Figure 8.
Type “K” trap (Vocarb
TM
3000 trap)
showing the various layers.
6cmCarboxen 1000
(for gases)
1cmCarboxen 1001
(to prevent breakthrough)
12
Choosing theRight Trap forYourAnalysis
Type “K”Trap (Vocarb™ 3000Trap):
Themost effective trap on themarket is the
Vocarb™ 3000 or type “K” trap (Figure 8). This trap has exceptional ability to retain highly
volatile compounds like difluorodichloromethanewithminimal bleed, activity, or break-
down, yet it workswell for trapping higher boiling compounds like naphthalene and
trichlorobenzene. The trap resists adsorption of water andmethanol, and virtually eliminates
the need formoisture control systems (MCS) and the dry purge step on the concentrator.
Because this trap contains Carboxen™ 1000 adsorbent (described on page 11), which has a
surface area of over 1200m
2
/g, a desorb temperature of 245°C is requiredwhen using
Tekmar purge and trap instruments. For OIAnalytical sample concentrators, such as the
Model 4560, the desorb temperature should be 220°C or lower. The lower temperaturewill
prevent overshooting themaximum temperature of the trap, whichwould damage the pack-
ingmaterials (caused by the rapid trap heating rate, 800°C/min., of theOI system).
3
When
using this trap be sure to verify performance. Non-linear response for chloromethane is a
sign of breakthrough and an indication that the trapmust be changed.Another indication of
a defective “K” trap is loss in response for acrolein.
Type “J”Trap (BTEXTRAP™Trap):
The “J” trap is excellent for concentrating gasoline
range organics (GRO) because it retains lesswater andmethanol compared to the “K” trap,
and canwithstand higher temperatures than theTenax
®
/silica gel trap. BecausemanyGRO
samples have high concentrations of gasoline components, it is necessary to dilute the sam-
ple inmethanol, and this trap can accept a heavy sample loadwith percent levels of
methanol while still passing continuing calibration check criteria. The disadvantage of the
“J” trap is its limited ability to retainmore polar analytes like the ethers and alcohols.
Laboratories analyzing for
tert
-butyl alcohol will attain lower detection limits by using the
“K” trap, compared to the “J” trap. For GRO samples containingmethyl-
tert
-butyl ether
(MTBE), trap selectionwill depend on the samplematrix.When analyzing highly contami-
nated soils forMTBE, it is best to use the “J” trap. For cleaner samples, the “K” trap pro-
vides better sensitivity.
Type “B”Trap (Tenax
®
/SilicaGel Traps):
Tenax
®
/silica gel traps are used for a variety of
VOCmethods. These traps exhibit better recoveries of polar analytes than the “K” trap, but
the silica gel layer adsorbswater, methanol, and carbon dioxide. TheTenax
®
/silica gel trap
also has better lot-to-lot reproducibility compared to the “K” or “I” traps. For laboratories
that are not trying to achieveMDLs for gaseousVOCs at concentrations above 10ppb, these
trapswill workwell. To achieve detection limits for gases at concentrations below 10ppb,
the lower water andmethanol retention of the “K” trap is recommended.
Type “F”Trap (OV
®
-1 /Tenax
®
/SilicaGel Traps):
Although these traps are recommended in
manyEPAmethods, they exhibit more bleed and activity than theTenax
®
/silica gel trap,
with no significant improvement in performance. This suggests the bleed originates from the
OV-1 (methyl silicone)material.
4
Therefore, laboratorieswishing to adheremore closely to
the EPAMethod Protocol should chooseTenex
®
/silica gel trapswithout OV
®
-1.
Type “I” trap (Vocarb 4000™):
The “I” trap is used for increased response for less volatile
compounds such as the chloronaphthalenes andmethylnaphthalenes. Generally, it is used
only for applications involving analytes of largermolecular size and is not the first choice
for ketones or alcohols. Common desorb times of two to fourminutes should be increased
with the “I” trap, to optimize sensitivity for compounds having high boiling points.
10cmCarbopackB
(formid- and later-eluting volatiles)
Purge Flow
Vent
3. OI Analytical, “VolatileOrganics Analysis: Building a State-of-the-Art Purge andTrapGC/MS
system”ApplicationNote 02971294.
4. OI Analytical, “Proper Trap Selection for theOI AnalyticalModel 4460APurge andTrap Sample
Concentrator”ApplicationNote 12851098.
