23 February 2012 New photoacoustic cell with diamond window for mid-infrared investigations on biological samples
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We present a new photoacoustic (PA) cell, which is sealed on the sample side with a 163 μm thick chemical vapor deposition (CVD) diamond window. The investigation of samples containing volatile compounds with an openended PA cell leads to varying conditions in the PA chamber (changing light absorption or relative humidity) and thus causes unstable signals. In contrast the diamond cover ensures stable conditions in the PA chamber and thereby enables sensitive measurements. This is particularly important for the investigation of biological samples with a high water content. Due to the high thermal conductivity of CVD diamond (1800 W/mK) strong PA signals are generated and the broad optical transmission range (250 nm to THz) renders the cell useful for various applications. The performance of the cell is demonstrated by tracking glucose in aqueous keratinocyte solutions with an external-cavity quantum cascade laser (1010-1095 cm-1). These measurements yield a detection limit of 100 mg/dl (SNR=3). Although glucose measurements within the human physiological range (30-500 mg/dl) are feasible, further improvements are needed for non-invasive glucose monitoring of diabetes patients. First in vivo measurements at the human forearm show an additional PA signal induced by blood pulsation at a frequency around 1 Hz and a steadily increasing relative humidity in the PA chamber due to transepidermal water loss if the cell is neither closed with a diamond window nor ventilated with N2.
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Jonas Kottmann, Julien M. Rey, Markus W. Sigrist, "New photoacoustic cell with diamond window for mid-infrared investigations on biological samples", Proc. SPIE 8223, Photons Plus Ultrasound: Imaging and Sensing 2012, 82231A (23 February 2012); doi: 10.1117/12.905973; https://doi.org/10.1117/12.905973

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