Page 162 - Illustrated Pocket Dictionary of Chromatography

REEQUILIBRATION 163

reduced pressure, P r In supercritical fluids the ratio of the
system pressure, P, to the fluid’s critical pressure, P c:
P r = P P c

reduced temperature, T r In supercritical fluids the ratio of the
system temperature, T, to the fluid’s critical temperature, T c, is the
reduced temperature:
T r = T T c

reduced velocity, Calculated from the linear velocity, u, the dif-
fusion coefficient of the analyte, D, and the packing material particle
diameter, d p:

v = ud p D

reduced linear velocity, v The rate at which an analyte moves
one particle diameter along a column due to flow with respect to
the rate due to diffusion. Mathematically reduced linear velocity is
expressed as:
◊
v = u d p D m
where u is the linear flow velocity (distance/time), d p is the particle
2
diameter (distance), and D m is the diffusion coefficient (distance /
time) of the analyte in the mobile phase. Note that is unitless and is
the primary variable in the Knox equation, which is used to deter-
mine the reduced plate height for a system (as a function of chang-
ing ).

reduction The process of a compound (or atom) gaining
electrons.

re-equilibration For any analysis that changes elution conditions
during the course of a separation, the system must return to the fully
equilibrated initial conditions. This process is called re-equilibration.
The time needed to reach initial conditions is called the re-equilibra-
tion time and can be quite lengthy. For example, it may take 10–15min
for a GC column temperature to return to the starting temperature,
and for LC it may require similar lengths of time to reach the initial
mobile phase composition and reestablish column equilibrium with
the mobile phase.