Quantification of impurities in carbon nanotubes: Development of ICP-MS sample preparation methods

Carbon nanotubes (CNTs) are one of the prospective nanomaterials to be used in consumer products. Commercially available carbon nanotubes contain significant amounts of metal impurities which are added during synthesis as a catalyst. The presence of metal impurities in CNTs influences the physicochemical properties of materials and its applications. The quantification of metal impurities in CNTs is essential to conduct environmental and health risk assessments. This study aims to develop a method which can be used effectively in laboratories to quantify multi-element impurities in CNTs using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Prior to the analysis, samples were prepared by conventional microwave acid digestion (CMAD) and microwave oxygen combustion (MOC). In addition, the ICP-MS method was validated by determining multiple analytical figures of merit such as linear calibration range, system suitability, method detection limit (MDL), limit of quantitation (LOQ), and accuracy. The developed CMAD and MOC methods were suitable to digest metal impurities in carbon nanotubes and the ICP-MS results were in good agreement (80-110% of recovery) with the certified values. In comparison of two methods, CMAD is a simple and straightforward two-step procedure with safe experimental conditions, and requires only 5 mg of sample. Moreover, CMAD overcomes the disadvantage of the pre-existing CNT digestion methods such as the requirement of multi-step acid digestion and drying processes.