Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 260

temperature stability), gaseous or liquid samples can be directly introduced into a
heated region at the head of the column. An inlet is the device on the GC that accepts
the sample and transfers it to the column. For packed columns, a sample is
introduced directly into the column (on-column injection), or introduced into the
heated region where it is vaporized and carried into the column (flash vaporizer).
Four inlets are in common use in capillary GC: split, splitless, on-column, and
programmed-temperature vaporization. A split/splitless injector is used when
sample quantity should be reduced at the split mode. Care should be exercised not to
overload the column, because sample size (liquid) is typically very small in GC
(e.g., 0.2-20 L for capillary columns).

Thecolumnisheldinanoven between two other separate heaters for
an injector port and a detector. These three components have independent
programmed temperature control. Since the key role of columns in chromato-
graphic separation has been discussed in the previous section, we will now focus
on the appropriate temperature of these three components, which is the key
variable in GC. For the injection port, a general rule is to have the temperature at
50 C higher than the boiling point of the sample. This temperature should be hot
enough to vaporize the sample rapidly, but low enough not to thermally
decompose the analytes. Column temperature is the easiest and most efficient way
to optimize separation. The control of column temperature within 0.5 Cis
essential. Raising the column temperature speeds both the elution (higher vapor
pressure) and the rate of approach to equilibrium between the mobile and
stationary phases. In GC, the change in temperature as the separation proceeds is
called a temperature gradient, which can range from the ambient to 360 C. Higher
column temperature should be avoided, since it will cause column bleeding where
the stationary phase itself can be vaporized and/or decomposed and the material is
then passed along the column and eluted.
The detector temperature depends on the type of detector used in the GC.
As a general rule, however, detector temperature from the column exit must be
hot enough to prevent condensation of the sample and/or liquid phase. The
detector is where the chemical signal is sensed in the form of a chromatogram.
GC detectors commonly used in environmental analysis will be described in
Section 10.3.1.

m L for packed columns and 0.01-3 m

10.2.2 High Performance Liquid Chromatography
(HPLC)

Although variations exist, a typical HPLC system with the basic components is
schematically shown in Figure 10.4. From the beginning of the solvent flow to the
end of waste effluent, an HPLC system includes the following basic components:
(a) solvent reservoirs and a degassing unit, (b) a solvent pump, (c) a sample injection
system, (d) a column (guard column and analytical column), (e) a detector, and (f) a
data acquisition system. The purpose, positioning, and functioning of the major
components are described below. Since mobile phases (solvents) are crucial in