Capillary Chromatography - History of Capillary Columns 1
History of Capillary Columns
There were no commercial capillary columns available in 1958 and all operational columns were 'home made'. In fact, the basic characteristics of capillary columns and their operating conditions were determined, almost exclusively, using these home made columns. The first columns were made from copper capillary 1/16 in. O.D. and 0.010 I.D. which was readily available commercially and relatively inexpensive. The standard length then was 100 ft and the stationary phase was coated on the walls of the column from a solution in a volatile solvent. It was soon found that only dispersive stationary phases (see book 7 of this series) such as Apiezon Grease or squalane could be made to form a stable film on the copper surface particularly at higher temperatures. Polar stationary phases such as dinonyl phthalate or polyethylene glycol would not form a stable film and degenerated into tiny drops on the column wall particularly as the column temperature was raised. These drops dramatically increased the high resistance to mass transfer in the stationary phase and greatly reduced the efficiency. The next type of column material to be examined was cupro-nickle but, although satisfactory for dispersive stationary phases, also produced unstable films with polar materials. Stainless steel proved little better and the surface of the stainless steel exhibited catalytic activity resulting in some substances being oxidized and others to suffer molecular rearrangement. At that time only dispersive and some semie-polar substances could be used as stationary phases. In 1957 Scott (2) demonstrated the effective use of Nylon columns for use as capillary columns. He found that they coated well with squalane and dinonyl phthalate and could be used in extremely long lengths (e.g. 1000 ft) to produce very high efficiencies. A column 10000 ft long 0.020 in. I.D. was shown to give efficiencies of up to one million theoretical pates. Nylon, although an interesting material and due to its ready availability was a simple way of producing long columns and high efficiencies, is not a suitable material for general use due to its operating temperature being limited to below 120ûC. Nevertheless, some startling separations were obtained from the Nylon Columns. The separation of a mixture of hydrocarbons is shown in figure 1 originally presented on a suitably modified cathode ray oscilloscope.
The column was 274 cm long, 0.020 in. I.D., carrying a film of squalane 0.47 mm thick and operated at room temperature (ca 23ûC). n Pentane was eluted in 13 seconds. and the total scan period was 26 second. It is seen that under certain circumstances the Nylon column can be very effective for fast separations and, in fact, this separation was one of the first fast GC separations of its kind.