Page 125 - Tandem Techniques

Page 107

physical properties of the solute and mobile phase. Golay's' original equation assumed an average
velocity through the entire length of tube but, due to the compressibility of the gaseous mobile phase,
this assumption leads to significant error when practical data was compared with that predicted
theoretically. The equation, as already discussed in chapter 1, was modified by Ogan and Scott [2] to
take into account the compressibility of the mobile phase, and their modified Golay equation is given as
follows:

where (H) is the variance per unit length of the column or the height equivalent to a theoretical plate.
(uo) is the velocity of the mobile phase at the column outlet,
(Dm) is the diffusivity of the solute in the mobile phase measured at atmospheric pressure and
the column temperature,
(Ds) is the diffusivity of the solute in the stationary phase, at the column temperature,
(r) is the radius of the column,
(k') is the capacity factor of the eluted solute,
(df) is the film thickness of the stationary phase,
and (g) is the inlet/outlet pressure ratio of the column.

Now, for all practical columns, (r) will take values close to 0.25 mm and (df) values close to 0.25 µm.
In addition, Dm will be approximately 0.1 cm /sec and Ds about 0.000015 cm /sec. It follows that the
2
2
function involving (r ) will be one to two orders of magnitude greater than the function involving (df )
2
2
and thus, to a first approximation,

Equation (3.2) will apply to both connecting tubes (where k'= 0, that is when there is no stationary
phase coating on the internal walls of the tube) and to GC coated capillary columns If equation (3.2) is
differentiated and