University of North Florida
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Contact Info

Stuart Chalk, Ph.D.
Department of Chemistry
University of North Florida
Phone: 1-904-620-1938
Fax: 1-904-620-3535
Email: schalk@unf.edu
Website: @unf

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Raluca Ioana Stefan

Abbrev:
Stefan, R.I.
Other Names:
Raluca-Ioana Stefan
Address:
Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa
Phone:
+27 124 20 2510
Fax:
+27 123 62 5297

Citations 13

"Simultaneous Determination Of -thyroxine (-T4), -thyroxine (-T4), And -triiodothyronine (-T3) Using A Sensors/sequential Injection Analysis System"
Talanta 2004 Volume 64, Issue 1 Pages 151-155
Raluca-Ioana Stefan, Jacobus F. van Staden and Hassan Y. Aboul-Enein

Abstract: 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 -thyroxine (-T4) or both -T4 and -triiodothyronine (-T3) are formulated for the dysfunctions of thyroid. A sequential injection analysis system that can use two amperometric immunosensors (for the assay of -T3 and -T4) and an amperometric biosensor (for the assay of -thyroxine, -T4) 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.

"Immunosensor For The Determination Of Azidothymidine: Its Utilization As Detector In A Sequential Injection Analysis System"
Talanta 2003 Volume 59, Issue 5 Pages 883-887
Raluca-Ioana Stefan, Rahel Girmai Bokretsion, Jacobus Frederick van Staden and Hassan Y. Aboul-Enein

Abstract: 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.

"Simultaneous Detection Of S And R Captopril Using Sequential Injection Analysis"
Talanta 2000 Volume 51, Issue 5 Pages 969-975
Raluca-Ioana Stefan, Jacobus (Koos) F. van Staden and Hassan Y. Aboul-Enein

Abstract: A simultaneous detection sequential injection analysis (SIA) system is proposed for the determination of S and R captopril using a potentiometric, enantioselective membrane electrode based on maltodextrin (DE = 14-17) for the assay of S-captopril and an amperometric biosensor for the assay of R-captopril. The proposed SIA system can be utilized reliably for the on-line simultaneous detection of the enantiomers in the synthesis process at a rate of 38 samples per hour in the following linear concentration ranges: 100-1000 nmol/l (R-captopril) and 1-1000 µmol/l (S-captopril) with a RSD better than 0.009% (n = 10).

"Biosensors For The Determination Of Ortho-acetyl-L-carnitine. Their Utilization As Detectors In A Sequential Injection Analysis System"
Prep. Biochem. Biotechnol. 2003 Volume 33, Issue 3 Pages 163-172
Raluca-Ioana Stefan; Rahel Girmai Bokretsion; Jacobus F. van Staden; Hassan Y. Aboul-Enein

Abstract: 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.

"Bienzymatic Amperometric Sensor For Protein Assay In Milk"
Prep. Biochem. Biotechnol. 2002 Volume 32, Issue 2 Pages 135-142
Stefan Raluca-Ioana, Makhapa A. Makhafola, Jacobus (Koos) F. van Staden

Abstract: 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 µmol/L, linear concentration range between 1.8 and 2.8 µmol/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 µmol/L and linear concentration range between 2 and 3.5 µmol/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.

"On-line Assay Of The S-enantiomers Of Enalapril, Ramipril And Pentopril Using A Sequential Injection Analysis/amperometric Biosensor System"
J. Pharm. Biomed. Anal. 2004 Volume 36, Issue 4 Pages 889-892
Raluca-Ioana Stefan, Jacobus F. van Staden, Camelia Bala and Hassan Y. Aboul-Enein

Abstract: 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 RSD values better than 0.1% (n = 10).

"Simultaneous Determination Of - And -carnitine Using A Sequential Injection Analysis/amperometric Biosensors System"
J. Pharm. Biomed. Anal. 2003 Volume 33, Issue 2 Pages 323-328
Raluca-Ioana Stefan, Rahel Girmai Bokretsion, Jacobus F. van Staden and Hassan Y. Aboul-Enein

Abstract: A sequential injection analysis (SIA) system is described for the simultaneous determination of -and -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 -amino acid oxidase and horseradish peroxidase (HRP) for the assay of - carnitine, and -amino acid oxidase and HRP for the assay of -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.

