Capillary Chromatography - History of Capillary Columns 3
Figure 2. The Separation of the Isomeric Heptanes on a Soft Glass Capillary Column
The column was 50 ft long, 0.005 in. I.D. coated with squalane as the stationary phase and operated at 20ûC. Hydrogen was used as the carrier gas at a linear velocity of 95 cm/s. and an inlet pressure of 57 p.s.i. The efficiency of the solute n-heptane (peak 10) was 17,600 theoretical plates (see Book 6 of this series for the meaning of theoretical plate) and all the isomers of heptane were separated in less than 1 minute. The separation shown in figure 2 employing 0.005 in. diameter soft glass column is at last an order of magnitude better in performance than that obtained from the Nylon column and being glass, can be used readily at elevated temperatures and is thus, far more versatile. Even 43 years later, the separation shown in figure 2 is still probably the fastest separation of this particular mixture that has so far been obtained .
Despite the apparent practical success of the soft glass capillary it still had certain disadvantages. The tube was rigid and. thus, difficult to fit to the chromatographic system (e.g., the sample injector and the detector) and it also possessed significant surface activity that caused solute oxidation, molecular rearrangement together with peak tailing. The rigidity of the column evoked the use of a rather clumsy procedure fro straightening the ends of the column. In addition, glass column were still difficult to coat with and even film of stationary phase. There followed the development of a series of different methods for coating glass capillaries which extended over nearly 20 years. A number of those introduces were quite effective (including etching the surface with acid) but some, to say the least, were exotic and appeared to need the services of occult to be successful. nevertheless, during the period 1960-1979 methods were found to coat most stationary phases on glass capillaries and the columns that wee produced were, in general , chromatographically satisfactory.