Two potential disadvantages of temperature programming are the inevitable
delay between consecutive chromatographic runs while the oven is cooled
down and a stable starting temperature re-established, and the possible
decomposition of thermally-labile compounds at the higher temperatures.
Computer-controlled systems improve the reproducibility of temperature
programming, and the oven can be automatically force-cooled between runs to
save time.Although GCis primarily a technique for analyzing mixtures of volatile solutes,
there are many compounds and materials that are either nonvolatile, have
volatile components in a nonvolatile matrix, or are thermally labile. In some
instances, samples may contain solutes at such low levels that they must be pre-
concentrated prior to analysis.
Special procedures have been developed for handling such samples, e.g.● Nonvolatile and thermally labile materialscan either be pyrolyzedor
chemically derivatizedto yield volatile products that can be successfully
chromatographed.
Small samples of paints, plastics, polymers and many ionic compounds
can be pyrolyzed (thermally decomposed) in a modified injection port under
controlled conditions to yield characteristic lower molecular mass and
volatile products that are swept onto the column. The resulting pyrograms
can be used as fingerprints of the original materials for identification
purposes (Fig. 2). Compounds of very limited volatility and/or thermal
sensitivity containing hydroxyl, carboxyl and amino functional groups can
be readily reacted with appropriate reagents to convert these into much less
polar methyl, trimethylsilyl or trifluoroacetyl funtionalities. Fatty acid, carbo-
hydrate, phenol and aminoacid derivatives can be chromatographed, but
often HPLC (Topics D6 and D7) is the preferred technique.
● Headspace analysisinvolves chromatographing the vapors derived from a
sample by warming it in a partially filled vial sealed with a septum cap. After
equilibration under controlled conditions, the proportions of volatile sample
components in the headspaceabove the sample are representative of those in
the bulk sample. The headspace vapors, which are under slight positive
pressure, are sampled by a modified and automated injection system or gas
syringe, and injected onto the column (Fig. 3(a)). The procedure is useful for
mixtures of volatile and nonvolatile components, such as residual monomers
in polymers, alcohol or solvents in blood samples (Fig. 3(b)), and flavors and
perfumes in manufactured products, as it simplifies the chromatograms and
protects the column from contamination by nonvolatile substances.
● Thermal desorptionis a procedure where solutes can be collected on a solid
sorbent in a pre-concentration step, then thermally desorbed by rapid
heating in a unit linked to a modified injection port and through which the
carrier gas is flowing. Sorbents, such as activated charcoal or one of the gran-
ular packings used in packed column GC, are normally contained in a small
tube with which polluted industrial or urban atmospheres can be sampled by
allowing passive diffusion through the tube over a prolonged period or
drawing the air through over shorter periods. Thermal desorption can also
be used in conjunction with headspace analysisto pre-concentrate volatile
solutes, and to purge and trapvolatile solutes in liquid samples using a
stream of gas (Fig. 3(a)).Special
procedures used
in GC
D5 – Gas chromatography: procedures and applications 151