Chemical speciation by sequential injection analysis: an overview.

The simplicity of the sequential injection (SIA) manifold and its low need for maintenance makes it an ideal tool in speciation. As miniaturization and reduction of reagent consumption are also ultimate goals in chemical sensing, it is useful to review the use of combined injection and programmed flow as a central issue in designing SIA systems with chemical sensors and structurally simplified chemical analysers. This overview gives an insight into the current state, analytical scope and performance characteristics of sequential injection systems as analytical tools for speciation. The suitability of SIA for speciation analysis is illustrated by the methods used in the conduits of sequential injection systems for the chemical conversion of different chemical forms into detectable chemical species. Configurations of the basic sequential injection speciation analysis systems were designed around a multi-syringe-time-based-injection system with one detector, direct and indirect speciation of different forms using a single detector including diode array detection and direct speciation of different forms using multiple detection. Examples showing the use of SIA for the simultaneous determination or speciation of metal ions, inorganic anions and organic compounds are given with some recent results from our research groups.

Spectrophotometric determination of bromate by sequential injection analysis.

A simple and rapid on-line spectrophotometric method for the determination of bromate is proposed. The method is based on the reaction of bromate and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-bromo-PADAP) with SCN(-), a red complex is formed and monitored at 550nm. The linear range found is between 0.18 and 3.00mgl(-1) with a detection limit of 0.15mgl(-1). The sampling rate was calculated to be 45 samples per hour. The proposed method has a precision of less than 0.8%.

On-line speciation of bromine and bromide using sequential injection analysis with spectrophotometric detection.

An on-line procedure for the simultaneous determination of bromine and total bromine (bromine+bromide oxidised to bromine) is proposed, which lead to the determination of bromide by subtraction. Phenol red was used as chromogenic reagent for bromine and total bromine after bromide was oxidised to bromine by Chloramine T. The linear range found is 1-10 mg L(-1) with a detection limit of 0.6 mg L(-1) for bromine, and a linear range of 0.8-15 mg L(-1) with a detection limit of 0.4 mg L(-1) for total bromine. The calculated RSD for bromine is less than 0.8% and for total bromine less than 0.7%. The system is fully computerised and able to run 30 samples per hour with an automated rinsing step that eliminates sample carry-over. The results for both bromine and bromide from the proposed sequential injection analysis (SIA) system compare favourably with standard manual methods and statistical evaluation proves no significant difference between the results of the proposed SIA system and the standard method at the 95% confidence level. The presence of other halides was found not to interfere.

Maltodextrins as new chiral selectors in the design of potentiometric, enantioselective membrane electrodes.

Maltodextrins (dextrose equivalent (DE) 4.0-7.0, 13.0-17.0, and 16.5-19.5) are proposed as novel chiral selectors for the construction of potentiometric, enantioselective membrane electrodes. The potentiometric, enantioselective membrane electrodes can be used reliably for the assay of S-captopril as raw material and in pharmaceutical formulations such as Novocaptopril tablets, by use of direct potentiometry. The best response was obtained when maltodextrin with higher DE was used for construction of the electrode. The best enantioselectivity and time-stability was achieved for the lower DE maltodextrin. L-proline was found to be the main interferent for all the proposed electrodes. The surface of the electrodes can be regenerated by simply polishing; this furnishes a fresh surface ready for use in a new assay.

Amperometric biosensors/sequential injection analysis system for simultaneous determination of S- and R-captopril.

Two amperometric biosensors based on L- and D-amino acid oxidase, respectively, are proposed for the simultaneous detection of S- and R-captopril in a sequential injection analysis system (SIA). The linear concentration ranges are: 0.4-1.6 micromol/l (S-captopril) and 120-950 nmol/l (R-captopril) with detection limits of 0.2 and 15 nmol/l, respectively. The biosensors/SIA system can be used reliably on-line in synthesis process control, for the simultaneous assay of S- and R-captopril with a frequency of 34 samples/h.

Immunosensors in clinical analysis.

Clinical analysis requires methods of high reliability. The importance of immunosensors in clinical analysis is discussed. Due to the fact that the construction of immunosensors plays a very important role in their response characteristics, different types of immunosensor designs are highlighted. The performances of immunosensors are discussed taking into account the type of transducer utilized for construction.

Amperometric biosensor based on D-aminoacid oxidase for the R-perindopril assay.

