New microdialysis-electrochemical device for simultaneous determination of ascorbic acid and 5-hydroxyindole-3-acetic acid in rat striatum

A new device combining microdialysis with electrochemical microsensor was developed. It can be applied to monitor the biomolecules in the brain for biological and pharmaceutical research. In this paper, the device was applied to simultaneously determine ascorbic acid (AA) and 5-hydroxyindole-3-acetic acid (5-HIAA) in rat striatum. The microsensor used for the device was poly (sulphosalicylic acid) microsensor, which exhibited a good electrocatalytic effect on oxidization of AA and 5-HIAA. The oxidation currents measured by differential pulse voltammetry (DPV) were linear for AA in the range of 0.02-1.0 mmol l⁻¹, and for 5-HIAA from 0.5 to 10.0 μmol l⁻¹ (r=0.9998 and 0.9991, respectively). The detection limits were calculated to be 0.01 mmol l⁻¹ for AA and 0.25 μmol l⁻¹for 5-HIAA (S/N=3). Studies also showed that co-existing substances in biological fluids did not interfere with AA and 5-HIAA determination when using this microsensor. Since, the substances in the microdialysate are easily oxidized by air, the microdialysate in this device was under the protection of N₂. It was found that the concentrations of AA and 5-HIAA in rat striatum were 215±5 and 6.21±0.61 μmol l⁻¹ (mean±S.E.M., n=7), respectively with this device under the protection of N₂. In addition, the effect of the nitric oxide donor, sodium nitroprusside (SNP), on 5-HIAA in the rat striatum was investigated. It was found that a high concentration of SNP (1.0 mmol l⁻¹) resulted in a 34% increase in 5-HIAA level.     

Electrochemical determination of maltol in beverages with glassy carbon electrode and its silica sol-gel modified electrode

The electrochemical behavior of maltol on a glassy carbon (GC) electrode was investigated. The results were applied to differential pulse voltammetric determination of maltol in beverages pretreated by ultrafiltration. Under the optimum experimental conditions, the linear range is 1×10⁻⁵ to 6×10⁻⁴ mol l⁻¹ maltol and the relative standard deviation for 0.4 mmol l⁻¹ maltol is 0.6% (n=9). The detection limit was 5 μmol l⁻¹. Furthermore, silica sol-gel film on GC electrode could be used as suitable selective membrane, which integrated selective membrane on the electrode and substituted for the pretreatment of ultrafiltration. Under the above conditions, maltol was determined by semi-differential linear sweep voltammetry at a silica sol-gel modified GC electrode in the concentration range of 5×10⁻⁶ to 5×10⁻⁴ mol l⁻¹. The detection limit was 2 μmol l⁻¹ and the relative standard deviation for 0.1 mmol l⁻¹ maltol was 0.7% (n=7). The proposed method is of sensitivity, simplicity, rapidness and no contamination. It had been applied to the direct determination of maltol in beverages such as grape wines, drinks and beers without any pretreatment. The results obtained with the present method were satisfactory with those obtained by spectrophotometry. It could be used as a simple and practical method for the determination of the flavor enhancer maltol in beverages.     

Post-column terbium complexation and sensitized fluorescence detection for the determination of norepinephrine, epinephrine and dopamine using high-performance liquid chromatography

