Thick-film columns

GC peak broadening Thick film column is used. Thick film should... 

Figure 8.19 Two-diaenslonal separation of the components of a coal derived gasoline fraction using live switching. Column A was 121 n open tubular column coated with poly(ethelene glycol) and column B a 64 m poly(dimethylsiloxane) thick film column. Both columns were temperature programmed independently taking advantage of the two oven configuration. Peak identification 1 acetone, 2 2-butanone, 3 > benzene, 4 isopropylmethylketone, 5 isoprop-anol, 6 ethanol, 7 toluene, 8 => propionitrile, 9 acetonitrile, 10 isobutanol, 11 — 1-propanol, and 12 = 1-butanol. (Reproduced with permission from Siemens AG).

The medium-film thickness is about 0.3-0.6 pm and generally offers the best compromise of sample capacity, retentivity, and phase stability. The phase ratio determines the capacity of the column and influences its retentivity of solutes. The phase ratio (j8) can be defined as the ratio of the inner column radius to that of the product of twice the stationary-phase film thickness or 0 = r/2df. We can now also use phase ratios to group film thicknesses and now say that thick-film columns have phase ratios of less than about 80. (In capillary SFC the typical stationary-phase film thicknesses are 0.1-0.3 pm.) The effective phase ratio can change in capillary SFC, depending on the characteristics of the stationary phase and the operating density [57]. The change in phase ratio can be attributable to a swelling of the stationary phase under certain SFC conditions. 

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. 

Fig. 4 illustrates the results obtained under these conditions with the 5.0 im thick film column mentionad previously.. 4gain a retenzion gap was used just prior to the analytical colunm. 

It is particular concern in the analysis of aqueous samples, because the solvent effects involved during the direct aqueous injection (DAI) allow the direct analysis of levels ()ig l) of trihalomethanes and related compounds in water, using well deactivated thick film columns and E(D detection [19, 20 ]. 

It is interesting that while hm n is smaller for low-k solutes in open tubular columns of conventional film thickness, in these very thick film columns the reverse is true h, is now smaller for high-k solutes (Figure 6). 

Another point of contention relates to column bleed in general, thick film columns have higher bleed rates, and some argue that the bleed rate is directly proportional to the stationary phase film thickness (11). This generalization was once correct, but columns prepared with high purity polymers that are extremely clean, and bonded to properly deactivated tubing, do not exhibit this limitation unless the columns are abused. Some bleed problems can be associated with residues from "dirty" samples that remain on the column, while others are attributable to... 

A more detailed look shows that with Increasing film thickness the efficiency increases, but at the same time the selectivity decreases. This is particularly so for the separation of peaks B and C. However, the resolution is increased for volatile substances when the film thickness is increased. With rather thick film columns it is possible to separate the main constituents of earth gas as is demonstrated in "Figure 7". 

The relationship between capillary column inside diameter, column efficiency, expressed as the number of theoretical plates per meter (N/L), and film thickness is illustrated in Figure 7. These curves represent the theoretical limits of column efficiency for a given column inside diameter and stationary phase coating. The very thin-film, narrow-bore columns will be the most efficient and their efficiency changes markedly as the film thickness increases. The efficiency of a 0.25-mm i.d. column with a stationary phase film thickness of less than 1 pm is shown to be limited by mass transfer in the mobile phase (designated by Cg from the van Deemter equation), whereas mass transfer in the stationary phase (CL) becomes predominate in thick-film columns. The point at which Cg = CL is shown for narrow- and wide-bore capillary columns in Figure 7. 

Figure 5.9 (f) Sulphur compounds using a flame photometric detector and direct injection onto a wide bore thick film column. Rtx-1, 60m, 0.53mm i.d., 7.0ixm, 50°-200°C, 15°Cmin, He. 

The test is performed under optimized conditions of carrier gas flow and temperature program rates, which are adjusted for column length and carrier gas viscosity. Table 2.16. To obtain a correct value for the gas hold-up time for thick-film columns (df > 0.7 p,m) it should be measured at 100°C (methane is considerably retained at... 

Another general advantage of immobilization is that it enables thick-film columns to be prepared to optimize the phase ratio for different applications. It is difficult to prepare conventionally coated columns with films thicker than 0.5 pm. Immobilized films of 1-8 pm are easily prepared using cross-linking reactions. 

Thick film columns should be used when higher capacity and longer column lifetime is desirable. Normally, thick films are used with wide-bore eolumns. 

A thick-film column (which inherently are more inert) should be utilized for samples having a range of solute concentrations. Thicker films of stationary phase (>1 p.m) should be used for analysis of more volatile solntes. Very thick films (>5 ttm) should be selected for analyses to be performed at room temperamre. 

Keywords air SUMMA canister thermodesorption volatile halogenated hydrocarbons cryofocusing water removal thick film column. 

Dimensions 60 m length X 0.25 mm ID X 1.0 pm film thickness The thick film columns DB-1 or DB-5 guarantee chromatographic separation even at start temperatures just above room temperature if cryofocusing is used... 

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