Instrumental aroma analysis needed

In order to obtain more detailed information and to increase the selectivity and meaningfulness of the analyses, it is often necessary to separate the VOCs using gas chromatography systems (GC). Only this enables the qualitative and quantitative detection of individual VOCs (so-called marker substances). The systems consist of an injector for sampling, which is often automated and coupled with an autosampler, a chromatographic separation column for separating the individual substances and a detector. Additional concentration of the substances using an upstream adsorber unit (e.g. SMPE, ITEX) is possible to reduce the detection limits. The substances in the chromatogram that is obtained are identified by measuring reference substances or through a database comparison of the retention indices calculated from the retention times.

In the ‘HERACLES NEO’ from Alpha MOS, which is based on the ultra-fast principle of flash chromatography, two GC columns connected in parallel with different polarities (hydrophobic/hydrophilic) are used to clearly assign the substances. In this process, detection is carried out with a flame ionisation detector (FID). The Arochem Base software then enables the respective molecule to be identified as needed with the associated odour attributes or descriptors familiar from human sensory analysis. The database contains more than 88,000 components with over 1,900 sensory attributes and descriptors. As in the multi-sensor concept, statistical methods are additionally used in this case to evaluate the chromatograms along the lines of non-target analysis. This system is particularly used in the beverage and packaging industries.

A significantly lower detection limit can be achieved through mass spectrometry, by using an ion mobility spectrometer (IMS), for instance. Combinations of GC with IMS (GCxIMS) and also GC with mass spectrometers (GCxMS) have more than doubled in practical use, particularly in quality monitoring in food processing. The technology’s potential has been further refined in the area of VOC measurements in the gas space above solid and liquid samples.

A variety of food fraud projects have already been undertaken using the ‘FlavourSpec®’ from GAS-Gesellschaft für analytische Sensorsysteme mbH. Besides the classification of olive oils of various qualities, palm oil quality controls have also been undertaken, fake honeys have been determined and the ‘diacetyl’ marker or off-note has also been identified successfully, quickly and reliably in the final brewing process.

IMS essentially consist of an ionisation and a drift chamber. The gaseous analytes enter the ionisation chamber from the chromatographic column in gas form and are ionised there at atmospheric pressure in a two-stage process using a radiation source (the activity of the radiation source lies below the statutory exposure limit). The ions enter the drift chamber via a periodically opening grille and flow along an electric field counter to the direction of flow of the drift gas in the direction of the Faraday plate that is used as a detector. Due to collisions with the drift gas, the various ions achieve different mean drift speeds and enable identification via the specific drift time, which is determined by means of the mass and structure, in addition to detection via the retention index. Special software programs that supplement the analysis system, such as e.g. VOCal from GAS mbH, additionally enable the quantification of individual substances. Alternatively, the IMS from GAS mbH can also be coupled to a high-resolution (HR) GC benchtop system as a detector module (HR-GCxIMS). This enables further optimisation with respect to the selection of the substances by means of program-controlled regulation of the gas chromatographic column’s temperature. In this process, the three-dimensional data sets are also evaluated in-line with the aforementioned statistical methods.

The previous methods enable the identification of the analytes based on database comparisons of retention indices or by measuring reference substances in each case. Clearer detection can be enabled by mass spectrometry methods that use the ‘soft ionisation’ technique as an ion source and are therefore able to selectively detect individual substances in complex matrices without time-consuming chromatographic pre-separation. As the mass-to-charge ratio of the formed ions is measured, the substances can be identified directly.

Depending on the issue, both quadrupole MS and high-resolution time-of-flight MS can be used as mass analysers. Ionisation is the key component in this: in its SICRIT® plug-and-play system, Plasmion GmbH has developed ionisation sources based on dielectric barrier discharge, a cold plasma through which the analytes flow. This enables the sensitive detection of several hundreds of substances in < 1 s in the trace range. Photonion GmbH uses vacuum UV sources and resonance-enhanced multi-photon ionisation (REMPI). Again, data evaluation is carried out as in the other methods here. Numerous other potential applications are currently emerging with respect to automated odour analysis, particularly in in-line use, to support human sensory panellists in their often high quality monitoring workload and to specifically and efficiently prevent food fraud.