Ambient air sampling for tritium--determination of breakthrough volumes and collection efficiencies for silica gel adsorbent.

Abstract

Ambient air samples for tritium (as HTO) can be collected using the solid adsorbent silica gel. The purpose of this study was to determine the maximum practical sampling volume and overall collection efficiency for water vapor collected on silica gel columns and to demonstrate the use of an impinger-based system to load water vapor onto silica gel columns. Breakthrough volumes (Vb) were measured and chromatographic efficiencies (expressed as the number of theoretical plates, N) were calculated for a 20 degrees C to 50 degrees C temperature range, with the relative humidity at approximately 30%. The tests yielded relative breakthrough volumes (air volume/adsorbent depth, m3 cm(-1)) of 0.36 for 20 degrees C, 0.20 for 30 degrees C, 0.15 for 40 degrees C, and 0.077 for 50 degrees C. For 18-cm columns, the average tritium tracer recoveries at 20 degrees C were 71% with no observed breakthrough for air volumes up to 5 m3, while at 40 degrees C mean tritium tracer recoveries dropped from 75% for volumes < or = 3.0 m3, to 0% for a volume of 5.0 m3. Frontal chromatographic profiles were measured for water vapor migrating through silica gel columns that were divided into 5 segments. The chromatographic efficiency of the silica gel columns was determined by graphical evaluation of the chromatography profiles. At a sampling rate of 0.25 L min(-1) and 30% relative humidity, the number of theoretical plates per adsorbent depth were 0.55 N cm(-1) at 20 degrees C, 0.68 N cm(-1) at 30 degrees C, 0.51 N cm(-1) at 40 degrees C, and 0.30 N cm(-1) at 50 degrees C. Chromatographic theory was used to estimate the overall collection efficiency of the silica gel columns as a function of the ratio of the sampling volume to breakthrough volume and the chromatographic efficiency. For a 9.5 m3 sample volume, 30% relative humidity, 0.25 L min(-1) sampling rate, and a 54-cm column, the overall collection efficiency was above 99.9% at 20 degrees C, above 95% at 30 degrees C, just below 80% at 40 degrees C, and <<80% at 50 degrees C.