Flavor Analysis - American Chemical Society

Chapter 11

Thermo Desorption as Sample Preparation Technique for Food and Flavor Analysis by Gas Chromatography

Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0705.ch011

M . Rothaupt Givaudan Roure Research Ltd., Ueberlandstrasse 138, CH-8600 Duebendorf, Switzerland

Sample preparation is one of the important steps in flavor and food analysis. Thermo extraction of volatile substances simplifies sample preparation by separating volatile compounds from non-volatile compounds. A temperature vaporising program allows an additional pre -fractionation of the analytes. Solvent venting supports the chromatography by removing a large amount of solvent. Solvent venting also helps eliminate interfering and problematic ingredients from the sample. Sample preparation by supercritical fluid extraction (SFE) will be compared to thermo extraction; some specific advantages and disadvantages of both techniques will be discussed.

Existing extraction methods like SFE, solid phase micro extraction (SPME), classical liquid/liquid extraction, and accelerated solvent extraction (ASE) give a variety of different sample preparation choices. Sample preparation via thermo extraction is discussed in this paper. This technique has emerged as a powerful technique for the analysis of food and flavors. It offers a convenient and selective alternative for the preparation of samples for gas chromatographic analysis. Thermo desorption (TO) is the extraction of compounds by their vapor pressure from a non-volatile, complex matrix. In a thermo desorption experiment, the sample is placed into a specifically designed glass tube. This tube is then inserted into the thermo desorption system (TDS ), where the temperatures can be varied from -150 °C to +350 °C by the thermostatic controls. Carrier gas flowing over the sample delivers the volatiles into the G C injection liner. The liner can be maintained at the same temperature as the TDS, or it can be filled with adsorbent material and cooled in order to trap the analytes quantitatively. This additional focusing mechanism is called a cooled injection system (CIS). The key components of a typical TDS are shown in Figure 1.

116

©1998 American Chemical Society In Flavor Analysis; Mussinan, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

117


Instrument Configuration Temperature and carrier gas flow are the most important parameters for thermo desorption. Higher temperatures can be used even when thermally sensitive substances are present. Temperature can be set so that thermally sensitive compounds are desorbed at low temperatures; the desorption temperature can then be increased to facilitate desorption of molecules with low vapor pressure. Flow can be set to such high rates that even compounds with poor volatility, i.e., those with very low vapor pressure, can be transferred to the injector. Desorption of polar compounds with high boiling points, e.g. glycerol (b.p. = 290 °C), is possible with high carrier gas flow. Addition of a solvent venting (SV) feature can reduce chromatographic interference from solvents while the analytes are retained in the CIS. Solvent venting is useful for liquid or complex matrices where solvents can interfere with the gas chromatographic separation of the compounds of interest (see Figure 2). The split valve, of the CIS, is open in solvent venting mode. Thermally extracted volatiles enter the liner, and the analytes are trapped in the CIS, typically at a very low temperature, while the solvent is vented through the split outlet. Venting of the solvent is dependant on temperature and carrier gas flow. Flow rates that are too high will result in loss of analytes. In case the trapping temperature cannot be set low enough, an adsorbent (e.g. Tenax, Porapak) may be placed into the liner to trap the analytes. This action allows analytes to be trapped at a higher (e.g. ambient) temperature (4 5). A further enhancement to solvent venting is solvent venting with stop flow. In stop flow mode the carrier gas flow to the analytical column is suppressed by an open back pressure regulator. The difference between analyses with and without solvent venting can be significant. The presence of water in a sample can be reduced to a level that the interference to the chromatographic separation is minimised. The CIS temperature has to be kept at temperatures above 0 °C when water is present, because at temperatures below 0 °C the liner will be blocked by ice. The range of applications for TDS can be enlarged to include samples with high water content by using an off-line thermo - extraction instrument with a pre-extraction step. This instrument offers the possibility to vent out all problematic solvents in an off line venting procedure. The sample is placed into a larger tube, which can also be used for larger sample volumes (6). The extraction takes place by heating up the thermo extraction tube. The extracted volatiles are then transferred to a TDS tube, which is filled with an adsorbent material. The trapped analytes can then be analysed by the thermo - desorption system as described above. After the desorption process is completed, the "injection" is carried out by heating the liner to transfer the sample to the analytical column. Mild desorption conditions can be achieved by heating in a suitable temperature gradient. The desorption is also very mild when other compounds are present that can form azeotrops with the analytes. If desorption temperature gradients are too mild, peak broadening may occur and has to be taken into consideration when choosing the cryotrap parameter. t

In Flavor Analysis; Mussinan, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

118


pressure]

column

FIGURE 1:. Configuration of a Thermo Desorption Unit

Figure 2. Diagram of a Cooled Injection System (CIS)


