Sensitive infrared spectroscopy of isoprene at the part per billion level using a quantum cascade laser spectrometer

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.