Chapter 5
63
Operation
Figure 5.4. Phosphorus Emission and Gate Settings for Maximizing
Phosphorus
(Note: The PMT voltage was set to 400 V.)
Recommended
Phosphorus Gate
Maximizing Sulfur Response
Sulfur exhibits a quadratic chemiluminescence response (i.e., the response is a
function of the square of the sulfur concentration). For detecting trace levels, the
objective is to increase sensitivity by maximizing the sulfur concentration within
the combustion just as the flame begins to propagate through it. Conversely, for
high concentrations, the objective is to decrease the overall sulfur concentration in
the combustion tube during flame propagation.
For trace-level analysis, sulfur concentration within the combustor and the PFPD’s
detectivity for sulfur can be increased in the following way:
1. Use the 2-mm combustor. (See “Installing the Combustor and Reassembling
the PFPD” in Chapter 4.)
2. Set the detector temperature to 250°C. Higher temperatures can be used for
some applications if needed, resulting in a small decrease in sensitivity.
3. Set the combustor gas composition to the optimum H
2
-rich mixture.
4. Set the fine adjust needle valve so that the needle valve is half a turn counter-
clockwise from the point in which the PFPD goes out of tick-tock.
5. In the Gate Parameters screen, set start and stop times of the integration
gates (recommend 6 ms and 24.9 ms respectively) to optimize sulfur
detectivity. If the gas flow rates are properly set, the sulfur emission should
extend to 25 ms or beyond (for 2-mm combustors) (see Figure 5.5).
Summary of Contents for 5380
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