Chapter 5 61
Operation
Using Multiple Manual Injections
If a permeation device is not available, making multiple isothermal injections of a
test analyte can also be used to optimize the PFPD’s detectivity.
1. Inject a sample containing the analyte of choice into the GC column. With
most solvents, the flame may not propagate when the solvent elutes from the
column due to excessively rich fuel conditions, but as the solvent concentra-
tion decreases in the PFPD to a sufficiently low level, the flame will be
reinitiated.
2. Monitor the height of the chromatographic peaks of the analyte on the data
integration device (e.g., software such as ChemStation).
3. With each subsequent injection, increase the H
2
flow, monitoring both the
background emission and the sulfur emission. The emission intensities will
increase to a maximum then start decreasing. The H
2
to Air 1 gas mixture at
maximum peak height represents the optimum combustor gas mixture.
Quenching can be reduced by setting this mixture to slightly air rich.
4. Use WinPulse to archive data for comparison purposes when making multiple
manual injections. The optional PFPDView program must be used to observe
the stored data.
5. After the optimum H
2
flow has been established and set, check that the
detector pulse rate is still in the 3–4 Hz range.
6. When the optimal gas setting has been attained, verify that the PFPD is close
to tick-tock by closing the fine adjust needle valve (clockwise) until the PFPD
just goes into tick-tock, as shown in Figure 5.3. Open the fine adjust needle
valve one-half turn from the tick-tock position.
Note: To reduce the time required to optimize the combustor detector flow using
the multiple injection method, injections can be made in rapid succession.
To use this technique successfully, it is necessary to initially determine the
difference in the elution times of the solvent and analyte of choice. The
subsequent multiple injections must be completed within that time period
or spaced in time so that the analyte does not elute simultaneously with the
solvent from a subsequent injection.
7. Optimize the PMT voltage and trigger level. See step 14 of “Starting the
PFPD System” in this chapter.
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