Introduction to Soil Chemistry

gas chromatography 181
Table 9.3. Common Detection Methods for Chromatography
Gas Thermal conductivity (TC or TCD)
Flame ionization (FID)
Mass spectrometry (MS or GC/MS)
High-precision-liquid Ultraviolet or visible (UV–Vis)
Refractive index
Conductivity
Thin-layer Fluorescence-indicator-impregnated
Visualization reagents
Charring with acid and heat
Electrophoresis Staining
Ultraviolet or visible (UV–Vis)
Conductivity
9.2.4. Detection
There are three main types of detectors used in gas chromatography: thermal
conductivity (TC), also known as a hot-wire detector, flame ionization (FID),
and quadrupole mass spectrometer (MS) (Table 9.3). The TC detector consist
of coils of high-resistance wire in a detector block where the carrier gas from
the gas chromatograph exits the column and flows over the wire. The coils of
wire are arranged in a Wheatstone bridge with two arms receiving gas exiting
the column and other two receiving the pure carrier gas. The heat capacity of
the carrier gas changes when a compound in the carrier gas is exiting the
column; this changes the resistance characteristics of the coil, which are
recorded. TC is a universal detector but it is also the least sensitive of the
common detectors.
The flame ionization detector has a small hydrogen flame into which the
carrier gas exits. There is a voltage across the flame, which is nonconducting.
When an organic compound exits the column, it is burned and produces ions,
which conduct electricity. The electrical signal thus produced is recorded. FID
is a highly sensitive detector although not the most sensitive. It will respond
only to organic compounds, or compounds that will burn, and exiting
compounds are thus destroyed in the process.
The quadrupole mass spectrometer detector or mass filter, as it is sometimes
called, is extremely sensitive and allows the identification of compounds
exiting the gas chromatograph. The compounds are destroyed as they exit the
chromatograph and enter the mass spectrometer (shown in Figure 9.5), where
they are ionized, fragmented, and analyzed. Other sensitive but also more
selective detectors are available and may be called for in certain analyses.
9.2.5. Gas Chromatography Applied to Soil Analysis
Soil extracts are most commonly characterized by gas chromatography and
gas chromatography–mass spectrometry. For example, the composition of soil