Open tubular columns advantage

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).

Packed columns, being easier to prepare than semi-packed columns, offer a more feasible way to avoid many of the problems of open tubular columns while still retaining the advantages of... 

Another advantage of open tubular columns is that most separations can be performed at a lower temperature than with packed columns. 

Open tubular columns enable the chromatographer to effect separations impossible by other techniques, for example, separation of isomers. Their use permits a smaller inventory of columns and a smaller selection of stationary phases. Their main advantage is their high efficiency and their disadvantage is their small load capacity. 

The need to measure concentrations in very small volumes is not restricted to biological systems. For example, open tubular columns for liquid chromatographic separations offer the advantage of increased resolution, but because their internal diameters may be as small as 15 pm, the amount of material in the eluted peaks is very small. Thus, the use of these columns requires detectors that can be used with low concentrations in small volumes. Jorgenson and co-workers showed that this could be accomplished by the insertion of a 10-pm-diameter, cylindrical electrode made from a carbon fiber into the end of the column [4]. The close fit between the column wall and the fiber ensured that a large fraction of the eluting molecules were electrolyzed. When the electrochemical data were collected in a voltammetric mode, the resolved compounds could be classi-... 

Despite many advantages, CEC columns packed with microparticulate sorbents do have some limitations such as the relatively large void volume between the packed particles and the slow diffusional mass transfer of solutes into the stagnant mobile phase present in the pores of the separation medium [83,84]. Alternative approaches to alleviate the problem of mass transfer and intraparticular void volume are the concepts of monolithic chromatographic beds and open-tubular columns. In mono-... 

For packed columns, the most important variable thai affects column efliciency is the diameter of the particles making up the packing. For open tubular columns, the diamelcr of the column itself is an important variable. The effect of particle diameter is demonstrated by the data shown in Figure 26-11 for GC. A similar plot for LC is shown in Figure 28-2. To take advantage of the effect of column diameter, narrower and narrower columns have been used in recent years. 

The retention index system has the advantage of being based on readily available reference materials that cover a wide boiling range. In addition, the temperature dependence of retention indexes is relatively small. In 1984 Sadtier Research Laboratories introduced a library of retention indexes measured on four types of fused-silica open tubular columns. The computerized format of the database allows retention index searching and possible identity recall with a desktop computer. Measurement of retention indexes is the basis of the Rohrschneider-McReynolds scheme for classification of stationary phases in GC (see Section 27C-4),... 

---