Page 195 - Illustrated Pocket Dictionary of Chromatography

198 THERMAL CONDUCTIVITY DETECTOR (TCD)

N = C(t r w x ) 2

where C is a constant derived from which the peak width, t wx , is taken.
Here the subscript x refers to the percent peak height. Manufacturers
report this figure in plates per column or plates per meter. For
example, if a 25cm column has a peak that generates 4000 theoreti-
cal plates, the the corresponding plates per meter would be: (4000
plates/25cm) ¥ (100cm/m) = 16,000 plates/m.
thermal conductivity detector (TCD) A nondestructive con-
centration detector used in GC. The detector measures the difference
in the conductivity generated by the sample as compared against the
carrier gas along. This difference causes a resistive imbalance on the
filaments of a Wheatstone bridge system and a signal is generated
from the change in resistance. The TCD is most effective for nonor-
ganic analytes such as NH 3, CO 2, H 2O, etc. The lower detection limits
are quite high compared with other GC detectors, in the 10ppm range.
This leads to use with packed-bed columns. These columns can
tolerate high sample loadings as compared with capillary columns.
4
The linear range of the TCD is about 10 .
thermospray ionoization (TS) Uses a volatile charged compo-
nent of the mobile phase to initiate the ionization of the analyte. A
common reagent is ammonium salt:
+
+
+
A + NH 2 Æ[ A NH 2 ] Æ [ AH ]+ NH 3
◊
Ionization occurs in an ion source after a aerosolized spray is formed
by passing the sample through a capillary inlet. After passing through
the source and becoming charged, the ions are then accelerated to
the detector. This is a ionization process that is “gentle” and can
effectively form molecular ions.
thin-layer chromatography (TLC) A technique that uses a
sorbent applied to a planar support, called a plate, for separation.
The sample is applied to the sorbent that is contained in a sealed con-
tainer called a developing tank. The solvent moves up the support
through capillary action and causes the analytes to migrate up the
sorbent based in the differential affinity between the solvent and the
sorbent. Standard TLC uses a sorbent that is prepared from 10- to
60-mm particles that is applied to the plate in a thickness of ~250mm.
The standard plate dimensions are in the 10cm ¥ 10cm to 20cm ¥
20cm range.