Anal. Chem. 2000, 72, 5338-5347
Measurement of Atmospheric Hydrogen Peroxide and Hydroxymethyl Hydroperoxide with a Diffusion Scrubber and Light Emitting Diode-Liquid Core Waveguide-Based Fluorometry Jianzhong Li and Purnendu K. Dasgupta*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
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 longlife 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 temperaturecontrolled 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. Hydrogen peroxide is an important analyte in clinical chemistry. It also plays a central role in atmospheric studies due to its ability to oxidize dissolved SO2.1 Hydrogen peroxide, methyl hydroperoxide (MHP), and hydroxymethyl hydroperoxide (HMHP) are of particular interest and detailed reviews are available.2 In the aqueous phase, H2O2 is the primary analyte of interest since MHP displays a much lower Henry’s law constant3 and HMHP T H2O2 + HCHO exhibits a dissociation constant of 6 mM at 25 °C.4 Luminol chemiluminescence (CL) can be very sensitive and has been exploited for the measurement of marine * Corresponding author: (e-mail)
[email protected]. (1) Navas, M. J.; Jimenez, A. M.; Galan, G. Atmos. Environ. 1999, 33, 22792283. (2) Gunz, W. G.; Hoffman, M. R. Atmos. Environ, 1990, 24A, 1601-1634. Jackson, A. V. Crit. Rev. Environ. Sci. Technol. 1999, 29, 175-228. (3) O’Sullivan, D. W.; Lee, M.; Noone, B. C.; Heikes, B. G. J. Phys. Chem. 1996, 100, 3241-3247. (4) Zhou, X.; Lee, Y.-N. J. Phys. Chem. 1992, 96, 265-272.
5338 Analytical Chemistry, Vol. 72, No. 21, November 1, 2000
H2O2.5 Many oxidants generate luminol CL; direct measurement of H2O2(g) by luminol CL is difficult. Alternative CL techniques for H2O2(g) have been developed.6 Few have attempted to measure gaseous peroxides electrochemically7 because it is not sufficiently sensitive. Introduced by Lazrus et al.,8 fluorometry dominates present atmospheric peroxide measurements. There are pedagogical merits to fluorometric H2O2 measurements as well.9 Typically, a nonfluorescent substrate is oxidized by H2O2 to a fluorescent product mediated by a peroxidase enzyme, typically horseradish peroxidase (HRP).8-10 Cost and stability issues of HRP have led to attempts to use photocatalysis11 and other nonenzymatic reactions.12 Synthetic metalloporphyrins have been extensively studied as HRP mimics13 and shown to be effective.14 Phthalocyanine complexes are effective catalysts in both acidic and basic solutions.15 Several Fe-porphine compounds, inexpensive commercial products from animal blood, display peroxidatic activity.14 Bovine hematin displays a greater peroxidatic activity per unit mass than any commercial HRP preparation at
