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In the quiescent condition, prior to injecting a sample, both legs of the bridge are exposed
to pure carrier gas. In this condition, the bridge is balanced and the bridge output is
electrically nulled.
The analysis begins when the sample valve injects a fixed volume of sample into the
column. The continuous flow of carrier gas moves the sample through the column. As
successive components elute from the column, the temperature of the measurement
element changes.
The temperature change unbalances the bridge and produces an electrical output
proportional to the component concentration.
The differential signal developed between the two thermistors is amplified by the
preamplifier. Figure 1-3 illustrates the change in detector electrical output during elution of
a component.
Detector output during component elutionFigure 1-3:
In addition to amplifying the differential signal developed between the two thermistors,
the preamplifier supplies drive current to the detector bridge.
The signal is proportional to the concentration of a component detected in the gas
sample. The preamplifier provides four different gain channels as well as compensation for
baseline drift.
The signals from the preamplifier are sent to the electronic assembly for computation,
recording on a printer, or viewing on a PC monitor with MON2020.
Introduction
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