restek-tnx14 - page 177

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AdsorbentMaterials
Tenax® Adsorbent (surface area: 50m 2/g): Tenax® adsorbent is excellent for trapping non-
polar compounds and is hydrophobic so it does not retainwater; however, it does have some
disadvantages.Very volatile compounds are not retainedwell andmust be trapped on a
stronger adsorbent material. In addition, polar compounds like alcohols are poorly retained
on this adsorbent. Tenax ® adsorbent also has limited thermal stability; the 2,6-diphenyle-
neoxide polymer thermally decomposes into toluene, benzene, and other aromatics. The par-
ticlesmelt together and permanently adhere to the trap; this then restricts carrier gas flow.
As the adsorbent degrades, there often is a loss in response for brominated compounds.
There are two grades of Tenax ® adsorbent used as a trappingmaterial: Tenax® GC and
Tenax® TA (TrappingAgent) adsorbents. Common background contaminants inTenax® GC
adsorbent include benzene and toluene. Tenax® TA adsorbent is a purer form and ismore
commonly recommended for thermal desorption applications. Themanufacturer’s recom-
mended operating temperature is 230°C but, realistically, thematerial performs best when
kept below 200°C. Samples that contain organic acids can degradeTenax® adsorbent. This
effect ismore pronounced at higher temperatures; for longer trap life and better consistency
do not use traps containing this adsorbent at temperatures above 200°C.
SilicaGel (surface area: 200-800m2/g): Silica gel is a stronger adsorbent thanTenax®
adsorbent. Silica gel is commonly used in conjunctionwithTenax® adsorbent as a trap for
volatile organic pollutants. It is an excellent trappingmaterial for polar and highly volatile
compounds that are gases at room temperature; however, silica gel is extremely hydrophilic
andwill retain large amounts of water. Be aware that if a trap contains silica gel, dry purg-
ingwill not reduce thewater content.
Coconut Charcoal (surface area: 900m 2/g): Coconut charcoal is another strong adsorbent
material. It is commonly used in series after silica gel for trapping very volatile compounds
that might break through the gel. Coconut charcoal is hydrophobic, and does not retain sig-
nificant amounts of water. It does, however, trap carbon dioxide (CO2 ) purged from the sam-
ple, and it has been reported that charcoal is a source of CO 2, which can interferewith the
quantitation of early-eluting compoundswhen usingGC/MS systems.
GraphitizedCarbonBlack or Carbopack ® Adsorbent (surface area: 10-100m2/g):
Graphitized carbon black (GCB) is an alternative toTenax® adsorbent. GCB is available in
many pore sizes and is effective in trapping volatile organics in the same range as Tenax®
adsorbent. GCB is hydrophobic and has excellent thermal stability, making it ideal for purge
and trap techniques. Highly volatile compounds are not retainedwell onGCB andmust be
trapped on stronger adsorbent materials such as carbonmolecular sieves.
CarbonMolecular Sieves (surface area: 50-800m2 /g): Carbonmolecular sieves such as
Carbosieve™ -SIII are alternatives to silica gel and charcoal. High surface areasmake these
materials ideal for trapping highly volatile compounds. They are commonly used in series
after GCB because they retain compounds that break through theGCB. Carbonmolecular
sieves are hydrophobic and have excellent thermal stability.
Carboxen ®-1000Adsorbent (surface area: 1200m2/g): Carboxen® -1000 adsorbent is a
strong adsorbent designed to be used as the innermost adsorbent bed in the trap (Figure 8,
page 12). Thismaterial traps Freon ® compounds, permanent gases, and light hydrocarbons.
It has characteristics very similar to those of Carbosieve ® S-III packingmaterial. Carboxen®-
1000 adsorbent is stable to temperatures of 300°C. Its only shortcoming is the adsorption of
CO 2, which can interferewith early-eluting compounds.2 Carboxen® -1001 andCarboxen®-
1002 are similarmaterials.
2. Mosesman, N.H.,W.R. Betz, and S.D. Corman. "Alternate TrappingMaterials for Purge-and-Trap
Analysis of VolatileCompounds." Proc.-WaterQual. Technol. Conf. Adv.Water Anal. Treat..
14 (1987): 245-50.