Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 272

Table 10.6 Applicability of various GC detectors for the analysis of five analytes §

TCD FID ECD PID NPD Hall MS

Benzene X X N X N N X

DDT X X X X N X X

a-BHC X Poor X N N X X

Chlordane X Poor X X N X X

TCE X Poor X N N X X

X: Applicable; N: Not applicable

depends on the number of carbons, and its sensitivity is low for non-hydrogen
containing organics. Consequently, FID will be sensitive enough for benzene and
DDT, but unlikely for a-BHC, chlordane, and TCE. Hall detector, particularly ECD,
is very sensitive to halogen-containing compounds. Hence, the non-halogen benzene
cannot be measured through these two detectors. Since PID uses UV to ionize the
compounds, two saturated halogens (a-BHC and TCE) will be very unlikely to be
ionized by UV irradiation.

10.3.2 Detectors for High Performance Liquid
Chromatography

Compared to various GC detectors, the numbers of available HPLC detectors are
limited. An earlier survey in 1982 indicated three HPLC detectors most commonly
used in workplace, 71% of which are UV detectors, 15% are fluorescence detectors,
and 5.3% refractive index detectors. Apparently, mass selective detector coupled
with HPLC has recently been shown to be a powerful tool that is also complimentary
to GC-MS. When LC-MS becomes increasingly affordable, routine use in the future
is likely in environmental labs as well. We introduce below these three common
HPLC detectors; the details on LC-MS will be given in Chapter 12.

UV Detectors

For a UV detector to be responsive, the analyte should have strong UV absorbing
functionalities as introduced in Chapter 8. In principle, UV detectors used in HPLC
are the same as the UV detectors in the normal UV-VIS spectrometers. The marked
difference between these two is the size of the sample cells. In UV-VIS
spectrometers, a typical size of sample cells is 1 mL with a light path length of
1 cm. The typical UV cell volumes in HPLC, however, are limited to 1-10 L and

cell lengths to 2-10 mm. The much smaller cell volume is essential to avoid peak
broadening for an improved resolution. The most powerful UV spectrophotometric
detectors currently used are the photodiode array (PDA) detectors, which permit
simultaneous detection of spectral data in a range of wavelengths. Thus, spectral
data for each peak can be rapidly collected and stored as a sample passes through to a
flow cell from the column.

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