residual fraction (F4): Residue from step three extracted with a hydrochloric/nitric acid (1:1, V/V) mixture and digest under pressure and temperature in closed vessel in a microwave digestion
system. The resultant solutions were subsequently used to determine the heavy metals. A blank was also run at the same time and no detectable concentration was observed when aliquots of the sequential extraction reagents were processed and analyzed with the samples. The concentrations of Cr, Ni, Cu, As, Cd and Pb in different fractions were determined by using inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700 series)