APPLICATION OF INDUCTIVELY COUPLED PLASMA SPECTROMETRY TO THE DETERMINATION OF POTENTIALLY TOXIC ELEMENTS IN WHEAT AND MISWAK FROM SAUDI ARABIA
The accumulation of potentially toxic elements (PTEs) in plants from anthropogenic activities represents an important environmental pollution problem when these concentrations exceed the permissible levels. A method based on electrothermal vaporization coupled to inductively coupled plasma (ICP) optical emission spectrometry was developed for the direct analysis of miswak (natural toothbrush) in 77 s, as a first step in the risk assessment process. This method eliminates the need for digesting solids and the associated issues of contamination and loss of analyte. However, risk assessment cannot be based solely on total concentrations as only the bio-available fraction (i.e. the fraction entering the blood stream) can exert a toxic effect. For a more realistic risk assessment, bio-accessibility (the fraction dissolved in the gastrointestinal tract), which is equal to bio-availability in the worst case scenario, was measured using an on-line leaching method simulating the human gastrointestinal tract. This method afforded the continuous monitoring of elements by ICP mass spectrometry as they were released from wheat and miswak. The bio-accessibility data obtained using this dynamic model were reproducible and comparable with those obtained by a conventional batch model despite taking 15 min instead of over 4 h. The bio-accessibility results were also used as part of a human health risk assessment where they were compared to tolerable daily intake values. Application of the on-line leaching method to freshly collected miswak as well as miswak exposed to vehicular traffic for a few days revealed the contamination that resulted from this source of pollution. Moreover, multiple peaks in the temporal profiles measured during on-line leaching confirmed an additional source of contamination, which was further confirmed by correlations between the leaching profiles of different elements and between Pb isotopes. In fact, the resulting 206Pb/207Pb isotopic ratio matched that of Pb previously added to gasoline as an anti-knocking agent, suggesting that leaded gasoline is still in use in Saudi Arabia. Application of the same method to wheat from different regions of Saudi Arabia similarly revealed this source of Pb in wheat from Safeer. It also indicated coal combustion as a source of Pb in wheat from Safeer and Qassim.