Automated sampler for the measurement of non-methane organic compounds

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.