Elemental determination in carbon nanotubes by inductively coupled plasma optical emission spectrometry after a greener and simple microwave-assisted digestion method

Abstract

A method for the determination of elemental contaminants and metal catalyst residues (Al, Ca, Co, Cr, Fe, La, Mg, Mo, Ni, and Zn) in carbon nanotubes (CNTs) by inductively coupled plasma optical emission spectrometry (ICP OES) was developed by optimizing a greener digestion protocol for sample preparation. The feasibility of using a diluted nitric acid solution combined with H2O2 and high air flow-rate outside the digestion vessel during the microwave-assisted heating was evaluated. Complete digestion of up to 100 mg of CNT was possible using 6 mL of 1 mol L−1 HNO3 combined with 2 mL of 30% w w−1 H2O2, and 125 m3 h−1 of air flow-rate outside the digestion vessel during microwave heating. Comparison of the results obtained using the proposed method with those using a comparative method described in the literature was carried out. Accuracy was also evaluated by comparison of results with those by neutron activation analysis (NAA), and no statistical difference was observed. Limits of quantification (LOQ) ranged from 0.86 (Co) to 4.12 μg g−1 (Ca). Digests obtained using the proposed method were adequate for reliable determinations by ICP OES, and can be alternatively used for routine quality control of metal catalyst residues or impurities in CNTs. Moreover, the proposed method agrees with the recommendations of green analytical chemistry, bringing an important contribution to the sample preparation of CNTs, which normally involves hard conditions and time-consuming protocols and only allows digestion of very limited sample masses (up to 25 mg).