119


Thermal Extraction Applications The analysis of fragrance in an air freshener stick, which contained a wet soapy matrix, was analysed without any sample preparation except thermo desorption (Figure 3). This analysis would require extensive sample clean up if carried out by classical extraction. Sample preparation consisted of weighing the sample and putting it into the TDS tube. Weighing was done quickly, because recovery rates are influenced by the time of exposure to air. This parameter was found to have a stronger influence on the loss of highly volatile compounds than break through from the CIS. The fragrance of a shampoo was analysed and results were compared to those from the T D of the shampoo, which used the fragrance. It was observed that most of the fragrance compounds were extracted quantitatively from the shampoo, but a few compounds were hardly recovered. This may have several reasons. Venting time was too long, trap temperature was too high, or incompatibility of the adsorbent material with these molecules. Incompatibility here means that the compounds are not retained on the adsorbent in the trap and are lost through the solvent split, or they are not released from the adsorbent because a strong binding interaction takes place between the compounds and the trapping material. Isolation of autoxidation products in moistening creams, which contain a considerable amount of water, can be carried using the TDS. In this case, the sample is placed into the TDS tube and thermally desorbed using solvent venting to reduce interference from compounds like water. The desorption temperature (80 °C) was low enough not to desorb the fat, and only the analytes were transferred to the injection system. The low content of these analytes could be overcome by using a "large volume" of oil in order to get enough analytes to be determined. The analysis of fresh ginger roots by TDS is an example of the moderate conditions that are applied during sample extraction by thermo desorption (Figure 4). Conventional sample preparation for the analysis of essential oils is carried out by steam distillation. This distillation exposes the analytes to high temperatures for quite a long time in the distillation vessel. Degradation of flavor compounds can easily take place in the distillation flask. Thermo extraction can be used to remove most of the water content in a short period of time. This is necessary because the analysis is carried out on a polar (wax) column. The desorption is so mild that thermally sensitive compounds can be detected at much higher concentrations than expected, based on reports in the literature. Thermo desorption does not require knowledge of the physicochemical properties of the analyte and in most cases leads more quickly to good results. For each single compound extraction efficiency and recovery rate of TDS and SFE have to be determined by spiking the sample with the compound of interest. The amount of spiking material must be in the same range as the compound in the sample.



120

8CG00O-

15.00

Tim«->

20.C0

25.00

joJ. 30.00

35 00

Figure 3. Chromatogram of Air Freshener by TDS

TIC: O I N O E R . D

Abundance

1OOOOO

VJU 15.00

20.00

25.00

Figure 4. TDS Analysis of essential oil


121


Comparison to S F E Supercritical fluid extraction (SFE) is an extraction method comparable to TDS in that it minimises solvent consumption. SFE is particularly effective in extracting hydrophobic compounds (1, 2). Even non-volatile compounds are extractable under very mild conditions. Recoveries are comparable to results obtained with thermo desorption for nonpolar compounds. To extract polar compounds, it is necessary to add a modifier to the supercritical fluid. The modifier is usually the same solvent that is used to rinse the analytes from the traps. Recovery of polar compounds using thermo desorption is also comparable, on a order of magnitude, to SFE. Extracts prepared by SFE are compatible with several separation techniques. Changes in density of supercritical fluids influence the extraction of nonpolar compounds of different molecular weight. These compounds can be fractionated in a short period of time by the correct choice of density. In addition, hydrophobic compounds can be extracted from complex matrices containing sugar, proteins, and fat in almost quantitative amounts using SFE. To illustrate this point, a chromatogram of a SF extract of cookies is shown in Figure 5. Although, the recovery of polar molecules is typically poor for SFE, addition of a modifying solvent significantly improves the recovery of these compounds. Addition of a modifying eluent, e.g. acetonitrile or methanol, opens up a new dimension of compounds that can be extracted by SFE. The selection of substances by polarity can be used as an additionalfractionationparameter. Modifiers are typically added in two ways. First, the modifier can be added directly into the extraction thimble. Second, the modifier can be added by a modifier pump that delivers a constant flow rate and improves extraction efficiency and reproducibility. Modification can also mean addition of an adsorbent to the sample. The adsorbent retains specific substances which make problems for the analysis. For example, extraction of flavors from a fatty matrix can be achieved by adding cellulose powder to the sample. The extraction takes place at low densities. The supercritical carbon dioxide extracts the flavor compounds without any of the problematic sample matrix compounds i.e. fatty acids or triacylglycerols. Analysis of the SF extract can be carried out on any analytical instrument like GC, HPLC, or a capillary electrophoresis. The recovery of terpenes from a hydrophobic matrix isolated by SFE can yield rather high values (80 %). Conclusion TDS as a new powerful sample preparation technique was presented and compared to SFE. The decision for either of the presented sample preparation methods is dependant on the compounds which have to be quantified and also on the sample matrix. With these presented techniques a wide range of compounds can be determined in a reliable and reproducible way.


122

TIC: B N S F E 0 4 . D

Abundance

450000 400000 350000


300000 250000 200000 150000 100000 50000

Tlme->

1

0

.00

15.00

20.00

25.00

30.00

35.00

40.00

Figure 5. SFE Extract of Cookies with high Water Content

Literature Cited 1. Antinescu G.; Doneanu C.; Radulescu V. Flavour and Fragrance Journal 1997, 12, 173 2. Coleman W. M.; Lawrence B. M . Flavour and Fragrance Journal 1997, 12, 1. 3. Reverchon E.; Delia Porta G.; Gorgoglione D. Flavour and Fragrance Journal, 1997,12, 37. 4. Grob K. Analytical Chemistry, 1994, 66 no.: 20, 1009 - 1018 5. Borgerding A. J., Wilkerson C. W., Anal.Chem. 1996, 68, 2874. 6. Mol H. G.-J.; Althuizen M.; Janssen H.-G.; Cramers C. A. J. High Resol. Chromatogr., 1996, 19, 69.


Flavor Analysis - American Chemical Society

Recommend Documents