IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 56, NO. 2, APRIL2007 305 Towards Realization of Reactive Gas Amount of Substance Standards Through Spectroscopic Measurements Pamela M. Chu, David D. Nelson, Jr., Mark S. Zahniser, J. Barry McManus, Quan Shi, and John C. Travis Abstract—This paper discusses the use of high-precision direct absorption spectroscopy to realize the amount of substance stan- dards for ozone (O 3 ), a reactive species which cannot be delivered through traditional gravimetrically based gas cylinder standards. The O 3 amount of substance is linked to the known concentra- tions of nitric oxide (NO) and nitrogen dioxide (NO 2 ) through a gas-phase titration reaction and spectroscopic measurements us- ing room-temperature pulsed quantum-cascade lasers. Ultimately, this approach will allow the determination of an O 3 transition line strength linked to the primary gravimetric standards of NO and NO 2 . The line strength can be used in future measurements to determine O 3 concentrations of unknown samples. Index Terms—Absorption spectroscopy, gas reference stan- dards, ozone, traceability. I. I NTRODUCTION T HERE is an ongoing interest in monitoring the ambient levels of reactive gaseous species such as ozone (O 3 ), in part, due to concerns over health effects as well as their important role in atmospheric chemical processes. O 3 is a pow- erful oxidant and is toxic to humans, animals, and vegetation at high concentrations. Ambient O 3 is typically present in the troposphere at mixing ratios of 30–60 nmol mol -1 . These levels are heavily influenced by the overall air quality and often serve as an indicator of the general pollution level of a given airshed. To understand the overall atmospheric chemistry and promote effective regulations, accurate measurements of O 3 and other related chemical species are critical. To support research and regulatory measurement require- ments, the National Institute of Standards and Technology (NIST) has been providing gravimetrically based primary-gas- mixture standard-reference materials for calibrating instrumen- tation used to measure various components of gas mixtures. These primary gas mixtures are supplied in an aluminum-alloy cylinder, but it is difficult to deliver the amount of substance standards for certain species in this manner because reactions might degrade the quality of the sample. An alternative approach for delivering known-gas- concentration mixtures is to use the absorption of electromag- netic radiation to determine the analyte concentration as it is Manuscript received August 1, 2006; revised October 30, 2006. P. M. Chu and J. C. Travis are with the National Institute of Standards and Technology, Gaithersburg, MD 20899 USA. D. D. Nelson, Jr., M. S. Zahniser, and J. B. McManus are with the Aerodyne Research, Inc., Billerica, MA 01821 USA. Q. Shi is with the Sionex Corporation, Bedford, MA 01730 USA. Digital Object Identifier 10.1109/TIM.2007.891151 being delivered in real time. The absorption process directly probes intrinsic properties of the chemical species and will follow the well-established Beer–Lambert law. Assuming that there are no other competing nonlinear processes and accurate knowledge of the spectroscopy, the absorbency will scale predictably with the analyte concentration at a given temper- ature and pressure. The O 3 standard-reference photometer maintained at NIST uses the Beer–Lambert law to determine known concentrations of O 3 in purified air from 1 nmol mol -1 to 1 μmol mol -1 from absorbency measurements at 253.7 nm [1]. The uncertainty of the O 3 standard was initially thought to be dominated by a 1% stated uncertainty in the O 3 absorption coefficient [2]. Recent studies, however, have identified other potential biases within the instrument [3]. The growing interest in global O 3 measure- ments and international standards, as well as recent advances in spectroscopic-measurement capabilities has encouraged a re- examination of the O 3 standard-reference photometer. In this paper, we present the ongoing efforts at NIST to further improve the delivery of the amount of sub- stance standards for O 3 by using simultaneous high-resolution direct-absorption measurements of nitric oxide (NO), nitrogen dioxide (NO 2 ), and O 3 and using the newly developed room- temperature pulsed quantum-cascade (QC) lasers. The absolute nature of direct-absorption measurements provides a major advantage over other common spectroscopic techniques such as the chemiluminesence detection of NO and laser-induced fluorescence detection of NO 2 . By using high-resolution spec- troscopy in the mid-infrared region, specific rovibrational tran- sitions of a given chemical species can be probed, allowing greater selectivity compared to probing unresolved electronic transitions in the UV region. The initial measurement of the fundamental molecular- absorption parameters requires well-characterized samples of known concentrations. This can be a major challenge, espe- cially for reactive species. Absorption cross-sectional measure- ments of O 3 often use the sample pressure to infer the O 3 concentration [4]. An alternative approach which provides a more specific determination of the O 3 concentrations is to relate the O 3 concentration to NO and NO 2 primary gravimetric standards through the gas-phase-titration reaction shown in the following expression: NO + O 3 → NO 2 + O 2 k NO+O 3 (298 K)=1.9 × 10 -14 cm 3 mol -1 s -1 (1) 0018-9456/$25.00 © 2007 IEEE