Page 268 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

9 CAS CHROMATOCRAPHV
2. support-coated open tubular (SCOT), which have a finely-divided layer of
solid support material deposited on the inner wall on to which the stationary
phase is then coated These SCOT columns are not as efficient as WCOT
columns but have a higher sample capacity, which enables them to be used
without a stream splitter.
Capillary columns are fabricated from thin-walled stainless steel, glass, or
high-purity fused silica tubing (the last is preferred for its inertness). Typical
dimensions of the columns, which are coiled, are 25-200m long and 0.2-0.5 mm i.d.
Excellent open tubular columns may now be purchased, providing a number
of stationary phases of differing polarity on WCOT and SCOT columns, and
whose efficiency, greatly improved sample detectability, and thermal stability
surpass those exhibited by packed columns; their chief disadvantage is that they
have a lower sample capacity than packed c~lumns.~~.~~
4. The detector. The function of the detector, which is situated at the exit of
the separation column, is to sense and measure the small amounts of the
separated components present in the carrier gas stream leaving the column. The
output from the detector is fed to a recorder which produces a pen-trace called
a chromatogram (Fig. 9.lb). The choice of detector will depend on factors such
as the concentration level to be measured and the nature of the separated
components. The detectors most widely used in gas chromatography are the
thermal conductivity, flame-ionisation and electron-capture detectors, and a
brief description of these will be given. For more detailed descriptions of these
and other detectors more s~ecialised texts should be con~ulted.~~-~~
Some of the important properties of a detector in gas chromatography are
briefly discussed below.
(a) Sensitiuity. This is usually defined as the detector response (mV) per unit
concentration of analyte (mgmL-'). It is closely related to the limit of
detection (MDL) since high sensitivity often gives a low limit of detection.
Since, however, the latter is generally defined as the amount (or concentration)
of analyte which produces a signal equal to twice the baseline noise, the
limit of detection will be raised if the detector produces excessive noise. The
sensitivity also determines the slope of the calibration graph (slope increases
with increasing sensitivity) and therefore influences the precision of the
analysis [see also (b) below].
(b) Linearity. The linear range of a detector refers to the concentration range
over which the signal is directly proportional to the amount (or concentration)
of analyte. Linearity in detector response will give linearity of the calibration
graph and allows the latter to be drawn with more certainty. With a convex
calibration curve, the precision is reduced at the higher concentrations
where the slope of the curve is much less. A large linear range is a great
advantage, but detectors with a small linear range may still be used because
of their other qualities, although they will need to be recalibrated over a
number of different concentration ranges.
(c) Stability. An important characteristic of a detector is the extent to which
the signal output remains constant with time, assuming there is a constant
input. Lack of stability can be exhibited in two ways, viz. by baseline noise
or by drift, both of which will limit the sensitivity of the detector. Baseline
noise, caused by a rapid random variation in detector output, makes it