Page 99 - Illustrated Pocket Dictionary of Chromatography

H

head pressure, Dp The measure of the resistance to mobile-phase
flow at the head of the column. This term includes all sources of resist-
ance to flow after the column as well (e.g., connector tubing, detec-
tor, etc). Compare with backpressure. Head pressure is measured by
a pressure transducer that is placed in the mobile phase path just
before the injector. It is a function of flow rate, mobile-phase tem-
perature, mobile-phase composition, column length and diameter, and
packing material characteristics.

headspace analysis All headspace techniques involve the extrac-
tion, collection, and analysis of volatile components in a liquid sample.
Analysis is often coupled to a GC. Common types of headspace analy-
ses include static and purge and trap.

heart cutting A purification technique used in semipreparative
and preparative LC that takes a slice of the peak near the peak maxi-
mum in order to obtain the highest purity fraction from the peak.

Henderson–Hasselbach equation This equation is derived
from acid-base dissociation equilibrium constants, K a:
-
+
K a = [ A ][ H ] [ HA ]
+
-
where [A ], [H ], and [HA] are the solution concentrations of the con-
jugate base, hydronium ion, and undissociated acid, respectively.
Rearrangement yields the final form of the Henderson–Hasselbach
equation:
pH = pK a + log A [ ( - ] [ HA ])

+
where pH and pK a are the -log of [H ] and K a, respectively.
n-heptane Molecular weight: 100.2; boiling point: 98.4°C; refrac-
tive index (20°C): 1.3876; density (20°C): 0.68g/mL; viscosity (20°C):
o
0.41cP; UV cutoff: 200nm; eluotropic strength (e ) on alumina—0.01,
Illustrated Pocket Dictionary of Chromatography, by Paul C. Sadek.
ISBN 0-471-20021-2 Copyright © 2004 John Wiley & Sons, Inc.
95