Measurement of atmospheric formaldehyde with a diffusion scrubber and light-emitting diode-liquid-core waveguide based fluorometry

A new automated instrument for the near-realtime measurement of atmospheric formaldehyde is described, The chemistry involves the cyclization reaction of formaldehyde with 1,3-cyclohexanedione (CHD) in the presence of ammoniu m ions to form a fluorescent dihydropyridine derivative. A GaN-based light- emitting diode (LED) emitting in the near UV was used as the excitation sou rce in a miniature flowthrough fluorescence detector based on a transversel y illuminated liquid-core waveguide. The instrument is configured to operat e in a periodic autozero mode where the exhaust from the sampling pump is c hemically treated to provide zero gas for automated periodic checks of the baseline. The liquid-phase portion of the system provides a S/N = 3 limit o f detection (LOD) of 10-nM aqueous formaldehyde, A thermostated Nafion (R) -membrane-based diffusion scrubber is used to collect atmospheric formaldeh yde into pure water with an absorption efficiency of similar to 70%, which results in an LOD of 30 pptv HCHO. (In cases where the H2O2 to HCHO ratio i s very high, as in background polar atmospheres, the LOD will deteriorate m arkedly.) Design, performance details, and illustrative results from a 1999 field campaign (Atlanta Supersite Study) are presented. Interference from H2O2 is discussed in detail. (C) 2001 John Wiley & Sons, Inc.