Terbium sensitized fluorescence was used as a post-column detection system to develop a simple, sensitive and rapid high-performance liquid chromatographic method for the simultaneous determination of catecholamines norepinephrine (NE), epinephrine (E) and dopamine (DA).Catecholamines were separated by an ion-pair reversed-phase chromatography on a BDS-Hypersil analytical column with a mobile phase of methanol and 50 mmol l⁻¹ acetate buffer (pH 4.7) containing 1.1 mmol l⁻¹ SOS and 0.11 mmol l⁻¹ EDTA (15+85 v/v).Catecholamines and the internal standard (3,4-dihydroxybenzylamine, DHBA) were post-column derivatized by the addition to the eluent of an alkaline solution containing a stoichiometric mixture of terbium(III) chloride and EDTA. Fluorescence detection (λ_ex=300 nm, λ_em=545 nm) is based on the sensitization of terbium ion fluorescence after complexation with catecholamines.The chemical compatibility between the eluent and the post-column reagent was studied and the analytical characteristics of the method were established. Detection limits found were 1.0×10⁻⁸, 4.0×10⁻⁸ and 7.0×10⁻⁸ mol l⁻¹ for NE, E and DA, respectively. The method has been successfully applied to the determination of catecholamines in urine samples after solid-phase extraction (SPE) pre-treatment. Recoveries from urine spiked with NE (4.0×10⁻⁷, 2.0×10⁻⁶ and 4.0×10⁻⁶ mol l⁻¹), E (8.2×10⁻⁸, 4.1×10⁻⁷ and 8.2×10⁻⁷ mol l⁻¹) and DA (1.0×10⁻⁶, 5.0×10⁻⁶ and 1.0×10⁻⁵ mol l⁻¹) varied from 98 to 100% (mean=99.3%), from 106 to 107% (mean=106.3%) and from 98 to 101% (mean=99.3%), respectively. The between-run precision (relative standard deviation, R.S.D.) for the method for three urine samples at different concentration levels of each catecholamine varied from 3.6 to 7.0%.     

Determination of phosphoamino acids by micellar electrokinetic capillary chromatography with laser-induced fluorescence detection

A micellar electrokinetic capillary chromatography (MEKC) method with laser-induced fluorescence detection (LIF) was developed for analyzing three phosphoamino acids including phosphotyrosine (P-Tyr), phosphoserine (P-Ser), and phosphothreonine (P-Thr). 3-(2-Furoyl)quinoline-2-carboxaldehyde (FQ), a fluorogenic dye, was employed for derivatization of these phosphoamino acids. Results indicated that the complete baseline resolution of each phosphoamino acid was obtained within 10 min, using 20 mmol l⁻¹ sodium borate buffer (pH 9.35) containing 20 mmol l⁻¹sodium deoxycholate (SDC) and 10 mmol l⁻¹ Brij35. Other common amino acids, especially Glu and Asp, did not disturb the assay of these phosphoamino acids. There was a linear relationship between the peak area for analyte and its concentration, with correlation coefficients in the range of 0.9966-0.9996. The concentration detection limits (signal-to-noise = 3) for P-Tyr, P-Ser, and P-Thr were 10, 40, and 75 nmol l⁻¹, respectively. The developed method was successfully applied for determining phosphoamino acids in the hydrolysis sample of a phosphorylated protein kinase.     

Extraction of arsenic compounds from lichens

Different extraction procedures were applied to improve the extraction efficiency of arsenic compounds from lichens. Two lichen species were chosen from an arsenic-contaminated environment: epiphytic Hypogymnia physodes (L.) Nyl. and terricolous Cladonia rei Schaer. Samples were extracted with water at temperatures of 20, 60 and 90 °C, using mixtures of methanol/water (9:1, 1:1 and 1:9), Tris buffer and acetone and the extracts speciated. Water and Tris buffer showed the best extraction efficiency of all extractants used; however, the extraction efficiency was still less than 23%. Since a major fraction of arsenic appeared to be associated with trapped soil particles, a sequential extraction procedure originally designed for soils (extraction steps: (1) 0.05 mol l⁻¹ (NH₄)₂SO₄; (2) 0.05 mol l⁻¹ (NH)₄H₂PO₄; (3) 0.2 mol l⁻¹ NH₄-oxalate buffer, pH 3.25; (4) mixture of 0.2 mol l⁻¹ NH₄-oxalate buffer and 0.1 mol l⁻¹ ascorbic acid, pH 3.25; (5) 0.5 mol l⁻¹ KOH) was applied and found to remove 45% of the total arsenic from H. physodes and 83% from C. rei. The lipid-soluble fraction of arsenic was estimated by k₀-INAA analysis of diethylether extracts and was found to be negligible. An HPLC-UV-HGAFS system was used to determine the arsenic compounds extracted. In both lichen species, arsenous acid, arsenic acid, monomethylarsonic acid, dimethylarsinic acid, arsenobetaine, trimethylarsine oxide and glycerol-ribose were detected. In addition, phosphate-ribose was found in H. physodes.     

