Microbore Columns—Design, Construction, and Operation

The factors that determine the length and radius of a liquid chromatographic column are examined theoretically. It is shown that for a given reduced plate height and k' value operating at the optimum practical mobile phase velocity (in this case, 0.15 cm/sec), the column length is determined by the separation ratio of the substances to be separated, whereas the column radius is determined by both the separation ratio of the two solutes and the extra column variance due to the apparatus. In fact, it is shown that the column radius is the column parameter that is used to compensate for poor equipment design, and at the same time, sacrificed mass sensitivity and solvent economy. Three commercially available chromatographs are examined, and it is shown that in all cases the apparatus variance is completely unsuitable for use with microbore columns and even unsatisfactory for use with conventional 4.6 mm i.d. columns for solutes eluted at k' values less than 2. It is suggested that apparatus be designed to provide extra-column variances of less than 1 μl2 (i.e., 0.2 μl2), and such equipment will permit columns of 0.5 mm i.d. to be used and thus increase the mass sensitivity of the column by two orders. At the same time, the solvent consumption will be reduced by the same factor.