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Precision agriculture (PA) strongly relies on spatially differentiated sensor information. Handheld instruments based on laser-induced breakdown spectroscopy (LIBS) are a promising sensor technique for the in-field determination of various soil parameters. In this work, the potential of handheld LIBS for the determination of the total mass fractions of the major nutrients Ca, K, Mg, N, P and the trace nutrients Mn, Fe was evaluated. Additionally, other soil parameters, such as humus content, soil pH value and plant available P content, were determined. Since the quantification of nutrients by LIBS depends strongly on the soil matrix, various multivariate regression methods were used for calibration and prediction. These include partial least squares regression (PLSR), least absolute shrinkage and selection operator regression (Lasso), and Gaussian process regression (GPR). The best prediction results were obtained for Ca, K, Mg and Fe. The coefficients of determination obtained for other nutrients were smaller. This is due to much lower concentrations in the case of Mn, while the low number of lines and very weak intensities are the reason for the deviation of N and P. Soil parameters that are not directly related to one element, such as pH, could also be predicted. Lasso and GPR yielded slightly better results than PLSR. Additionally, several methods of data pretreatment were investigated.

In this work, ion mobility (IM) spectra of more than 50 aromatic compounds were recorded with a laser-based IM spectrometer at atmospheric pressure. IM spectra of PAH in the laser desorption experiment show a high complexity resulting from the occurrence of monomeric, dimeric, and oligomeric cluster ions. The mobilities of all compounds were determined in helium as drift gas. This allows the calculation of the diffusion cross sections (Omega(calc)) on the basis of the exact hard sphere scattering model and their comparison with the experimentally determined diffusion cross sections (Omega(exp)). Extended Omega(exp)/Omega(calc) and Omega(exp)/mass correlations were performed in order to gain insight into conformational properties of cationic alkyl benzenes and internal rotation of phenyl rings in aromatic ions. This is demonstrated with some examples, such as the evaluation of the dihedral angle of the ions of 9,10-diphenylanthracene, o- and m-terphenyl, and 1,2,3- and 1,3,5-triphenylbenzene. Furthermore, sandwich and T-structures of dimeric PAH cations are discussed. The analysis was extended to oligomeric ions with up to nine monomer units. Experimental evidence is presented suggesting the formation of pi-stacks with a transition toward modified pi-stacks with increasing cluster size. The distance between monomeric units in dimeric and oligomeric ions was obtained.

The halogen radicals that react with azaarenes are produced by the photooxidation of halogenide anions with hydroxyl and sulfate radicals and exist as complexes of the radical and the respective halogenide anion in the aqueous phase. The main reaction products of the reactions are identified, and in the case of the bromine radicals, the second order rate constants are determined. Oxidation takes place according to the different redox potentials of the two reactants and is especially observed for chlorine radicals. A typical product spectrum comparable with that in reactions with hydroxyl and sulfate radicals has been found. The formation of some oxidation products in reactions of bromine radicals is in contradistinction to the oxidation potentials of the azaarenes and can be understood only by the reaction of their excited states. The halogenation is the main reaction of the azaarenes. Halogenation products of both, the benzene and the pyridine/diazine rings, have been found. The halogenation of the pyridine/diazine ring again requires the reaction of excited states. The majority of derivatives is halogenated in substitution reactions, but in the reaction of benzo[h]quinoline, addition is also observed. The resonance energy per electron is responsible for the change in the halogenation mechanism from substitution to addition.

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