AADCO 737 Series Operating Instructions Manual page 21

9.0 PURIFICATION REACTORS
9.1 AADCO Instruments, Inc. offers four basic types of purification reactors,
designated "A", "B", "C", and "D".
The "A" PURIFICATION REACTOR produces air of purity outlined in Section
9.2
11.0 and with an oxygen concentration of 20.8% ± 0.3%. In addition, the CO
concentration will be that of the ambient environment of the user's locale (~350-
ppm). This purification reactor should be specified if the operator wishes to
avoid calibration disparities where it is essential that the carbon dioxide level
for both the "zero" air and the sample be the same; e.g., use of the flame
photometric detector or nondispersive infrared. The "A" model is the most
universal purification reactor for air monitoring applications and is usually
supplied when advised of this application.
The "B" PURIFICATION REACTOR is factory set to produce air with the same
9.3
purity as the "A" model but with an oxygen concentration of 37.0% ± 0.5%, at
specified conditions. By in-house experimentation, this concentration has been
found to produce a greatly increased response for most commercial flame
ionization detectors over that response experienced with cylinder air. This has
since been field proven and has become the purification reactor of choice when
high sensitivity FID is required. It has also been found to decrease the noise
level of the flame photometric detector when used in conjunction in a gas
chromatography mode. Use of this purification reactor will eliminate both the
oxygen and air cylinders when operating this detector for that application.
9.4 It should be noted that output flows less than 50% of the rated output should be
avoided with any model pure air generator which contains a "B" reactor. This is
the lower threshold for maintaining the oxygen output at 37%. Flows below this
level will produce an oxygen enrichment greater than 37% oxygen. This higher
oxygen level causes the flame to become too hot with a consequent increase in
noise. This increased noise can be nullified, without loss of sensitivity, by
decreasing the hydrogen flow slightly.
9.5 Those chromatographers actively using flame ionization detectors with pure air
generators having "B" reactors experience a response from three to ten times
greater than the response of the same detector with cylinder air, particularly if
nitrogen is used as the carrier gas. The magnitude of this increased response
will depend upon the particular detector. The substitution of nitrogen for helium
carrier gas will also improve resolution within the GC column. The more dense
the carrier gas the better the resolution.
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