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.