The reactors that were prepared pre - sented high stability for at least eight hours under intense use in flowing solutions, when submitted to the consecu - tive injection of 3.0 × 10 −4 to 1.1 × 10 −3 mol L −1 captopril. To examine the efficiency of the solid-phase reactor containing immobilized AgSCN in the polyurethane resin, experiments were performed with consecutive injections of captopril solutions. The transient signals are shown in Figure 8. The relative standard deviations (RSDs) obtained were lower than 2% (n = 12) for both solutions and a sample throughput of 40 h –1 was attained. The precision of the flow system was evaluated in two concentration levels (6.0 × 10 −4 and 1.0 × 10 ‐3 mol L ‐1 ). Typi - cal transient signals corresponding to a linear calibration graph for captopril are shown in Figure 7. The observed quantification limit (tenfold blank standard deviation/slope) was 1.0 × 10 −4 mol L −1 and the detection limit (threefold blank standard deviation/slope) was 8.0 × 10 −5 mol L −1. Under optimum ex- perimental conditions, the flow-injection system showed a linear response to captopril in the concentration range from 3.0 × 10 −4 to 1.1 × 10 −3 mol L −1 (A= 0.0714 + 281.25 C r = 0.998, where A is the absorbance and C the concentra - tion of captopril in mol L −1 ). The recovery results obtained suggest an absence of the matrix effect in the determina- tion of captopril in those samples. In this study, 2.0 × 10 −1, 4.0 × 10 −1 and 6.0 × 10 −1 mmol L −1 of captopril were added to each product. Recoveries between 97.5% and 103% of captopril from five pharmaceutical formulations ( n = 3) were ob - tained by using the flow-injection procedure. Table I presents the optimization of the chemical and flow-injection parameters studied in this work. A flow rate of 1.1 mL min −1 was then selected taking into account the magnitude of the analytical signal, stability of the baseline, and low reagent consumption. For higher flow rates, the sensitivity decreased due to the short contact time between the sample zone and the silver thiocyanate particles in the solid-phase reactor. Best results were obtained with a flow rate of 1.1 mL min −1. The effect of the total flow rate was investigated in the range of 0.6–4.4 mL min −1. Consequently, a length of 50 cm for reactor coil B was selected. The analytical signal increased up to 50 cm, while higher lengths led to a decrease in the analytical signal because of sample dispersion. The analytical signal was investigated in the 30–100 cm range. The influence of the reactor coil length (B) is shown in Figure 2, was also evaluated. Consequently, the volume of 500 μL was chosen for the sample, based on the most effective balance of repeatability and magnitude of the analytical signal. The analytical signal increased from 100 to 500 μL for captopril, remain - ing almost constant at higher volumes. Figure 6 shows the effect of the volumes of captopril inserted in the flow system by varying the volumes of the loop (L) between 100 and 700 m L, keeping the concen- tration of captopril at 1.0 × 10 −3 mol L −1. Therefore, a 1.0 × 10 −2 mol L −1 Fe(III) solution was chosen for further experiments. As can be seen in Figure 5, it was observed that the height of the peaks increased with the increase in Fe(III) concentration up to 1.0 × 10 −2 mol L −1 and remained practically constant for higher concentrations of this re - agent. signal was evaluated in the range from 3.0 × 10 −3 to 5.0 × 10 −2 mol L −1 for a 1.0 × 10 −3 mol L −1 reference capto- pril solution.
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