ceivable that in sampling air nearly saturated with water vapor, the silica gel might become inactive after exposure to a relatively small sample. This can be eliminated by the inclusion of a trap cooled to dry ice temperature before the detector tube. Substances which form colored compounds with the reagent are more difficult to remove.Any substance having a negligible vapor pressure at -78°C. can be removed in the U-tube trap, or, if present in sufficiently small quantity, by the glass beads in the detector tube preceding the gel layer. Un-
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