Optimizing the Analysis of Volatile Organic Compounds

Broad Peaks: Peak broadening is another problem often experienced when analyzing VOCs
by purge and trap methods. Broad peaks are caused either by poor sample transfer from the
purge and trap to the GC or by dead volume within either system. Broad peaks frequently
result from dead volume in the connection between the purge and trap unit and the GC system.
(See pages 18-21 for connection methods.) Because trap desorption is a relatively slow
process, the sample band transferred to the GC can be very wide. To reduce this bandwidth,
the sample must be transferred quickly and/or refocused at the inlet of the GC column
through a secondary cold trapping technique such as cryofocusing or subambient cooling.
The transfer time can be reduced by using the desorb preheat feature. During this step, the
trap is heated to 5°C below the desorption temperature, and the valve is positioned so no
flow passes through the trap. This helps the compounds trapped on the adsorbents to rapidly
migrate from the trap when backflushing begins.
The desorb flow rate also will affect the bandwidth. If the desorb flow is too low (9mL/min.) in conjunction with long, thick-film columns, the bandwidth can be
reduced enough so that secondary trapping is not required. Ideally, desorbing at a flow rate of
20-30mL/min. yields a very narrow bandwidth. However, when using narrow bore columns, it
might be necessary to split the flow at the injection port to maintain column efficiency.
Foaming Samples: Analysts deal with foaming samples in two primary ways: by dilution or
by addition of an anti-foaming agent. Diluting the sample compromises the detection limit,
but in the end may save instrument downtime. Anti-foaming agents such as polydimethylsiloxane
and silicon dioxide methylcellulose are designed to reduce foaming of surfactants in
a liquid matrix. These are effective at preventing a sample from foaming, but they generally
produce artifact peaks that can interfere with the target analytes. An anti-foam blank must be
run prior to samples to determine the contribution of artifact peaks from the anti-foaming
agent. If dilution or anti-foaming agents do not reduce foaming or if samples have not been
screened for surfactants, use a 5 or 10mL sample in a 25mL purge vessel to prevent the bubbles
from entering the fittings and, ultimately, the trap. If you are running an unattended
autosampler, you can insert a plug of deactivated fused silica or glass wool into the top of
the purge vessel to prevent foam from entering the purge and trap lines. If all else fails consider
switching to a fritless sparge tube and increasing the purge time to effectively remove
the volatile analytes.
Figure 12.
A low desorb flow can produce tailing peaks, as in this example, desorbed at 9mL/min.
EPA Method 8020, Rtx
1
3
1. benzene
2. α,α,α-trifluorotoluene (SS)
3. toluene
4. ethylbenzene
5. m-xylene
6. p-xylene
7. o-xylene
2
® -5Sil MS column, 40m, 0.45mm ID, 1.5µm (cat. #12798)
Carrier gas: 9mL/min. @ constant pressure
GC: Finnigan 9001
Detector: FID
Oven temp.: 40°C (hold 2 min.)
85°C @ 4°C/min.
(hold 1 min.) to
225°C @ 40°C/min.
(hold 2 min.)
min. 3.50 4.50 5.50 6.50 7.50 8.50 9.50 10.50 11.50
4
5,6
7
Wastewater samples commonly
contain surfactants and other
material that can contaminate
the concentrator.
17
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