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

A new automated instrument for the near–real-time measurement of atmospheric formaldehyde is described. The chemistry involves the cyclization reaction of formaldehyde with 1,3-cyclohexanedione (CHD) in the presence of ammonium ions to form a fluorescent dihydropyridine derivative. A GaN-based light-emitting diode (LED) emitting in the near UV was used as the excitation source in a miniature flowthrough fluorescence detector based on a transversely illuminated liquid-core waveguide. The instrument is configured to operate in a periodic autozero mode where the exhaust from the sampling pump is chemically 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 of detection (LOD) of 10-nM aqueous formaldehyde. A thermostated Nafion®-membrane–based diffusion scrubber is used to collect atmospheric formaldehyde into pure water with an absorption efficiency of ∼70%, which results in an LOD of 30 pptv HCHO. (In cases where the H₂O₂ to HCHO ratio is very high, as in background polar atmospheres, the LOD will deteriorate markedly.) Design, performance details, and illustrative results from a 1999 field campaign (Atlanta Supersite Study) are presented. Interference from H₂O₂ is discussed in detail. © 2001 John Wiley & Sons, Inc. Field Analyt Chem Technol 5: 2–12, 2001