A New Photoionization/Flame-Ionization Series Detector for Gas Chromatography0 pages

A New Photoionization/Flame-Ionization
Series Detector for Gas Chromatography
Application Note 00541095

Keywords
Gasoline
Hydrocarbon
Kerosene
Natural Gas Well
PID/FID

Introduction
Class determination of hydrocarbon species is of interest since structure
plays an important role in reactivity in photochemical reactions (1).
Saturated, low molecular weight hydrocarbons (e.g., methane and ethane)
are not photochemically reactive. Branched alkanes, alkenes, and aromatics, however, tend to be very reactive in atmospheric photochemical
reactions that form irritating oxidants (such as ozone), PAHs, and smog.
A GC detection method has been developed that is based on analyzing
the effluent from a chromatographic column with two detectors and then
comparing the relative responses for each analyte. The technique of
evaluating response ratios was introduced over 30 years ago by Grant (2)
and first applied to PID/FID applications by Driscoll (3). The relative
response of the FID is similar for many types of carbon compounds (e.g.,
aromatics, alkenes, and alkanes); therefore, it is used to measure the
relative levels of a particular hydrocarbon regardless of the degree of
saturation. In contrast, the PID’s relative response differs greatly depending on a compound’s degree of saturation. The PID has low sensitivity
for alkanes but is very sensitive for aromatics.
A tandem detector has been developed that takes advantage of the sensitivity differences between the PID and FID. The detector set presented
here takes advantage of a unique design that allows the FID to interface
directly to the PID without a transfer line; previous PID/FID detector
systems have required splitters and/or transfer lines (4, 5) between the
detectors to achieve an interface. The design used in this study eliminates
the need for nonstandard fittings and the possibility of dead volumes
associated with those fittings. It also eliminates cold spots associated
with non-heated transfer lines. Series operation is possible since the PID
is a nondestructive detector.
Determine hydrocarbon class by evaluating the normalized response ratio
obtained by dividing the PID response by the FID response, then
normalize to an internal reference compound.
Experimental
A cross section of the dual detector set used, a Model 4450 PID/FID (OI
Analytical, College Station, TX), is shown in Figure 1. The PID was
equipped with a standard 10.0 eV lamp. In this series detector, the sample
stream flows through the PID’s ionization chamber where it is
continuously irradiated with high energy ultraviolet light. As compounds