Characteristics of the sample: All the components of the mixture have to be volatile at the temperature of the injector (300 °C, approx.), except that the sample wants to analyse using the technique of space in head. For this reason it will not be able to contain metals, salts, acids or inorganic bases and other non volatile components like polymers of long chain, with the exception of the samples analysed for space in head. It be necessary to inform of the solvent of the sample and, whenever it is possible, of the possible components of this. The usual rank of concentration of the anàlits of the sample is 1 µg/ml.

The samples dissolved in water, DMSO, DMF and other little volatile solvents will have to be analysed with the technique of space head-first.

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 the separation of the components of a mixture according to its boiling point and the different bachelor's degree of interaction of the components of the mixture with the stationary phase of the column. The coupling with the spectrometry of masses allow to detect individually 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.

Sources of ionisation

UEM has two different sources of ionisation:

Electronic ionisation (EI): It 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 be produced. The very stable molecular ions will have little trend to fragment and will be very abundant. For the specificity and robustness of the fragmentation in the registered spectres, its study provides relevant structural information. Besides, by computer comparison of the registered spectre with a library of spectres, the identification of the substance can be achieved. The UEM NIST of more than 190,000 spectres has a commercial library.

Chemical ionisation (CI): In this type of ionisation the education of ions of the sample implies 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. It is of the components of the sample for this that often is used to determine the molecular weights. In chemical ionisation, besides the sample and of the carrying gas, great amounts of reaction gas are introduced to the chamber of ionisation. 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 among them causing several processes until reaches an equilibrium. 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 gas of reaction.

Them applications more usual of this technique they entail the analysis, the quantification and the identification of mixtures of volatile or semivolatile compounds with a high sensitivity.

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

The technique of GC-MS allows to analyse volatile products. When these are in an aqueous or non volatile matrix the volatile fraction introducing the sample in a hermetically closed recipient and analysing the space head-first can analyse, him to say the gas fraction. The pikes obtained to the chromatogram can be associated with concrete molecules making use of bookshops.
Concrete example: Determination of the products of degradation of the toluene through GC-MS.
More 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 not wished by-products. Often the commercial chemical products bring not wished 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 mecanístics studies through isotopic labeling

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. In this way, in the case to have used reagents marked isotopically, we will be able to know if these have ended up being part of the product of the reaction and in which it measures. The products marked isotopically are those that contain atoms with a different isotopic distribution to the one found in nature. For example, the products that they contain are it ¹⁸Or, since the most abundant isotope of the oxygen is the¹⁶O. Therefore, the presence of¹⁸O with higher levels than 0.2% implies participation in the reaction of the isotopically marked products introduced. The studies of isotopic labeling are a very important source of mecanísitica information.
More information: "Highly Stereoselective Epoxidation with H₂O₂ Catalyzed by Electron-Rich Aminopyridine Manganese Catalysts". Org. Lett., 2013, 15(24), 6158-6161.

• Pamphlet of the Agilent Technologies 5975C
• Specifications for the Agilent Technologies GC-MS 5975C