C. Bouigeon et al., LONG PACKED-COLUMN SUPERCRITICAL-FLUID CHROMATOGRAPHY - INFLUENCE OF PRESSURE-DROP ON APPARENT EFFICIENCY, Analytical chemistry, 68(20), 1996, pp. 3622-3630
Supercritical fluid chromatography, when performed on a packed column,
is a powerful and fast separation technique, To enhance the number of
theoretical plates (TP) available, long packed columns (>1 m) have be
en used successfully, despite controversy over the effect on column ef
ficiency of the density gradient induced by the pressure drop, Peak br
oadening and deformation were reported, and packings with larger parti
cle diameter than those used in liquid chromatography (10 mu m instead
of 3-5 mu m) were advised in order to reduce the column pressure drop
, Velocity gradient induced by the density gradient was reported to re
duce efficiency, This paper presents the results of investigations on
the influence of density gradient on the apparent efficiency obtained
on a series of four 25-cm x 4.6-mm-i.d. Nucleosil C18 columns connecte
d in series (particle size, 5 mu m), Apparent column efficiency is fou
nd to vary from less than 10 000 TP to more than 100 000 TP versus the
density and the density gradient. The higher the density gradient, th
e higher the efficiency loss, A model is presented which accounts for
the effect of linear velocity and density gradients on peak broadening
, It confirms that it is the linear velocity variation rather than the
variation of the density which causes band broadening and allows pred
iction of conditions for which apparent efficiency loss occurs, To red
uce the density gradient induced by column pressure drop, one can comp
ensate for pressure gradient by a superimposed temperature gradient (m
ultitemperature control of the mobile phase via three column ovens), I
t allows one to obtain the highest efficiency and to use CO2 at lower
density without any loss of efficiency. When methanol is added to the
CO2, no pressure drop compensation is required in order to obtain the
highest apparent efficiency. As density gradient compensation via mult
itemperature control of the mobile phase provides higher apparent effi
ciency and, consequently, higher resolution than in isothermal operati
on, it is especially useful for separation of complex oil samples.