Optimizing the Analysis of Volatile Organic Compounds

12
Figure 8.
Type “K” trap (Vocarb TM 3000 trap)
showing the various layers.
Vent
Purge Flow
10cm Carbopack B
(for mid- and later-eluting volatiles)
6cm Carboxen 1000
(for gases)
1cm Carboxen 1001
(to prevent breakthrough)
www.restekcorp.com
Choosing the Right Trap for Your Analysis
Type “K” Trap (Vocarb 3000 Trap): The most effective trap on the market is the
Vocarb 3000 or type “K” trap (Figure 8). This trap has exceptional ability to retain highly
volatile compounds like difluorodichloromethane with minimal bleed, activity, or breakdown,
yet it works well for trapping higher boiling compounds like naphthalene and
trichlorobenzene. The trap resists adsorption of water and methanol, and virtually eliminates
the need for moisture 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 required when using
Tekmar purge and trap instruments. For OI Analytical sample concentrators, such as the
Model 4560, the desorb temperature should be 220°C or lower. The lower temperature will
prevent overshooting the maximum temperature of the trap, which would damage the packing
materials (caused by the rapid trap heating rate, 800°C/min., of the OI 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 trap must 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 less water and methanol compared to the “K” trap,
and can withstand higher temperatures than the Tenax ® /silica gel trap. Because many GRO
samples have high concentrations of gasoline components, it is necessary to dilute the sample
in methanol, and this trap can accept a heavy sample load with percent levels of
methanol while still passing continuing calibration check criteria. The disadvantage of the
“J” trap is its limited ability to retain more 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 containing methyl-tert-butyl ether
(MTBE), trap selection will depend on the sample matrix. When analyzing highly contaminated
soils for MTBE, it is best to use the “J” trap. For cleaner samples, the “K” trap provides
better sensitivity.
Type “B” Trap (Tenax ® /Silica Gel Traps): Tenax ® /silica gel traps are used for a variety of
VOC methods. These traps exhibit better recoveries of polar analytes than the “K” trap, but
the silica gel layer adsorbs water, methanol, and carbon dioxide. The Tenax ® /silica gel trap
also has better lot-to-lot reproducibility compared to the “K” or “I” traps. For laboratories
that are not trying to achieve MDLs for gaseous VOCs at concentrations above 10ppb, these
traps will work well. To achieve detection limits for gases at concentrations below 10ppb,
the lower water and methanol retention of the “K” trap is recommended.
Type “F” Trap (OV ® -1 /Tenax ® /Silica Gel Traps): Although these traps are recommended in
many EPA methods, they exhibit more bleed and activity than the Tenax ® /silica gel trap,
with no significant improvement in performance. This suggests the bleed originates from the
OV-1 (methyl silicone) material. 4 Therefore, laboratories wishing to adhere more closely to
the EPA Method Protocol should choose Tenex ® /silica gel traps without OV ® -1.
Type “I” trap (Vocarb 4000): The “I” trap is used for increased response for less volatile
compounds such as the chloronaphthalenes and methylnaphthalenes. Generally, it is used
only for applications involving analytes of larger molecular size and is not the first choice
for ketones or alcohols. Common desorb times of two to four minutes should be increased
with the “I” trap, to optimize sensitivity for compounds having high boiling points.
3. OI Analytical, “Volatile Organics Analysis: Building a State-of-the-Art Purge and Trap GC/MS
system” Application Note 02971294.
4. OI Analytical, “Proper Trap Selection for the OI Analytical Model 4460A Purge and Trap Sample
Concentrator” Application Note 12851098.