Werner Schrön

Investigation of phosphorus release from different compounds in electrothermal vaporization inductively coupled plasma atomic emission spectrometry in the absence and presence of modifiers and its application to plant analysis

Abstract Inductively coupled plasma atomic emission spectrometry (ICP-AES) combined with electrothermal vaporization (ETV) was applied to the determination of phosphates in aqueous solutions. The sensitivity for phosphorus was found to depend on the cations in the sample. To understanding this phenomenon the thermal behavior of H 3 PO 4 , (NH 4 ) 2 HPO 4 and KH 2 PO 4 was studied. There were significant differences in the thermal release of phosphorus from these compounds. To explain these phenomena, different reaction mechanisms were calculated using thermodynamic data. La(NO 3 ) 3 and Pd(NO 3 ) 2 were successfully applied as modifiers for the stabilization of phosphates during the thermal pre-treatment step and to establish an uniform thermal behavior. The application of Pd(NO 3 ) 2 as modifier led to uniformity in the thermal behavior and to stabilization of different phosphates tested. This resulted in a good correspondence between the certified and the measured concentrations of total phosphorus in decomposed plant reference materials. The stabilization of using La(NO 3 ) 3 modifier bases on the formation of thermally stable LaPO 4 . This explanation has been supported by comparing the experimental results with thermodynamic calculations considering different reactions for the thermal decomposition of the phosphate. Unfortunately, there are no thermodynamic data for palladium phosphates and phosphides available.

Gas equilibria in spectrochemical analysis

Abstract The formation, modification and atomization of gaseous halides (or MeX-compounds, X = F, Cl) in thermal plasmas for spectrochemical analysis can be described in terms of thermochemical equilibria. The differences in stability of MeX compounds in the d.c. arc, flames and graphite furnaces can be reduced to the dependence of equilibrium constants on temperature and to the influence of the composition of the plasma. The present paper shows results of systematic thermodynamic calculations in the form of a simple trend analysis. The calculations cover 42 elements and more than 150 of their halides. Resulting values of the reaction conditioned pressure p MeX differed by more than 25 orders of magnitude. Most metals form chlorides more easily than fluorides, but the elements Si, B, Al, P, As, Ti, Ta, Be preferentially form gaseous fluorides. Atomization always increases with temperature. Little or no atomization at all occurs in the cases of BF, GeS, BaCl, AlF, BCl, BaF, GaCl, InBr, BeF, AlCl, KC1, CaCl, CaF, NaCl, and BeCl, whereas strong atomization is typical of CuCl, SnCl 2 , MgF, FeCl 2 , GeCl, MgCl, AgCl, NiCl 2 , PbCl 2 , ZnCl 2 , CdCl 2 , HgCl 2 , AsCl 3 , and BiCl 3 (in this order). In general, atomization directly depends on the partial pressure ratio P H 2 / P HX 2 (where H = hydrogen). The presented results provide a new basis for experimental investigations and indicate new possibilities for improved control of the processes occurring in thermal plasmas used for spectrochemical analysis.

Improvement of the detection limits in pure Al2O3 powder by thermochemical reactions in a d.c. arc plasma and non-linear blank correction

Abstract Thermochemical reactions with halides were employed in particular applications of optical emission spectrography (OES), i.e. d.c. arc excitation, to enhance weak signals and to improve the detection limits. For trace element determinations in Al 2 O 3 with special graphite electrodes, the detection limits of Fe, Mn, Ni, Co and Cu, were improved by one or two orders of magnitude with sodium chloride as a thermochemical reagent. These results indicated good agreement with the results of chemical-thermodynamic trend analysis. The trace elements Fe, Mn, Ni, Co and Cu form chlorides in preference to Al. Part of the matrix element Al also reacts with chlorine to form AlCl 3 . Most of this chlorine is available again at higher temperatures because AlCl 3 decomposes to AlCl at about 3300 K. In this way the alumina has an effect as an additional carrier for chlorine and improves the trace element evaporation. With this procedure for trace element determinations in alumina the blank signals of the elements Fe, Mn, Cu, Ni and Co were detectable in the Al 2 O 3 calibration matrix and in the NaCl suprapur additive. These blank values were eliminated by a combination of nonlinear blank correction and a calibration using weighted nonlinear regression. From this chemometric treatment and the thermochemical reactions resulted an improvement of the limits of detection by up to two orders of magnitude.

Thermodynamic considerations on halogenating reactions in spectrochemical analysis of SiC powders for advanced ceramics

Abstract. A systematic thermodynamic trend analysis has been carried out for the direct spectrochemical analysis of SiC powders with halogenating additives. As the experimental results are in a good agreement with the theoretical calculations predictions for the selection of the best additives with regard to the performance of the analytical method have been enabled.