Instrumentation for Gas Chromatography - Forensic Applications of Gas Chromatography - page 20

developing a separation. For faster analyses, a shorter column may be benefi-
cial, provided the compounds are either well separated or few in number. In
contrast, a longer column (60 m) may be required when separation of com-
pounds is not possible by using a smaller internal diameter column, using a
different stationary phase, or altering the column temperature.
2.5.4 Thickness of the Stationary Phase
The thickness of the stationary phase of a capillary column normally varies
between 0.1 and 5 μm. Typically, increasing the film thickness (i.e., thickness
of the stationary phase) will result in more retention of the compounds, as
well as more sample capacity but with an overall lowering in column effi-
ciency (see Section 3.2.2). In general terms, a thin film thickness is good
for separating high boiling point compounds leading to decreased analy-
sis times. In contrast, a thicker film thickness is best for low boiling point
compounds resulting in improved resolution of early eluting compounds but
with increased overall analysis times. A good starting column for method
development would have a film thickness of 0.25 μm.
2.5.5 Overall Description of a Capillary Column
Finally, it is typical to describe a capillary GC column using the following
nomenclature:
DB-5 30 m × 0.25 mm i.d. × 0.25 μm film thickness
This is the manufacturer (as identified by the letters at the start), followed by
the number that identifies the stationary phase composition of the polysilox-
ane, followed by the column length × the internal diameter of the capillary
column × the dimensions of the film thickness (i.e., thickness of the station-
ary phase) as described by the manufacturer and numerical code.
In addition, the use of either isothermal or temperature-programmed
GC will also influence the separation. In isothermal analysis, the retention of
compounds is more dependent on the column length such that a doubling of
column length will double the analysis time. However, doubling the column
length increases the resolution by 41% (see Section 3.2.4).
In contrast, in temperature-programmed GC, the retention is more depen-
dent on temperature such that doubling the column length marginally increases
analysis time. However, the chromatographic temperature-programmed oper-
ating conditions need to be optimised to achieve an optimum separation.
For details of the influence on chromatographic separation of varying
the stationary phase, column internal diameter, film thickness and column
length, see Chapter 4.