Abstract
A high-sensitivity light-emitting diode (LED)-based photoacoustic NO2 sensor is demonstrated. Sensitive photoacoustic gas sensors based on incoherent light sources are typically limited by background noise and drifts due to a strong signal generated by light absorbed at the photoacoustic cell walls. Here, we reach a sub-ppb detection limit and excellent stability using cantilever-enhanced photoacoustic detection and perform a two-channel relative measurement. A white-light LED is used as a light source, and the spectrum is divided into two wavelength channels with a dichroic filter. The photoacoustic signals generated by the two wavelength channels are measured simultaneously and used to solve the NO2 concentration. The background signal is highly correlated between the two channels, and its variations are suppressed in the relative measurement. A noise level below 1 ppb is reached with an averaging time of 70 s. This is, to the best of our knowledge, the first time a sub-ppb detection limit is demonstrated with an LED-based photoacoustic NO2 sensor. As LEDs are available at a wide selection of emission wavelengths, the results show great potential for development of cost-effective and sensitive detectors for a variety of other trace gasses as well.
Highlights
A high-sensitivity light-emitting diode (LED)-based photoacoustic NO2 sensor is demonstrated
As opposed to most other sensing techniques based on optical absorption, PA detectors have great potential toward miniaturization because the PA signal is inversely proportional to the sample volume.[3]
PA-based NO2 sensors have been demonstrated using light-emitting diode (LED) light sources, but the detection limits have been in the range of tens of ppb at best.[11−14] LED light sources offer good stability, high efficiency, low cost, and wide availability of emission wavelengths.[15]
Summary
The dichroic mirror divided the spectrum so that the reflection channel contained the narrow peak at. The standard deviation over the 10 min measurement of the largest concentration in the calibration set (1.042 ppm) was 2.6 ppb This is similar to the noise level of the blank measurement as given by the Allan−Werle deviation and shows that the measurement precision is not impaired much by the presence of the analyte or the higher signal level. Water can have a complex interference to the concentration signal since it can affect the acoustics of the photoacoustic cell as well as the relaxation rate of excited states in air.[9,10] To evaluate the effect of water concentration, the humidity of the sample gas was varied by passing part of the synthetic air flow over the air space above the distilled water. Interference from water absorption could be reduced by filtering a narrower portion of the LED spectrum, but this would be done at the cost of sensitivity due to reduced optical throughput
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