Time-resolved ICP-MS measurement of part-per-trillion level of analyte ions adsorbed onto carbon nanotubes

Abstract

Metal ions adsorbed onto individual nanoparticles are measured using time-resolved inductively coupled plasma - mass spectrometry (ICP-MS). The mixture of the test elementnanoparticles in the original sample solution is introduced into the ICP by conventional solution nebulization. The adsorbed analyte ions on each particle produce a plume of gaseous analyte ions in the ICP. The plume was detected as a current spike in the mass spectrometer. The signal-to-background ratios (SBR) are significantly improved. The sample modulation method improves the ICP-MS detection limits by at least one order of magnitude [1]. In our previous study, part-per-trillion levels of Ba2+, Cd2+ and La3+ ions were adsorbed onto 100-nm Fe(OH)3 particles of concentration of part-per-billion. In this study, carbon nanotubes (CNTs) were used as the adsorbent. CNTs are chemically more stable than the Fe(OH)3 particles. The carbon matrix is also lighter than the Fe matrix of the Fe(OH)3 particles. The CNTs were treated in concentrated nitric acid at elevated temperature to oxidize the CNTs partially by insertion of oxygen-containing functional groups (e.g., hydroxyl and carboxyl groups) onto the particles [2]. The treated CNTs can be dispersed in water readily. ICP-MS spikes were readily observed for trace metal ions of concentration of parts-per-trillion (ppt). The spike intensity varied linearly with analyte concentration up to 50 ppt. Experimental conditions of analyte preconcentration (equilibration time, temperature, and solution pH) and ICP-MS operating parameters (sampling depth and carrier gas flow rate) will be discussed. Time-resolved inductively coupled plasma-atomic emission spectrometry (ICPAES) was also used to investigate the vaporization process of the analyte-adsorbent particles in the ICP. The details of the experimental set-up and signal characteristics will be discussed.\r\n\r\n[1] M.H.P. Yau, W.T. Chan, A novel detection scheme of trace elements using ICP-MS, J. Anal. At. Spectrom., 20, 1197-1202 (2005).\r\n[2] A.G. Rinzler, J. Liu, H. Dai, P. Nikolaev, C.B. Huffman, F.J. Rodriguez-Macias, P.J. Boul, A.H. Lu, D. Heymann, D.T. Colbert, R.S. Lee, J.E. Fischer, A.M. Rao, P.C. Eklund and R.E. Smalley, Large-scale purification of singlewalled carbon nanotubes: process, product, and characterization, Appl. Phys. A., 67, 29-37 (1998).