If you are using the Vocarb3000 trap and desorbing above 250C, there is a warning in Method 5030b about breakdown of the trap material contributing to a background of bromomethane. This could cause problems when looking at such low levels in samples.
4.6.5.5 Alternate Trap Materials
A number of hydrophobic carbon molecular sieve and graphitized carbon black
materials have been developed. Various combinations of these materials have been
shown to provide retention properties similar to the Tenax\Silica gel\Carbon trap.
Alternate trap construction with such materials is allowed, provided that the adsorption
and desorption characteristics obtained achieve equivalent or better method sensitivity
and precision in comparison to the performance documented in the Determinative
Method.
4.6.5.5.1 The following alternatives have been shown to be viable
for most analytes of concern:
7.6-cm CarbopackTM B/1.3-cm CarboseiveTM S-III
VOCARB 3000 - 10.0-cm CarbopackTM B/6.0-cm Carboxin TM 1000/1.0-cm
CarboxinTM 1001
VOCARB 4000 - 8.5-cm CarbopackTM C/10.0-cm Carbopack TM B/6.0-cm
CarboxinTM 1000/1.0-cm CarboxinTM 1001
These combinations require rapid heating to desorption temperatures of 245EC
to 270EC (follow manufacturer's instructions). At these increased temperatures,
catalytic and thermal decomposition of analytes has been reported. The
VOCARB 4000 combination has also been demonstrated to catalytically break
down 2-chloroethyl vinyl ether, and to partially decompose 2,2-dichloropropane.
Bromoform and bromomethane have shown some thermal decomposition.
4.6.5.5.2 The amount of thermal decomposition products formed
must be routinely tracked by daily monitoring of the formation of chloromethane
and bromomethane. A daily check standard containing surrogates, internal
standards, and 20 μg/L bromoform must be analyzed prior to the analysis of the
daily check standard. If levels of chloromethane or bromomethane exceed 0.5
μg/L, then the trap may be too contaminated with salts or tightly bound
contamination for analysis to continue. The trap must be replaced and the
system recalibrated.
NOTE: Even newly constructed traps may have become
contaminated prior to their first use from airborne vapors.
These highly adsorptive materials must be kept tightly
sealed in an area of minimum organic vapor
contamination.
