23
Detection Systems
VOCs can be analyzed using a variety of detection systems, includingGC/MS, GC/PID, and
GC/ELCD. Here, we discuss consequences of using each of these systems, and present tips
formaintenance and troubleshooting.
ColumnConfigurations
SingleColumn:
Environmental engineers characterize a contaminated site usingMS or
dual-columnGC, or theymight monitor the site solelywith single-columnGCmethods.
Injections of standards on a single column, delivered to the detector, can provide tentative
identification and quantification. Retention times for analytes listed in a givenmethod are
established by injecting a check standard containing all of the target compounds. Retention
times for analytes in site samples are compared against retention times for the standard, to
verify if unknown compoundsmatch known targets.A single column configurationworks
well with characterized samples, but retention times are not unique for every analyte, espe-
cially for early-eluting compounds that spend little time in the stationary phase (e.g., Freon
®
compounds). In environmental laboratories coelutions from non-target compounds also are
very common, creating very complex chromatograms that are difficult to interpret using a
single-column design.
Dual Columns:
In a dual-column configuration, the sample passes through a fused silica
guard column, then is split between two analytical columns of differing selectivity.
Standards are injected to establish retention times on both columns simultaneously. One dis-
advantage to this configuration is the 50% loss in sensitivity resulting from splitting the
sample. This loss can be overcome by increasing the sample volume or by optimizing the
detector. Flow rates for the two columns should agreewithin 20% because uneven splitting
will further affect sensitivity.
Detector Configurations
Detectors can be connected in parallel, in series, or in tandem, to double the amount of
information about the sample.
Parallel System:
In a parallel system the sample is split equally between the two detectors,
allowing both detectors to be destructive (e.g., ELCD/FID). This detection systemworks
well but is unsuitable for a dual-column analytical configuration because the sample already
will have been split between the two columns.
Series System:
Series detection involves connecting two detectors in sequence, using a short
length of deactivatedmetal or fused silica tubing. The sample passes through the first detec-
tor, whichmust be non-destructive (e.g., PID), then through the second detector. This pro-
duces two sets of information about the samplewith no loss in sensitivity because the sam-
ple volume does not change from the first detector to the second. The only disadvantage is
dead volume, which can broaden the peaks.Minimize dead volume byminimizing the
length and ID of the line connecting the detectors.
TandemSystem:
The tandem configuration connects two detectorswithout the dead volume
associatedwith a series system. The non-destructive first detector is the base for the second
detector. The units can be connected to a single detector port on oneGC. Thismakes it pos-
sible to use a dual-column configuration, with each column connecting to tandem detectors,
producing four sets of data per analysis. This approach is used inEPAMethod 8021.
9
Detectors:
Method requirements determine the choice of detector(s). The current shift
toward analysis by performance-based criteriamakes it possible to use detection other than
that listed in amethod if it can be shown that performance is similar to, or better than, what
would be attained by following the guidelines in themethod. Themost commonGC detec-
tors are the PID, the FID, and the ELCD. GC/MS eliminates the need for a confirmation
column.
9. R.D. Braun. Introduction to Instrumental Analysis. McGraw-Hill BookCompany. NewYork. 1987.
pp. 915-916.
