Page 417 - Instrumentation Reference Book 3E

400 Chemical analysis: gas analysis

To power supply
heater
T upstream temperatur ownstream temperature sensor
1

5 Vdc

Figure 18.21 Principle of operation of electronic mass-flow controller. Courtesy Brooks Instrument Division, Emerson
Process.
of the sample should remain constant from one dency to bleed, especially at high pressure or
injection to the next. temperature.

18.4.3 Chromatographic column 18.4.4 Controlled temperature enclosures
The packed separating columns used in process Many components of the gas chromatograph,
chromatographs are typically 1-2 m lengths of including the injection valve, columns, and detec-
stainless steel tubing, 3-6 mm outer diameter, tors, are required to be kept at constant tempera-
wound into a helix for convenient housing, and tures or in environments whose temperature can
packed with a solid absorbent. Separation of per- be altered at known rates. and separate tempera-
manent gases is normally carried out on columns ture-controlled zones are usually provided in the
packed with molecular sieve. These are synthetic instrument.
zeolites, available in a range of effective pore Two general methods are used to distribute
diameters. Porous polymeric materials have been heat to maintain the temperature-sensitive com-
developed which are capable of separating a wide ponents at constant temperatures (&O. 1 K or better)
range of organic and inorganic molecules, and and to minimize temperature gradients. One uses
use of these proprietary materials gives much an air bath, and the other metal-to-metal contact
more predictable column performance than when (or heat sink). The former depends on circulation
liquid coated solids are used. In addition the of heated air and the latter upon thermal contact
polymeric materials are thermally stable and do of the temperature-sensitive elements with heated
not suffer from “bleed” or loss of the liquid sta- metal.
tionary phase at high temperatures which can An air bath has inherently fast warm-up and
give rise to detector noise or drift in the baseline comparatively high temperature gradients and
of the chromatogram. offers the advantage of ready accessibility to all
One or more columns packed with these mater- components within the temperature-controlled
ials can normally be tailored to the needs of most compartment. The air bath is most suitable for
process analyses. However, in certain cases it may temperature programming and is the usual
be necessary to include valves to switch between method for control of the temperature of the chro-
columns or detectors during the analysis, or to matographic column.
divert the carrier gas to waste to prevent a certain Metal-to-metal contact has a slower warm-up
component, for example a solvent present in high but relatively low temperature gradients. It has
concentration, from reaching the detector. These the disadvantage of being a greater explosion
switching operations are referred to as backflush- hazard, and may require the analyzer to be
ing, or heart cutting if the unwanted peak occurs in mounted in an explosion-proof housing resulting
the middle of the chromatogram. in more limited accessibility and more difficult
Capillary columns are also used for chromato- servicing. The detectors are often mounted in
graphy. They use a solvent coated to walls of the heated metal blocks for control of temperature.
column and can therefore have a high plate count The choice of the method of heating and tem-
with low pressure drop. The downside to capil- perature control may depend on the location
lary columns are their fragility and greater ten- where the instrument is to be used. Instruments