When relating gas chromatography to catalytic transformations of bio­mass, it can be stated that GC (Figure 2) provides qualitative and quantita­tive determination of organic components such as extractives, hemicel-

lulose building blocks, organic acids, etc. The derivatized and vaporized products are introduced to the column for separation and identified in a detector, whose response is recorded as a chromatogram. Capillary col­umns made of fused silica with a stationary phase as a thin film of liq­uid or gum polymer on the inside of the tube are mainly used. The most commonly utilized stationary phases are siloxane polymer gums with dif­ferent substituents providing different polarity. The polymers are usually cross-linked in the column by photolytic or free-radical reactions, bringing strength to the polymer films. Wall-coated open-tubular columns with a liquid phase coated directly on the inner walls, as well as support-coated open-tubular columns are applied. In the latter case a stationary phase is coated on fine particles deposited on the inner walls. Among non-polar columns, HP-1, DB-1, etc., based on dimethyl, polysiloxane could be mentioned. HP-5 with 95% dimethyl polysiloxane and 5% phenyl groups is slightly more polar. Still more polar columns employ polyoxyethylene or polyester liquid phases.

Capillary columns are available in a wide range of internal diameters, lengths and liquid film thicknesses (Figure 2). Although longer columns provide better separation, they have an increased analysis time which is usually undesired. In addition, longer columns lead to higher pressure and thus to problems with the injection. Columns with thicker films have higher capacity, but usually require higher temperature, while thin-film columns are suited for large molecules with low volatility. In principle, analysis of components with up to 60 carbon atoms is possible.

Different types of injection systems are used in GC. Split mode, where the injected material after evaporation is split between the column and an outlet, affords rapid volatilization and homogeneous mixing with the car­rier gas. Most of the sample will pass out through the split vent and only a small proportion will flow into the column. Splitless systems provide a more reliable quantification allowing analysis of even such high-molec­ular mass compounds as triglycerides and steryl esters. Flame ionization detectors, which are of destructive nature, have high sensitivity to hydro­carbons, but are not able to detect water. On-line coupling of capillary columns with mass spectrometers is routine nowadays and enables conve­nient structure identification.

An important but sometimes forgotten issue is the fact that the sen­sitivity for different compounds is varying for a detector; thus, different peak areas are in proportion to the weight concentration. Knowledge of re­sponse factors is therefore necessary and calibration for components espe­cially with various functional groups should be properly done. Commonly, internal standard compounds are applied, e. g., compounds which are not present in the sample itself are purposely added. Chemically they should be similar to the sample compounds with close retention time, however, with no peak overlapping (Figure 3).

In addition to such advantages of GC as accurate quantification based on internal standards, a possibility to be combined with a mass spectrom­eter and complete automation regarding injection and analytical runs, the very high resolution should also be mentioned. On the other hand only molecules up to about 1000 mass units can be analyzed, as they should be stable at high temperatures. Therefore, sometimes samples should be processed before the analysis. The last point is important for polar com­pounds, like for example acids, which should be derivatized. GC and GC-MS analysis in the vapour phase require volatile derivatives that do not adsorb onto the column wall. Different derivatizations for different substances are recommended, e. g., silylation or methylation for extrac­tives, methanolysis and silylation for carbohydrates. Silyl derivatives of R-O-Si(CH3)3 type containing a trimethylsilyl group (TMS) are formed by the displacement of the active proton in — OH, — NH and — SH groups. Thus, protic sites are blocked, which decreases dipole-dipole interactions and increases volatility. Common silylation reagents are listed in Figure 4.

Methylation relies on the following reactions: utilization of diazo- metane (CH2N2): R-COOH + CH2N2 = RCOOMe + N2; acid-catalyzed esterification: R-COOH + ROH => RCOOR’, as well as on-column es­terification using tetra — methyl ammonium salts R-COOH + N+Me4OH — => RCOOMe.

One of the variants of GC is associated with coupling pyrolysis to it (Figure 5). In this arrangement the sample is thermally degraded in an inert atmosphere. The degradation products are introduced to GC or GC — MS for separation and identification allowing qualitative and quantitative determination of semi-volatile and non-volatile components, such as ex­tractives, polymers, paper chemicals, and lignin, etc.