Mode-mismatched photothermal lens-based trace furfural detection using a quantum cascade laser as excitation source

In this paper, a mode-mismatched photothermal lens liquid sensing system using a distributed feedback quantum cascade laser (DFB-QCL) as the excitation source for the detection of furfural in insulating oil. The mid-infrared absorption spectrum of furfural in an insulating oil matrix was characterized by Fourier transform infrared (FTIR) spectroscopy, identifying the optimal absorption peak at 1703 cm⁻¹ . Temperature and current tuning of the DFB-QCL were performed on pure oil and a 500 mg/L furfural/insulating oil sample to determine the optimal temperature and current values, ensuring maximum sensitivity during light excitation of the oil samples. Frequency-domain investigation of the PTL signal response revealed an exponential decrease in signal amplitude with increasing modulation frequency. The PTL signal amplitude exhibited a linear correlation with furfural concentration in the insulating oil. The sensing system has a low limit of detection (LOD) for furfural in insulating oil down to 5.9 mg/L, with a linear concentration range extending up to 500 mg/L. This method does not require pretreatment of the oil sample and can be used for real-time in-situ monitoring.