11
Questions&Answers
Whywere the adsorbents packed by bed-volume versus
bed-weight?
Because the density range of the adsorbents tested varied
significantly, packing the adsorbents at the same bed-weight
was not feasible. For example, if we would have packed the
adsorbentsall at thesamebed-weights, someof theadsorbents
would have extended past the heated zone of the thermal
desorber.Other tubeswouldhavehad too littleadsorbent in the
tube for the tests.Theactual bed-weightsandmeshsizeof each
adsorbent canbeseen in
Table1
.Theadvantageof packing the
tube by bed-volume for this research is that the bed-length of
3.7cm occupies about half of the average heated- zone of most
thermal desorbers. This allows at least twodifferent adsorbents
to be packed in most thermal desorption tubes. By using the
same bed-volume of adsorbent as conducted in this research,
theuser canexpect similar performance from theadsorbentsby
using the colored charts.
What mesh sizewere the adsorbents?
The mesh size of the adsorbent ranged from 20/40 mesh to
60/80mesh. It isvirtually impossible toacquire theadsorbentsall
at onemesh size.
Whywasnitrogenused insteadofairtochallengethetubes?
Nitrogen was used because of its purity compared to com-
pressed air. If compressed air would have been used the
adsorbentswouldhaveconcentrated theslightestcontaminants.
Also there is a significant amount of water inmost air systems,
which would have required extensive efforts to reduce the
moisture content.
Whywas 50mL/min chosen as the sampling flow rate?
The flow rate used during the challenges remained constant at
50mL/min. TheUSEPATO-1method (3) recommends that the
linear flow velocity through an adsorbent tube be 50-500cm/
minute. UsingEquation 1, the calculated linear velocity through
a 4mm sampling tubes used in this studywas 398cm/min).
Were any test analytes retainedon the glass frit at the
inlet of each tube?
No, not any of theseanalytes.We tested this by spiking thegas
mix on to the empty fritted glass tubes and analyzed them right
away. No significant quantity of any analytewas detected
Whywas an Internal Standardnot used?
An internal standardcouldnotbeused,becausenooneorgroup
of analytes could have been retained on all the adsorbents. For
example, there were only a few analytes retained on the glass
beads.So ifwehadusedahighboilingpointanalyte for theglass
beads, thesameanalytewouldnot havebeen released from the
Carboxen(s)/CarbosieveS-III.Aseparate internalstandardwould
have been needed for each of the adsorbents, thusmaking the
use of this technique not very helpful.
How canwe assume 100% recovery from theCarbotrap
300 used for the calibration?
For this research, all we could dowas assume 100% recovery.
Other models could have been researched, but the important
thing to keep in mind that performance charts are meant to
illustrate the relativedifferencebetween thevariousadsorbents.
We do not attempt to say the recoveries are absolute.
Could the desorption temperature have an affect on
recovery?
Yes, the desorption temperature could have both positive and
negativeaffectson recovery. For this research, our attemptwas
to choose the highest temperature typically used.
What is thedifferencebetweenCarbopacksandCarbotrap?
Theonlydifference is themeshsizeof theadsorbents.Carbotraps
are20/40mesh,andCarbopacksare40/60meshorsmaller.The
performancechartscanalsobeused incomparing theCarbotrap
adsorbents.
References
1. Method2549VolatileOrganicCompounds,NIOSHManual of Analytical
MethodsFourthedition1996
2. CompendiumofMethods forDeterminationof ToxicOrganicCompounds in
Ambient Air EPATO-17Determinationof VOCs inAmbient AirUsingActive
SamplingontoSorbent TubesSecondEdition1997
3. CompendiumofMethods forDeterminationof ToxicOrganicCompounds in
Ambient Air EPATO-1Determinationof VOCs inAmbient AirUsingTenax
AdsorptionandGC/MSpageTO-1 thru9
Acknowledgement
Theauthorwould like to thankGERSTEL for theuseof their equipment for this
research. Theautomatedabilityof TDSAeased theburdenofmethoddevelop-
ment for this research.
Patents
CarbosieveAdsorbent—GermanPatentNo1935500. PatentHolder—Badishe
Anilin-&Soda-FabrikAktiengesellschaft.
Carboxen-564Adsorbent—USpat.No. 4,839,331
Trademarks
CeliteCorp. -Chromosorb
CrawfordFittingCo. -Swagelok
EnkaResearch InstituteArhem - Tenax
Gerstel GmbH -GERSTEL
HayesSeparations Inc. -HayeSep
Lab-Line -Multi-Blok
MicrosoftCorporation -Excel
Sigma-Aldrich -Carbopack,Carbotrap,Carboxen
WatersAssociates. Inc. -Porapak
What ConcentrationDoes
Challenge Volume
20mLGasVolumeRepresent
0.2 Liters
100ppb
1 Liter
20ppb
5 Liter
4ppb
10 Liter
2ppb
20 Liter
1ppb
100 Liter
0.2ppb
Equation 1
B = linear velocity (cm/min)
Q = flow rate (mL/min)
= 3.14
r
2
= inside radius of the tube (cm)
What does a 20mL syringe volume of the 1000ppb gasmix
relate to in a real world sample?
The table below illustrates what the ppb concentration of the
20mL syringe volumewould represent based on if the contents
were released into the corresponding volumes. Example: If the
20mLsyringevolumeof the1000ppb test gasmixwere released
into a 5-Liter sealed volume, the concentration of the gas mix
would be diluted to 4ppb.
Q
B=
r
2
