures with a high sensitivity.

Identification of solvent and other volatile products in solid or aqueous samples

The GC-MS technique allows us to analyse volatile products.When these are in an aqueous or non-volatile matrix, an analysis of the volatile fraction can be made by putting the sample in an air-tight recipient and analysing the headspace, or gas fraction.The peaks obtained in the chromatogram can be associated with specific molecules making use of libraries.
Specific example: Determination of the toluene degradation products through GC-MS.
Further information: "Robust Iron Coordination Complexes with N-Based Neutral Ligands As Efficient Fenton-Like Catalysts at Neutral pH", Environ. Sci. Technol., 2013, 47 (17), 9918–9927.

Identification and quantification

Identification and quantification of unwanted by-products.Commercial chemical products often have unwanted impurities.These impurities can be a big problem in later stages because they can behave in an unexpected way.The chromatography of gases coupled to spectrometry of masses can be of great help in order to identify and quantify the impurities that the sample contains.

- Quantification of products and mechanistic studies through isotopic labelling

In this case, besides quantifying the products obtained in a reaction, the GC-MS has been used to study the isotopic composition of the products obtained.That way, where isotopically marked reagents have been used, we will be able to find out whether these have ended up being part of the product of the reaction and to what extent.The products marked isotopically are those that contain atoms with a different isotopic distribution to the one found in nature.For example, the products they contain are ¹⁸Or, since the most abundant isotope of oxygen is ¹⁶O.Therefore, the presence of¹⁸O with higher levels than 0.2% implies participation in the reaction of the isotopically marked products introduced.Isotopic marking studies are a very important source of mechanistic information.
Further information: "Highly Stereoselective Epoxidation with H₂O₂ Catalyzed by Electron-Rich Aminopyridine Manganese Catalysts". Org. Lett., 2013, 15(24), 6158-6161.

• Agilent Technologies 5975C pamphlet
• Specifications for the Agilent Technologies GC-MS 5975C