Volatiles from liquids
Profiling of volatiles and detection of contaminants
A wide range of volatile organic compounds (VOCs) contribute to the characteristic flavours of beverages such as fruit juice and wine. As for food analysis, understanding the VOC profiles of beverages can help manufacturers and suppliers to identify the factors affecting customer perception, and so make improvements to production processes.
Aroma profiling is also valuable to help identify undesirable characteristics such as off-odours and contaminants, whether in the final product or the raw materials – an area that also has relevance to monitoring of water.
However, liquids can be troublesome to analyse because of the need to separate the volatile components from bulk components such as water, ethanol or acetic acid, in order to avoid overload of the analytical system.
Extraction and distillation were for a long time standard approaches to this problem, but these are labour-intensive, and are increasingly being superseded by more convenient sampling techniques (which also lessen the risk of losing the most volatile components).
What Markes can offer
Thermal desorption systems
Sampling vapours onto sorbent tubes, with analysis by thermal desorption (TD), is a useful method for analysing volatiles from liquids, because it provides the high level of concentration enhancement needed for detailed chemical analysis. It also involves minimal sample preparation and is readily automated, and so is a convenient alternative to conventional sample preparation methods.
In addition, Markes’ UNITY-xr and TD100-xr thermal desorbers are particularly well-suited to the challenges of analysis of the volatiles in liquid samples, for the following reasons:
- The short, inert flow path ensures compatibility with reactive compounds such as mercaptans, amines, and oxygenated species, which often have very low odour thresholds.
- The cryogen-free focusing trap allows selective elimination of unwanted, high-concentration interferents including water, ethanol and acetic acid, which might otherwise compromise detection of trace-level target compounds.
- Sorptive extraction – Sorptive extraction, for example using Markes' HiSorb probes, is a good way of extracting the less volatile compounds from aqueous samples. The probes are simply lowered into the liquid sample, agitated for a set time, rinsed with distilled water, dried, and analysed by direct thermal desorption from a TD tube. This has advantages over the related method of solid-phase microextraction (SPME), which is based on adsorption of analytes onto a polymer-coated fibre. This method is convenient, but the small size of the fibre means that it is less sensitive than sorptive extraction.
- Direct desorption – Direct desorption is useful to analyse non-water-based liquids, by extraction of the volatiles from a small sample. The samples is placed on a small piece of glass or quartz wool, which is then inserted into an empty TD tube (using a tube liner if desired). Heating the tube in a stream of inert gas sweeps the volatiles onto the focusing trap of the thermal desorber, for analysis by GC in the usual manner. Applications of this technique include the odour profiling and quality control of cooking oils or essential oils.
|Detecting compounds in complex matrices || |
Aroma profiling of beverages often requires detailed interrogation of complex chromatograms, to confirm the presence (or absence) of key target components, such as trace-level odour-active or taint compounds. It is also often necessary to determine the non-target compounds present, for instance when comparing similar products.
Markes’ TargetView software can help reduce the amount of time spent on these tasks. Through a combination of ‘intelligent’ background compensation, compound deconvolution and library matching, it does away with the need to manually process complex GC–MS profiles, producing a simple report that lists the analytes found in the sample.