GC analysis

Gas chromatographic analysis of FFAs isolated from dairy products has been preformed by a number of different techniques. Examples of 

TAGs can transfer to the lower phase and be converted to methyl esters, giving false high results. 

The methods described above have been used principally to quantify FFAs in cheese, but can be used for other milk products with some slight modifications. All the above methods use internal standards (typically FFAs which are not present in milk fat), and the recovery of all FFAs is based on the recovery of these internal standards. It is best to use both volatile and non-volatile FFAs as internal standards. Currently, the International Standard for the extraction of lipids and lipo-soluble compounds from milk and milk products is ISO 14156 (ISO, 2001) and involves solvent extraction. Determination of the fatty acid composition of milk fat involves the preparation of fatty acid methyl esters (FAME) by transesterification (ISO, 2002a), followed by quantification by GC (ISO, 2002b). 

---

Caffeine in coffee, tea, and soda is determined by a solid-phase microextraction using an uncoated silica fiber, followed by a GC analysis using a capillary SPB-5 column with an MS detector. Standard solutions are spiked with G3 caffeine as an internal standard. 

Gas chromatography (gc) has been used extensively to analyze phenoHc resins for unreacted phenol monomer as weU as certain two- and three-ring constituents in both novolak and resole resins (61). It is also used in monitoring the production processes of the monomers, eg, when phenol is alkylated with isobutylene to produce butylphenol. Usually, the phenoHc hydroxyl must be derivatized before analysis to provide a more volatile compound. The gc analysis of complex systems, such as resoles, provides distinct resolution of over 20 one- and two-ring compounds having various degrees of methylolation. In some cases, hemiformals may be detected if they have been properly capped (53). 

Owing to poor volatihty, derivatization of nicotinic acid and nicotinamide are important techniques in the gc analysis of these substances. For example, a gc procedure has been reported for nicotinamide using a flame ionisation detector at detection limits of - 0.2 fig (58). The nonvolatile amide was converted to the nitrile by reaction with heptafluorobutryic anhydride (56). For a related molecule, quinolinic acid, fmol detection limits were claimed for a gc procedure using either packed or capillary columns after derivatization to its hexafluoroisopropyl ester (58). 

To analyze pesticides from the sample, several GC techniques were used GC with FID and EC detectors and GC/MS with external standards. Pesticides are mostly analyzed using split/splitless technique where higher amount of injected solution exits the gas chromatograph without decomposing therefore by quantification of the several pesticides in the filter, we found out how harmful is exposing analysts to pesticide compounds during the GC analysis. 

Phenols hold an important place among organic pollutants, which need to be constantly monitored in waters and in places of militai y activities. Sampling of phenol matrix is conducted with solid face extragents (SPE) with further HPLC or GC analysis. Application of the known SPE usually is ineffective as it doesn t give the possibility to provide full extraction of the analyt (microcontents) in the matrix media. Therefore SPE application needs further progress in their selectivity. 

The container was sealed with a serum cap and thermostatted at 25°C. 100 mg aliquots of the solute were added sequentially to the mixture by means of a hypodermic syringe. After each addition, the container was shaken, thermal equilibrium allowed to become established over a period of about 30 minutes and then a 5 pi sample of the solvent taken for GC analysis. Corrections were made for... 

The amount of gas employed in a GC analysis is not usually important, particularly where open tubular columns are used. In LC, however, solvent use presupposes a solvent disposal difficulty if not a toxicity problem and, thus, solvent consumption can be extremely important. 

M. Biedermann, K. Grob and W. Meier, Partially concunent eluent evaporation with an early vapor exit detection of food uxadiation tltiough coupled LC-GC analysis of the fat , 7. High Resolut. Chromatogr. 12 591-598 (1989). 

K. Grob and J. M. Stoll, Loop-type interface for concuirent solvent evapoi ation in coupled HPLC-GC. Analysis of raspbeiry ketone in a raspbeiry sauce as an example , 7. High Resolut. Chromatogr. Chromatogr. Commun. 9 518-523 (1986). 

Figure 3.5 Two-dimensional GC analysis of tobacco essential oil using non-polar primary and polar secondary separ-ations. The top tr-ace indicates the primary separ-ation, with the four resulting heart-cut cliromatograms shown below being obtained on the transfer of approximately 1-2 min fractions of primary eluent. Reproduced from B.M. Gordon et al. J. Chwmatogr. Sci. 1988, 26, 174 (23).

Figure 4.13 GC X GC analysis of vetiver essential oil column 1, BPX5 column 2, BPX50 (0.8 m in length). The lower trace presents the pulsed peaks obtained from the modulation process, and shows such peaks in a manner that represents the normal cliromatograpliic result presentation. Tliis nace is many times more sensitive than a normal GC trace. In the upper plot, the 2D separation space shows that the BPX50 column is not very effective in separating components of the oils based on polarity, since all the components are bunched up along the same region of 2D time.

There are reported to be a number of important characteristics of GC X GC that permit more reliable peak response quantitation over single-column GC analysis. These are as follows ... 

