Detectors 5
Advanced User Guide 97
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gas flows
The hydrogen fuel gas and makeup gas mix at the EPC module,
and enter the bottom of the detector where the column enters
the transfer line. See Figure 4. This gas mixture eliminates any
dead volume near the column entrance before mixing with air in
the emission block for combustion.
A second flow of makeup gas is used to sweep the volume
between the thin-walled tube (that carries the column sample
stream) and the transfer line walls. This gas flow also enters the
emission block but is separate from the sample stream. The
purge flow rate is not measured directly, but is controlled by a
fixed restrictor and the makeup flow setting. A makeup flow
setting of 60 mL/min will result in about 12 mL/min of purge
flow.
The total flow from the detector is:
column flow + makeup flow + purge flow = total flow
However, the total flow that carries the sample is equal to just
the column flow plus the makeup flow.
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linearity
Several mechanisms produce sulfur emission. The excited
species is diatomic, so that emission intensity is approximately
proportional to the square of the sulfur atom concentration.
The excited species in the phosphorus mode is monatomic,
leading to a linear relationship between emission intensity and
atom concentration.
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Lit Offset
The default Lit Offset is 2.0 pA.
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photomultiplier protection
The PMT is extremely sensitive to light. Always turn the PMT
voltage off (which turns off the high voltage to the PMT) before
removing the PMT housing or opening the emissions chamber.
Failing to do this can destroy the PMT.
Even with the PMT voltage off, protect the PMT from room light.
Cap the housing when removed, place it end down to exclude
light, reduce room light level before exposing the PMT, and so
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