Evaluation and Comparison between a Conventional Acid Digestion Method and a Microwave Digestion System for Heavy Metals Determination in Mentha Samples by ICP-MS.

Mentha is the most common plant species grown in Saudi Arabia's Madinah city. Locally, the two popular types of mint are commercially called (Hasawi or Habaq) and (Mograbi). Both species are used in herbal teas, alone or as spice mixtures for many foods to offer aroma and flavour. In this research Hasawi mint samples were collected from three different locations and digested by two methods (acid digestion method and a microwave digestion system). Then, they were analysed for their metal contents by ICP-MS. The results from both digestion methods revealed that the following 10 metals were presented: V, Cr, Mn, Co, Ni, Cu, Zn, As, Cd and Pb. The microwave digestion system was the best for all studied heavy metals in term of sample preparation procedures, linearity (≥ 0.999), recovery (97%- 101%, n=2) and precision (R.S.D% ≈ 1-2 %, n=2). Mint samples from first location contained 7 metals within the permissible limit while the remaining three metals (Cr, Cu and Zn) had higher levels. The second location contained 9 metals within the permissible limit but only one metal (Cu) had a higher level. The third location contained 6 metals within the permissible limit while it had 4 metals (Cr, Ni, Cu, Cd) with higher level. All locations contained As, Cd and Pb within permissible limit except the third location which had a cadmium amount of (0.6 mg/kg) while the cadmium permissible level is (0.3 mg/kg). Some farmers use composts (from inorganic sources), fertilizers and plant nutrients in relatively large amounts that are rich in some metals and this might explain some of the higher value of metals in the mint. Moreover, contamination from industrial activities nearby mint farms might also be expected.