Appendix C
107
The PFPD provides two separate analog signal output channels. Thus the phospho-
rus mode can be assigned to Channel 1 and the sulfur mode to Channel 2. When
setting the Alpha value in each mode to zero (see Figure C.2), Channel 1 provides
the output in the phosphorus mode (i.e., Gate P output), and Channel 2 provides the
output in the sulfur mode (i.e., Gate S output). Thus the output signals from
phosphorus and sulfur in Channels 1 and 2 are directly proportional to the concen-
tration of each species in the combusted analyte and the average emission coeffi-
cient of each species within Gate P and Gate S, respectively. Figure C.3 shows
Channels 1 and 2 chromatograms derived using the Figure C.2 gate parameter
values together with a pesticide test standard containing tributyl phosphate (TBP)
and tetrahydrothiophene (THT). Since the Alpha values were set to zero in this
example, the PFPD’s interheteroatom selectivity has not been optimized using dual
gate enhancements.
The first (left) peak in each chromatogram in Figure C.3 represents the compound
with only sulfur (i.e., THT, which contains no phosphorus). The reason why a
sulfur peak appears in the phosphorus mode is that the phosphorus gate (P Gate,
which is output in Channel 1) contains part of the overlap between the phosphorus
and sulfur emission lifetimes (shaded area in Figure C.1). Similarly, the appear-
ance of the phosphorus peak in the sulfur mode is due to the fact that the sulfur
gate (S Gate, which is output in Channel 2) also partly includes the overlap be-
tween the phosphorus and sulfur emission lifetimes.
Quantifying Gate Scaling Factors
The optimal value of the Gate B scaling factor (Alpha) values for the phosphorus
and sulfur modes can be determined using either GC iteration (trial and error) as
described by Amirav and Jing (1995 - see bibliography) or empirically using the
following procedure:
1. Using the WinPulse Gate Parameters screen, specify the phosphorus and
sulfur mode shown in Figure C.2. Set the Alpha value for each mode to zero.
Figure C.2. Gate Parameters Window Showing Gate Settings
for Phosphorus (P) and Sulfur (S)
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