Water-Soluble Humic Acid Quantification Using a Flow-Injection System with and without Sample Pretreatment

Two automated methods for the determination of humic acids (HAs) in natural water samples are presented. Flow injection systems were developed based on the formation of a complex between humic acids and toluidine blue dye. When the complex is formed, the dye absorbance signal decreases at selected operating conditions. Method A, without sample pretreatment, presents a linear calibration curve over HA concentration range 1.14-35.0 mu g mL(-1), with a limit of detection (LOD) of 0.4 mu g mL(-1) (3 Sy/x) and a sample throughput of 80 h(-1). To determine lower HA concentration levels, a preceding separation/preconcentration step was included (method B). This method has a linear range of 1.5-20.0 mu g mL(-1) and a sample throughput of >8 h(-1) and could detect humic substances at a concentration of 0.05 mu g mL(-1) after a 4-min retention time. The RSDs for methods A and B calculated using the slopes of seven independent calibration graphs obtained on different days and with different conditions (standard solution, reagent solution, etc.) were 4.5% and 4.2% respectively. Both methods were satisfactorily applied to natural water samples, with recovery percentages in the 95-107% range, and compare very well with other methods.