Anal. Chem. 2006, 78, 6320-6326
Isotopic Analysis of Atmospheric Formaldehyde by Gas Chromatography Isotope Ratio Mass Spectrometry Andrew L. Rice*
Department of Physics, Portland State University, Portland, Oregon 97207-0751 Paul D. Quay
Department of Oceanography, University of Washington, Seattle, Washington 98195-5351
Little is known about the isotopic composition of formaldehyde in the atmosphere, a chemical intermediate in hydrocarbon oxidation. Here, we present a promising new method to analyze the carbon (δ13C) and hydrogen (δD) isotopic composition of atmospheric formaldehyde. The direct isotopic analytical technique described uses continuous-flow gas chromatography-isotope ratio mass spectrometry, which provides flexibility for either isotopic analysis without correction for derivative functional groups. Current levels of precision of measurement are (1.1 and (50‰ (1σ) for δ13C and δD analyses, respectively. Concentration of formaldehyde in ambient air is also determined, coincident with isotopic measurement, to a precision of (15%. The method has the required sensitivity for analyses of formaldehyde in urban air on relatively small volume grab samples of whole air (10-70L STP), potentially providing high temporal resolution. This is particularly advantageous for studying formaldehyde given its short lifetime and large variability in the atmosphere. A key photochemical intermediate in atmospheric hydrocarbon oxidation is formaldehyde (HCHO).1 Processes that generate HCHO are generally initiated by the hydroxyl (OH) radical and ozone (O3) during the daytime and nitrate (NO3) at night. Losses due to wet and dry deposition processes will often bypass the formaldehyde intermediate. Mechanisms of these reaction chains have been detailed from the simplest hydrocarbons (e.g., methane2) to higher molecular weight volatile organic compounds (e.g., isoprene3). As one of the longest-lived intermediate species in hydrocarbon reaction sequence, HCHO thus serves as a tracer for reactive hydrocarbon chemistry in the atmosphere. It is important to study HCHO as a source of OH, CO, and H2 in polluted and clean atmospheres. The lifetime of HCHO (hours) is controlled by photolysis, which occurs through two pathways: a radical channel (λ
