Background overcorrection problems for lead in the presence of phosphate with various metals in Zeeman graphite furnace atomic absorption spectrometry

Phosphate in various chemical forms has been widely used as a chemical modifier for the determination of Pb by graphite furnace AAS. When Pb was determined in bone digestate, with NH₄H₂PO₄ modifier and a transversely-heated furnace and a longitudinal Zeeman background correction system, low recoveries of Pb were found. This was found to be caused by a background overcorrection problem associated with the matrix of phosphate plus Ca and/or Mg. The overcorrection problem caused erroneously low Pb concentrations to be predicted when using aqueous Pb standards for calibration. The overcorrection problem was not reproduced with a transverse Zeeman correction system. While the magnitude and shape of the background signal changed with different phosphate compounds, the important features remained constant. We suggest that the background overcorrection problem was caused by molecular absorption of PO formed during atomization in the presence of any of several metals, most specifically the alkaline earth elements, since the PO molecular absorption bands appear to be subject to Zeeman-effect splitting.