Optimizing the Analysis of Volatile Organic Compounds

Troubleshooting Common Problems
Associated with Purge and Trap Units
Water: The most common problem with VOC analysis is water in the sample. Water does
not create problems with the purge and trap system, but it will create problems with the GC
system. A large quantity of water can quench the PID response, causing a negative dip in the
baseline. Water also can saturate the MS and create interference with early eluting gases.
Analysts using an MS can observe the water band eluting from the column as a large baseline
rise. Since VOC samples typically are aqueous or contain large amounts of water, water
vapor will be purged along with the target compounds. Although water cannot be completely
prevented from collecting on the trap, the amount transferred to the GC system can be
minimized by using a trap that contains hydrophobic adsorbents (see Adsorbent Materials
and Traps, page 10). A dry purge step also may remove water from the adsorbent surface
(see Sequences and Flow Paths, page 8). Many new purge and trap systems employ water
management to minimize the amount of water reaching the GC system, but be aware that
these systems can have adverse effects on the recovery of polar compounds such as ketones
(page 13). Many laboratories purge 5mL-10mL of sample in 25mL purge vessels (see
photo); water condenses on the inner wall of the vessel, reducing the amount of moisture
that ultimately gets onto the trap.
Leaks and Active Sites: Another common problem in purge and trap systems is reduced
sensitivity caused by leaks or active sites in the system. Reduced sensitivity for all compounds
normally indicates a leak. To test for leaks in the purge and trap system, perform a
pressure decay test by capping off the purge vent during the purge cycle. The bubbles passing
through the purge vessel should stop within 2 to 10 minutes. If the bubbles do not stop,
there is a leak in the purge system. To locate the leak, use a leak detector. Start checking for
leaks at the purge vessel and work back to the inlet line on the back of the instrument.
Leaks most commonly occur at the purge vessel and at the trap fittings.
Reduced sensitivity for specific compounds usually indicates the presence of active sites in
the system. Poor response for bromoform or other brominated compounds is a good indicator
of active sites in the purge and trap unit or transfer line. However, poor bromoform
response also can be caused by high transfer line temperatures (>130°C). Reduce the transfer
line temperature and determine if bromoform recoveries improve. Another component
that decomposes due to active sites is 1,1,2,2-tetrachloroethane (Figure 11). 6 To reduce or
eliminate sources of activity, clean or replace sources of contamination, including internal
gas lines and the transfer line. Inert Silcosteel ® -treated tubing is an excellent choice for replumbing
purge and trap gas lines. Tekmar’s newest purge and trap concentrator, Model
3100, incorporates Silcosteel ® treatment on all tubing and internal surfaces.
Ghost Peaks: Ghost peaks typically are caused by carryover from sample components that
collect within the purge and trap system. This problem is most common when performing
total petroleum hydrocarbon (TPH) analysis because these samples often contain high
molecular weight components. If the valve oven and transfer line temperatures are set too
low, high molecular weight compounds can condense in the line, then bleed onto the column.
To eliminate ghost peaks, temporarily increase the purge and trap valve oven and
transfer line temperatures to bake out the contaminants. The heated mount feature on some
purge and trap instruments can reduce carryover by up to 50%, but this also will increase
the amount of moisture entering the trap. The standard mount temperature is 40°C; increasing
the temperature to 70°C significantly reduces sample carryover. For severe contamination,
steam cleaning or methanol rinsing can be performed (see instructions on page 11).
Unlike in other cleaning procedures, here we do not recommend using water after methanol
rinsing because it is very difficult to remove water from the purge and trap system. Ghost
peaks also are caused by adsorbent contamination or degradation. Tenax ® can break down to
toluene, benzene, styrene, naphthalene, and other aromatic compounds (see Adsorbent
Materials and Traps, page 10). This normally is an indication of trap overheating. To prevent
this problem, do not expose a trap containing Tenax ® adsorbent to temperatures above 200°C.
6. Tekmar-Dohrmann, Purge and Trap Concentrator Course, 1989. Cincinnati, Ohio.
Purging a 5mL sample in a 25mL
purge vessel, to reduce water
transfer to the trap.
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