13
MoistureControl Systems—Water andMethanolManagement
Water andmethanol can cause the biggest problems in purge and trap concentration. During
the desorb step, water andmethanol that accumulated on the trap are released into the chro-
matographic system.Asmuch as 10µL of water can accumulate on a trap containing silica
gel during a purge; this expands to 12mL of water vapor during desorption.
5
Interference
caused by excesswater is a problem during detection. For example, water vapor passing
through a PID can cause a negative dip in the baseline.Water also can saturate a PID,
decreasing its sensitivity and interferingwith the identification of compounds that coelute
withwater. Detector saturation also can occur withMS systems.Although the lower end of
the scan range typically is adjusted above themolecular weight of water, interference can
still occur. If thewater plug is very large, the peaks for analytes that elute in the
water/methanol regionwill broaden and sensitivitywill be reduced.
In similar fashion, methanol also causes interferenceswith target analytes. The PID gives a
positive signal formethanol as a broad, flat-topped peak that usually interfereswith 2-
methylpentane, 3-methylpentane, and vinyl chloride.Adjusting theMS scan range to start
above 35amu canminimize the effects ofmethanol (mass/charge ratio of 31amu).When
using anRtx
®
-VMS column, or a cyanopropylphenyl “624”-type column, methanol and
chloroethanewill elute simultaneously. This can affect sensitivity and linearity for
chloroethane (Figure 9).When using a “502.2” phase column, methanol coeluteswith bro-
momethane. Spiking higher concentration intermediate standards into the purge vessel or
autosampler vials canminimizemethanol interference.Also note that an increase in
methanol added to the purge standard also inevitably increases the amount of water that
purges into the system.
In recent years, muchwork has gone into developing hydrophobic adsorbents that minimize
water collection on the trap. Extensive studies recommend incorporating a dry purge cycle
to removewater from the trap prior to desorption. Current designs of purge and trap systems
have added features to eliminatewater prior to delivering the sample to the chromatograph-
ic system.Moisture control systems (MCS) removewater by condensation, prior to the des-
orb step. Such systems typically are composed of a piece ofmetal tubing that is heated dur-
ing purge and then cooled to 30°C. The sample, desorbed from the heated trap, travels
through theMCS, where a large portion of thewater is condensed from the saturated carrier
gas. These systems are very effective for GCmethods that do not have polar/active com-
pounds, such as ketones, in the analyte list.An older purge and trap system that does not
have anMCS can be retrofittedwith one. Restek offers anMCS bypass line for Tekmar
3000 and 3100 purge and trap concentrators, to increase response andmaintain linearity for
ketones, alcohols, and acetates (Figure 10, page 14).When analyzing samples for ketones or
other polar compounds, theMCS should be bypassed tomaintain linear calibration for these
compounds.
Table II.
Compositions and characteristics of common types of traps.
Description
Trap Designation
Dry Purge
Preheat (°C)
Desorb (°C)
Bake (°C)
24cmTenax
®
A
yes
175
180
200
15cm Tenax
®
/8cm silica gel
B
no
175
180
200
8cm Tenax
®
/7.7cm silica gel/7.7cm charcoal
C
no
175
180
200
16cm Tenax
®
/7.7cm charcoal
D
yes
175
180
200
1cmOV
®
-1/7.7cm Tenax
®
/7.7cm silica gel/
E
no
175
180
200
7.7cm charcoal
1cmOV
®
-1/15cm Tenax
®
/7.7cm silica gel
F
no
175
180
200
1cmOV
®
-1/ 23cm Tenax
®
G
yes
245
250
260
7.6cmCarbopack
®
B/1.3cmCarbosieve
®
S-III
H
yes
245
250
260
8.5cmCarbopack
®
C/10cmCarbopack
®
B/
I (Vocarb
™
4000)
yes
245
250
260
6cmCarboxen
®
1000/1cmCarboxen
®
1001
7.7cmCarbopack
®
C/1.2cmCarbopack
®
B
J (BTEXTRAP
™
)
yes
245
250
260
10cmCarbopack
®
B/6cmCarboxen
®
1000/
K (Vocarb
™
3000)
yes
245
250
260
1cmCarboxen
®
1001
5. OI Analytical, “OI AnalyticalModel 4560 SampleConcentrator RapidTrapHeating”
ApplicationNote 04521297.
