Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 267

electronic devices. The following discussions give the reader an overview of
common chromatographic detectors with a focus on their analytical principles and
applications.

10.3.1 Detectors for Gas Chromatography

It is estimated that more than 60 types of GC detectors have been developed. For
most environmental applications, only a few are commonly used, including thermal
conductivity detector (TCD), flame ionization detector (FID), and electron capture
detector (ECD). We will describe these three most common GC detectors. A brief
comparison will then be made by the inclusion of four additional GC detectors that
are of some environmental applications. The mass selective detector or mass
spectrometer (MS), because of its uniqueness in both qualitative and quantitative
analysis, will be introduced in Chapter 12.

Thermal Conductivity Detector

The thermal conductivity detector was developed prior to the existence of GC. In
TCD, two gas streams (carrier gas only or carrier gas with sample) pass over two
separate heated resistors or filaments (Fig. 10.7). The resistors are made of materials
whose resistance is temperature sensitive. At constant electrical power, the
temperature of electrically heated filaments depends upon the thermal conductivity
of the surrounding gas. As carrier gas containing analytes passes through the cell, a
change in the filament current occurs. The current change is compared against the
current in a reference cell with carrier gas only. The difference is measured and a
signal is generated. The resistances of the twin-detector pairs are usually compared
by incorporating them into two arms of a simple Wheatstone Bridge circuit (the
classical method for measurement of resistance).
The thermal conductivities of helium and hydrogen are roughly 6-10 times
greater than those of most organic compounds. Thus, in the presence of even small
amounts of organic materials, a relatively large decrease in the thermal conductivity

Signal output

Out Out

Filaments

From column
(Helium

Reference
(Helium gas)

+ Sample)

Figure 10.7 Side view of a typical thermal conductivity detector (TCD). From a top view
(omitted), an arrangement of two sample detector cells and two reference detector cells can be seen.
The filaments in four cells are incorporated into a Wheatstone bridge circuit used to measure electrical
resistance