"All flows, all changes". Heraclides
"It's not difficult to know, it is difficult to get familiar". Skovoroda The theory of gas chromatography, presented by equation of Van Deemter,
(1) shows that for each chromatographic separation there exists an optimal flow rate above and below which column efficiency is reduced.
In practice, linear speed of sample zone moved by carrier gas, is changed continuously and increases as it approaches outlet, which results in a non-effective use of a part of column.
In this case equation (1) characterizes optimal separation process only in that section of chromatographic column through which sample passes at an optional speed.
In equation (1) are constant values and is speed of moving zone of sample which changes continuously in time. Thus equation (1) gets following form:
(2) where is speed of sample at a distance from beginning of column and changes - increases - toward outlet. This shows that HETP is not always optimal in all sections of chromatographic column. Let be length of chromatographic column, pressure at inlet of column and pressure at outlet of column, distance of a point on chromatographic column from its inlet. It is evident that pressure at point on column can be determined from correlation
The flow rate at any point on chromatographic column depends on pressure at that point, as well as on inlet pressure and outlet pressure of column. But pressure at any point on column depends on inlet pressure and outlet pressure of chromatographic column and on distance from column inlet. Thus flow rate at any point on column in case of fixed physical parameters of chromatographic column can be considered as a function of pressure at this point and at ends and of distance of this point from column inlet;
Consequently whole process in chromatographic column is characterized by a multiple of equations (3).
When at inlet and outlet of column pressure is kept constant, each section of chromatographic column, through which sample zone moves, is characterized by its equation from multiple (3).
However linear speed of sample zone moving with carrier-gas can be kept constant by programming pressure gradient movement along column in time, realizing function: pressure - location - time by keeping constant pressure difference ∆ p at ends of chromatographic column during whole cycle of analysis (Russia Patent "Chromatograph of A. S. Hayrapetyan").
The sample travels through chromatographic column in time
where is length of chromatographic column and optimal speed of sample zone.
The pressure at inlet and outlet of chromatographic column, undergoing a change at equal intervals of time, is expressed by
where - sample injection time, and - elution time.
We mark by
the sections of chromatographic column travelled by sample zone with an optimal velocity of carrier-gas at corresponding moments of time,