## 2 Resonance Properties of Spectrophotones  
The spectrophotometer SP1 employs a standard 32.7 kHz quartz tuning fork (QTF) coupled with two 4.4 mm long tubes having inner diameters of 0.6 mm and positioned 50 \(\upmu\)m distant from the QTF surface. This resonator tubes configuration has represented the reference spectrophotone structure for QEPAS experiments until 2013, allowing a signal-to-noise ratio (SNR) enhancement up to a factor of 30 compared to a bare QTF [19]. Spectrophotone SP2 is composed of a QTF with T-shape prongs coupled with two 12.4 mm long resonator tubes having inner diameters of 1.59 mm and positioned 200 \(\upmu\)m distant from the QTF. The prongs spacing is 800 \(\upmu\)m, 2.7 larger than the standard QTF. Enlarging of the prong spacing from 300 \(\upmu\)m to 800 \(\upmu\)m allowed the reduction of the 1\(\sigma\) noise level by one order of magnitude when operating with mid-infrared quantum cascade laser sources and focusing the optical beam through the spectrophotometer by using a single lens. A sketch of SP1 and SP2 is shown in Figure 1.  
The acoustic properties of both spectrophotones were firstly measured. QTF vibration can be electrically induced by providing a sinusoidal voltage excitation. Thus, the QTF piezo-current can be converted in a voltage signal by means of a transimpedance amplifier and demodulated at the excitation frequency by means of a lock-in amplifier. The SP1 and SP2 spectrophotones are mounted in two acoustic detection modules (ADM1 and ADM2, respectively) composed of a vacuum-tight gas cell equipped with two ZnSe wedged 1/2 inch windows, with an antireflection coating in the 1.65-3 \(\upmu\)m spectral range, and gas-in and gas-out connectors. Each ADM consists of a preamplifier with a gain-bandwidth optimized for the related QTF operating frequency. Resonance curves of both spectrophotones can be retrieved by varying the excitation frequency step-by-step. These measurements were performed in air, at atmospheric pressure. The resonance curves were acquired at the respective  
Figure 1: Schematics of spectrophotones SP1 (**left**) and SP2 (**right**) employed in this article. The main geometrical parameters are also reported.  
fundamental in-plane flexural mode resonance frequencies and normalized to the peak values are reported in Figure 2.  
To determine the resonance properties of both spectrophotones, namely the resonance frequency and the related quality factor, the resonance curves can be fitted by a Lorentzian function. The resonance peak broadening is proportional to the energy losses occurring in the vibrating prongs. Furthermore, the larger the broadening, the higher the losses [40]. The quality factor Q was calculated as the ratio between the resonance frequency and the full-width-half-maximum (FWHM) value of the resonance curve. For ADM1 a resonance frequency of 0.81 Hz and a quality factor of ~5080 was measured while SP2 is characterized by a fundamental resonant mode frequency of 0.67 Hz with a quality factor of ~9930. ADM2 exhibits a higher Q-factor with respect to ADM1 despite a lower resonance frequency. Both aspects are beneficial for QEPAS: low resonance frequencies are beneficial for slow relaxing gases and high quality factors positively affecting the QEPAS signal [41].