Optimizing the Analysis of Volatile Organic Compounds

Adsorbent Materials
Tenax ® Adsorbent (surface area: 50m 2 /g): Tenax ® adsorbent is excellent for trapping nonpolar
compounds and is hydrophobic so it does not retain water; however, it does have some
disadvantages. Very volatile compounds are not retained well and must 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-diphenyleneoxide
polymer thermally decomposes into toluene, benzene, and other aromatics. The particles
melt 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 trapping material: Tenax ® GC and
Tenax ® TA (Trapping Agent) adsorbents. Common background contaminants in Tenax ® GC
adsorbent include benzene and toluene. Tenax ® TA adsorbent is a purer form and is more
commonly recommended for thermal desorption applications. The manufacturer’s recommended
operating temperature is 230°C but, realistically, the material performs best when
kept below 200°C. Samples that contain organic acids can degrade Tenax ® adsorbent. This
effect is more pronounced at higher temperatures; for longer trap life and better consistency
do not use traps containing this adsorbent at temperatures above 200°C.
Silica Gel (surface area: 200-800m 2 /g): Silica gel is a stronger adsorbent than Tenax ®
adsorbent. Silica gel is commonly used in conjunction with Tenax ® adsorbent as a trap for
volatile organic pollutants. It is an excellent trapping material for polar and highly volatile
compounds that are gases at room temperature; however, silica gel is extremely hydrophilic
and will retain large amounts of water. Be aware that if a trap contains silica gel, dry purging
will not reduce the water 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 significant
amounts of water. It does, however, trap carbon dioxide (CO2) purged from the sample,
and it has been reported that charcoal is a source of CO2, which can interfere with the
quantitation of early-eluting compounds when using GC/MS systems.
Graphitized Carbon Black or Carbopack ® Adsorbent (surface area: 10-100m 2 /g):
Graphitized carbon black (GCB) is an alternative to Tenax ® 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 retained well on GCB and must be
trapped on stronger adsorbent materials such as carbon molecular sieves.
Carbon Molecular Sieves (surface area: 50-800m 2 /g): Carbon molecular sieves such as
Carbosieve -SIII are alternatives to silica gel and charcoal. High surface areas make these
materials ideal for trapping highly volatile compounds. They are commonly used in series
after GCB because they retain compounds that break through the GCB. Carbon molecular
sieves are hydrophobic and have excellent thermal stability.
Carboxen ® -1000 Adsorbent (surface area: 1200m 2 /g): Carboxen ® -1000 adsorbent is a
strong adsorbent designed to be used as the innermost adsorbent bed in the trap (Figure 8,
page 12). This material traps Freon ® compounds, permanent gases, and light hydrocarbons.
It has characteristics very similar to those of Carbosieve ® S-III packing material. Carboxen ® -
1000 adsorbent is stable to temperatures of 300°C. Its only shortcoming is the adsorption of
CO2, which can interfere with early-eluting compounds. 2 Carboxen ® -1001 and Carboxen ® -
1002 are similar materials.
2. Mosesman, N.H., W.R. Betz, and S.D. Corman. "Alternate Trapping Materials for Purge-and-Trap
Analysis of Volatile Compounds." Proc.-Water Qual. Technol. Conf. Adv. Water Anal. Treat..
14 (1987): 245-50.
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