Direct determination of phosphate in urine by sequential-injection analysis with single on-line dilution-calibration method and photometric detection.

In this work we report a sequential-injection (SI) analysis method for the direct determination of phosphate in urine with on-line dilution and photometric detection. The developed SI manifold incorporated an additional dilution coil (DC) connected to one of the ports of the selection valve. On-line dilution was performed by aspirating a zone of sample with volume V(S) from the sample line into the holding coil (HC), transferring volume V(T) of that zone into the DC, and, finally, aspirating a portion of V(T) with volume V(A) back into the HC. This dilution introduces an additional dispersion of the initial sample zone represented by the effective dispersion coefficient D(eff) which can be tailored to the desired value by appropriate selection of the volumes V(S), V(T), and V(A). Actual detection of phosphates was performed by the molybdenum blue method with detection at 690 nm. By employing an on-line dilution step with a D(eff)= 100, the calibration curve for phosphate was linear in the range 0.05 x 10(-2) to 3 x 10(-2) mol L(-1) which covers the actual phosphate concentration in urine samples. The regression coefficient was r2 = 0.996, the relative standard deviation was sr = 3.9% at the 1 x 10(-2) mol L(-1) level (n = 10), and the injection frequency was 30 injections h(-1). The method was applied to urine samples with recoveries > 97%.

Flow-injection manifold for the simultaneous spectrophotometric determination of Fe(II) and Fe(III) using 2,2'-dipyridyl-2-pyridylhydrazone and a single-line double injection approach.

A simple and rapid flow-injection (FI) method is reported for the simultaneous spectrophotometric determination of Fe(II) and Fe(III) in pharmaceutical products. The method is based on the reaction of Fe(II) with 2,2'-dipyridyl-2-pyridylhydrazone (DPPH) in acidic medium to form a water-soluble reddish complex (lambdamax=535 nm). Fe(III) reacts with DPPH under flow conditions only after its on-line reduction by ascorbic acid (AsA). Both analytes were determined in the same run via a double-injection valve, which enabled the simultaneous injection of two sample volumes in the same carrier stream (,,single-line double-injection" approach). The two well-defined peaks produced corresponded to total iron [Fe(II)+Fe(III)] and Fe(II). Speciation of the analytes in their mixtures was achieved by multiple regression analysis. The calibration curves obtained were linear over the ranges 0-30 and 0-50 mg L(-1) for Fe(II) and Fe(II), respectively, and the precision [s(r)=1.0% for Fe(II) and 1.5% for Fe(III)] was satisfactory. The method proved to be selective and adequately sensitive (cL=0.25 and 0.17 mg L(-1) for Fe(III) and Fe(II), respectively, in mixtures). Application of the method to the analysis of pharmaceutical samples resulted in excellent accuracy; the percent mean recoveries were in the range 99.0-102.0% for both Fe(II) and Fe(III) and the mean relative error was e(r)=1.0%.

Direct and selective flow-injection method for the simultaneous spectrophotometric determination of calcium and magnesium in red and white wines using online dilution based on "Zone Sampling".

The present work reports a selective and simple flow injection method for the direct and simultaneous determination of calcium and magnesium ions in red, rose, and white wines. Both ions react with methylthymol blue (MTB) at a strongly basic medium to form colored complexes that are monitored spectrophotometrically (lambda(max) = 610 nm). The simultaneous determination is achieved by online masking of magnesium by 8-hydroxyquinoline (8-HQ). Incorporating an online dilution mode based on the "zone sampling" technique in the FI system, the determination of both analytes was achieved without any pretreatment of the samples, in the range 0-350 mg L(-1) and 0-200 mg L(-1) for Ca(II) and Mg(II), respectively. The 3 sigma detection limits were quite satisfactory (2.1 and 1.8 mg L(-1) for Ca(II) and Mg(II) respectively), and the precision was 1.2% (at a mixture of 100.0 mg L(-1) Ca(II) + 100.0 mg L(-1) Mg(II), n = 12). A detailed study of interferences proved that the proposed method is highly selective. The application of the method to the direct analysis of red, rose, and white wines yielded excellent results compared with those obtained by using FAAS as a reference method (e(r) < 2.8%).

Rapid flow injection spectrophotometric determination of monofluorophosphates in toothpastes after on-line hydrolysis by alkaline phosphatase immobilized on a cellulose nitrate membrane.

