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Frequency-Domain Detection for Frequency-Division Multiplexing QEPAS.

Xiang Chen^1,2, Hao Liu^2,3, Mai Hu^2,3

¹Jinlin Institute of Technology, Nanjing 211169, China.

Sensors (Basel, Switzerland)

|June 10, 2022

Summary

This study introduces a frequency-domain detection method for quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors, enabling precise multi-gas measurements with a narrow 0.6 Hz modulation frequency interval. The method achieved low detection limits for methane and carbon dioxide simultaneously.

Keywords:

frequency-division multiplexing frequency-domain signal photoacoustic spectroscopy quartz tuning fork

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Area of Science:

* Spectroscopy
* Analytical Chemistry
* Sensor Technology

Background:

* Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a sensitive gas detection technique.
* Multi-gas measurements using QEPAS often require broader modulation frequency intervals.
* Frequency-division multiplexing (FDM) allows simultaneous detection of multiple gases but can be limited by frequency resolution.

Purpose of the Study:

* To develop and demonstrate a frequency-domain detection method for QEPAS sensors.
* To achieve precise multi-gas measurements with a narrow modulation frequency interval using FDM.
* To investigate the performance of this method for detecting methane (CH4) and carbon dioxide (CO2).

Main Methods:

* Employed a frequency-domain detection approach for QEPAS sensors.
* Utilized a narrow modulation frequency interval of 0.6 Hz for CH4 and CO2 detection.
* Obtained frequency-domain 2f signals with a 0.125 Hz resolution using a real-time frequency analyzer.
* Applied multiple linear regressions to analyze gas mixture signals.

Main Results:

* Achieved simultaneous detection of CH4 and CO2 with a narrow 0.6 Hz modulation frequency interval.
* Demonstrated small deviations within 2.5% and good linear relationships for gas detection.
* Obtained simultaneous detection limits of 0.6 ppm for CH4 and 2.9 ppm for CO2.
* Observed increased QEPAS signal amplitudes due to closer modulation frequencies to the QTF resonant frequency.

Conclusions:

* The developed frequency-domain detection method enables precise multi-gas measurements in FDM mode with narrow modulation frequency intervals.
* This approach enhances QEPAS sensor performance, particularly under low-pressure conditions.
* The method has the potential for detecting more gas species simultaneously with multiple lasers.