MECHANISMS OF CHLORIDE INTERFERENCES IN ATOMIC-ABSORPTION SPECTROMETRY USING A GRAPHITE-FURNACE ATOMIZER INVESTIGATED BY ELECTROTHERMAL VAPORIZATION INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY .1. EFFECT OF MAGNESIUM-CHLORIDE MATRIX AND ASCORBIC-ACID CHEMICAL MODIFIER ON MANGANESE

The interference by magnesium chloride with the atomization of manganese in electrothermal atomic absorption spectrometry (ETAAS) has been investigated using electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). The ETV-ICP-MS allows the direct observation of the loss of manganese during the charring step, and thereby allows differentiation between the manganese loss during charring and the loss due to formation of molecular species during atomization. The mechanism of interference by magnesium chloride is dependent on the charring temperature. At temperatures above 700-degrees-C, the manganese is lost during charring; this loss occurs as the magnesium chloride matrix undergoes hydrolytic decomposition, and the manganese is carried away from the graphite furnace with the hydrogen chloride gas generated by the hydrolysis reaction. At charring temperatures lower than 700-degrees-C the suppression in the manganese atomic absorption signal is due to a vapour-phase interference caused by formation of manganese chloride during atomization. The addition of ascorbic acid, as a chemical modifier, removes interferences in both the higher and the lower charring temperature regions. The results obtained by ETV-ICP-MS show that, for charring temperatures above 700-degrees-C, ascorbic acid prevents the loss of manganese during charring. This effect is explained by retardation of hydrolysis of the magnesium chloride matrix by the chemical modifier, ascorbic acid.