A new, rapid flow injection (FI) method is reported for the spectrophotometric determination of monofluorophosphate (MFP) ions in toothpastes. MFP ions are hydrolyzed on-line by alkalinephosphatase (APase) immobilized on a cellulose nitrate membrane, prior to injection in the FI system. The yielded orthophosphate ions are determined spectrophotometrically (lambda(max) = 690 nm) using the molybdenum blue approach. The chemical and FI variables that affected the enzymatic reaction were studied and optimized. A study of interferences was also carried out. The proposed method is very precise (s(r) = 0.7% at 1.0 x 10(-4) mol l(-1) MFP, n = 12), fast (sampling rate of 72 h(-1)) and allows the determination of MFP ions in the range of 4.0 x 10(-5) to 6.0 x 10(-4) mol l(-1) with a satisfactory 3sigma detection limit of 4.0 x 10(-6) mol l(-1). The application of the proposed FI method to toothpaste samples yielded accurate results (e(r) < 2.0%) compared with a potentiometric reference procedure.

Simple, rapid reagent-injection spectrophotometric determination of fluorides in pharmaceutical formulations.

A new simple and rapid reagent-injection method is reported for the determination of fluorides in pharmaceutical formulations. The method is based on the inhibitory effect of fluoride ions upon the Fe(III) catalytic oxidation of 2,4-diaminophenol (DAP) by H2O2. Free fluoride ions form stable complexes with Fe(III), thus reducing its catalytic effect. The decrease in the absorbance of the oxidation product is monitored spectrophotometrically at 500 nm. The various chemical and physical variables of the FI system were optimized and a study of interfering ions was also carried out. A linear calibration graph was obtained from 0 to 750 mg x l(-1) for F- ions. The precision was very good (sr=0.3%) and the 3sigma detection limit was satisfactory (cL=0.471 mg x l(-1)). The sampling rate was 90 injections x h(-1). The method has been successfully applied to the determination of F- ions in pharmaceutical formulations.

Flow injection manifold for the direct spectrophotometric determination of bismuth in pharmaceutical products using Methylthymol Blue as a chromogenic reagent.

A new simple and rapid flow injection method is reported for the direct determination of bismuth in pharmaceutical products. Methylthymol Blue (MTB) was used as a chromogenic reagent and the absorbance of the colored Bi(III)-MTB complex produced was monitored at 548 nm. The various chemical and physical variables were optimized and a study of interfering ions was also carried out. Linear calibration graphs were obtained from 0 to 100 mg l-1 Bi(m) (120 injections per hour). The precision was very good (sr = 1.3%) and the limit of detection was cL = 0.150 mg l-1. The average accuracy was also very good (er = 0.75%) and was evaluated by comparison of the results obtained with those claimed by the manufacturers. The method was found to be adequately selective, considering the ions that the samples contain.

On-line dilution flow injection manifold for the selective spectrophotometric determination of ascorbic acid based on the Fe(II)-2,2'-dipyridyl-2-pyridylhydrazone complex formation.

A new simple and rapid FI method for the accurate and precise spectrophotometric determination of ascorbic acid (AsA) in pharmaceutical formulations is reported. The method is based on the reduction of Fe(III) to Fe(II) by the analyte, and the subsequent reaction of the produced Fe(II) with 2,2'-dipyridyl-2-pyridylhydrazone (DPPH) in acidic medium (pH ca. 2.5) to form a colored complex (lambda(max)=535 nm). An on-line dilution mode using a binary inlet static mixer (BISM) was incorporated in the FI system, allowing the determination of the analyte in a wide concentration range. The calibration graph was linear in the range of 5.7-600.0 mg l(-1) AsA, at a sampling rate of 120 injections h(-1). The method was found to be very precise [s(r)=0.1% at 300 mg l(-1) AsA (n=12)] and the 3delta detection limit (c(L)=1.7 mg l(-1)) was quite satisfactory. The detailed study of various interferences confirmed the high selectivity of the proposed method. Its application to a variety of pharmaceuticals produced excellent results, with a mean relative error of e(r)<1.0%.

Normal and differential demasking flow-injection manifold for the direct spectrophotometric determination of zinc(II) in biological materials and pharmaceutical formulations.

A normal and a differential demasking flow-injection (FI) manifold were developed and optimized for the spectrophotometric determination of microamounts of Zn(II) in biological materials and pharmaceutical formulations. The reported method is very sensitive, rapid, simple and it is based upon the reaction of Zn(II) with 2,2'-dipyridyl-2-pyridylhydrazone (DPPH) in a strongly basic medium to form a yellow-coloured complex (lambda max = 448 nm). By using the differential demasking manifold, the tolerance of the method to many cations was enhanced by a mean factor of 25. The obtained calibration graphs were linear in the range of 0-10 mg l-1 Zn(II), at a sampling rate of 120 injections h-1 in both cases. The precisions of both manifolds were very good (RSD = 0.6 and 0.8%, respectively) and the 3 sigma detection limits were quite satisfactory (cL = 4 and 6 ng ml-1 respectively). The method has been successfully applied to the determination of Zn(II) in serum, human hair and pharmaceutical formulations with recoveries ranging between 98.0 and 101.6%. The obtained results were also in excellent agreement with flame atomic absorption spectrometry (FAAS), since the mean relative error was er = 0.9%.