"Determination Of (+)-3,3',5-Triiodo-L-thyronine (L-T3) From Serum Using A Sequential Injection Analysis/Immunosensor System"
J. Immunoassay Immunochem. 2004 Volume 25, Issue 2 Pages 183-189
Raluca-Ioana Stefan; Jacobus Frederick van Staden; Hassan Y. Aboul-Enein

Abstract: 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-T 3 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.

"On-line Monitoring Of R-captopril Using An Amperometric Biosensor/sequential Injection Analysis System"
Instrum. Sci. Technol. 2002 Volume 30, Issue 3 Pages 243-250
Raluca-Ioana Stefan, Jacobus (Koos) F. van Staden, Hassan Y. Aboul-Enein

Abstract: An automated system for the on-line monitoring of R-captopril, based on the concept of sequential injection analysis with an amperometric biosensor (based on D-amino acid oxidase) as detector, is described. The sampling rate is 80 samples per hour with an RSD < 0.22% in the linear range between 0.2 and 1.0 µmol/L. The detection limit is 160 nmol/L.
Automation

"Electrochemical Sensor Arrays"
Crit. Rev. Anal. Chem. 1999 Volume 29, Issue 2 Pages 133-153
Raluca-Ioana Stefan, Jacobus F. Van Staden and Hassan Y. Aboul-Enein

Abstract: 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.
Environmental Food Biological Sensor Electrochemical analysis Chemometrics Non-immobilized enzyme Review

"Design And Use Of Electrochemical Sensors In Enantioselective High Throughput Screening Of Drugs. A Minireview"
Comb. Chem. High Throughput Screen. 2000 Volume 3, Issue 6 Pages 445-454
Raluca - Ioana Stefan, Jacobus (Koos) F. van Staden, Hassan Y. Aboul-Enein

Abstract: The importance of reliable detection systems for enantiomeric assays increases with the necessity of high throughput screening analysis of raw materials for the pharmaceutical industry. The utilization of electrochemical sensors in enantioselective analysis is an accurate and precise alternative to chromatographic techniques. The reliability of the response characteristics as well as of the analytical information obtained by using electrochemical sensors is strictly correlated with the design of the sensors. The designs evaluated for sensors have been based on PVC, imprinting polymers and carbon paste matrices. Among these, carbon paste sensors have been the most reliable and have been utilized for the construction of potentiometric, enantioselective membrane electrodes as well as for amperometric biosensors, and immunosensors. There are two ways to use the electrochemical sensors in enantioselective screening analysis: Selective binding and catalyst selectivity. A molecule with a special chemical architecture is required for selective binding: A lock for a key. The high reliability of analytical information obtained using these sensors has made possible the automation of potentiometric and amperometric techniques by integration of enantioselective sensors as detectors in flow injection analysis and sequential injection analysis techniques.

"Simultaneous Determination Of - And -methotrexate Using A Sequential Injection Analysis/amperometric Biosensors System"
Biosens. Bioelectron. 2003 Volume 19, Issue 3 Pages 261-267
Raluca-Ioana Stefan, Rahel Girmai Bokretsion, Jacobus Frederick van Staden and Hassan Y. Aboul-Enein

Abstract: A sequential injection analysis (SIA) is proposed for the simultaneous determination of - and -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 -amino acid oxidase (-AAOD) or/and -glutamate oxidase (-Glox) and horseradish peroxidase (HRP) for the assay of -Mtx and -amino acid oxidase (-AAOD) and HRP for the assay of -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 - and -Mtx with a frequency of 34 samples per hour.

"On-line Simultaneous Determination Of S- And R-perindopril Using Amperometric Biosensors As Detectors In Flow Systems"
Anal. Chim. Acta 2002 Volume 467, Issue 1-2 Pages 189-195
Raluca-Ioana Stefan, Jacobus F. van Staden, Ludwig Vusimuzi Mulaudzi and Hassan Y. Aboul-Enein

Abstract: Two types of flow systems were selected for the simultaneous assay of S- and R-perindopril (pdp): flow injection analysis (FIA) and sequential injection analysis (SIA). The SIA system was more efficient, because of the highest precision and accuracy, and the lower consumption of sample and buffer. The amperometric biosensors used as detectors in the flow systems were based on L- and D-amino acid oxidase (AAOD). The linear concentration ranges are in the nmol L-1 range, from 120 pmol L-1 to 40 nmol L-1 (3 x S.D.), with very low detection limits. The biosensors/flow system can be used reliably online in synthesis process control, for the simultaneous assay of S- and R-pdp with a frequency of more than 30 samples per hour. (C) 2002 Elsevier Science B.V All rights reserved.