Laser-induced breakdown spectroscopy for soil diagnostics

Laser-optical measurements and fibre optics are potentially attractive tool s for applications in soil science because of their great sensitivity and s electivity and their capabilities for on-line and in situ analysis. We have investigated laser-induced breakdown spectroscopy (LIBS) for the quantitat ive detection of metal ions on the surface of natural soil samples from two sites (Hohenschulen and Oderbruch, Germany). The LIBS technique allows the spatially resolved investigation of adsorption and desorption effects of i ons in sail. A frequency doubled (532nm) and Q-switched Nd:YAG laser with a pulse duration of 8ns is focused on the soil surface and induces a plasma. Typical power densities are 150 mJ mm(-2). The plasma emission is recorded in time and spectrally resolved by a gateable optical multichannel analyse r (OMA). A delay time of about 500 ns between laser pulse and OMA gate was used to resolve single atomic and ionic spectral lines from the intense and spectrally broad light that is emitted by the plasma itself. The dependenc y of the LIBS signal of a single spectral line on the amount of water in th e sample is investigated in detail. The results indicate that quenching of water in the plasma plume reduces the line intensities, while the interacti on with aquatic colloids increases the intensity. The two processes compete with each other, and a non-linear correlation between measured line intens ities and the amount of water in the sample is obtained. This is verified b y a simple computer simulation and has to be taken into account for the qua ntitative interpretation of LIBS signals, e.g. when absolute concentrations are estimated. In the present investigation natural calcium concentrations < 2 <mu>g kg(-1) were measured with the LIBS technique in the samples for the two test sites. In addition, measurements were made with dry and water- saturated BaCl2 mixed soil samples, and no significant difference in the de tection limit for barium was obtained.