A new amperometric biosensor based on D-aminoacid oxidase is described for the assay of R-perindopril. R-perindopril can be determined in the 400-20 nmol/L concentration range; the detection limit is 10 nmol/L. The selectivity was checked with S-perindopril, D- and L-proline, and polyvinylpyrrolidone. The main interfering species was D-proline. An automated system for the assay of R-perindopril based on the concept of flow injection with an amperometric biosensor (based on D-aminoacid oxidase) as detector is also described. The system is suitable for the on-line monitoring of R-perindopril at a sampling rate of 72 samples/h, in the linear range: 100 nmol/L -20 nmol/L with an RSD better than 0.09% (n = 10).

Simultaneous detection of S and R captopril using sequential injection analysis.

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 mumol/l (S-captopril) with a RSD better than 0.009% (n=10).

Evaluation of different SIA sample-buffer configurations using a fluoride-selective membrane electrode as detector.

The peak profiles of four different buffer-sample SIA configurations, e.g. buffer-sample; sample-buffer; buffer-sample-buffer and sample-buffer-sample with the last two in the sandwich mode were evaluated with a fluoride-selective membrane electrode as detector. The best response characteristics and peak shapes as well as recovery and precision values were obtained for the buffer-sample configuration. For low concentration levels, sandwich SIA configurations are more suitable, when optimum buffer and pH are used. The utilisation of a cheap electrolyte with a minimum consumption of buffer and a cheap, robust instrumentation made the SIA system suitable for on-line determination of fluoride.

S-perindopril assay using a potentiometric, enantioselective membrane electrode.

A potentiometric, enantioselective membrane electrode based on graphite paste (graphite powder and paraffin oil) has been constructed. The graphite paste is impregnated with a 10(-3) mol/L 2-hydroxy-3-trimethylammoniopropyl-beta-cyclodextrin (as chloride salt) solution. The potentiometric, enantioselective membrane electrode can be used reliably for enantiopurity tests of S-perindopril using a chronopotentiometric (zero current) technique, in the 10(-5)-10(-2) mol/L concentration range (detection limit 5 x 10(-6) mol/L), with an average recovery of 99.58% (RSD = 0.33%). The enantioselectivity was determined over R-perindopril and D-proline. The response characteristics of the enantioselective, potentiometric membrane electrode were also determined for R-perindopril. It was shown that L-proline is the main interfering compound. The surface of the electrode can be regenerated simply by polishing, obtaining a fresh surface ready to be used in a new assay.

Biosensor for enantioselective analysis of S-cilazapril, S-trandolapril, and S-pentopril.

The purpose of this research was to construct an amperometric biosensor on L-amino acid oxidase (L-AAOD) supported on a carbon paste for the enantioselective analysis of S-cilazapril (I), S-trandolapril (II), and S-pentopril (III). A chemically modified carbon paste was constructed with L-AAOD. The proposed amperometric biosensor proved reliable for the purity of I, II, and III. The linear working ranges obtained for drug assay were 0.001-100 mumol/l (I), 0.02-10 mumol/l (II), 0.08-50 mumol/l (III) over the pH ranges 7.0-7.4 (I), 6.8-7.4 (II), and 7.0-7.4 (III). The low limits of detection obtained were 5 pmol/l (I), 15 mumol/l (II), and 5 mumol/l (III), respectively. The selectivity of the biosensors was checked by both mixed and separate solution methods. Polyvinylpyrolidone and D-proline did not interfere with the assay of the studied drugs, whereas L-proline presented a potent interfering species. The relative standard deviation values (< 0.2%) make the biosensors suitable for direct amperometric assay of these angiotensin-converting enzyme (ACE) inhibitors. The proposed amperometric biosensor that was based on L-AAOD proved reliable in the analysis of the above-mentioned ACE inhibitors and can be used for the quality control of these drugs.

Biosensor for the enantioselective analysis of S-perindopril.

Because S-perindopril enantiomer is the eutomer which is responsible for the angiotensin-converting enzyme inhibition activity, it is necessary to develop a reliable method for its assay from its distomer, the R-enantiomer. For this purpose, an amperometric biosensor was developed based on L-amino acid oxidase. The working range of the described biosensor was 20pmol/L-10 micromol/L on the 7.0-7.4 pH range, with a detection limit of 2pmol/L. The low enantioselectivity for R-perindopril, as compared with S-enantiomer, was demonstrated by both mixed solutions and separate solutions methods (amperometric selectivity coefficient is 1.0 x 10(-4)). The biosensor was also selective towards D-proline and polyvinylpyrrolidone. The amperometric biosensor can be used for enantioselective analysis of S-perindopril in raw material, with an RSD < 1%. The life time (t95%) of the biosensor is three weeks.

