Thin-film columns

Thick-film, narrow-bore columns in Table 24-6 provide a good compromise between resolution and sample capacity. They can be used with most detectors (but usually not thermal conductivity or infrared) and with compounds of high volatility. Retention times are longer than those of thin-film columns. Thick-film, wide-bore columns are required for use with thermal conductivity and infrared detectors. They have high sample capacity and can handle highly volatile compounds, but they give low resolution and have long retention times. 

The first application of GLC in carbohydrate chemistry was to separate fully methylated methyl glycopyranosides of simple pentoses and hexoses (10), and much literature now covers these derivatives (II, 12, 13, 14). Shortly after this work, acetate derivatives were examined by various workers (15, 16), and a little later another development was the use of these acetates with thin-film columns containing liquid phases of high thermal stability such as silicone polymers (e.g., SE-30) and fluoro-alkyl silicone polymers (e.g., QF-1) (17). 

Thin film columns (below 0.20 xm) may give poor peak shapes for strongly adsorptive chemicals... 

The stationary phase film thickness of capillary columns range from about 0.1-10 pm and can be divided into three film thickness ranges. Thin-film columns are usually 0.1-0.2 pm and offer the greatest stability. They have smaller sample capacity as compared to the thicker films but are the best for use with high temperatures. 

Fig. 3 shows a 500 juL injection of a test solution (Table 1). There is not a recognizable difference between the results obtained with or without liquid nitrogen, chromatogram 3A 3B respectively. It should be noted that thick film columns result in lower sensitivity and resolution power than thin film columns. 

The most important feature of the capillary column, which makes it differ from other types of columns, is that they have no carrier in their configuration. There are two types of capillary columns thin film columns and thin layer columns. 

The inner wall of the thin film column is covered with a thin film of the liquid phase. The thickness of this film can vary from 1 to 3 im. One example of the inner arrangement of the thin film column is shown in Figure 2.2. 

Generally, the smallest amount of sample injected will give the best separation performance on thin-film columns. Thin films of stationary phase are easily destroyed or their polarity altered by overheating or by injecting "dirty samples that contain nonvolatile and/or particulate residues. However, many workers point out that it is easier and faster to break off the first few coils of the column and reinstall it rather than to do extensive sample cleanup. For best separation efficiency, good retention time reproducibility, quantitative accuracy and precision, and to... 

Figure 5.9 (i) Free fatty acids analysed on a wide bore very polar thin film column using helium carrier gas and direct injection. Stabilwax-DA, 30m, 0.53mm i.d., 0.25pm, KKfC, 2min, to... 

Figure 5.9 (j) Basic drugs analysed on a narrow bore thin film column using helium and polar stationary phase. Rtx-200, 30m, 0.25mm i.d., 0.25pm, 100-325°C, 4°Cmin, He. 

Equation [4] is very useful for deducing the general operating requirements of high-speed analysis. The use of a short column, a higher than usual carrier gas velocity, and relatively small retention factors (which can be easily achieved by using high temperature and/or thin film columns), can reduce analysis times... 

Thin film columns are useful for high-boiling compounds, trace analysis, and for instanees when column bleeding must be minimized at high temperatures. 

Thin-film columns provide higher resolution of high-boiling solutes but lower resolution of more volatile components under any set of column temperature conditions. 

The sample capacity of thin-film columns may be inadequate and require cryogenic temperature control of the column oven. 

A solute will exhibit a lower elution temperature as film thickness decreases thus, thin-film columns are ideal for high-boiling petroleum fractions, triglycerides, and other compounds. 

Figure 5.2 shows plots of H versus u for 10-m-long, thin-film columns using hydrogen carrier gas at 50°C and assuming a retention factor of 2.0. A Dq value of 0.4 cm /s was used for all plots. Plots are shown for column diameters of... 

FIGURE 5.3 Golay plots for 0.20-mm-i.d thin-film columns of various lengths using hydrogen as carrier gas. A binary diffusion coefficient of 0.4 cm /s and a retention factor of 2.0 are assumed. 

FIGURE 5.13 Temperature-programmed chromatograms of an n-alkane mixture (n-C7-n-C19) with a programming rate of 50°C/min (a) 25-m-long, 0.25-mm-i.d. thin-film column with an average carrier-gas velocity of 100 cm/s (b) 4.0-m-long, 0.25-mm-i.d. thin-film column with a gas velocity of 200 cm/s. 

An example is shown in Figures 5.21 and 5.22 where a 20-component mixture is separated isothermally in about one minute using a 12-m-long, 0.25-mm-i.d. thin film column ensemble consisting of 6.0 m of a nonpolar dimethyl polysilox-ane column Ca followed by 6.0 m of a polar polyethylene glycol column Cb. The plots of solute band position versus time were obtained by spread sheet calculations, which use as input retention factors for all components on the individual columns as well as the column dimensions, the inlet, outlet (1.0 atm) and junction-point pressures and the carrier-gas (hydrogen) viscosity at the column temperature. The slight curvature in the plots is the result of carrier-gas acceleration from inlet to outlet. 

Figure 5.24 shows the high-speed separation of a 20-component pesticide mixture (plus one impurity peak). The 14-m-long, 0.18-imn-i.d. thin film column ensemble consists of 7.0 m of a trifluoropropylmethyl polysiloxane column followed 7.0 m of 5% phenyl dimethyl polysiloxane column segment. For chromatogram (a), no stop-flow pulses were used, and component pairs 2,3 and 10,11 coelute. For chromatogram (b), a single 2-s wide stop-flow pulse was used to enhance the resolution of peak pair 2,3. Note that the peak pattern and resolution... 

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