Development of methods for fertilizer analysis by inductively coupled plasma atomic emission spectrometry

Abstract

Developing the ICP-AES method to a standard method of determination was performed for five nutrients (B, Mg, Fe, S and Ca) in commercial fertilizers. The procedure included finding the best analytical wavelengths on the basis of calibration and interference studies and testing the sample preparation and measurement procedures in collaborative studies. The collaborative studies were done for the determination of water-soluble B, Mg and S and acid-soluble B, Mg, S and Fe by ICP-AES and the accuracy of the method used was in the range 3,7-12% independent of the measurement wavelength used. To find the best wavelengths for determining of the above-mentioned elements, the calibration lines at three or four wavelengths of each element were analyzed for linearity, detection limits and quantification limits. The regression of the calibration line should be >0.9998, at which point the limits of detection and quantification will be low enough. Interference studies were done at those wavelengths to find the interference-free determination wavelength. The most important aspect of the interference studies was the mathematically independent combination of the matrix elements (seven potential interfering elements at five concentration levels). Analyzing this subset for the selected element, it is possible to evaluate the interferences of the different matrix elements on the examined element. Multiple linear regression proved a valuable technique in achieving this and it was able to be used because calibration was confined to within a linear range. The observed interferences may be regarded either as an interelement effect or as a spectral effect. The effect is considered a spectral effect when the wavelength of the interfering element is within ± 0.2 nm of the analyte line; if not, the effect is considered an interelement effect. Typically the interelement effects were caused by easily ionizable elements (Ca, K, Na and Mg) and the spectral effects by P, S, B and Fe. The observed interferences agreed in great measure with earlier known interferences, although many new interferences were also found. The best wavelengths according to the calibration and the interference studies were 208.959 nm for B, 383.826 nm for Mg, 238.204 nm for Fe, 180.731 nm for S and 396.847 nm for Ca. The outlines of the SFS-standards for the determination of water-soluble boron and magnesium and acid-soluble magnesium have already been accepted as new SFS standards by the Chemical Industry Federation of Finland.