15
Troubleshooting Common Problems
Associatedwith Purge and Trap Units
Water:
Themost common problemwithVOC analysis iswater in the sample.Water does
not create problemswith the purge and trap system, but it will create problemswith theGC
system.A large quantity of water can quench the PID response, causing a negative dip in the
baseline.Water also can saturate theMS and create interferencewith early eluting gases.
Analysts using anMS can observe thewater band eluting from the column as a large base-
line rise. SinceVOC samples typically are aqueous or contain large amounts of water, water
vapor will be purged alongwith the target compounds.Althoughwater cannot be complete-
ly prevented from collecting on the trap, the amount transferred to theGC system can be
minimized by using a trap that contains hydrophobic adsorbents (seeAdsorbentMaterials
andTraps, page 10).A dry purge step alsomay removewater from the adsorbent surface
(see Sequences and FlowPaths, page 8).Many new purge and trap systems employwater
management tominimize the amount of water reaching theGC 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 ofmoisture
that ultimately gets onto the trap.
Leaks andActive 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 com-
pounds 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 pass-
ing through the purge vessel should stopwithin 2 to 10minutes. 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 andwork back to the inlet line on the back of the instrument.
Leaksmost 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 indica-
tor 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 trans-
fer 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 re-
plumbing 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 ismost commonwhen 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, highmolecular weight compounds can condense in the line, then bleed onto the col-
umn. To eliminate ghost peaks, temporarily increase the purge and trap valve oven and
transfer line temperatures to bake out the contaminants. The heatedmount feature on some
purge and trap instruments can reduce carryover by up to 50%, but this alsowill increase
the amount ofmoisture entering the trap. The standardmount temperature is 40°C; increas-
ing the temperature to 70°C significantly reduces sample carryover. For severe contamina-
tion, steam cleaning ormethanol rinsing can be performed (see instructions on page 11).
Unlike in other cleaning procedures, herewe do not recommend usingwater aftermethanol
rinsing because it is very difficult to removewater 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 (seeAdsorbent
Materials andTraps, page 10). This normally is an indication of trap overheating. To prevent
this problem, do not expose a trap containingTenax
®
adsorbent to temperatures above 200°C.
Purging a 5mL sample in a 25mL
purge vessel, to reducewater
transfer to the trap.
6. Tekmar-Dohrmann, Purge andTrapConcentrator Course, 1989. Cincinnati, Ohio.
