2
ExperimentalDetails
Adsorbents Tested
We tested twenty-four different adsorbents. Carboxen(s),
CarbosieveS-III,andCarbopack(s)areexclusive toSupelcoand
havebeenused in the fieldof thermal desorptionandpurgeand
trap for years.We also chose adsorbents such as Tenax, silica
gel, and glass beads because of their traditional use in the field
of thermal desorption. Porapak
®
, Chromosorb
®
and HayeSep
®
arealsoused insome thermal desorptionapplications.Coconut
and petroleum charcoal predominately have been used for
solvent desorptionapplications, but someuses of thesemateri-
als do exist in thermal desorption applications.
For this research, onlyone lot per adsorbent was tested.
Table1
shows the listofadsorbents testedand thephysical propertiesof
theadsorbentssuchas themeshsize, packingdensity, andbed
weights.
AnalytesUsed as theTest Probe
Theanalyteschosenas testprobes for this researchareasubset
of the EPA Hazardous Pollutant list, and are also common to
many industrial hygiene samplingmethods.We used a gasmix
containing the 43 analytes listed in
Table 2
. Thismix contained
a broad spectrum of volatile organic analytes with physical
properties that range from (50 to 260) inmolecular weight, and
(-30 to 215°C) in boiling point. The gas mix is available as a
Supelco stock product Catalog #500429. The concentration of
each analyte in the gas mix is 1000ppb. We introduced a 20-
milliliter undiluted volume of this gas mix to each adsorbent.
(
Table2
shows thecalculatedmassofeachanalytecontained in
the 20mL volume).
Wechose thegasmix forseveral reasons.First, theanalytesare
in the gas phase to simulate a real world sample. Second, if we
haduseda liquidsolventmix, suchasmethanol, it couldalter the
resultsbecause it toomayoccupy theporesitesof theadsorbent.
Thiscouldcreateacompetition forsorptionsiteswith theanalytes
of the test mix. Third, the use of a solvent would interfere in the
detection of the very volatile analytes. This is due to the chro-
matographicconditions thatwechose tooptimize the transfer of
the analytes to the capillary column.
AnalyticalEquipment
Thermal Desorber
GERSTEL
®
loaned the thermal desorptionunit used in thisstudy
toSupelco. TheGERSTELTDSA, shown in
Figure1,
provided
the means to automate the analysis of the adsorbents. The
TDSA interfaceswith theGERSTELCIS4 Inlet thatservesas the
cryo-focusing trap for the desorption of the adsorbents.
Cryo-FocusingTrap
The GERSTEL CIS 4 inlet was used to re-focus the analytes
desorbed from theadsorbents.The injectionport linerof the inlet
contained twodifferentmaterials to facilitate the retentionof the
very volatile analytes in the test mix.We used liquid nitrogen to
cool the inlet liner to -150°C during the desorption of the adsor-
bent tubes.Wedesorbed the inletat350°C.Weusedastandard
inlet liner (available fromGERSTEL GC07540 10) and packed
the inlet with the following adsorbents:
●
CarbotrapC 20/40mesh: 10mm bed length
(25milligrams)
●
Glass Beads 60mesh: 6mm bed length (25milligrams)
This inlet configuration was determined after we performed
several experiments to optimize the chromatography of the gas
mix.
Figure2
shoesanexampleof thechromatographyachieved
with this set-up. (Notice the resolution of the first five analytes).
Figure2. TheResultsof theTestGas
Desorbed fromaCarbotrap300
GasChromatograph
Supelco used a Hewlett Packard 6890 GC with a 5973 mass
selective detector (Turbo Pump System) for the study. The
capillary column was a 60 meter x 0.25mm ID, 3.0µm film
SPB-1 column.
Other Equipment Used
●
Supelco’s prototype “Adsorbent Tube Injector System”
served as the device to transfer the gasmix onto the
adsorbent packed tubes.
●
DynathermModel 60 Six-TubeConditioner served as a
means to condition the packed adsorbent tubes. A
second unit served as away to control the flow rate
throughmultiple tubes simultaneously for the following
volume challenges: 1, 2, 5, 10, 20, and 100 liters.
●
Mettler Balancemodel AE100 served as away to
determine the actual bed-weights of each packed
adsorbent tube.
Table 3
shows the operating conditions for the equipment.
The largeCO
2
isconcentratedonto the refocusing trapduring theprocessof the
TDSA loading theadsorbent tube into thedesorber oven.
Figure1. GERSTELTDSACoupled
toaHP6890GC/5973MSD
CO
2
