AUTOMATED HIGH-SPEED ANALYSIS OF SELECTED ORGANIC-COMPOUNDS IN URBAN AIR BY ONLINE ISOTOPIC DILUTION CRYOFOCUSING GAS-CHROMATOGRAPHY MASS-SPECTROMETRY

An automated environmental air monitor has been developed to measure s elected organic compounds in urban air. The instrument is based on a c ryofocusing-thermal desorption gas chromatographic mass spectrometry t echnique where the mass spectrometer is a slightly modified residual g as analyzer (RGA). The RGA was chosen as a detector because the whole system must be robust for long periods, with 24-h continuous air monit oring. RGA are extremely simple and seemed the most reliable mass spec trometers for this purpose. Moreover, because they have no physically limited ion source, contamination is considerably reduced, so maintena nce intervals are longer. The gas chromatograph is equipped with a com puter-controlled six-way sampling valve, with a 100-mL sampling loop a nd thermal desorption cold trap injector. Environmental air is enriche d with an isotopically labeled internal standard in the sampling line. This internal standard is added with a validated, custom-made, permea tion tube device. The ''on-line'' internal standard provides for high quality quantitative data because all variations in instrument sensiti vity in cryofocusing or in thermal desorption efficiency are taken int o account. High repetition rates (down to 5 min for a full analytical cycle) are obtained with the use of an isothermal gas chromatography p rogram, microbore capillary column, and environmental air sampling dur ing the gas chromatography run.