Teledyne GC-Pro/TCD - Gas Chromatograph - Manual (Page 24)

Operational Theory GC-Pro TCD
Teledyne Analytical Instruments 10
sensitivity. And since the sampling valve is designed to inject a precise
and known volume of sample at a fixed temperature, the actual
concentration of the compound of interest can be determined.
A stainless steel packed column containing Chromosorb Diatomite
or other material depending on the application, is held at a constant
temperature. The temperature as well as the carrier gas and its pressure
are critical and depend on the application. The actual temperature
setpoint and compound separation for your instrument have been
determined at the factory. They are listed in the Addendum and Testing
Results section of Appendix C.
Using a specific carrier gas to inject the sample gas into a known in
volume, the specific component of interest elutes from the column
within a few minutes. The exact timing is characteristic of the material
and has been determined at the factory during testing. Additionally, a
clear separation is observed from other components in the sample gas
however each has a characteristic timing so the instrument can select
only the peak associated with the compound of interest for processing.
All necessary information regarding carrier gas type, sample loop
volume, temperature and pressure settings, and internal timing settings
are given in the Addendum and Testing Results section of Appendix C.
2.2 Modes of Operation
The analyzer has 2 modes of operation depending on the position of
the GC Sampling Valve (See Piping Diagram in Figure 2-2). They are:
Sample Mode (position A) and Analysis Mode (position B).
1. Valve Position A—Sampling Mode
In this mode the analyzer configures the operational valve to
back flush the column and charge the sample loop. The Sample
Mode is programmed to continue for a 7 minute period by
factory default; however, it is usually modified to match the
needs of a particular process. The specific timing interval for
your system is listed in the Addendum and Testing Results
section of Appendix C. It is possible to set the Sample Mode
duration up to 25 minutes.
2. Valve Position B—Analysis Mode
In this mode the analyzer configures the sampling valve to feed
the gas in the sample loop through the column and to the
detector. The eluted sample from the column is fed to the TC