# Simultaneous Detection of 12CH4, 13CH4, and Related Isotope Ratio Exploiting a Frequency-Multiplexed Mid-Infrared Quartz-Enhanced Photoacoustic Sensor

## TL;DR

A new sensor detects two methane isotopes at once using infrared light, enabling precise real-time analysis for environmental and industrial applications.

## Contribution

A dual-gas QEPAS sensor enables simultaneous detection of 12CH4 and 13CH4 with high precision using frequency-multiplexed lasers.

## Key findings

- The sensor achieved a precision of 1‰ in detecting isotopic delta ratio variations for 1% CH4 mixtures.
- The system demonstrated a linear response to varying 13CH4 concentrations in methane-based samples.
- The sensor is suitable for real-time, high-precision methane isotopic analysis in environmental and industrial settings.

## Abstract

We report the development of a dual-gas Quartz-Enhanced
Photoacoustic
Spectroscopy (QEPAS) sensor operating in the mid-infrared range for
the simultaneous detection of 12CH4 and 13CH4. The sensor employs a frequency-modulated
multiplexing scheme using two distributed-feedback quantum cascade
lasers to independently excite the fundamental (fo) and
overtone (f
1) vibrational modes of a quartz
tuning fork coupled with resonator tubes. The f
0-demodulated signal is dedicated to monitoring 12CH4, while the f
1-demodulated
signal selectively quantifies 13CH4, enabling
the analysis of the isotopic composition of methane samples. Calibration
measurements demonstrated a linear response of the QEPAS signal to
varying 13CH4 concentrations in CH4-based samples diluted in N2, with a precision of 1‰
in detecting isotopic delta ratio variations for 1% CH4 mixtures at 0.8 s integration time. The proposed system is suitable
for real-time, high-precision isotopic methane sensing aimed at applications
such as environmental monitoring, geochemical tracing, and industrial
process control.