Measurement of atmospheric hydrogen peroxide and hydroxymethyl hydroperoxide with a diffusion scrubber and light emitting diode-liquid core waveguide-based fluorometry

We describe a new automated instrument for measuring gas- and aqueous-phase H2O2. The chemistry relies on the hematin-catalyzed oxidation of nonfluorescent thiamine to fluorescent thiochrome by H2O2; this reaction is 25-fold more selective for hydrogen peroxide than its nearest alkyl hydroperoxide congener, CH3HO2. The optical characteristics of the fluorescent product are such that it is ideally excited by newly available GaN-based light emitting diodes emitting in the near-UV. A stable long-life miniature flow-through fluorescence detector based on a transversely illuminated liquid core waveguide is thus used for this purpose. The limit of detection (LOD, S/N = 3) for liquid-phase H2O2 is 11 nM. A temperature-controlled high-efficiency Nafion membrane diffusion scrubber is used to collect gaseous hydrogen peroxide with near-quantitative efficiency with an S/N = 3 LOD of 13.5 pptv. The system responds to hydrogen peroxide and hydroxymethyl hydroperoxide but not to methyl hydroperoxide. The use of very inexpensive and stable reagents, highly sensitive detection, benign chemistry, and a fluorescence detector using a solid-state illumination source results in a particularly affordable automated instrument. Design and performance details and illustrative results from a 1999 field campaign (Atlanta Supersite Study) are presented.