GC using chiral columns coated with derivatized cyclodextrin is the analytical technique most frequently employed for the determination of the enantiomeric ratio of volatile compounds. Food products, as well as flavours and fragrances, are usually very complex matrices, so direct GC analysis of the enantiomeric ratio of certain components is usually difficult. Often, the components of interest are present in trace amounts and problems of peak overlap may occur. The literature reports many examples of the use of multidimensional gas chromatography with a combination of a non-chiral pre-column and a chiral analytical column for this type of analysis. 

On-line LC-GC has frequently been used as a clean-up technique for the analysis of trace levels of contaminants (pesticides, plasticizers, dyestuffs and toxic organic chemicals) in water and food products. Several different approaches have been proposed for the analysis of contaminants by on-line LC-GC. Since pesticide residues occur at low concentration in water, soil or food, extraction and concentration is needed before GC analysis is carried out. 

Analogous to HPLC-HRGC, the combination of packed column SEC and capillary column GC can be used for the analysis of complex samples. The advantage of SEC/GC with respect to HPLC/GC is the absence of problems associated with the evaporation of the HPLC mobile phase prior to the GC analysis. 

Figure 11.12 GC analysis of (a) urine sample spiked with opiates 3 p.g/ml) and (b) blank urine sample. Peak identification is as follows 1, dihydrocodeine 2, codeine 3, ethylmor-phine 4, moipliine 5, heroin. Reprinted from Journal of Chromatography, A 771, T. Hyotylainen et al., Determination of morphine and its analogues in urine by on-line coupled reversed-phase liquied cliromatography-gas clrromatography with on-line derivatization, pp. 360-365, copyright 1997, with permission from Elsevier Science.

Figure 12.5 Effect of shifting the time window for the ti ansfer. Operation in the SEC-GC analysis of polymer additives in a poly styene matrix, shown foi the following fractions ...

The preseparation utilized a 5 pim cyano column (250 cm X 4.6 mm i.d.) and a 5 p.m silica column (250 cm X 4.6 mm i.d.) in series, followed by GC analysis on an SE-54 column (25 m X 0.2 mm i.d., 0.33 p.m film thickness). The SFC system separated the aviation sample into two peaks, including saturates and single-ring aromatics as the first peak, and two-ring aromatic fractions as the second peak. These fractions were selectively cut and then transferred to the GC unit for further analysis. (Figure 12.20). 

Figure 12.20 SFC-GC analysis of a sample of aviation fuel (a) SFC separation into two peaks (b and c) coixesponding GC ttaces of the respective peaks (flame-ionization detection used throughout). Reprinted from Journal of High Resolution Chromatography, 10, J. M. Levy et ah, On-line multidimensional supercritical fluid chromatography/capillary gas chromatography , pp. 337-341, 1987, with permission from Wiley-VCH.

Figure 12.22 SFC-GC analysis of aromatic fraction of a gasoline fuel, (a) SFC trace (b) GC ttace of the aromatic cut. SFC conditions four columns (4.6 mm i.d.) in series (silica, silver-loaded silica, cation-exchange silica, amino-silica) 50 °C 2850 psi CO2 mobile phase at 2.5 niL/min FID detection. GC conditions methyl silicone column (50 m X 0.2 mm i.d.) injector split ratio, 80 1 injector temperature, 250 °C earner gas helium temperature programmed, — 50 °C (8 min) to 320 °C at a rate of 5 °C/min FID detection. Reprinted from Journal of Liquid Chromatography, 5, P. A. Peaden and M. L. Lee, Supercritical fluid chromatography methods and principles , pp. 179-221, 1987, by courtesy of Marcel Dekker Inc.

Liquid chromatography (LC) is a good alternative to GC for polar or thermolabile eompounds. While polar eompounds need to be derivatized for GC analysis, this is therefore not neeessary for LC analysis. 

Chlorophenoxy acids are relatively polar pesticides which are usually determined by LC because volatile derivatives have to be prepared for GC analysis. This group of herbicides can be detected by multiresidue methods combined with automated procedures for sample clean-up, although selectivity and sensitivity can be enhanced by coupled-column chromatographic techniques (52). The experimental conditions for Such analyses are shown in Table 13.1. 

A more sophisticated method, giving a much more detailed characterization, involves the on-line coupling of EC and GC (LC-GC). Analysis schemes for middle distillates (kerosine, diesel and jet fuels) combining EC and GC have been reported by various authors (25-31). However, only Davies et al. (25) andMunari et al. (27) have reported on the required automatic transfer of all of the individual separated fractions from the EC unit the GC system. Davies used the loop-type interface and Munari the on-column interface. Only Beens and Tijssen report a full quantitative characterzation by means of LC-GC (31). 

LC-GC, therefore, shows promise for forensic science applications, reducing sample handling and preparation steps by essentially using an on-line LC column in place of one or more extraction steps. This is followed by a traditional high resolution GC analysis. The methods described here for pesticides and hormones could be readily adapted to a variety of analyses, especially those involving fatty matrices. Such as tissues, food or blood. 

Figure 15.8 shows the multidimensional GC analysis of urinary aeids, following lyophilization and derivatization by methyl ehloroformate. In this figure, ehromatogram (a) shows the eomplexity of the urine matrix and the need for a seeond separation dimension. A heart-eut is taken over a small range at about 45 min. The... 

---