Evaluation of sample pretreatment methods for multiwalled and single-walled carbon nanotubes for the determination of metal impurities by ICPMS, ICPOES, and instrument neutron activation analysis

Abstract

The manufacture of carbon nanotubes (CNTs) relies on the use of transition metal catalysts. The presence of metals in CNTs has been shown to critically affect the physical, chemical and surface properties of the material for applications in areas such as gas sensors and microcolumns. Once CNTs are released into the environment, the bioavailability of the metals is of concern, in the context of potential human toxicity. In the present study, methods were developed to determine the metals' concentrations in single-walled and multiwalled CNTs (SWCNT and MWCNT, respectively). The metals' concentrations in the SWCNT and MWCNT were determined by inductively coupled plasma optical emission spectrometry (ICPOES) and mass spectrometry (ICPMS), after the CNTs had been pretreated with one of the three extraction/digestion methods: water extraction, dilute acid (1% HNO3) extraction, and microwave acid digestion. The total metal concentrations were determined by instrumental neutron activation analysis (INAA). The metals in CNTs were found to have poor solubility in water and dilute acid, suggesting that the role of CNT metals in cytotoxicity may be limited due to their limited bioavailability, and that metals encapsulated in the CNTs could have potential use as tracers for CNTs, in biological or toxicological studies. Microwave acid digestion can achieve a near-complete extraction of metals from the CNTs, and thus is a suitable cleaning method, when high-purity CNTs are desired. Microwave acid digestion followed by ICPOES analysis produced results closer to those obtained by INAA than to those obtained by ICPMS; the latter method was subject to nonspectral interference induced by carbon residues in the sample solution.