Determination of isoxsuprine hydrochloride by sequential injection visible spectrophotometry.

An automated sequential injection (SI) spectrophotometric method for the determination of isoxsuprine hydrochloride is described. The method is based on the condensation of aminoantipyrine with phenols (isoxsuprine hydrochloride) in the presence of an alkaline oxidizing agent (potassium hexacyanoferrate) to yield a pink colored product, the absorbance of which is monitored at 507 nm. Chemical as well as physical SI parameters that affect the signal response have been optimized in order to get better sensitivity, higher sampling rate and better reagent economy. Using the optimized aforesaid parameters, a linear relationship between the relative peak height and concentration was obtained in the range 1-60 mg l-1. The detection limit (as 3sigma value) was 0.3 mg l-1 and precision was 1.4% and 1.6% at 5 and 10 mg l-1, respectively. As compared to previous reports, wide linear range, low detection limit, and highly economical reagent consumption are the advantages of this automated method.

Simultaneous determination of L- and D-methotrexate using a sequential injection analysis/amperometric biosensors system.

A sequential injection analysis (SIA) is proposed for the simultaneous determination of L- and D-methotrexate (Mtx) using amperometric biosensors as detectors. A SIA system is proposed due to the highest precision and accuracy and lower consumption of sample and buffer. The amperometric biosensors used as detectors in SIA system were based on L-amino acid oxidase (L-AAOD) or/and L-glutamate oxidase (L-Glox) and horseradish peroxidase (HRP) for the assay of L-Mtx and D-amino acid oxidase (D-AAOD) and HRP for the assay of D-Mtx were selected. The linear concentration ranges are of pmol/l to nmol/l magnitude order, with very low limits of detection. The SIA/biosensors system can be used reliably on-line in synthesis process control, for the simultaneous assay of L- and D-Mtx with a frequency of 34 samples per hour.

On-line assay of the S-enantiomers of enalapril, ramipril and pentopril using a sequential injection analysis/amperometric biosensor system.

A sequential injection analysis/amperometric biosensor system is proposed for the enantioselective analysis of the S-enantiomer of enalapril, ramipril and pentopril. The amperometric biosensor used as detector in the sequential injection analysis was designed by immobilization of l-amino acid oxidase in carbon paste. The proposed SIA system can be utilized reliably for the enantioanalysis of the S-enantiomer from the raw materials as well as from their pharmaceutical formulations, with a rate of 75 samples per hour and R.S.D. values better than 0.1% (n = 10).

Simultaneous determination of L- and D-carnitine using a sequential injection analysis/amperometric biosensors system.

A sequential injection analysis (SIA) system is described for the simultaneous determination of L-and D-carnitine using amperometric biosensors as detectors. The SIA system was used, because of its high precision, accuracy and low sample and buffer consumption. The biosensors were designed using physical and chemical immobilization of L-amino acid oxidase and horseradish peroxidase (HRP) for the assay of L- carnitine, and D-amino acid oxidase and HRP for the assay of D-carnitine. The linear concentration ranges are in the pmol/l to nmol/l magnitude order, with very low limits of detection. The biosensors/SIA system was used reliably for on-line process control of the enantiopurity of carnitine with a frequency of 34 samples per hour.

Enantioselective potentiometric membrane electrodes based on alpha-, beta- and gamma-cyclodextrins as chiral selectors for the assay of L-proline.

Novel enantioselective, potentiometric membrane electrodes based on carbon paste impregnated with alpha-, beta- and gamma-cyclodextrins are reported. Response characteristics showed that the proposed electrodes could be reliably used in the assay of l-proline (l-pro), with the best enantioselectivity and time-stability exhibited by alpha-cyclodextrin. The enantioselective, potentiometric membrane electrodes based on the proposed unsubstituted cyclodextrins (CDs) showed lower detection limits (in the region of 10(-10) to 10(-9)mol L(-1)) than the one previously studied, based on beta-cyclodextrin derivative [R.I. Stefan, J.F. van Staden, H.Y. Aboul-Enein, Anal. Lett. 31 (1998) 1787-1794]. The surfaces of the electrodes are easily renewable by simply polishing on an alumina paper.

