Octadecyl Immobilized Surface for Precipitate Collection with a Renewable Microcolumn in a Lab-on-Valve Coupled to an Electrothermal atomic Absorption Spectrometer for Ultratrace Cadmium Determination

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Abstract
Octadecyl immobilized surface was, for the first time, proved to be a superb precipitate-collecting medium. Surface charge effect was assumed to dominate the adsorption of cadmium hydroxide precipitate, facilitated by electrostatic interaction between the negatively charged C18 bead surface and positively charged cadmium hydroxide clusters. Residual silanol groups on the C18-immobilized silica surface did not contribute to precipitate adsorption. A novel procedure for ultratrace cadmium preconcentration was proposed by incorporating a renewable microcolumn in a lab-on-valve system. Cd(OH)2 precipitate was adsorbed onto the C18 surface, which was afterward eluted with 20 μL of nitric acid (1%) and quantified with detection by electrothermal atomic absorption spectrometry. An enrichment factor of 28 and a limit of detection of 1.7 ng L-1, along with a sampling frequency of 13 h-1 were obtained with a sample consumption of 600 μL within the concentration range of 0.01−0.2 μg L-1, achieving a precision of 2.1% RSD at the 0.05 μg L-1 level. The enrichment factor was further enhanced to 44 by increasing the sample volume to 1200 μL. The procedure was validated by analyzing cadmium in three certified reference materials, that is, river sediment (CRM 320), sea lettuce (CRM 279), and frozen cattle blood (GBW 09140). Good agreement between the obtained results and the certified values was achieved.

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