Paris D. N. Svoronos and Thomas J. Bruno
The pressure drop across an open tubular or capillary column is often important for optimization of gas chromatographic analyses (Refs. 1,2). Column performance is typically assessed by the height equivalent to a theoretical plate (HETP), which is based on the average linear carrier gas velocity. As the average linear velocity increases, the head pressure and carrier gas flow rate increase as well. One may express the pressure drop across the column as:
Δp = pi - po
where Δp is the pressure drop, pi is the inlet or head pressure, and po is the outlet pressure. The head pressure is typically a gauge pressure measured electronically, while the outlet pressure is the barometric pressure, which can be measured (a) electronically, (b) with a mercury barometer, or (c) with an aneroid barometer. Concern for the spillage of mercury has caused an increase in the number of laboratories now employing an electronic measure for the outlet pressure. In relation to the average carrier gas velocity:
Δp = 8ηLu/rc2
where η is the carrier gas viscosity, L is the column length, u is the carrier gas velocity, and rc is the column internal radius. For helium carrier gas at 100 °C, the following tables provide the pressure drop Δp as a function of carrier velocity u, in units of kPa, for 10 m columns (Table 1), 25 m columns (Table 2), and 50 m columns (Table 3) of various diameters d = 2rc. Column definitions for Tables 1-3 are as follows.
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