Evaluation of various sample preparation procedures for the determination of chromium, cobalt and nickel in vegetables

Three methods for the determination of Cr, Co and Ni in vegetables were tested with a view to the evaluation of various sample preparation procedures involving dry ashing, the direct slurry technique and continuous flow microwave-assisted digestion, NIST SRM 1572 Citrus Leaves was used for this purpose, For the determination of the analytes by AAS, the temperature programmes of the graphite furnace were optimized and the use of some chemical modifiers was studied, Direct slurry analyses of vegetables (in samples prepared at a 2% m/v concentration in 1% HNO3 containing 10% H2O2) were generally accurate and precise for Co and Ni, but gave low recoveries (about 85%) for Cr, Continuous digestion of the samples in a microwave oven was necessary for the reliable determination of Cr (about 100% recovery) by use of 5% HNO3 as the digesting reagent, The best method for each application was used for the determination of Cr, Co and Ni in a variety of vegetables, using direct calibration with aqueous standards.