Determination of diphenhydramine by capillary electrophoresis with tris(2,2'-bipyridyl)ruthenium(II) electrochemiluminescence detection

Tris(2,2′-bipyridyl)ruthenium(II) electrochemiluminescence detection in a capillary electrophoresis separation system was used for the determination of diphenhydramine. In this study, platinum disk electrode (300 μm in diameter) was used as a working electrode and the influence of applied potential and buffer conditions were investigated. Under optimal conditions: 1.2 V applied potential, pH 8.50, 15 kV separation voltage and 10 mmol l⁻¹ running buffer, the calibration curve of diphenhydramine was linear over the range of 4×10⁻⁸ to 1×10⁻⁵ mol l⁻¹. This technique gave satisfactory precision, and relative standard deviations of migration times and chemiluminescence peak intensities were less than 1 and 6%, respectively. The technique was applied to animal studies for determination of diphenhydramine extracted from rabbit plasma and urine samples, and the extraction efficiency were between 92 and 98.5%.     

Synthesis of cross-linked chitosan possessing N-methyl-d-glucamine moiety (CCTS-NMDG) for adsorption/concentration of boron in water samples and its accurate measurement by ICP-MS and ICP-AES

A chitosan resin derivatized with N-methyl-d-glucamine (CCTS-NMDG) was synthesized by using a cross-linked chitosan (CCTS) as base material. The N-methyl-d-glucamine (NMDG) moiety was attached to the amino group of CCTS through the arm of chloromethyloxirane. The adsorption behavior of 59 elements on the synthesized resin was systematically examined by using the resin packed in a mini-column, passing water samples through it and measuring the adsorbed elements in eluates by ICP-MS. The CCTS-NMDG resin shows high ability in boron sorption with the capacity of 0.61 mmol ml⁻¹ (= 2.1 mmol g⁻¹). The sorption kinetics of this resin was faster than that of the commercially available resins. Other advantages of the synthesized resin are: (1) quantitative collection of boron at neutral pH regions; (2) complete removal of large amounts of matrices; (3) no loss of efficiency over prolonged usage; (4) effective collection of boron in wide range concentration using a mini column containing 1 ml resin; (5) complete elution of boron with 1 mol l⁻¹ nitric acid. The resin was applied to the collection/concentration of boron in water samples. Boron in tap water and river water was found to be in the range of 6-8 μg l⁻¹. The limit of detection (LOD) of boron after pretreatment with CCTS-NMDG resin and measurement by ICP-MS was 0.07 μg l⁻¹ and the limit of quantification (LOQ) was 0.14 μg l⁻¹ when the volume of each sample and eluent was 10 ml.     

A sensitive post-column photochemical derivatization/fluorimetric detection system for HPLC determination of bisphosphonates

