A typical GC Bench layout is shown in Fig. 13. This reflects a ‘single’ analysis cell. Variations on
this will be on the length of cell or the number of cells.
The length of the cell is dependent on the concentration of the sample measured and may vary
from 1mm (at very high concentration) up to about 250mm (at low concentration).
Infrared (heat) radiation from a small source is directed through a rotating filter wheel, a
collimating lens, an optical (Thin-Film) filter, the sample cell, a focussing lens and on to a solid
state detector. The wheel is rotated at 1100 RPM, to give a modulation frequency of 18.3 Hz.
Each cell element is sealed at both ends by a window that does not optically interfere with the
radiation.
The heart of the analyser is the gas filter wheel, which contains a sealed sample of gas of the
type to be measured, and a non-absorbing gas. As the wheel rotates it provides a ratio of
sample and reference signals, in a sequence detected by a wheel position sensor. The ratio
signals, together with the selective transmission of the optical filter, sensitises the analyser to
respond to that region of the infrared spectrum corresponding to the measured gas in the
sample.
When the gas to be measured enters the analysis cell, it absorbs some radiation, and alters the
ratio of the sample and reference signals. It is this change in energy level that is amplified to
give the output signal.
The optical path between the source and the first cell window, and between the last window
and the detector, will normally contain room air, or a gas from which any interfering gases have
been stripped. This air is referred to as ‘Purge’ air, and if a chemical stripper is used, it will be
shown on the gas circuit. As Purge air has the properties required for the Zero Gas, it may be
used as such.
Whilst the bench can be stripped down to individual parts and parts replaced, this process will
require re-calibration of the bench and possibly some re-adjustments in the Pre-amplifier
and/or amplifier associated with the detector. Since this requires special training and
equipment, the User should not undertake re-calibration unless such training and equipment
has been approved.
6.3 SB Bench
The layout of an SB (single beam) infrared bench is shown in Fig. 14, and like the GC bench, is
a non-dispersive comparator. Unlike the GC type however, it has no built in reference and
operates as an absorptiometer.
The source block contains an infrared (heat) source and a simple rotating, stepper-motor driven
shutter. This rotates at 200RPM to give 3.3Hz modulation of the IR beam, which passes
through the cell, and onto a solid state detector, via a suitable thin film optical filter in close
proximity.
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