Abstract: In this article, a flow system developed for the amperometric determination of a great variety of pharmaceuticals that are known to lead the rapid poisoning of the working electrode surface is described. The referred system was made up of two parallel flow channels that shared the voltammetric detector of tubular configuration, whose movement in the manifold followed the concept of multi-site location of detector. In this way, after each measurement, the conditioning of the working electrode was possible through the passage by its surface of a regeneration solution without implying the alteration of the carrier that flowed in the analytical channel of the manifold. The methodology proposed was evaluated through the determination of two drugs belonging to two distinct therapeutic groups: an antihypertensive (diltiazem) and a non-steroid anti-inflammatory (nimesulide). The results obtained after evaluation of various pharmaceutical formulations on the Portuguese market were in the case of diltiazem compared with those supplied by the reference US Pharmacopoeia XXIV method, with no statistically significant differences having been observed for a confidence interval of 95%. In the case of nimesulide, since no official reference method exists, a series of recovery experiments were proceeded with and a mean value of 101.1% with a RSD of 0.7% was obtained.

Abstract: An amperometric multisite detection flow injection analysis (FIA) system was developed for sequential determination of 2 analytes with a single sample injection and single detector. Tubular composite carbon electrodes with an inner diameter similar to that of the FIA manifold tubing were constructed so that measurements could be made without impairing the sample plug hydrodynamic characteristics. The electrochemical behavior of the tubular voltammetric cell in a low-dispersion FIA manifold and the behavior of the FIA system incorporating this type of voltammetric cell intended for multisite detection were evaluated by performing measurements with potassium hexacyanoferrate(ii). Feasibility of the approach was demonstrated in the sequential determination of salicylic and acetylsalicylic acids in pharmaceutical products at a fixed potential of 0.98 V. The system allows sequential determination of salicylic acid concentrations ranging from 1.0 x 10^-5 to 5.0 x 10^-5 M and acetylsalicylic acid concentrations between 1.0 x 10^-3 and 5.0 x 10^-3 M with good precision on both detection sites and with relative standard deviations (RSDs) greater than or equal to 1.5 % (n = 10) and 2.1 % (n = 10), respectively. A comparison of these results with those of the U.S. Pharmacopeia procedure showed RSDs <5.0 and 1.0% for salicylic acid and acetylsalicylic acid, respectively. The proposed method enables 15 determinations per hour, which corresponds to the analysis of approximately 8 samples per hour. The detection limits of the methodology were approximately 3.5 x 10^-6 and 1.1 x 10^-5M, respectively, for the first and second monitoring sites.

Abstract: In this work a new strategy for the automation of the standard addition method by FIA with voltammetric detection is described, based on the concept of multi-site detection. In the system developed, a single voltammetric detector with mobility in the FIA manifold, carried out sequential measurement of the sample plug before and after addition of the standard solution. The construction and evaluation of the voltammetric detector of tubular configuration and internal diameter similar to the tubing of the FIA manifold, whose incorporation in the system allowed the measurement process to be carried out without change of the hydrodynamic characteristics of the sample plug, is also mentioned. To evaluate the, usefulness of the proposed multi-site voltammetric detection system, this was used in the amperometric determination of ascorbic acid in fruit juices where the standard addition method is frequently required to overcome the matrix effect. The results obtained with the FIA system were compared with those from the reference procedure proposed by AOAC. No statistical difference between methods was found at the 95% confidence level. The relative deviations between both methods were always less than or equal to 3.8%. The sampling rate was about 55 samples per hour (110 measurements).

Abstract: This work describes the construction of a Nafion coated glassy carbon tubular electrode and its use, coupled to a multi-commutated flow system, for the voltammetric determination of acetaminophen in serum and pharmaceutical formulations. The modification of the electrode enhanced the analytical signal intensity and, simultaneously, prevented the electrode surface fouling. The multi-commutated system conferred high versatility to the manifold, allowing it to be easily adjusted to each determination without the need to introduce any physical reconfiguration. The on line enzymatic hydrolysis of acetaminophen, giving rise to 4-aminophenol, allowed the problem of interferences resulting from the oxidation of the matrix serum constituents to be overcome.The method presented a linear range up to 5.0 x 10^-4 mol L-1 with a detection limit of 1.7 x 10^-5 mol l-1, a sampling rate of 24 determinations per hour and a repeatability (expressed in relative standard deviation) always lower than 3%. The method was applied to serum samples and pharmaceutical formulations and no statistically significant difference between the results obtained by the proposed method and by the reference methods was found, for a confidence level of 95%.