An automated sampler has been constructed for the unattended collection of
whole-air samples in Summa passivated stainless steel canisters. The sample
r consists of a Viton diaphragm pump; a differential-pressure flow controll
er; a 2-position, 3-port valve; a 10-port multiposition valve; and a digita
l Valve sequence programmer that controls the sequence and timing of the el
ectrically actuated valves. All connecting tubing was constructed from Silc
osteel tubing. Two configurations of the automated sampler and a passive co
llection system were evaluated by comparing the combined sampling precision
s of the three systems with the analytic precision, derived from replicate
analyses of an ambient air sample. The analytic precision was generally < /-5%, with higher values observed for analytes with mixing ratios in the te
ns of pptv. However, analytic precision values for methanol and ethanol wer
e poor, greater than +/-20%. Values for sampling precision were greater by
about a factor of 2 than the analytic precision. The poor results for the l
ight oxygenated hydrocarbons could be caused by sorption of the analytes in
the preconcentration and sampling systems and difficulties in peak integra
tion.