Optimizing the Analysis of Volatile Organic Compounds

Detection Systems
VOCs can be analyzed using a variety of detection systems, including GC/MS, GC/PID, and
GC/ELCD. Here, we discuss consequences of using each of these systems, and present tips
for maintenance and troubleshooting.
Column Configurations
Single Column: Environmental engineers characterize a contaminated site using MS or
dual-column GC, or they might monitor the site solely with single-column GC methods.
Injections of standards on a single column, delivered to the detector, can provide tentative
identification and quantification. Retention times for analytes listed in a given method 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 compounds match known targets. A single column configuration works
well with characterized samples, but retention times are not unique for every analyte, especially
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 disadvantage
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 agree within 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 system works
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 deactivated metal or fused silica tubing. The sample passes through the first detector,
which must be non-destructive (e.g., PID), then through the second detector. This produces
two sets of information about the sample with no loss in sensitivity because the sample
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 by minimizing the
length and ID of the line connecting the detectors.
Tandem System: The tandem configuration connects two detectors without the dead volume
associated with 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 one GC. This makes it possible
to use a dual-column configuration, with each column connecting to tandem detectors,
producing four sets of data per analysis. This approach is used in EPA Method 8021. 9
Detectors: Method requirements determine the choice of detector(s). The current shift
toward analysis by performance-based criteria makes it possible to use detection other than
that listed in a method if it can be shown that performance is similar to, or better than, what
would be attained by following the guidelines in the method. The most common GC detectors
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 Book Company. New York. 1987.
pp. 915-916.
23
www.restekcorp.com