A new construction for a potentiometric, enantioselective membrane electrode--its utilization to the S-captopril assay.

A novel potentiometric, enantioselective membrane electrode based on graphite paste (graphite powder and paraffin oil) has been constructed. The graphite paste is impregnated with 2-hydroxy-3-trimethylammoniopropyl-beta-cyclodextrin (as chloride salt) solution, 10(-3) mol l(-1). The potentiometric, enantioselective membrane electrode can be used reliable for S-captopril assay as raw material and from Novocaptopril tablets, using a chronopotentiometry (zero current) technique, in the 10(-6)-10(-2) mol l(-1) concentration range (detection limit 2 x 10(-7) mol l(-1)), with an average recovery of 99.99% (RSD=0.05%). The enantioselectivity was determined over R-captopril and D-proline. The response characteristics of the enantioselective, potentiometric membrane electrode were determined also for R-captopril. It was shown that only L-proline is the main interfering compound. The surface of the electrode can be regenerated by simply polishing, obtaining fresh surface ready to be used in a new assay.

Simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional ion-selective electrodes in series.

An on-line automated system for the simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional calcium-selective and fluoride-selective membrane electrodes as sensors in series is described. Samples (30 mul) are injected into a TISAB II (pH=5.50) carrier solution as an ionic strength adjustment buffer. The sample-buffer zone formed is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. The system is suitable for the simultaneous on-site monitoring of calcium (linear range 10(-5)-10(-2) mol l(-1) detection limit 1.94x10(-6) mol l(-1) recovery 99.22%, RSD<0.5%) and fluoride (linear range 10(-5)-10(-2) mol l(-1) detection limit 4.83x10(-6) mol l(-1) recovery 98.63%, RSD=0.3%) at a sampling rate of 60 samples h(-1).

Biosensors for the enantioselective analysis of S-enalapril and S-ramipril.

S-Enalapril, and S-ramipril are angiotensin-converting enzyme (ACE) inhibitors which are used for treatment of hypertension. Due to the fact that only the S enantiomer possesses the ACE inhibiting activity, it is necessary to develop an enantioselective analytical method for its discrimination from the less active R-enantiomer. An amperometric biosensor, based on L-amino acid oxidase, was developed and proved reliable for the analysis of the S-enantiomer of these ACE inhibitors. The working range of the biosensor for S-enalapril assay (A) is 0.4-120 mumol/L, and for S-ramipril assay (B) is 0.2-100 mumol/L, with a limit of detection of 163 nmol/L (A) and 107 nmol/L (B), respectively. It is of interest to mention that the biosensors demonstrated enantioselectivity versus D-proline (1.4 x 10(-3) mol/L(A), 5.3 x 10(-3) mol/L(B) and also the selectivity versus the polyvinylpyrolidone (3.0 x 10(-3) mol/L(A), 3.2 x 10(-3) mol/L(B), respectively. The working pH ranges are: 6.8-7.4 (A), and 6.2-7.0 (B), respectively. The RSD < 1% assured by using the amperometric biosensors for S enantiomers assay in raw materials, in tablet formulations, and their suitability for the analysis of these drug enantiomers.

Moclobemide-selective membrane electrode and its pharmaceutical applications.

A liquid membrane electrode prepared with moclobemide-dipicrylamine ion-pair complex, dissolved in nitrobenzene as solvent, was studied for analytical performance. The linear response covers the range 10(-3)-10(-6) M moclobemide solution, with a slope of 50.7 mV decade(-1) (pH range 3.5-8). The detection limit is 3 x 10(-7) M. The electrode shows stability, good reproducibility and fast response. The selectivity of the electrode is good. There are two important interfering ions: mianserin and thiamine (Vitamin B(1)). The compression excipients (such as Mg(2+), starch, talcum powder) do not interfere. These characteristics of the electrode enabled it to be used for the determination of moclobemide in drugs and as an active substance, via indirect and direct potentiometric methods. Via an indirect potentiometric method moclobemide, as an active substance, can be determined with an average recovery of 99.96% and a relative standard deviation of 0.85%, and this method can also be used for its determination in drugs with a relative standard deviation of < 2%. The electrode is useful for the determination of the dissolution rate of moclobemide tablets. The physical processes are numerically simulated by typical equations. The apparent first-order rate constants for disintegration and dissolution were calculated.