Trace metal impurities determination in high-purity polyimide by plasma-based techniques

Abstract

Polyimides are widely employed in the industry to produce several components that require resistance to temperature and chemical reagents. Due to the high thermo-oxidative stability and chemical inertness, the digestion of this kind of polymer is a challenging step, but mandatory for further analysis by plasma-based techniques, as inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). In the present work, a single method based on microwave-induced combustion (MIC) is proposed for polyimide digestion and further determination of 18 analytes (As, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, Pb, Sb, Sn, Ti and V) by ICP-OES and ICP-MS. The use of NH4Cl as volatilization aid mixed with polyimide powder before the combustion was required for Cr recovery. As absorbing solution, a mixture containing HNO3 and HCl was employed providing quantitative recoveries for all analytes. The accuracy of the proposed method was evaluated by spike recovery tests, by results comparison with reference values (SEMI F48-0600 protocol) and by analysis a certified reference material (CRM) of polyethylene (EC680K – low density polyethylene). Spike recoveries were from 94 to 108%, while the results using MIC and reference values were in an agreement (better than 94%). Regarding to CRM analysis, no statistical difference was observed (t-test, 95% of confidence level) between MIC and certified values. The proposed method allows the combustion of relative high sample mass (600 mg) with high digestion efficiency (dissolved carbon lower than 100 mg L−1) that prevents eventual interferences during the determination by plasma-based techniques, caused by carbon in digests. It was possible to use diluted acids as absorbing solution, reducing the blank values and, consequently, the limits of detection that were in ng g−1 range even for ICP-OES analysis. The digests were suitable for analytes determination by ICP-OES and ICP-MS and no additional dilution was required.