Enantioanalysis of l-2-hydroxyglutaric acid in urine samples using enantioselective, potentiometric membrane electrodes based on maltodextrins.

Quantitative assay of l-2-hydroxyglutaric acid (l-2-HGA) is important for the diagnosis of l-2-hydroxyglutaric aciduria. Three enantioselective, potentiometric membrane electrodes (EPMEs) based on maltodextrins with different dextrose equivalent (DE) (DE: 4.0-7.0 (I), 13.0-17.0 (II), 16.5-19.5 (III)), were designed for the enantioanalysis of l-2-HGA. The enantioselective, potentiometric membrane electrodes can be used reliably for enantiopurity assay of l-2-HGA using the direct potentiometric method in the ranges of 10(-9)-10(-5), 10(-6)-10(-3) and 10(-8)-10(-5)molL(-1) for the enantioselective, potentiometric membrane electrodes based on maltodextrins I, II and III, respectively, with very low detection limits. A high reliability was obtained when the electrodes were used for the assay of l-2-HGA in urine samples.

Sequential injection spectrophotometric determination of ritodrine hydrochloride using 4-aminoantipyrine.

A sequential injection spectrophotometric determination of ritodrine hydrochloride is described. The method is based on the condensation of aminoantipyrine with phenols in the presence of an alkaline oxidizing agent to yield a pink coloured product the absorbance of which is monitored at 503nm. Different sequential injection analysis (SIA) parameters including reagent concentrations have been optimised and used to obtain the analytical figures of merit. A linear concentration range of 3.1-123.5mumolL(-1) and a detection limit (as 3sigma-value) of 1.0mumolL(-1) were obtained. The precision was 2.4 and 2.3% relative standard deviation (R.S.D.) at 6.2 and 15.4mumolL(-1), respectively. This method is superior over previously reported ones in terms of linear range, short analysis time, high sample throughput, excellent reagent economy and minimum waste generation.

Diamond paste based immunosensor for the determination of azidothymidine.

An amperometric immunosensor, based on diamond paste (diamond powder and paraffin oil), has been constructed for the assay of azidothymidine (AZT). The diamond paste is impregnated with anti-AZT. The immunosensor can be used reliably for the assay of azidothymidine in its pharmaceutical formulations. The potential used for azidothymidine assay was + 240 mV vs. Ag/AgCl electrode. The surface of the immunosensor can be regenerated by simply polishing, thereby obtaining fresh immunocomposite ready to be used in a new assay. The new amperometric immunosensor, based on diamond paste, gives reliable results for the assay of AZT as raw material and from its pharmaceutical formulation.

Monocrystalline diamond paste-based electrodes and their applications for the determination of Fe(II) in vitamins.

A new class of electrochemical sensors, namely, electrodes based on diamond paste, was designed using monocrystalline diamond (natural diamond 1 microm and synthetic diamond, 50 microm (synthetic-1) and 1 microm (synthetic-2)) powder and paraffin oil. The characterization of the electrodes was performed using cyclic voltammetry and differential pulse voltammetry. Fe(II) was determined by differential pulse voltammetry (DPV) at 75 mV (vs Ag/AgCl) using all diamond paste-based electrodes. The linear concentration range was between 10(-8) and 10(-4) mol/L for both the natural diamond and synthetic-2 with detection limits of 10(-10) and 10(-9) mol/L, respectively, whereas the linear concentration range for synthetic-1 was between 10(-7) and 10(-3) mol/L with a detection limit of 10(-8) mol/L Fe(II) was determined successfully from four types of pharmaceutical products. The recovery values of Fe(II) in the pharmaceutical products were higher than 98.00% with relative standard deviation values < 5%.

Biosensor for the enantioselective analysis of the thyroid hormones (+)-3,3',5-triiodo-L-thyronine (T3) and (+)-3,3',5,5'-tetraiodo-L-thyronine (T4).

