Page 306 - gas transport in porous media

Chapter 18: Measurement of Vapor Concentrations
305
When a mixture of components is injected into a chromatograph equipped with an
appropriate column, the components travel down the column at different rates and
therefore reach the end of the column at different times. A detector is positioned at
the end of the column to quantify the concentrations of individual components of the
mixture being eluted from the column. Several different types of detectors can be
used with chromatographic separation and are discussed later.
Gas chromatography may be classified into two major divisions:

• Gas-liquid chromatography, where the sorbent (material which separates the mix-
ture into individual components) is a nonvolatile liquid called the stationary-liquid
phase, coated as a thin layer on an inert, granular solid support, and
• Gas-solid chromatography, where the sorbent is a granular solid of large sur-
face area.

The moving-gas phase, called the carrier gas, is an inert gas such as nitrogen or
helium which flows through the chromatographic column packed with the sorbent.
The mixture partitions itself between the moving gas phase and the sorbent and
individual components of the mixture move through the column at a rate dependent
upon its partition coefficient in the liquid phase (gas-liquid chromatography) or upon
its adsorption coefficient on the packing (gas-solid chromatography) and the carrier-
gas flow rate. Open tubular glass or stainless steel capillary tubes of 0.005–0.02 in.
(0.1–0.5 mm) inside diameter and length often as great as 300 ft (90 m), coated on
the inside walls with a nonvolatile stationary-liquid phase, are also widely used in
the separation of complex mixtures.
The apparatus used in gas chromatography consists of four basic components: a
carrier-gas supply and flow controller, a sample inlet system providing a means for
introduction of the sample, the chromatographic column and associated column oven,
and the detector system.
Although the carrier gas is most commonly nitrogen or helium, other gases such as
carbon dioxide, argon, xenon, and hydrogen are occasionally used. Use of a carrier
gas of higher molecular weight will improve column efficiency. Therefore nitrogen
or perhaps a gas of even higher molecular weight is preferred to helium if a detector
other than thermal conductivity is being used.
Arotometer may be used in the carrier-gas system to give an approximate indication
of flow rate. A rotometer consists of a graduated tube with slowly increasing inside
diameter and a glass or metal ball that is suspended in the gas flow within the tube at
a height dependent upon the flow rate. Since the position of the ball is a function of
both the flow rate and the column back pressure when positioned at the column inlet,
a rotometer can be used only for rough approximations of flow rate. A soap-bubble
flowmeter is used for more accurate measurements.
Sample inlets are of two general types depending upon whether the sample is
gaseous, liquid, or solid. Liquid samples are generally injected by means of a cali-
brated hypodermic syringe through a silicon rubber septum into a metal or glass-lined
metal injection port, while gaseous samples are introduced by means of a valve and