Sensitive infrared spectroscopy of isoprene at the part per billion level using a quantum cascade laser spectrometer
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Sensitive infrared spectroscopy of isoprene at the part per billion level using a quantum cascade laser spectrometer Jacob T. Stewart1 · Jacob Beloin1 · Melanie Fournier1 · Grace Kovic1 Received: 24 February 2020 / Accepted: 7 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract A quantum cascade laser-based infrared spectrometer equipped with a multipass cell has been used to perform sensitive infrared spectroscopy of isoprene at part per billion (ppbv) concentration levels. The instrument was used to measure the absorption of the strong Q-branch of the ν26 vibrational mode of isoprene near 992 cm−1 to determine isoprene concentrations in gas-phase samples. The response of the spectrometer is highly linear in the concentration range measured (0.3–10.5 parts per million by volume) and the lowest noise-equivalent concentration determined for the spectrometer is 3.2 ppbv at an optimal averaging time of 9 s when performing measurements at atmospheric pressure. At reduced pressure (190 Torr), the lowest noise-equivalent concentration increases to 9 pbbv, but the reduced pressure decreases spectral interference caused by absorption peaks from other chemical species, namely ammonia, methanol, and carbon dioxide. The spectrometer was used to directly measure the isoprene concentration in breath samples from a volunteer without any sample processing, showing the potential real-world application of the present approach.
1 Introduction Trace gases play an important role in the chemistry of Earth’s atmosphere, making it essential to develop methods to measure their concentrations. Isoprene ( C5H8) is a trace gas that is the most abundant biogenic volatile organic compound (BVOC) emitted into Earth’s atmosphere, comprising roughly 50% of total BVOC emissions globally [1, 2]. Isoprene is readily oxidized by ozone and hydroxyl radical, leading to a relatively short atmospheric lifetime, on the order of hours, and a low atmospheric concentration, on the order of a few parts per billion by volume (ppbv) [1]. Oxidation of isoprene has been shown to contribute to the formation of secondary organic aerosols [3], particularly in the presence of anthropogenic oxidants such as nitric oxides [4]. The oxidation of isoprene in the presence of nitric oxides also leads to the formation of tropospheric ozone, a harmful pollutant that is a major component of smog [5]. The ozone-forming potential of isoprene is particularly important in major urban centers, where there is a high flux of
* Jacob T. Stewart [email protected] 1
Department of Chemistry, Connecticut College, New London, CT 06320, USA
anthropogenic nitric oxides, and prompted a recent study of isoprene emission in Beijing [6]. In addition to its importance in atmospheric chemistry, isoprene is also one of the most abundant hydrocarbons in human breath. Breath isoprene has been linked to the cholesterol biosynthesis pathway and there is interest in measuring isoprene as a way to perform noninvasive monitoring of patients [7]. For example, a
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