The determination of Cr, Cu and Mn in seawater with transversely heated graphite furnace atomic absorption spectrometry

The atomization signals of Cr, Cu and Mn in the presence of seawater salts have been investigated using a new transverse heated atomic absorption spectrometer and longitudinal Zeeman-effect background correction system. In unmodified seawater, the vaporization of the chloride matrix at a relatively low background absorption signal level may induce errors of correction which cannot be neglected at low metal concentration levels. Even without charring, seawater salts do not interfere with the recovery of Cr; however, due to temporal overlap of analytes and chloride vapours, seawater salts interfere severely with the recovery of Cu and Mn. For diluted seawater solutions (S < 3.5 parts per thousand) a charring step at about 500 degrees C almost suppresses the chloride chemical interference effect in the case of Cu and Mn, Mn being sufficiently stabilized through MgCl2 hydrolysis to permit the elimination of NaCl at 1000 degrees C before atomization. However, for more saline solutions, due to mass effect the hydrolysis of MgCl2 is not complete; the recovery of Mn is lowered to about 80% and the recovery of Cu is less than 20%, In the presence of nitric acid (more than 0.15 M) or oxalic acid (more than 0.1 M) which hydrolyses MgCl2 to MgO and stabilizes Cr, Cu and Mn in their oxide forms, the recovery for Cr, Cu and Mn is good and a charring step at about 1150 degrees C permits the elimination of the major part of the matrix (Na species) before atomization at 2150 degrees C; these conditions are almost optimal for the simultaneous determination of Cr, Cu and Mn. The detection limits obtained for 20 mu l seawater 0.15 M in HNO3 or 0.1 M in oxalic acid are respectively 0.042 mu g 1(-1) for Cr, 0.075 mu g 1(-1) for Cu and 0.026 mu g 1(-1) for Mn (3 sigma).