Standard additions: 0.1 g of the reference material wheatflour GBW 08503 and 0.05 g of Triton X-100 were accurately weighed into a 50 mL volumetric flask, and (i) 25, (ii) 10 and (iii) 5 mL of the most concentrated aqueous standard and the appropriate amount of HNO3 was added (in total 6 mL HNO3/100 mL of slurry) and treated in ultrasonic bath for 15 min.