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columns (29). Because of their increased sample capacity, these wider diameter columns could be used with direct sample introduction (without any split) and even with standard thermal-conductivity detectors. In addition, because of the smaller capacity factors (due to the higher phase ratios), these columns were particularly suited for the rapid analysis of highboiling compounds. For example, Quiram could analyze n-cetyl alcohol in 8 min at 175 °C (i.e., 169 °C below its boiling point) using a 250-ft X 1.55-mm i.d. column and a flow rate as high as 800 mL/min (30). This result, of course, is not surprising; in fact, in 1960 Golay had pointed out (31) that the pipeline from Texas to Maine (which certainly does have a very high phase ratio) actually may be an excellent "open-tubular column" for highboiling compounds. Until now, I have surveyed only the development of columns with a wider diameter. The pioneering work on small-diameter (less than 0.2 mm) columns was carried out by Desty in 1960-61 (10, 32). Until recently, very little work was done in this field, mainly because of the unavailability of such tubing and problems with the introduction of extremely small samples. However, over the past five years, there has been renewed interest in such columns. With regard to the length of opentubular columns, for more than two decades most were made of relatively long tubes—usually 100-300 ft. Some were even longer. The record in column length belongs to Al Zlatkis; in 1959, he prepared and tested a onemile-long open-tubular column made of nylon tubing with an i.d. of 1.676 mm, and obtained an HETP of 1.61 mm (33). Shorter columns have also been used since the beginning of open-tubular column GC, although less frequently. I have already mentioned that Golay's first columns were less than 20 ft long. Desty has also reported on short (1-10 m) columns (10,32). In 1964 Marco demonstrated the analysis of C1-C12 alcohols on a 9-m X 0.50-mm i.d. column in less than 5 min (34). In our laboratories, Johansen carried out a detailed study in 1977 (35) that demonstrated some possibilities for short (10-15 m) open-tubular columns. They greatly reduced the analysis time while still providing adequate efficiency. Such short columns, especially those with 0.50-0.53-mm i.d., are of particular interest now as a replacement for packed columns in routine applications.
Coating technique and liquid phases Basically, two methods have been used to coat the inside wall of the col-
CIRCLE 32 ON READER SERVICE CARD 1428 A · ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
umn tubing with liquid phase: the dynamic and the static method. In both cases, the liquid phase is in a solution. In the dynamic method, the solution is passed through the tube, wetting the inside surface; in the static method, the tube is completely filled with the liquid phase and the solvent is allowed to evaporate slowly. The dynamic method was first mentioned by Dijkstra and De Goey (36); the static method was first used by Golay (4). Later, Bouche and Verzele (37) simplified the static method by using a vacuum to evaporate the solvent slowly from the filled column tube. Both the dynamic and static methods exist in many variations. Just as with packed columns, commonly available organic compounds, such as plasticizers, hydrocarbons, polyalcohols, and polyesters, were used at the beginning as the liquid phase for open-tubular columns. The basic problem with these, however, is their relatively low molecular weight (resulting in excessive bleeding) and the presence of impurities. Slowly, higher molecular weight polysiloxanes (silicones), made specifically for gas chromatography, replaced the older phases, thereby extending the usable upper temperature limit. However, an apparent contradiction soon became evident: To reduce bleeding and further extend the upper temperature limit, high-molecular-weight polymers were desired. On the other hand, the higher the molecular weight of the polymer, the poorer its solubility. The solution to this problem is to coat the column with a relatively low molecular weight substance and then repolymerize this prepolymer in the column, creating a high-molecularweight, cross-linked liquid phase. The final polymer coating can also be partially bonded to the Si-OH groups of the inside tube surface. Madani, in 1976 (38,39), was the first to prepare such columns, and he was soon followed by a number of other researchers including Blomberg, the Grobs, Lee, Lipsky, Sandra, and Schomburg. This development is still continuing.
Film thickness In the past, relatively little was done on controlled changes to the liquidphase film thickness. This was mainly for three reasons: • For a long time, the dynamic coating method was used most frequently. With this method, it is difficult to know the actual film thickness. • Until the advent of immobilized, cross-linked stationary phases, films thicker than about 1 Mm were unstable: The excess phase was soon lost through bleeding. • Metal columns had to be coated with a somewhat thicker film to re-
