Comparison of three wet digestion methods for the determination of plant sulphur by inductively coupled plasma atomic spectroscopy (ICP-AES)

Increased incidence of sulphur (S) deficiency in crops has led to a greater need for S determination in plant tissues. Among the methods available for measurement of S, inductively coupled plasma atomic emission spectrometry (ICP‐AES) has been proved to be accurate and rapid. This paper compares digestion with HNO3‐HClO4, fuming HNO3‐KNO3 and alkaline NaOBr prior to the S measurement by ICP‐AES. Both HNO3‐HClO4 and fuming HNO3‐KNO3 were found to give satisfactory results for five certified plant materials, with high accuracy and precision. The HNO3‐HClO4 method involved a rigid temperature control, with slow ramp rates and a final temperature (190°C) lower than the boiling point of HClO4, to minimise the possibility of gaseous losses of S. Both methods can also be used for the simultaneous determination of phosphorus (P), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn), and additionally potassium (K) with HNO3‐HClO4. In contrast, the alkaline NaOBr method was less suitable as a digestion procedure for S measurement by ICP‐AES, because it involved a large dilution factor and produced high blank values.