10
DiscussionofResults
The following comments are valid with respect to the analytes
andconditionsweused in this research. Thecommentsmaynot
hold true for other analytes and/or testing conditions.
General Observations onCarboxenAdsorbents
Asexpected the recoverywaspoor for thoseanalyteswithboiling
points higher than Benzene. This is because the Carboxen(s)
havesmall poresdesignedspecifically to retainand releaseonly
the analytes with low boiling points. The Carboxen(s) should
always be usedwith aweaker adsorbent bed placed in front. A
bedof oneormoreof theCarbopack(s)oraPorousPolymer can
beusedso thehigherboilingpoint analytesarekept fromgetting
in contact withCarboxen.
In the actual analysis, both Carbon Dioxide and Sulfur Dioxide
wereobserved inmost of theCarboxenadsorbent analyses (no
Sulfur Dioxidewas observed from theCarboxen-1016or 1018).
This is common tomost carbonmolecular sieves, and does not
present a problem unless the user is trying to sample for these
two analytes.
Carboxen-1016 isanewlydevelopedadsorbent bySupelco that
demonstratesexcellentperformanceacrossbothawide rangeof
analytesand sample volumes. This canbeobservedby review-
ing its performance chart. It is a good candidate for numerous
thermal desorption applications.
The recoveriesofTrichloroethanewerehigh (greater than145%)
for Carboxen-1000, 1002, 1003. Thiswasmost likely due to the
dehydrohalogenation of 1,1,2,2-Tetrachloroethane. The corre-
spondingrecoveryof1,1,2,2-Tetrachloroethane from thesesame
Carboxenswasvery low (less than10%).Thissituationwouldnot
occur if amulti-bed tubewasusedbecauseaweaker adsorbent
is placed in front of theCarboxenwhen sampling atmospheres
containing 1,1,2,2-Tetrachloroethane.
General Observationson theCarbosieveS-III
Itappears that theCarbosieveS-IIIperformancewasworse than
othercarbonmolecularsieves.Theporeshapeof theCarbosieve
isdifferent from theCarboxens.Carbosieveshaveclosedpores
that may have been blocked by the analytes with high boiling
points. This could have prevented some of the low boiling point
analytes from reaching the available pore sites. Like the
Carboxens, CarbosieveS-III must have aweaker bed of adsor-
bent, suchasoneof theCarbopacksorPorousPolymer, placed
in front, to prevent the analytes with high boiling points from
reaching theporesof thisadsorbentduringsampling.Carbosieve
S-III also releases Carbon Dioxide during desorption, but not
Sulfur Dioxide.
General Observations on theCarbopackAdsorbents
Theperformancecharts illustrate the increasingstrengthsof the
CarbopackswithCarbopackFbeing theweakest, followedbyC,
Y,B, andX inorderof increasingstrength.The rangeof theF,C,
andYwouldextend intohigher boiling point analytes not inves-
tigatedby thisresearch.Therecoveryof theveryvolatileanalytes
from theCarbopackXextendsbeyond that ofCarbopackB. The
recoveryof 1,3-Butadiene fromCarbopackXextendedwell into
20-Liter challenge volume. This is significant because no other
adsorbent in this research performed so well with this analyte.
TheCarbopackXcloses thegapbetween theotherCarbopack(s)
and the Carboxen(s)/Carbosieve S-III in respect to its ability to
retain the analytes across the challenge volumes. However,
CarbopackXshouldhaveaweakeradsorbentbedplaced in front
of it when sampling analyteswith very high boiling points. All of
theCarbopack(s)arevirtuallyhydrophobicandaregoodchoices
when sampling in an environment where high humidity exists.
General Observationson thePorousPolymers
None of the porous polymers could retain the very volatile
analytes.BothTenaxTAandTenaxGRperformedwell for those
analytes that had boiling points higher than Benzene. The
capabilities of Tenax TA and Tenax GR can be broadened if a
bed of Carboxen is place after the Tenax.
The Porapak N, Chromosorb 106, and HayeSep D all showed
similar patternswith the recoveries of themid to higher boiling-
pointanalytes.Thebackgroundgenerated from theseadsorbents
caused problems with obtaining clean blanks. The analytical
system had to be baked out to reduce the contamination level
between each analysis.
General Observationon theCharcoals
It is common knowledge that charcoal itself is not a good
adsorbent for thermal desorption for several reasons. The ad-
sorptive strength of charcoal can be too strong and heat alone
does not always cause the release of the analytes. This was
apparent in this research. First, the recoveries of almost all the
analytes from the first desorptionwerepoorwith theexceptionof
a few very volatileanalytes. Second, a significant amount of the
analyteswasalsoobserved from thesecond re-desorptionof the
tube. The same trend was seen on both the coconut and
petroleum based charcoals. However, there are applications
wherecharcoal isandcanbeusedasanadsorbent bed inmulti-
tube, to retain and release the very volatile analytes such as,
Halocarbon 12 and Chloromethane.
General ObservationsonSilicaGel
Silicagel showed fair recoveryof theveryvolatileanalytesat the
0.2-Liter challenge. Silicagel shouldalsohaveaweaker adsor-
bent bed placed in front of it when sampling analytes with high
boiling points. Silica gel may have applications where Carbon
Dioxidewould interfere in theanalysisof theveryvolatileanalytes,
since noCarbonDioxidewas observed in the analysis.
General Observations onGlassBeads
As expected the glass beads do not have the ability to retain
manyanalytes. Theyhaveapplications if usedas the first bed in
amulti-bed tube to prevent very high boilers to come in contact
with a stronger adsorbent.
Conclusion
The result of this researchprovides theusers of our adsorbents
and thermal desorption tubes with a new tool for choosing an
adsorbent(s) for their application. By using the colored perfor-
mance charts, one can compare and choose an adsorbent or
construct amulti-bed tube foraspecific rangeof analytesacross
various sample volumes. There is no one adsorbent available
that canboth retainand releaseall theanalytes.However, there
is clear evidence that some of our new adsorbents such as,
CarbopackXandCarboxen-1016will benefit the fieldof thermal
desorption.
