Teledyne GC-Pro/TCD - Gas Chromatograph - Manual (Page 23)

GC-Pro TCD Operational Theory
Teledyne Analytical Instruments 9
Operational Theory
2.1 Introduction
The GC-Pro TCD Analyzer uses a Thermal Conductivity (TC)
Detector and a Gas Chromatograph (GC) Column to separate and
analyze different factions of a volatile hydrocarbon sample. Using a
carrier gas and a microprocessor actuated switching valve, a fixed
volume of sample is pushed into the column maintained at a constant
temperature. The eluted gas is analyzed for the specific components
configured for your application using the TC detector and the column is
back flushed to accept the fresh sample for the next cycle. Actual
separating and detecting sequence may vary depending on compounds
of interest in the application for which the analyzer is configured.
The GC-Pro TCD Analyzer uses the thermal conductivity detector to
sense a variety of components that are separated through a column and
eluted to the sensor in a time sequence determined by their respective
retention time. It compares the difference in conductivity between a
known volume of injected sample driven by carrier gas at a fixed
temperature to a constant flowing reference gas. Thermal conductivity is
a fast, accurate and reproducible tool for detection and measurement
under certain conditions. In general, thermal conductivity measurements
are binary and non-specific in nature, that is, they respond to the total
difference in thermal conductivity between two samples. If the sample
contains more than two components, the thermal conductivity
measurement is ill defined and cannot be used to determine the
concentration of the mixture without extensive calibration. Similarly, if
the thermal conductivity difference between components of a binary
mixture is low, sensitivity is negatively impacted.
The GC-Pro TC Analyzer avoids these issues by coupling the TC
detector with a gas separation capability in the GC column. Therefore,
even if the sample stream contains more than one compound, at any time
during analysis only a binary mixture which includes the component of
interest based on its separation properties, plus carrier gas is delivered to
the detector. A suitable carrier gas can be selected that among other
properties, maximizes the thermal conductivity difference for enhanced