Paragraph 7 PROCEDURE

Prepare a blank solution by repeating the whole procedure from the extraction stage, omitting only the test sample of fertiliser.

From the working calibration solution prepared using the method given for each individual trace element, prepare in 100 ml volumetric flasks a series of at least five calibration solutions of increasing concentration within the optimum measuring range of the spectrometer. If necessary, adjust the concentration of hydrochloric acid to bring it as close as possible to that of the diluted test solution (6.2). Add 10 ml of the same lanthanum salt solution (4.3) as used in (6.2). Make up to volume with the 0.5 M hydrochloric acid solution (4.2) and mix thoroughly.

Prepare the spectrometer (5) for the determination and adjust to the wavelength given in the method for the individual trace element concerned.

• Spray three times in succession the calibration solutions (7.2), the test solution (6.2) and the blank solution (7.1), noting each result and flushing the instrument with distilled water between individual sprayings.

• Construct the calibration curve by plotting the average spectrometer reading for each calibration solution (7.2) along the ordinate and the corresponding concentration of the element, expressed in mg/ml, along the abscissa.

• From this curve, determine the concentrations of the relevant trace element in the test solution x_s (6.2) and in the blank solution x_b (7.1), expressing these concentrations in μg per ml.