Sensor using a photo-acoustic absorption cell with two perpendicular acoustic resonators to analyze multiple molecules.

Abstract

An ultra-high sensitivity multi-molecule sensor based on a photo-acoustic cell with two perpendicular acoustic resonators and a common microphone has been reported. In this work, a 4.5 µm distributed-feedback quantum cascade laser and a 1.5 µm external cavity diode laser (EC-DL) were used as optical excitation sources. Considering the spectral ranges of the lasers used, it is possible to analyze eight molecules (Q C L:N ₂ O and C O ₂, EC-DL: H ₂ O, H ₂ S, N H ₃, CO, C H ₄, and C ₂ H ₂). The N ₂ O molecule was used to evaluate the performance of the photo-acoustic spectroscopy (PAS)-based sensor. A sensitivity of 0.073 V/ppm and a linearity of 0.99 were found by analyzing the PAS signal as a function of N ₂ O concentration at 2237.656c m ^-1. The long-term performance of the sensor was determined by performing an Allan deviation analysis. A minimum detection limit of 9.8 ppb for 90 s integration time was achieved. The simultaneous multi-trace gas detection capability was verified by measurement of N ₂ O, C O ₂, and H ₂ O. Depending on the coarse/fine-tuning ranges of the lasers used, the number of molecules analyzed can be further increased. Such a sensor could provide simultaneous diagnosis of many diseases through an analysis of breath air and simultaneous monitoring of the most important greenhouse gases.