Highly improved sensitivity of TS-FF-AAS for Cd(ii) determination at ng L−1 levels using a simple flow injection minicolumn preconcentration system with multiwall carbon nanotubes

Abstract

A new method for cadmium determination at ng L⁻¹ levels is described. The method is based on the on-line coupling of a flow preconcentration system using multiwall carbon nanotubes (MWCNT) as sorbent with TS-FF-AAS determination. Cadmium preconcentration was at pH 4.9 onto an MWCNT minicolumn (30 mg) for 2 min at a 5.0 mL min⁻¹ flow rate. The elution step is performed by using 0.5 mol L⁻¹ HNO₃ and the cadmium desorbed is directly pumped to a TS-FF-AAS. All experimental parameters that play important roles in system performance were evaluated and optimized by means of fractional factorial designs and response surface methodology. The excellent characteristics of MWCNT as sorbent, mainly owing to its high surface area, make it possible to obtain a preconcentration factor of 51-fold, thus improving the detection and quantification limits in TS-FF-AAS, 11.4 and 38.1 ng L⁻¹, respectively. When the flow preconcentration system, FI-TS-FF-AAS, was compared with FAAS alone, an increase in the sensitivity of 640-fold was obtained. The calibration graph was linear with a correlation coefficient higher than 0.999 from 38.1 to 1250 ng L⁻¹. Repeatability of the measurements (n = 10), assessed as relative standard deviation (RSD), was found to be 6.5 and 2.1% for cadmium concentrations of 100 and 1000 ng L⁻¹, respectively. Important parameters to characterize the flow preconcentration system were also evaluated, the consumption index being 0.196 mL, the concentration efficiency 25.5 min⁻¹ and the sample throughput 20 samples per hour. In order to demonstrate the accuracy of the system, addition and recovery studies in water samples (mineral water, tap water and river water) and cigarette samples were carried out. Moreover, for the same purpose, cadmium was determined in certified biological materials (Bovine Liver and Rye Grass), giving an average result in close agreement with the certified value.