Branch Enhanced Photoacoustic Sensor for Comprehensive Sevoflurane Monitoring

Comprehensive sevoflurane monitoring (CSM, sevoflurane concentrations from parts per billion to percent) is essential for the therapeutic diagnosis of anesthetized patients and occupational exposure monitoring. Photoacoustic spectroscopy, with its advantages of high sensitivity, wide dynamic range, miniaturization, and real-time, holds unique potential for CSM. A highly sensitive resonator with a small volume is an optimal choice regarding the limited exhaled gas amount. Wavelength modulation is not suitable for detecting trace sevoflurane with broad absorption lines because the modulation depth is far from optimal. Sensitive detection can be achieved using chopper-based amplitude modulation; therefore, a relatively low resonance frequency is critical. By the introduction of flexible polyurethane tubes, a branched photoacoustic cell (BPAC) was developed to compensate for the contradiction between the miniaturized resonator and the low resonance frequency requirement. The resonance frequency of BPAC was as low as 1036 Hz at a compact capacity of only 2.7 mL. Taking advantage of the replaceable and flexible branches, the geometry, resonance frequency, and sensitivity of BPAC could be optimized. The BPAC decoupling of the excitation and absorption paths, therefore, avoided the degradation of thermal-acoustic coupling at high concentrations. The sevoflurane detection results demonstrated that the Branch Enhanced Photoacoustic Spectroscopy (BEPAS) sensor yielded a 1σ limit of detection of 1.61 ppb with a 3 s integration time, corresponding to a normalized noise equivalent absorption coefficient of 2.2 × 10^-9 cm^-1 WHz^-1/2. With only a 1 cm long absorption path and high thermal-acoustic coupling, the BEPAS sensor provided a wide dynamic range of 1.61 ppb to 8% (154 dB). Continuous on-site testing for CSM issues was performed, which demonstrated the stability and reliability of this sensor. The developed BEPAS may open new avenues for low resonance frequency, ultrasensitivity, wide dynamic range, and compact large-molecule gas detection.