A new reversed-phase ion-pair high-performance liquid chromatographic (HPLC) method has been developed for the determination of the following bisphosphonic acids: alendronic acid (ALEN), etidronic acid (ETID), ibandronic acid (IBAN) and risedronic acid (RISE). Separation was achieved on a C₁₈ column using a mixture of 50 mmol L⁻¹ borate buffer pH 9.0 containing 0.25 mmol L⁻¹ tetrabutylammonium chloride and 0.5 mmol L⁻¹ EDTA and acetonitrile (97:3) as the mobile phase. The sensitive detection of the above bisphosphonic acids was based on their oxidation to orthophosphate by the on-line peroxydisulfate-assisted photolysis followed by post-column reaction with molybdate to yield phosphomolybdate. This subsequently reacted with thiamine to generate thiochrome and, finally, the fluorescence of thiochrome was measured at 440 nm with excitation at 375 nm. The developed method is precise with a mean relative standard deviation of 1.3%, sensitive (with a detection limit at the nmol L⁻¹ level), accurate, specific, rapid (analysis time approximately 13 min) and inexpensive because to the low cost of the reagents. The assay was applied to the analysis of the four bisphosphonic acids in commercial dosage formulations, in which the excipients did not interfere with the determination. The method was also applied to the determination of etidronate, risedronate and ibandronate in human urine. Sample preparation involves precipitation of the analytes from urine along with endogenous phosphates such as calcium salts by addition of calcium chloride at alkaline pH and dissolution of the precipitate in 0.05 mol L⁻¹ ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid.     

Implementing stepwise solvent elution in sequential injection chromatography for fluorimetric determination of intracellular free amino acids in the microalgae Tetraselmis gracilis

The concept of sequential injection chromatography (SIC) was exploited to automate the fluorimetric determination of amino acids after pre-column derivatization with o-phthaldialdehyde (OPA) in presence of 2-mercaptoethanol (2MCE) using a reverse phase monolithic C₁₈ stationary phase. The method is low-priced and based on five steps of isocratic elutions. The first step employs the mixture methanol: tetrahydrofuran: 10 mmol L⁻¹ phosphate buffer (pH 7.2) at the volumetric ratio of 8:1:91; the other steps use methanol: 10 mmol L⁻¹ phosphate buffer (pH 7.2) at volumetric ratios of 20:80, 35:65, 50:50 and 65:35. At a flow rate of 10 μL s⁻¹ a 25 mm long-column was able to separate aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn), serine (Ser), glutamine (Gln), glycine (Gly), threonine (Thr), citruline (Ctr), arginine (Arg), alanine (Ala), tyrosine (Tyr), phenylalanine (Phe), ornithine (Orn) and lysine (Lys) with resolution >1.2 as well as methionine (Met) and valine (Val) with resolution of 0.6. Under these conditions isoleucine (Ile) and leucine (Leu) co-eluted. The entire cycle of amino acids derivatization, chromatographic separation and column conditioning at the end of separation lasted 25 min. At a flow rate of 40 μL s⁻¹ such time was reduced to 10 min at the cost of resolution worsening for the pairs Ctr/Arg and Orn/Lys. The detection limits varied from 0.092 μmol L⁻¹ for Tyr to 0.51 μmol L⁻¹ for Orn. The method was successfully applied to the determination of intracellular free amino acids in the green alga Tetraselmis gracilis during a period of seven days of cultivation. Samples spiked with known amounts of amino acids resulted in recoveries between 94 and 112%.     

Flow-injection chemiluminescence determination of fluoroquinolones by enhancement of weak chemiluminescence from peroxynitrous acid

A flow-injection chemiluminescence (CL) method is described for the determination of fluoroquinolones including ciprofloxacin, norfloxacin and ofloxacin. The method is based on the enhancement by these compounds of the weak CL from peroxynitrous acid. The linear ranges are 1.0×10⁻⁷ to 1.0×10⁻⁵ mol l⁻¹ for ciprofloxacin and norfloxacin, and 3.0×10⁻⁷ to 3.0×10⁻⁵ mol l⁻¹ for ofloxacin, respectively. The detection limits (S/N=3) are 4.5×10⁻⁸ mol l⁻¹ ciprofloxacin, 5.9×10⁻⁸ mol l⁻¹ norfloxacin and 1.1×10⁻⁷ mol l⁻¹ ofloxacin, respectively. The proposed method was applied to the determination of fluoroquinolones in pharmaceutical preparations.     

