Concerns about volatile chemicals, including the constituents of everyday food materials and food products, are growing. Of equal concern is the growing body of global government legislation pertaining to this area.
Referred to as volatile organic compounds, or VOCs, these chemicals are many and varied. Broadly speaking, VOCs can be desirable (such as certain chemicals that give individual products their distinctive flavors and aromas) or undesirable (such as migrants from packaging, or the unpleasant by-products of microbial action).
Although individual VOCs are present at vanishingly small levels — usually much less than one part per million — they can nevertheless vary greatly in quantity. Such differences can have a significant impact on the final product’s aroma and perceived quality.
Unfortunately, actual VOC levels depend on a bewildering array of factors, including everything from raw-ingredient production conditions to individual responses to levels of particular chemicals.
Analyzing all the factors involved in a product deficiency can be highly complex and requires understanding of the chemicals present in a product at the various stages prior to consumption.
Fortunately, such information is readily available by modern analytical techniques.
Used in conjunction with an appropriate sample preparation method and a suitable detection technique, analytical techniques based on gas chromatography (GC) answer some of the most challenging food-analysis problems.
Mass spectrometry (MS), as an analytically rigorous technique, is the best detection method for the most complex GC analyses. It creates a mass-spectral “fingerprint” for every GC peak, which can then be matched against a library of such “fingerprints” to provide confident identification.
The combination of the two techniques, referred to as “GC–MS,” can identify any sufficiently volatile organic chemical and has long provided useful information about a wide range of food types.
However, sample preparation for GC–MS, especially for foods, can be very time-consuming. VOCs must be extracted from the sample so as to ensure high sensitivity, consistent results from run to run, and VOC profiles representative of those experienced by customers.
Sample preparation challenge
Solid-phase micro-extraction (SPME), which captures VOCs using adsorbent material of fine fiber suspended above the food, is one popular approach to addressing this challenge. However, it is difficult to automate — an issue when high-throughput analysis is required.
An appealing alternative to SPME in such situations is headspace (HS) sampling. It confines the food sample in a small space, with the released vapors being flushed on to a tube packed with an adsorbent material (a “sorbent tube”). This is either done all at once (static or equilibrium HS) or as a continuous process (dynamic HS). The vapors are then released from the adsorbent material by a technique known as thermal desorption (TD), which passes them directly to the GC–MS instrument. This has the significant advantage of being easy to automate.
HS approaches are particularly relevant to sampling VOCs from packaged materials, because the packaging acts as a ready-made vessel to contain the vapors.
A meaty example
One example of this involves a manually operated “grab”-sampler used to capture volatiles from the headspace above a sirloin steak. The volatiles were collected directly onto a sorbent tube, which was then analyzed by TD–GC–MS analysis. The results are displayed in Figure 1.
Individual compounds were identified by a combination of their retention time on the GC instrument and the data associated with each peak obtainable using a modern mass spectrometer. This allows confident identification to be made in most cases.
More than 40 major compounds were identified in the meat headspace. Just a few of the most interesting are shown in the figure. A particular feature is the excellent peak shape (without peak splitting or tailing). This is often difficult to achieve in food analysis because of the high humidity of food samples, which causes problems in the analytical system.
However, the small sample taken, the choice of sorbents and the optimized TD conditions ensured that any water collected was removed before it passed into the GC–MS instrument.
There was no particular reason to believe this typical supermarket-purchased product was contaminated. Nevertheless, small quantities of compounds that would be of interest to the manufacturer were found, including toluene, benzene and dichloromethane. These are unlikely to have arisen from the meat itself and are more likely to have migrated from the packaging.
Also of interest were un-branched short-chain alkanes and alkenes such as butane and pentane, which, along with the odorous sulfur compound dimethyl sulfide, are possible artifacts of the irradiation often employed to sterilize meat products.
The amounts of these compounds in the sample were relatively low and would not necessarily give cause for concern, but they illustrate the sorts of compounds that food analysts would be interested in monitoring and are a striking example of the power of TD–GC–MS to pick up on small but potentially significant components of complex samples.
This was just one example application. An advantage of TD–GC–MS is that it is compatible with a number of sampling techniques — making it possible for the analyst to adapt the analysis to the sample in question. For example, the headspace from unpackaged solid samples such as cheese or strawberries is easily sampled using a micro-chamber apparatus. Liquids can be investigated using adsorbent cartridges, which are typically immersed in the sample before being washed and heated in an empty thermal desorption tube to desorb the volatile components.
In conclusion, TD–GC–MS is a powerful technique that allows the analyst to obtain high-quality information on the chemical composition of a variety of foods — information that can help manufacturers to ensure product consistency, develop new formulations, comply with safety regulations and above all improve their products for the benefit of their customers and their businesses.
Markes International is a specialist manufacturer of instrumentation for detection of trace-level volatile and semi-volatile compounds.