Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 249

columns will continue to be employed primarily for fixed gas analysis and separation
where high resolution is not required or not always desirable.

10.1.2 Common Stationary Phases: The Key
to Separation

The stationary phases mentioned above have many forms and thousands are
commercially available from various manufacturers. Since it is the stationary phase
that has the greatest influence on the separations obtained and the key to good
separation, choosing a good column with the right stationary phase becomes a
critical step in developing a chromatographic method. The discussions below are
intended to help the reader understand the chemistry and the major characteristics of
column stationary phases of GC and HPLC, which are the two most commonly used
nowadays. The stationary phases used in IC will be briefly discussed in Section
10.2.3, and size exclusion chromatography is omitted because it has limited utility
for most environmental contaminants with small molecular weight.

GC Column Stationary Phase

The gas-solid chromatography (GSC) has a stationary phase made of alumina
(Al 2 O 3 ) or porous polymers in the PLOT columns. The GSC is based on the
adsorption of gaseous chemicals on the stationary phase. Owing to the semi-
permanent and the nonlinear sorption nature, the applications of GSC are limited to
certain low-molecular-weight gaseous species, such as components in air, H 2 S, CS 2 ,
CO, CO 2 , and rare gases.
The partition-based gas-liquid chromatography (GLC) is commonly shortened
to gas chromatography (GC). GLC is based on the partition of an analyte between a
gaseous mobile phase and a liquid stationary phase, immobilized on the surface of
an inert solid. It is reported that the common stationary phases in GLC that are listed
in Table 10.1 can provide separation of almost 90% or more of the sample
commonly encountered.
Five of the six liquid stationary phases listed in Table 10.1 have the general
structure of polydimethyl siloxane, and the fifth entry in the table is a polyethylene
glycol. Their basic structures are shown below.

R R R

H 2
C

H 2
C

R Si OSi

R

OSi
R

R

OH

CH 2 O CH 2 OH

R

n

n

(a) Polydimethyl siloxane (b) Polyethylene glycol

For polydimethyl siloxane, theR groups are all hydrophobicCH 3 , giving the
liquid the least polarity. For other polysiloxanes shown in the table, a fraction of the