Investigation of Environmentally Relevant Colloids by Laser-Induced Breakdown Detection (LIBD)

An apparatus for laser-induced breakdown detection (LIBD) was established, which is aimed at in-situ detection of aquatic colloids of a particle size down to < 50 nm and a particle concentration down to the ppt range. LIBD is based on the generation of dielectric breakdown events on individual colloid particles by pulses of a focused laser (cf. [1]). A diode pumped Nd:YAG laser is used as light source and is operated at maximum power for stability reasons. The pulse energy is controlled by a combination of two Fresnel rhombs and a calcite polarizer. The beam is focused into a quartz cell containing the particle suspension to be analyzed. The breakdown events are simultaneously observed by a CCD camera and by piezoelectric detectors.

Polystyrene latex standards of different particle size were diluted with de-ionized water (Membrapure) to mass concentrations of the ppt and ppb range. These solutions were used for the calibration of the LIBD system.

Two particle systems of environmental relevance were studied by LIBD.
First, a nearly particle-free spring water of a small stream was investigated. Photon correlation spectroscopy, an alternative technique of particle detection, was too insensitive to find particles in this water. Both the raw sample of this stream water and centrifugates/filtrates of the water could be successfully investigated and consistent results were provided by LIBD. Second, uranium(IV) colloids were generated in a glove box (N2 atmosphere) by cathodic reduction of uranium(VI) followed by the successive increase of the pH by coulometric titration. The formation of these colloids was traced by LIBD.

[1] Walther, C., Bitea, C., Hauser, W., Kim, J.I. and Scherbaum, F.J.
Laser Induced Breakdown Detection for the Assessment of Colloid Mediated Radionuclide Migration. Nucl. Instr. and Meth. B 195 (2002) 374-388