Adsorption behavior of L-methionine immobilized on controlled pore glass for cadmium preconcentration

PABLO H. PACHECO, RAÚL A. GIL, ROBERTO A. OLSINA, LUIS D. MARTINEZ, PATRICIA SMICHOWSKI

Rio de Janeiro, Brasil

Simposio; Tenth Rio Symposium on Atomic Spectroscopy; 2008

A study was performed to determine the adsorption behavior of the relative non-polar L-methionine for cadmium preconcentration. To this end, L-met was immobilised on controlled pore glass (CPG), incorporated in a microcolumn and inserted in a flow injection system for Cd preconcentration from aqueous solutions. Aminoacids possess different functional groups with different metal binding capacities. They are able to bind different metals depending on these functional groups, including different species. Selectivity, strong binding capacity, easiness of synthesis and environmental innocuity are some properties that aminoacids present, turning them into ideal molecules for preconcentration and monitoring of trace elements in environmental matrices. The possibility of immobilization into solid supports like controlled pore glass (CPG) provides the opportunity of column packing. Under these conditions, aminoacids acquires a specific binding orientation in the presence of a complexing metal. The high specific surface of CPG enhances the number of immobilisated molecules, providing a higher metal retention with an easy release and reusability. Aminoacids act as bidentate ligands with coordination involving the carboxyloxygen and the nitrogen atom of amino group. Aminoacids are immobilized through its amino group to CPG, leaving a free carboxilate group to interact with metals. The objective of the present study is the study of the sorption behavior of the L-met-CPG system for Cd retention. Binding constant of the system were calculated and it turned to be of 1.99, with sites capacity of n = 3.12. The ratio of the moles of Cd bound at pH 9.0 to moles of L-met was calculated and it was 0.03:1. On-line breakthrough curves were used to study the effect of pH, analyte concentration and influent flow rate on Cd retention. The Cd breakthrough experiments revealed different binding sites that showed a dependence on influent concentration. A particular behavior was observed at pH 7.0 and an increment of the retention capacity over this pH value. A complementary pH study was added with a titration curve. When the influence of flow rate was assessed a decreased retention capacity at faster flow rates was observed. The system showed an optimal adaptation to changes of the influent concentration. Transient peak areas revealed that Cd stripping from the column occurred instantaneously. The analytical performance of the system was evaluated. The system achieve an enhancement factor of 1300 (130-fold for the preconcentration system and 10-fold for the USN), reaching a detection limit of 0.625 ng L-1 when 10 mL of the solution were passed through. The system was applied successfully to cadmium determination in the standard reference material (SRM), QC METAL LL2 metals in natural water, as a validation study.