Studying the distribution pattern of selenium in nut proteins with information obtained from SEC-UV-ICP-MS and CE-ICP-MS

In this work, size exclusion chromatography (SEC) with UV and inductively coupled plasma mass spectrometry (ICP-MS) detection was used to study the association of selenium to proteins present in Brazil nuts (Bertholletia excelsa) under five different extraction conditions. As expected, better solubilization of proteins was observed using 0.05 mol L⁻¹ sodium hydroxide and 1% sodium dodecylsulfate (SDS) in Tris/HCl buffer (0.05 mol L⁻¹, pH 8) as compared to 0.05 mol L⁻¹ HCl, 0.05 mol L⁻¹ Tris/HCl or hot water (60 °C). Due to non-destructive character of Tris-SDS treatment, this was applied for studying molecular weight (MW) distribution patterns of selenium-containing nut proteins. Three different SEC columns were used for obtaining complete MW distribution of selenium: Superdex 75, Superdex Peptide, and Superdex 200 were tested with 50 mmol L⁻¹ Tris buffer (pH 8), 150 mmol L⁻¹ ammonium bicarbonate buffer (pH 7.8), phosphate (pH 7.5), and CAPS (pH 10.0) mobile phases. Using Superdex 200 column, the elution of at least three MW fractions was observed with UV detection (200-10 kDa) and ICP-MS chromatogram showed the co-elution of selenium with the two earlier fractions. The apparent MWs of these selenium-containing fractions were respectively about 107 and 50 kDa, as evaluated from the column calibration. For further characterization of individual selenium species, the defatted nuts were hydrolyzed with proteinase K and analyzed by capillary electrophoresis (CE) with ICP-MS detection. The suitability of CE for the separation of selenite, selenate, selenocystine and selenomethionine in the presence of the nut sample matrix is demonstrated. Complete separation of the above mentioned selenium species was obtained within a migration time of 7 min. In the analysis of nut extracts with CE-ICP-MS, selenium was found to be present mainly as selenomethionine.     

Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0 × 10⁻⁶-1.0 × 10⁻⁴ mol L⁻¹ and 4.0 × 10⁻⁶-3.2 × 10⁻⁴ mol L⁻¹, with detection limits of 4.0 × 10⁻⁷ mol L⁻¹ and 1.2 × 10⁻⁶ mol L⁻¹ for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.     

A permeation liquid membrane system for determination of nickel in seawater

The use of a permeation liquid membrane system for the preconcentration and separation of nickel in natural and sea waters and subsequent determination by atomic absorption spectroscopy is presented. 2-Hydroxybenzaldehyde N-ethylthiosemi-carbazone (2-HBET) in toluene is used as the active component of the liquid membrane. A study strategy based on a simplex design has been followed. Several chemical and physical parameters were optimized. Maximum permeation coefficient was obtained at a feed solution pH of 9.4, 0.3 mol l⁻¹ of HNO₃ in the stripping solution and 1.66 mmol l⁻¹ of 2-HBTE in toluene as carrier. The precision of the method was 4.7% at 95% significance level and a detection limit of 0.012 μg l⁻¹ of nickel was achieved. The preconcentration procedure showed a linear response within the studied concentration range from 3 to 500 μg l⁻¹ of Ni in the feed solution. The method was validated with different spiked synthetic seawater and certified reference water samples: TMDA-62 and LGC 6016, without matrix interferences and showing good concordance with the certified values, being the relative errors −5.9% and −2.2%, respectively. Under optimal conditions, the average preconcentration yield for real seawater samples was 98 ± 5%, with a nickel preconcentration factor of 20.83 and metal concentrations ranging between 2.8 and 5.4 μg l⁻¹.     

