Page 47 - Illustrated Pocket Dictionary of Chromatography

42 CONTROLLED PORE GLASS

controlled pore glass A silica-based support that is used in the
synthesis of DNA and as an LC support of ion-exchange, size-exclu-
sion, and affinity separations. Controlled pore glass offers high and
reproducible surface areas, high mechanical strength, and narrow and
reproducible pore size distribution.
control limit An established value for a parameter used to indi-
cate when the sample value exceeds or falls below this value. Control
limits are effective when they are used to monitor critical parameters
of a product over long periods of time. These limits mark the accept-
able operational limits of a process or characteristics of a product and
are part of a control chart. In most cases control limits are set for
maximum acceptable values (upper control limit, UCL) and minimum
acceptable values (lower control limit, LCL). If any parameter falls
outside the control limits, the process is stopped. Some control limits
for HPLC and GC could be efficiency (number of theoretical plates,
LCL only), resolution (R s value, LCL only), or peak asymmetry (tailing
and fronting, UCL and LCL).
control sample A sample that has already been analyzed and is
subsequently stored so that the analyte concentration does not change
over time. It is run along with current samples as a way to inde-
pendently verify that the entire method, from sample preparation to
analysis, is working consistently over time. Also called a check sample.
convex Describes a nonlinear gradient having a positive slope that
ultimately tends toward zero (see figure under concave). A concave
gradient is, for the %B or strong solvent, mathematically described as:

n
-
%B =1 - (1 tT ) ¥ 100
where t is the time since the start of the concave gradient, T is
the total time for the gradient, and n defines the steepness of the
gradient. See concave.
o
corrected retention volume, V R Term most frequently used in
GC; the result of the fact that flow is not constant throughout the
o
system (see compressibility factor). V R is calculated as:
o
V R = j ¥ V R
where V R is the retention volume obtained directly from the
chromatogram.