Cavity-enhanced absorption spectroscopy with a deep UV-LED

Abstract

The motivation for this project work was to build a portable analyser based on a UV LED to carry out in situ measurements of trace gas species under ambient conditions applied to atmospheric monitoring and breath analysis. Acetone has gained vital importance as a volatile organic compound (VOC) which is found as a pollutant in the atmosphere. Acetone also serves as a biomarker for metabolic processes particularly in people on a ketogenic diet or patients suffering from diabetic ketoacidosis (DKA), a life-threatening condition that can be experienced by Type 1 Diabetic patients. The project work mainly consisted of two parts: design of a portable set-up and as a proof of principle that a UV-LED based acetone detector is feasible instead of the most commonly used laser-based detector. Acetone absorption measurements were carried out in nitrogen background using a 300 nm LED employing the technique of Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS). The design of the portable set-up is based on a cage-system to achieve portability, robustness, mechanical stability, insensitivity to temperature and pressure variations, and cost effectiveness. The designed portable set-up has potential applications as a detector providing in situ real-time detection of trace gas species in atmospheric chemistry and in medical diagnostics as a breath acetone analyser. An incoherent light source such as an LED requires a different approach to light collimation and guiding compared to a laser. In the UV range this poses additional challenges due to a lack of off-the-shelf optical components for this purpose. In this project, a stable optical cavity was successfully set-up to carry out acetone absorption measurements in nitrogen background using CEAS. A noise-equivalent absorption coefficient of 4.1 ± 0.3 x 10−6 cm-1Hz-1/2 (corresponding to ~2 ppmv of acetone) was achieved with the CEAS set-up.