A novel kinetic determination of dissolved chromium species in natural and industrial waste water

A highly sensitive, selective and simple kinetic method was developed for the determination of dissolved chromium species based on the catalytic effect of Cr(III) and/or Cr(VI) on the oxidation of 2-amino-5-methylphenol (AMP) with H₂O₂. The fixed time and initial rate variants were used for kinetic spectrophotometric measurements by tracing the oxidized product at 400 nm for 10 min after starting the reaction. Boric acid and Tween-40 exerted pronounced activating and micellar sensitizing effects on the studied redox reaction, respectively. The optimum reaction conditions were: 3.0 mmol l⁻¹ AMP, 0.45 mol l⁻¹ H₂O₂, 0.50 mol l⁻¹ boric acid, 4 v/v% Tween-40, 10 mmol l⁻¹ phosphate buffer and pH 6.45 ± 0.02 at 35 °C. Both Cr(III) and Cr(VI) ions exerted the same catalytic effect on the studied reaction. Linear calibration graphs were obtained for the determination of up to 6.0 ng ml⁻¹ Cr with detection limits of 0.054 and 0.10 ng ml⁻¹ Cr; following the fixed time and initial rate methods, respectively. The proposed method was successfully applied to the speciation and determination of trace levels of dissolved Cr(III) and Cr(VI) in natural and effluents of industrial waste water. The total dissolved Cr(III) and Cr(VI) species was determined first. In a second run, Cr(VI) was determined alone after precipitation of Cr(III) ions in presence of Al(OH)₃ collector, where Cr(III) is then determined by difference. Moreover, published catalytic-spectrophotometric methods for chromium determination were reviewed.     

Determination of adrenaline by using inhibited Ru(bpy)3 electrochemiluminescence

Adrenaline was found to inhibit strongly the electrochemiluminescence (ECL) from the Ru(bpy)₃²⁺/tripropylamine system when a working Pt electrode was maintained at 1.05 V (versus Ag/AgCl) in pH 8.0 phosphate buffer. On this basis, a flow injection (FI) procedure with inhibited electrochemiluminescence detection has been developed for determination of adrenaline. The method exhibited a good reproducibility, sensitivity, and stability with a detection limit (signal-to-noise ratio = 3) of 7.0×10⁻⁹ mol l⁻¹ and dynamic concentration range of 2×10⁻⁸ to 1×10⁻⁴ mol l⁻¹. The relative standard deviation was 2.2% for 1.0×10⁻⁶ mol l⁻¹ adrenaline (n=11). The method was successfully applied to the determination of adrenaline in pharmaceutical samples. Moreover, ECL emission spectra, UV-Vis absorption spectra and cyclic voltammograms of Ru(bpy)₃²⁺/tripropylamine/adrenaline were studied. The inhibition mechanism has been proposed as the interaction of electrogenerated Ru(bpy)₃^2+* and the o-benzoquinone derivatives, adrenochrome and adrenalinequinone, at the electrode surface.     

Determination of benorilate in pharmaceutical formulations and its metabolite in urine at carbon paste electrode modified by silver nanoparticles

Benorilate was determined by the differential pulse voltammetry (DPV) using a carbon paste electrode modified by silver nanoparticles in 1.25 × 10⁻³ mol l⁻¹ KH₂PO₄ and Na₂HPO₄ buffer solution (pH = 6.88, 25 °C) .The anodic peak potential was +0.970 V (versus SCE). A good linear relationship was realized between the anodic peak currents and benorilate concentrations in the range of 1.0 × 10⁻⁷ to 2.5 × 10⁻⁴ mol l⁻¹ with the detection limit of 1.0 × 10⁻⁸ mol l⁻¹. The recovery was 95.2-103.6% with the relative standard deviation of 3.6% (n = 9). The pharmaceutical preparations, benorilate tablets samples and its metabolite (salicylic acid) in urine were determined with the desirable results.     

