A versatile method for direct determination of iron content in multi-wall carbon nanotubes by inductively coupled plasma atomic emission spectrometry with slurry sample introduction

Carbon nanotubes (CNTs), due to their unique combination of physicochemical properties, are continuously considered as one of the most interesting materials in contemporary science and industry. Their applicability depends on physical properties, such as number of walls, morphology, diameter and length (aspect ratio), but also on the content of carbonaceous and metallic contaminants (typically present as encapsulated catalyst nanoparticles from catalytic Chemical Vapor Deposition). Therefore, especially for medical and biomedical applications, the concentration of metallic impurities or modifiers should be monitored. In this paper, an inductively coupled plasma atomic emission spectrometry (ICP-AES) technique with slurry nebulization is proposed for direct determination of iron in multi-wall carbon nanotubes (MWCNTs). Slurries of MWCNTs are prepared in a 1% solution of Triton X-100, ultrasonicated to ensure their homogeneity and measured by the ICP-AES apparatus. The optimal range of MWCNTs slurry concentration and plasma radiofrequency (RF) power are established as 40-500 mg L-1 and 1.0 kW, respectively. The precision of the method does not exceed 1.5% of the mean values. Validation of the method is based on the fact that Fe is partly leached from the slurry and the assumption that iron in the leachate gives the same response in ICP-AES as iron in the residual MWCNTs slurry. This behavior was proved by summing both results for Fe (determined in the leachate and in the slurry of postleaching nanotubes), which always yielded the same total amount of Fe for different degrees of leaching and this value was in full agreement with the iron content in the original, tested sample, as determined by the proposed slurry method. Taking into account all the examinations and obtained results, it can be concluded that the proposed method is applicable for fast, versatile, direct, and simple quantitative analysis of iron in MWCNTs and potentially other nanoallotropes produced by c-CVD.