An amperometric biosensor based on L-aminoacid oxidase is proposed for enantioselective assay of (+)-3,3',5-triiodo-L-thyronine (L-T3) and (+)-3,3',5,5'-tetraiodo-L-thyronine (L-T4), due to the fact that only the L enantiomer has the hormonal activity. The construction of the amperometric biosensor is simple and reproducible. The analytical information obtained from enantioselective analysis are reliable. The RSD <1% assured by using the amperometric biosensors for L enantiomers assay as raw materials, and from tablets, demonstrated their suitability for the analysis of T3 and T4 at ppb concentration levels.

Determination of (+)-3,3',5-triiodo-L-thyronine (L-T3) from serum using a sequential injection analysis/immunosensor system.

A sequential injection analysis/immunosensor system is proposed for the analysis of T3 in serum with a rate of 75 samples/hr. The immunosensor design is based on the physical immobilization of anti-T3 in carbon paste. The working concentration range of the immunosensor in a sequential injection analysis system is between 3.4 and 340 ng/mL with a limit of detection of 2.19 ng/mL. The system is very reliable and very easy to design and operate.

Biosensors for the determination of ortho-acetyl-L-carnitine. Their utilization as detectors in a sequential injection analysis system.

In order to determine ortho-acetyl-L-carnitine, two biosensors were proposed. The biosensors were designed using physical immobilization of L-amino acid oxidase (L-AAOD) and horseradish peroxidase (HRP). Electrode characteristics were obtained and compared for the two carbon paste (graphite powder and paraffin oil) biosensors. The linear concentration ranges for the proposed biosensors were in the ranges of fmol/L to nmol/L, magnitude order with low limits of detection. Due to their reliability, the biosensors were used as detectors in a sequential injection analysis system, and gave reliable results for on-line assay of ortho-acetyl-L-carnitine in synthesis process control with a frequency of 75 samples per hour.

Utilization of maltodextrin based enantioselective, potentiometric membrane electrodes for the enantioselective assay of S-perindopril.

Enantioselective, potentiometric membrane electrodes (EPMEs) based on carbon paste impregnated with different maltodextrins {dextrose equivalent (DE) 4.0-7.0 (I), 13.0-17.0 (II) and 16.5-19.5 (III)} as chiral selectors for the assay of S-perindopril is described. The proposed electrodes could be reliably employed in the assay of S-perindopril raw material and from its pharmaceutical formulation, Coversyl((R)) tablets. The electrode based on maltodextrin (I) showed the best enantioselectivity and time-stability. The surfaces of the electrodes are easily renewable by simply polishing on an alumina paper.

Simultaneous determination of l-thyroxine (l-T(4)), d-thyroxine (d-T(4)), and l-triiodothyronine (l-T(3)) using a sensors/sequential injection analysis system.

Requirements of high purity and enantiopurity for the raw materials of active substances used for the pharmaceutical formulations involved utilization of high reliable analytical techniques for the analysis of the active compound. Sequential injection analysis system with electrochemical sensors as detectors proved to be a very good alternative for the chromatographic methods, as it is more reliable, not expensive, and faster. Drugs containing only l-thyroxine (l-T(4)) or both l-T(4) and l-triiodothyronine (l-T(3)) are formulated for the dysfunctions of thyroid. A sequential injection analysis system that can use two amperometric immunosensors (for the assay of l-T(3) and l-T(4)) and an amperometric biosensor (for the assay of d-thyroxine, d-T(4)) as detectors is proposed for the purity and enantiopurity tests of the raw materials used for the formulation of the drugs for thyroid. The system proved to be very reliable. The three compounds can be determined on-line in synthesis process control with a frequency of 20 samples per hour.

Electrochemical Sensor Arrays.

The importance of sensor arrays in environmental, food and clinical analysis is discussed. The possible designs of sensor arrays is shown. The most reliable mathematical models for data processing are presented. The importance of different types of electrochemical sensor arrays in analytical chemistry as well as their performances are shown.

Diamond paste-based electrodes for determination of Cr(III) in pharmaceutical compounds.

