LIBS-LIF-spectroscopy for the in-situ analysis of heavy metals in soils

Volltext: Laser-based spectroscopic techniques such as laser-induced breakdown spectrometry (LIBS) and laserinduced fluorescence (LIF) have been applied successfully to the chemical analysis of various substances. In contrast to conventional analytical methods like 1CP-OES, AAS or SD-OES, they are predestined for online and in-situ applications, because they allow contactless measurements and have essentially reduced requirements for sample preparation. The determination of heavy metals in soils represents one application for which these characteristics are of great importance. Consequently, numerous publications can be found in the literature on the investigation of the LIBS technique with respect to this topic. In most cases, limits of detection (LOD) in the range of some æg/g to some 10 mu g/g are found [Wisbrun l994], which is sufficient for most of the heavy metals in comparison to regulatory limits. For some critical elements like Cd or Hg, however, LODs below 1 æg/g are required, which has not been demonstrated with LIBS so far. It has been shown that improved detection limits can be achieved by using the combined LIBS-LIF technique in principle [Niemax 1990], [Gornushkin 19971]. In our work, we have investigated this hyphenated technique with special regard to the requirements of the multi-elemental, in-situ analysis of heavy metals in soils. A Q-switched Nd:YAG-laser operating at 1064 nm is used for the ablation of sample material and plasma generation. For LIF, analyte atoms are selectively excited within the plasma plume with a second, tuneable laser source operating in the UV wavelength range. The plasma radiation as well as the fluorescence light are guided to a Paschen-Runge spectrometer via fiber optics. This spectrometer is equipped with photomultipliers for all the relevant spectral lines, the signals of which are finally processed by a gateable multichannel integrator and evaluated with a PC. The LIBS excitation parameters were optimised in such a way that as many elements as possible can be analysed simultaneously with LODs below the German regulatory limits for unpolluted soils merely by the evaluation of the LIBS signal. This was the case for As, Cr, Cu, Ni, Pb and Zn. Additionally, Cd was analysed by the resonant LIF excitation at a wavelength of 228.8 nm, We measured a LOD of 0.25 æg/g which is an improvement by a factor of 24 with respect to the result obtained from the evaluation of the corresponding LIBS signal. It should be emphasised that the measurements were performed at ambient pressure, which is very important for the implementation of the technique for in-situ measurement purposes. In summary, we have demonstrated the simultaneous determination of 7 heavy metals in soil samples with sufficient detection limits. Currently, the LIBS-LIF technique is investigated with regard to the analysis of Hg. Future work will aim at a reduction of the experimental apparatus, which is certainly inevitable for the introduction of an in-situ measurement system

Coherent television distribution systems

An experimental TV distribution network based on coherent optical systems is being set up as part of a Heinrich-Hertz Institute research project. It is designed for TV distribution on future public optical networks and demonstrates the opportunities for future single-mode optical networks