Feasibility of DS-GF AAS for the determination of metallic impurities in raw material for polymers production

A new procedure is proposed for the determination of metal contaminants (Cr, Fe, Mg, Mn, Na and Ni) in polymeric diphenylmethane diisocyanate (PMDI), a raw material used to produce polyurethane polymers. The feasibility of using Zeeman-effect background correction graphite furnace atomic absorption spectrometry (GF AAS) and direct sampling (DS) was evaluated. Calibration using aqueous standard solutions was feasible and chemical modifiers as well as reference materials for this purpose were unnecessary. Relatively high sample masses (up to 23 mg) were used allowing very low limits of detection ranging from 0.06 ng g^-1 to 1 ng g^-1 (Fe and Mg) up to 3  ng g^-1 (Ni), with relative standard deviation lower than 15%. The following parameters were evaluated: pyrolysis and atomization temperatures , sample mass, as well as the use of low sensitivity conditions (Zeeman effect background correction magnetic field strength adjustment and the use of a secondary wavelength for Fe and Na determinations, respectively). Results were compared with those obtained by microwave-assisted digestion and microwave-induced combustion with subsequent analytes determination by inductively coupled plasma optical emission spectrometry (ICP-OES) and by inductively coupled plasma mass spectrometry (ICP-MS). No significant difference was observed between the results obtained by DS-GF AAS, ICP-MS and ICP-OES after both digestion systems. The proposed DS-GF AAS method allowed the determination of six elements in PMDI (which is considered as a complex matrix) with limits of detection lower than those achieved by other methods. This new procedure can be used as quality control of polyurethanes industry for ultra-trace inorganic impurities.