Instrumental analysis of taints

4.6 Instrumental analysis of taints

  The technique of choice is GC-MS. This technique has advanced steadily since the early 1990s. Sensitivity and spectral quality have improved dra- matically. Electron impact mass spectral libraries have greatly increased in size. Advances in capillary gas chromatography columns with low bleed and high resolution have provided the ability to separate relatively easily a large number of chemical compounds. These compounds can then be reliably identified by computer comparison against library spectra. Identifications should still be confirmed by running a pure standard and also by compar- ing retention times and mass spectra. Occasionally the libraries are wrong or the spectrum obtained on the instrument is different to that in the library because it may have been obtained on a different type of instrument.

  A choice of instrument types is available: quadrapole or ion trap. Mass spectra obtained using different mass spectrometers are not always identical for a particular compound. Ion traps are more sensitive than quadrapoles and there is an ability to carry out MS-MS operation and observe the fragmentation of daughter ions. Chemical ionisation (CI) is a technique where a molecular ion can be obtained from easily fragmentable molecules such as ethers by the gentle ionisation obtained by using chemi- cal reagent gases such as ammonia or methane. It is useful for compounds that easily fragment where electron impact ionisation does not provide molecular ions or unique characteristic spectra.

  Infrared (IR) scanning detectors are also available. The detector is non- destructive and GC-MS-IR instruments can be purchased where the sample passes through the infrared detector first before passing into the mass spectrometer. This provides an additional means of identifying compounds. Unfortunately the gas phase IR spectra obtained are different from the commercially available spectral libraries, with the result that suitable reference libraries are limited. In addition, the sensitivity of the detector is not usually adequate. The IR detector also adds considerably to the cost of the equipment.

  If odours are to be investigated, it is essential that the equipment be fitted with an odour port. This involves the use of a split at the exit of the ana- lytical column before the entrance to the mass spectrometer. This is best achieved by the use of a zero volume T piece, situated inside the gas chro- matograph oven. One arm of the T piece is connected to the odour port and the other is connected to the mass spectrometer. A simple design is described in the literature (Acree et al., 1976). The art of GC olfactometry has been refined over the years (Acree et al., 1984; 1997). The odour port should consist of a heated deactivated silica connection to the T piece. Humidified air is mixed with the gas eluting from the analytical column and presented to the human nose in a glass pocket-shaped vessel. The balance of gas flow is an important consideration. Insufficient flow to the mass spec- trometer results in reduced sensitivity from the mass spectrometer.Too high

  Packaging materials as a source of taints 81

  a flow rate to the mass spectrometer and not enough to the odour port results in no odours being smelt. Although it is possible to install a variable splitter valve to adjust the split ratios, this can result in peak broadening and reduced peak resolution.The easiest approach is to allow the mass spec-

  trometer to have its optimum flow (typically 1 ml min -1 ) and split the rest of the flow to the odour port. This is achieved using a length of deactivated

  silica tubing as a restrictor into the mass spectrometer (1.4 m for a 0.150 mm ID (internal diameter) restrictor). The odour port is connected by means of wide bore tubing. Alternatively a 5050 split can be set up by connecting equal lengths of restrictor to the mass spectrometer and odour port. A make up gas can be introduced at the split to increase the velocity of the flow to the nose port. This can enable simultaneous detection at the odour port and mass spectrometer. This equipment can all be purchased off the shelf from laboratory equipment suppliers.

  The use of an odour port greatly assists an odour investigation. This is another reason why odour investigations are usually easier than taste prob- lems. Identification of the cause of a taste requires the food to be fortified with the suspected tainting compounds. Tasting chemicals in this manner raises issues of safety. The only other solution is comparison of concentra- tions to taste thresholds in the literature. Unfortunately there is a dearth of comprehensive taint threshold data.

  Figures 4.1 and 4.2 illustrate the basic steps in investigating undesirable tastes and odours. The process starts with a sensory panel test by experi- enced panellists to establish a reliable description of the taste. An accurate description is invaluable in choosing subsequent lines of investigation. Samples of non-odorous and odorous packaging are then prepared using such techniques as steam distillation (a standard technique here is the Lickens–Nickerson technique or solid phase microextraction (SPME). Preparation techniques are discussed in section 4.7. They can then be analysed by GC-MS to produce chromatograms which can be compared in order to isolate a tainting compound.