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Open resonators are instead used to allow the humidity to evaporate, minimize the effects of pressure and temperature fluctuations during measurement hence increasing the long-term stability of the measurements. This is particularly useful for the continuous PA measurement of blood glucose levels since closed resonators result in the build-up of humidity inside the cell.
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Windowless or open resonators have the additional advantage of enabling continuous exchange of fluids between resonator and environment.
SLIP LENGTH COMSOL 5.3 WINDOWS
To avoid the problem, authors have proposed reducing the background signal by placing the windows at the acoustical nodal points, attenuating the background signal by use of buffer volumes next to the window, making 2- f measurements and using windowless resonators which eliminates the problem. This background signal can vastly affect measurements of samples of low concentration where the PA signal can be weaker than the background signal. However, the sensitivity of measurements from closed resonators is limited by window absorptions which produce a coherent background signal.
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Conventional photoacoustic spectroscopy usually applies closed resonators (using windows): gaseous samples are filled into a tightly sealed resonator while solid samples are placed inside a closed resonator filled with a non-absorbing gas. The modulation frequency of the electromagnetic radiation is matched to an acoustic eigenfrequency of the resonator thus exciting the corresponding acoustic mode. The technique provides high sensitivity, high selectivity, large dynamic detection range making it suitable for trace gas applications, ,, ] and, unlike other optical techniques, can measure optically opaque samples, , ].Īcoustic resonators are used to further increase the sensitivity of the photoacoustic (PA) signal. Photoacoustic spectroscopy (PAS) is a technique that detects acoustic waves generated from thermal de-excitation of molecules upon absorption of modulated electromagnetic radiation.