Microwave-induced combustion of carbon nanotubes for further halogen determination

Abstract

A procedure for single and multi-walled carbon nanotubes digestion by microwave-induced combustion (MIC) is proposed for the first time for further halogens (Cl, Br and I) determination by inductively coupled plasma mass spectrometry (ICPMS) and ion chromatography (F, C, Br and I). Samples of carbon nanotubes (up to 500 mg) were pressed as pellets and combusted using 20 bar of oxygen and 50 μl of 6 mol l⁻¹ of ammonium nitrate as igniter. The following absorbing solutions were evaluated: H₂O, (NH₄)₂CO₃, NH₄OH, tetramethylammonium hydroxide (TMAH) and water-soluble tertiary amines (CFA-C). A reflux step was applied after combustion (5 min of microwave irradiation at 1400 W) in order to achieve better analyte recoveries. For Cl determination by ICPMS, a dynamic reaction cell was used with ammonia as reaction gas. The accuracy was evaluated using certified reference materials (CRM) of coal, spiked samples and also by comparison of results with neutron activation analysis. In spite of 1% (v/v) of TMAH or CFA-C being used as absorbing solution, 100 mmol l⁻¹ NH₄OH was preferred in view of lower blank values and quantitative recoveries (better than 97%). Agreement with CRM values for NIST 1632b, NIST 1632c and SARM 19 was higher than 96% for Cl and Br (ICPMS) and for F (IC). The limit of detection (LOD, 3σ) for Cl, Br and I obtained by ICPMS was 1.3, 0.02 and 0.002 μg g⁻¹, respectively. The LOD for F, Cl, Br and I determined by IC was 0.3, 1.1, 2.4 and 4.2 μg g⁻¹, respectively. The residual carbon content for MIC digests was always below 1%. As an advantage over conventional procedures, using MIC it was possible to digest up to eight samples in only 25 min, obtaining a single solution suitable for determination of all halogens by different techniques. According to the authors’ knowledge this is the first application for halogen determination in this kind of material.