Abstract
In this paper, we report on the progress of the auto-triggered quartz-enhanced photoacoustic spectroscopy (QEPAS) technique which operates without external frequency generators and ensures permanent locking to the current resonance frequency of the tuning fork. This is obtained by incorporating the tuning fork in an oscillator circuit that autonomously oscillates at the present resonance frequency that shifts with changing environmental conditions, e.g., density and viscosity of the surrounding gas, temperature, and pressure. Both, the oscillation amplitude as well as the frequency can be read from the oscillator circuit. The photoacoustic signal appears as an offset of the electrically induced signal amplitude. Since the sum amplitude depends on the phase relation between the electrical and photoacoustic driving forces, the phase is permanently modulated, enabling the extraction of the photoacoustic component by use of a second lock-in amplifier stage which is being referenced with the phase modulation frequency. The functionality of this method is demonstrated for methane detection in a carbon dioxide atmosphere in a concentration range from 0 to 100% and ammonia in synthetic air employing a pulsed mid infrared QCL around 1280 cm−1. The gas mixtures are motivated by the demands in biogas-analysis.
Highlights
Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a type of photoacoustic spectroscopy which employs a piezoelectric quartz tuning fork instead of a microphone as a transducer
Further progress in QEPAS has been made in terms of using custom tuning forks with larger gaps and lower resonance frequencies to use light sources with larger focal diameters such as LEDs or THz-QCLs or by incorporating the photoacoustic cell and transducer, called spectrophone, in a multipass cell to increase the power density and the sensitivity
Provided that the excitation laser addresses multiple species, the large dynamic range of photoacoustic spectroscopy enables the measurement of both, main components as well as trace gases of a gas mixture
Summary
Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a type of photoacoustic spectroscopy which employs a piezoelectric quartz tuning fork instead of a microphone as a transducer. For a simple QEPAS setup only a bare quartz tuning fork is required; in most spectrophones the tuning fork is equipped with acoustic resonators [5] The most common approach is to use two cylindrical pipe elements sandwiching the tuning fork in a way that the radiation is aimed through both segments and between the prongs of the tuning fork In this configuration best sensitivity is achieved when the length of a segment is between half and a quarter of the sound wavelength. Provided that the excitation laser addresses multiple species, the large dynamic range of photoacoustic spectroscopy enables the measurement of both, main components as well as trace gases of a gas mixture This is one demand for biogas analysis. In this paper a new detection scheme for using both electrical and photoacoustic driving of the tuning fork and phase modulation of these two driving forces is discussed
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