Raman scattering applied to human breath analysis

Abstract

The chemical composition of exhaled human breath can be strongly correlated to medical conditions such as lung cancer or gastrointestinal diseases. To establish these correlations and, most importantly, to use them in diagnostics, molecular gas detection needs to be performed at trace concentrations, typically at parts-per-million (ppm) levels or below, for many compounds simultaneously. Traditional methods such as gas chromatography, a workhorse in scientific laboratories, is ill-suited for the fast, inexpensive point-of-care diagnostics that would be needed to build statistically meaningful ensembles over large populations. With the increasing availability and decreasing cost of high-power diode lasers and of uncooled CMOS cameras, spontaneous Raman spectroscopy (SRS), a vibrational molecular fingerprinting tool, is emerging as an economic alternative. Although gas SRS scattering cross sections are much smaller than, e.g., Rayleigh scattering cross sections, considerable progress in the development of enhancement techniques has been made over the past decade. This work reviews SRS enhancement approaches in the context of established human breath tests and provides a comparison with alternatives. Already, numerous trace gases such as H₂, CH₄, ¹³CO₂, and volatile organic compounds like acetone can be rapidly quantified in breath at concentrations below 1 ppm with SRS. With improvements in resolution and design of enhancement systems, SRS-based sensors could be scalably deployed in, e.g., pharmacies, and non-invasively screen for dozens of analytes at the parts-per-billion level.