Optimizing the Analysis of Volatile Organic Compounds

Applications Using GC Detection Systems
Purge and Trap Applications Using Tandem PID-ELCD:
US EPA Methods 502.2, 601, 602, 8010, 8020, 8021B
EPA methods for GC analyses of volatile compounds require purge and trap units for concentrating
the contaminants in water, soil, or wastewater. While purge and trap concentration
significantly increases sensitivity, relative to other sample introduction techniques, it does
have a downside: early-eluting volatile compounds typically exhibit broad peaks, due to
inefficient sample transfer from the trap to the GC. This distorted peak shape decreases resolution
between closely eluting compounds, placing demands on the analytical system and
requiring optimized GC operating conditions. Although cryofocusing improves separations
of early eluting compounds, most environmental laboratories do not use this approach
because it increases costs.
EPA methods for monitoring volatiles by GC often recommend using a PID and an ELCD,
connected in tandem or series. Coelutions of target compounds are allowed, as long as they
are resolved by the detectors. 13 For example, in Figure 39 bromoform and styrene elute with
the same retention time, but bromoform elicits a response only from the ELCD and styrene
elicits a response only from the PID. Thus, the selective detectors resolve these two compounds.
Because it characteristically produces tailing peaks, the ELCD is the more problematic
of the two detectors; sensitivity can be increased, but not without a sacrifice in peak
shape. Optimization of an ELCD minimizes tailing and maximizes sensitivity.
Analysis Time: Several factors contribute to the total analysis time for volatiles separations,
including purge and trap cycle time, sample analysis time, and GC oven cool-down time
(time required for the oven to cool from the final temperature to the initial temperature for
the next analysis). Long purge and trap cycles are a product of long purge times, dry purges,
long desorb times, and long trap bake times. Long oven cycle times result from low initial
oven temperatures (i.e., subambient to 35°C) and slow temperature program rates. A column
that unnecessarily exceeds the length needed to resolve the analytes can increase analysis
time and cost without significantly adding to the data obtained.
An Rtx ® -VGC primary column paired with an Rtx ® -VRX confirmation column make a good
combination for analyzing the compounds listed in Figures 39A & B. The target list
includes unregulated but commonly analyzed compounds such as methyl-tert-butyl ether
(MTBE) and Freon ® 113 (1,1,2-trichloro-1,2,2-trifluoroethane). A 35°C starting temperature
is necessary to resolve Freon ® 113 from 1,1-dichloroethane. Figure 39A shows there are no
early-analyte coelution problems on the primary column when using PID/ELCD detectors in
tandem – the gases and the trihalomethanes are separated.
Figures 40A & B show the analysis of Method 8021A/502.2 compounds, without Freon ®
113, using an Rtx ® -VGC column and an Rtx ® -502.2 column. A 50°C initial oven temperature
can be used, which greatly reduces the time needed for the GC to complete the oven
cycle and return to the starting temperature (cycle time) and, therefore, increases throughput.
An Agilent 5890 GC oven will cool from 205°C to 35°C in 9 minutes; this time, added
to the 28-minute analysis time in Figure 39, produces the fastest cycle time for this analysis:
37 minutes. In the analysis in Figure 40, the starting temperature is 50°C, the final temperature
is 200°C, and the oven takes 4 minutes to cool. The total cycle time, less than 30 minutes,
is significantly faster than for other pairs of columns. For example, an Rtx ® -VRX column
requires a starting temperature of 40°C; this, combined with a 28 minute analysis time,
means the total cycle time cannot be faster than 35 minutes.
13. EPA Method 8000B, Determinative Chromatographic Separations; US EPA. U.S. Government
Printing Office: Washington, DC, 1996, Rev. 2.
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