An overview of important gas chromatography–mass spectrometry (GC–MS) techniques currently used in food analysis is described. Considerable attention is devoted to the use of the mass spectrometer, in relation to its potential for separation and identification. The importance of comprehensive two-dimensional GC (GC×GC) is also discussed.
Food products are usually of a highly complex nature and are composed of organic material (fats, sugars, proteins and vitamins) and inorganic material, (water and minerals). Apart from natural constituents, foods can contain xenobiotic compounds deriving from a variety of sources, including the environment, packaging, agrochemical treatments, etc. Many xenobiotic compounds can have a profound negative effect on human health — even at trace concentration levels.
A gas chromatography mass spectometry (GC–MS) analysis of a food can vary in scope. For example, a GC–MS method can be used for the qualitative/quantitative analysis of untargeted volatiles (for example, elucidation of an aroma profile) or targeted ones (such as pesticides). Furthermore, a GC–MS method can also be exploited for the generation of a chromatography profile (fingerprinting), with the aim of distinguishing between food samples of the same type (for example, to determine geographical origin). In such studies, the exploitation of statistical methods is almost obligatory. However, for almost any purpose, a GC–MS technique must be both sensitive and selective, as well as possessing a decent separation power and speed. The extent to which one or more of the aforementioned features prevails is dependent on the initial analytical objective.Current one-dimensional GC approaches are generally based on the use of a 30 m × 0.25 mm × 0.25 µm column, which generates peak capacities in the 400–600 range, and are the most commonly exploited tools for the separation of food volatiles. One can expect to fully-resolve around 80–100 analytes, using such a capillary column (compound more, compound less). However, because food samples are generally of moderate-to-high complexity, then the occurrence of solute co-elution at the column outlet is common, leading to difficulties or possible errors in the identification and quantification of specific components.
In the GC–MS field, most analysts are located in one of two groups. On the one hand, there are the many separation scientists who are mainly GC specialists and devote their time almost entirely to the optimization of the separation step, and tend to treat the MS instrument as a simple detector. Such an approach is fine if the ion source receives totallyisolated solutes, identified commonly by using dedicated MS databases. Problems arise when peak overlapping occurs, hence demanding a deeper exploitation of the MS step (for example, by using peak deconvolution methodologies, extracted ions or knowledge of MS fragmentation processes). On the other hand, several MS specialists pay little attention to the GC process, and prefer to circumvent a poor GC separation by exploiting a mass-analysing second dimension: multi-compound bands are transformed into a bunch of ions that are resolved and detected on a mass basis. It is obvious, however, that in the case of extensive co-elution, the reliability of the qualitative/quantitative results can be hampered. In truth, both analytical dimensions are complementary, and should be pushed to their full capacities.