A comparative study of the efficiencies of W, Mg, Mg + Pd, Ir + Mg and Ir + W + PO43- as chem. modifiers for the thermal stabilization of Pb and Sn in slurries was performed. The Ir + Mg modifier contributes more than the others to minimizing matrix effects and preventing double peak formation. The influences of slurry concn., amt. of modifier and pyrolysis step on the integrated absorbance signals for Pb and Sn in sediment slurry were studied. The potential of fractional factorial design was explored to evaluate the effect on the absorbance signals of different factors such as drying and pyrolysis times, pyrolysis and atomization temps., presence of modifier and incorporation of a cool-down step in the electrothermal program. The Pb and Sn integrated absorbance signals are increased by lower and higher pyrolysis and atomization temps., resp. The Ir + Mg modifier also increases the absorbance signal for Sn. The use of an Ir + Mg modifier and ultrasonic agitation in the direct introduction of slurries into the graphite atomizer provides a consistent performance with good sensitivity, precision and anal. recovery. [on SciFinder (R)]