General setup diagram for photoacoustic spectroscopy using an external cavity quantum cascade laser.
Diagram showing the open (a) and the closed (b) photoacoustic cell for in vivo measurement of the absorption spectra of human epidermis. The entry of the beam from below and the focus slightly below the sample are indicated in red. The dimensions for the two main cylindrical cavities are explained in the text. A and R indicates, respectively, the absorption and the resonance cylinder.
PA signal intensity vs. frequency measured for carbon black at a fixed wavenumber (1170 cm−1) measured using the windowless cell (blue line) and the closed PA cell (red line).
(a) PA signal vs. wavenumbers for carbon black for two different repetition frequencies, 51.7 and 53.8 kHz, respectively, each spectrum is the average of 50 laser scans. (b) Relative standard deviation of the spectra shown in (a).
Photoacoustic spectra of skin (left small finger) compared to the absorption spectra of the same sample measured by ATR-FTIR spectroscopy. The photoacoustic spectrum fits almost perfectly to the ATR measurements.
Intensity of the PA signal for carbon black at fixed repetition frequency measured for different temperatures using both PA cell configurations. As expected, the temperature has a stronger affect over the sensitivity in the closed configuration than in the windowless case.
(a) 60 photoacoustic spectra of skin (left hypothenar) during a continuous OGTT. Each of these spectra is the average 20 scans of the laser across its tuning range. (b) Change of the PAS intensity at 1123 cm−1 compared to the PA cell temperature.
(a) The scores of PC1 for the spectra measured during the OGTT fit to the Stratum corneum spectra features. (b) Loadings of PC1 compared to the temperature of the PA cell.
(a) Difference photoacoustic spectra of skin during an OGTT after temperature correction. (b) Change of the difference spectra at 1123 cm−1 compared to the glucose concentration change during the OGTT.
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