Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 258

retentive. This will save analytical time. The a value is okay (indicative of possible separation),
and the resolution (R ¼ 4:35) is large enough for a good separation of two chemicals. It is
perhaps unnecessarily too large from the analytical time standpoint, which is consistent with
the k values calculated above.
10.1.5 Use of Chromatograms for Qualitative
and Quantitative Analysis
In a chromatogram, qualitative information of all chromatographic techniques can
be obtained by comparing the retention time of an analyte to that of a pure
standard. This will provide tentative identification of unknown compounds if the
analyst has a short list of compounds in mind. Further confirmative identification
of the compounds, if needed, can be obtained by other techniques, such as GC-
MS, LC-MS, and NMR (Chapter 12). It should also be noted that, although
chromatograms may not lead to positive identification of chemical species, they
can often provide sure evidence of the absence of certain compounds. That is, if
the sample does not produce the peak with the same retention time as the standard,
then it can be assumed that the chemical in question is not present at the given
detection limit.
Quantitative chromatographic analyses are achieved by determining instru-
mental signals from the chromatograms. Signals are normally measured by peak
area or less commonly by peak height, either one of which should be in direct
proportion with the change in analyte concentrations. Peak areas are better because
areas are independent of broadening effects. Peak heights suffer from many
variations, such as column temperature, flow rate, and sample introduction rate.
A standard calibration curve (i.e., a regression between area and concentration, see
Chapter 2) must be performed first. In some cases, an internal standard method
(area ratio of analyte to internal standard) can be used as an alternative (Section
9.4). In many areas of environmental research, authentic standards are not
commercially available and the analyst would have to synthesize a standard
on their own. If this is impossible, definite concentrations cannot be reported
and relative amounts of the compound based on instrumental signal may be
permissible.
10.2 INSTRUMENTS OF CHROMATOGRAPHIC METHODS
This section describes the operating principles of GC, HPLC, and IC. SFC, a hybrid
of GC and HPLC, is only briefly discussed since the popularity of SFC is still limited
at the present time.
10.2.1 Gas Chromatography
A schematic diagram of a typical GC system is given in Figure 10.3. The basic
components of a simple gas chromatography include the following: (a) carrier gas