A compact quartz-enhanced photoacoustic sensor for ppb-level ambient NO
2
detection is demonstrated, in which a high-power blue laser diode module with a small divergence angle was employed to take advantages of the directly proportional relationship between sensitivity and power, hence improving the detection sensitivity. In order to extend the stability time, a custom grooved quartz tuning fork with 800-μm prong spacing is employed to avoid complex signal balance and/or optical spatial filter components. The sensor performance is optimized and assessed in terms of optical coupling, power, gas flow rate, pressure, signal linearity and stability. A minimum detectable concentration (1σ) of 7.3 ppb with an averaging time of 1 s is achieved, which can be further improved to be 0.31 ppb with an averaging time of 590 s. Continuous measurements covering a five-day period are performed to demonstrate the stability and robustness of the reported NO
2
sensor system.
It is very important to monitor and evaluate the growth of Staphylococcus aureus (S. aureus) under various conditions. However, so far there is no technology that can monitor S. aureus in real time. This work proves that the wavelength modulated absorption spectroscopy (WMAS) based on tunable diode lasers can quickly measure the concentration of carbon dioxide produced by microorganisms. We use a variety of fitting models to get the growth curves of S. aureus and calculated the relationship between the growth rate and time through the obtained growth curve. Using the proposed technology, we have studied the growth status of S. aureus at different temperatures (five points in the range of 26-43 C), and the results show that the maximum growth rate have a strong temperature dependence. Therefore, it is proved that WMAS monitoring S. aureus is a user-friendly, non-invasive, fast, and high signal-to-noise ratio technology, which can be used for rapid and accurate bacterial growth assessment in the industry.
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