Desenvolvimento de metodologia para análise de especiação de cromo em fertilizantes orgânicos

Abstract

In this work, a methodology was developed for the analysis of chromium speciation in organic fertilizer by liquid chromatography coupled to inductively coupled plasma mass spectrometry (LC-ICP-MS). For the evaluation of the methodology, different samples of organic fertilizer were used, with different contents of organic matter, where, firstly, the samples were submitted to an acid decomposition process for determination of total chromium and later conditions of extraction and determination of Cr(VI). For the initial optimizations of this work, the EPA 3060A Method was used as a reference and adaptations were made. This method consists of alkaline extraction (pH 11,5), using a solution of 0.28 mol L-1 Na2CO3 with 0.5 mol L-1 NaOH and heating at 90-95 ºC for 60 min. Speciation analysis was performed using the LC-ICP-MS technique, where liquid chromatography conditions were evaluated, such as the type of separation column, type and concentration of the mobile phase and flow rate. The dionex AG7 column and pre-column C18, the mobile phase 15 mmol L-1 HNO3 at a flow rate of 0.5 mL min-1 were optimized as the best conditions. For the validation of this methodology, some figures of merit were evaluated, such as linearity, limit of detection (LOD), limit of quantification (LOQ), precision (repeatability and intra-laboratory reproducibility), robustness and accuracy. The sample preparation conditions for the determination of Cr(VI) were also evaluated using 0.28 mol L-1 Na2CO3 with 0.5 mol L-1 NaOH, 0.02 mmol L-1 diphenylcarbazide with 2.4 mmol L-1 EDTA, 50 mmol L-1 EDTA and H2O as extracting solvents. The best extraction condition for Cr(VI) was 0.28 mol L-1 Na2CO3 with 0.5 mol L-1 NaOH, using 10 mL and 20 mg of sample. Concentrations of 0.14 mol L-1 Na2CO3 + 0.25 mol L-1 NaOH and 0.07 mol L-1 Na2CO3 + 0.125 mol L-1 NaOH were evaluated to avoid interference and possible damage to the chromatographic column. 0.07 mol L-1 Na2CO3 + 0.125 mol L-1 NaOH was the best condition, since there was no significant change between the different extractor solvent concentrations, making it suitable for use in the extraction of the other evaluations. As a comparative method, the same extracts were analyzed by ion chromatography-visible spectrophotometry (IC-Vis), for the determination of Cr(VI) based on the official EPA 218.7 method, in which a good agreement was obtained between the two techniques.