A new class of monocrystalline diamond paste-based electrodes is proposed for the determination of chromium(III) at trace levels in vitamins. Three types of monocrystalline diamond-natural diamond 1mu (natural diamond), synthetic diamond 50mu (synthetic-1), and synthetic diamond 1mu (synthetic-2)-were used for electrode construction. The linear concentration ranges are between 10(-10) and 10(-8); 10(-9) and 10(-7), and 10(-10) to 10(-8) mol L(-1), with limits of detection of 10(-12), 10(-12), and 10(-11) mol L(-1), when natural diamond, synthetic-1, and synthetic-2, respectively, are used as electrode materials. For electrodes based on natural diamond and synthetic-1 it was found that Cr(III) yields a peak at about +0.275+/-0.015 V (vs. Ag/AgCl) within a predetermined positive potential range situated between +0.4 and +0.2 V, while for the electrode based on synthetic-2 the peaks are found at +0.300+/-0.015 V (vs. Ag/AgCl). The proposed method is reliable for the determination of chromium(III) at trace levels in two vitamin tablets (RSD<0.2%).

Bienzymatic amperometric sensor for protein assay in milk.

A new bienzymatic amperometric sensor is proposed for the assay of the protein content of milk. The sensor is based on two enzymes: carboxypeptidase A and L-amino acid oxidase. The response characteristics obtained for this sensor (detection limit of 1.5 micromol/L, linear concentration range between 1.8 and 2.8 micromol/L), as well as high selectivity over possible interferences from milk, made it applicable as a detector in flow injection analysis (FIA). The response characteristics obtained in the non-equilibrium conditions (FIA system) are: detection limit of 1.5 micromol/L and linear concentration range between 2 and 3.5 micromol/L. Without FIA, the average recovery of proteins from milk and milk products is 99.06 +/- 0.07% and, by utilization of FIA, it increased to 99.73 +/- 0.03. The sensor proved a good reliability for the assay of proteins in milk and milk products.

Immunosensor for the determination of azidothymidine: its utilization as detector in a sequential injection analysis system.

An amperometric immunosensor based on graphite paste (graphite powder and paraffin oil) has been constructed for the assay of azidothymidine (AZT). The graphite paste is impregnated with anti-AZT. The immunosensor can be reliably used for the assay of AZT in its pharmaceutical formulation. The potential used for AZT assay was 435 mV vs Ag/AgCl electrode. The surface of the immunosensor can be regenerated by simply polishing, obtaining fresh immunocomposite ready to be used in a new assay. Due to its reliability, the immunosensor was successfully used as a detector in a sequential injection analysis system, and gave reliable results for on-line assay of AZT purity in raw material and AZT contents in pharmaceutical formulations.

Determination of L- and D-enantiomers of methotrexate using amperometric biosensors.

In order to determine the enantiopurity of methotrexate (Mtx), seven biosensors were proposed for the assay of L-Mtx and three biosensors for the assay of D-Mtx. The biosensors were designed using physical and chemical immobilization of glutamate oxidase and/or L-amino acid oxidase (L-AAOD) and/or horseradish peroxidase (HRP) for the assay of L-methotherexate, and D-amino acid oxidase (D-AAOD) and HRP for the assay of D-Mtx. Electrode characteristics were obtained and compared for the different carbon paste based biosensors. The linear concentration ranges for the proposed biosensors were in the ranges of fmol l(-1) to pmol l(-1), magnitude order with limits of detection in the fmol l(-1) to nmol l(-1) concentration range. All biosensors were successful for the determination of the enantiopurity of Mtx as raw material, and in its pharmaceutical formulations (tablets and injections).

Simultaneous determination of creatine and creatinine using amperometric biosensors.

In order to determine creatine and creatinine amperometric biosensors were proposed. A bienzymatic biosensor based on creatinase (CI) and sarcosine oxidase (SO) was used for the assay of creatine and a trienzymatic biosensor based on CI, SO and creatininase (CA) for the assay of creatinine. The linear concentration ranges are of pmol l(-1) to nmol l(-1) magnitude order, with very low limits of detection. The biosensors proved high reliability for determination of creatine and creatinine as raw material, and in the pharmaceutical formulation.