Optical Absorbance Measurements of Opaque Liquids by Pulsed Laser Photoacoustic Spectroscopy

Abstract

In many relevant industrial applications, UV-vis online process monitoring is hampered by light scattering and opacity of the samples, whereas diluted and filtered samples are rarely available. Pulsed laser photoacoustic (PA) spectroscopy allows the measurement of both high and low absorptions without any need for sample preparation. An optimized detection geometry for absorption measurements in opaque liquids is described. The proposed PA sensor was realized by using two orthogonal detectors based on piezoelectric poly(vinylidene fluoride) (PVDF). Laser-induced pressure waves were sensed perpendicularly to (side-on mode) and along the axis of the laser beam (forward mode). Pressure waves generated by a single laser pulse, optical transmission and absorption, as well as the speed of sound in liquid samples were determined simultaneously using time-resolved detection. Evaluation of the PA signal permits the determination of absorption coefficients ranging from 0.1 to 1000 cm(-1). The influence of absorbing or scattering compounds on the signal was investigated in dye solutions and suspensions of TiO(2) particles.

Thermomechanical and Photophysical Properties of Crystal-Violet-Dye/H2O Based Dissolutions via the Pulsed Laser Photoacoustic Technique

“…In fact, we observe in this analysis an interesting and similar trend to the well-known Beer-Lambert law (according to (2)) rather than a power-curve (as described by (1)). This linear behaviour has been previously reported in scientific works for semitransparent liquid samples [6,7]. However, in our case, most of the studied dissolutions are heavily CV-loaded and fully opaque sample systems.…”

“…Nevertheless, by including the first point (corresponding to a 5.2 mg/L concentration) to evaluate the complete studied range, it would require an exponential fitting to better describe the physical behaviour of V in this particular system. In both cases, it is observed that the low CV-loaded dissolutions exhibit a slow V -parameter, tending to the standard H 2 O value reported in the literature (1490 m/s @ 20 ∘ C [6]). In contrast, for higher CV-loaded dissolutions, the speed of sound reaches extremely high V -values up to 2.000 m/s.…”

“…The generated acousticwaves result from the nonradiative electronic decay and the further fast thermal process induced by the optical absorption of a fraction of the pulsed light energy. According to a phenomenological analysis [5,6], for homogenous and isotropic media such as an organic-based dissolution, having a dye/chromophore concentration C, an optical path-length L, and an optical extinction coefficient , which is excited with a single laser wavelength and an input pulse intensity 0 , the amplitude of the photoacoustic signal is given by the following equation:…”

“…In a typical PLPA experiment, the acousticwaves travel at the characteristic speed of sound of the excited material to reach a coupled piezoelectric detector; this allows the mechanical-electrical experimental interface to collect the PLPA-data from the sample on a digital oscilloscope for subsequent numerical analyses. The experimental simplicity to obtain photoacoustic signals has led to the development of several PLPA-based measurement devices which are commonly used for the accurate evaluation of thermal and optical properties of different materials such as organic and inorganic crystals, semiconductor, and polymer systems [2][3][4][5][6][7][8][9]. In summary, several physical and structural material characteristics can be efficiently studied by means of diverse PLPA-based techniques and the adequate data analysis thereof.…”

“…Indeed, the M1-detector is closer to the acoustic wave source S (see Figure 1); hence, less attenuated and stronger RMS-amplitude signals can be detected/processed at this point. In other words, since most of the samples are highly opaque, the photoacoustic signals are generated within the first micrometres inside the liquid sample [6]. Then, the acoustic-waves travel a shorter distance through the liquid sample to reach the M1-detector.…”