Voltammetry determination of trace manganese with pretreatment glassy carbon electrode by linear sweep voltammetry

This paper described the determination of trace manganese using linear sweep voltammetry at a pretreatment glassy carbon electrode. The glassy carbon electrode pretreated by electrochemical method in the 0.1 mol l⁻¹ NaOH solution greatly improved the electrode responsibility in the determination of manganese(II). The barrier to the detection of low manganese concentration was overcome by means of autocatalytic effect of manganese oxide deposited on the electrode in advance. Under the optimum experiments condition (0.04 mol l⁻¹ NH₃-NH₄Cl buffer solution, pH 9.0), the linear range was 4×10⁻⁸ to 1×l0⁻⁶ mol l⁻¹ Mn(II) for linear sweep voltammetry and 1×10⁻⁹ to 4×10⁻⁸ mol l⁻¹ Mn(II) for convolution voltammetry. The relative standard deviation for 2×10⁻⁸ mol l⁻¹ Mn(II) is 3.4%. The proposed method is simple, rapid, sensitive and selective. It had been applied to the determination of trace manganese in samples with satisfactory results.     

New potentiometric sensors for creatinine

Three main types of creatinine potentiometric membrane sensors are described. They are based on the use of dibenzo-30-crown-10 (DB30C10) with potassium tetrakis(p-chlorophenyl)borate type (I), dibenzo-30-crown-10 alone type (II), and potassium tetrakis(p-chlorophenyl)borate alone type (III), incorporating in poly(vinyl chloride) matrix membrane plasticized with either o-nitrophenyl octyl ether or dioctylphthalate. The sensors are used for quantification of creatinine after soaking the membranes in 0.1 M creatinine solution for 2 days. The sensors show almost the same potentiometric response characteristics. Sensor type (I) exhibits Nernstian responses over a concentration range of 5.0 × 10⁻⁵ mol l⁻¹-1.0 × 10⁻² mol l⁻¹ creatinine with cationic slopes of 59.5 ± 0.1 and 60 ± 0.2 mV decade⁻¹ and detection limits of 1.1 × 10⁻⁵ mol l⁻¹ and 8 × 10⁻⁶ mol l⁻¹ creatinine, over the pH range of 3.5-6.5 and 3.5-7.0, for o-NPOE and DOP solvent mediators, respectively. Sensor type (II) displays Nernstian responses over a concentration range of 6.0 × 10⁻⁵ mol l⁻¹-1.0 × 10⁻² mol l⁻¹ creatinine with cationic slopes of 60.0 ± 0.1 and 65.0 ± 0.2 mV decade⁻¹ and detection limits of 1.5 × 10⁻⁵ mol l⁻¹ and 1.4 × 10⁻⁵ mol l⁻¹ creatinine over the pH range of 2.6-6.2 and 2.5-6.0, for o-NPOE and DOP solvent mediators, respectively. Sensor type (III) shows Nernstian responses over a concentration range of 7.0 × 10⁻⁵ mol l⁻¹-1.0 × 10⁻² mol l⁻¹ creatinine with cationic slopes of 60 ± 0.1 and 62.0 ± 0.2 mV decade⁻¹ and detection limits of 2.7 × 10⁻⁵ mol l⁻¹ and 2.0 × 10⁻⁵ mol l⁻¹ creatinine over the pH range of 2.5-6.0, for o-NPOE and DOP solvent mediators, respectively. The response times of the sensors for 10⁻³ mol l⁻¹ creatinine solution are instantaneous (4-10 s). The sensors show long-term stability with life span of ∼6 months. The sensors are used for determination of serum creatinine of rats (Rattus Norvigicus) with mean R.S.D. of 2.62%, and the results agreed well with the Jaffe kinetic method.     

A multicommuted flow system for sequential spectrophotometric determination of hydrosoluble vitamins in pharmaceutical preparations

A multicommuted flow system is proposed for spectrophotometric determination of hydrosoluble vitamins (ascorbic acid, thiamine, riboflavine and pyridoxine) in pharmaceutical preparations. The flow manifold was designed with computer-controlled three-way solenoid valves for independent handling of sample and reagent solutions and a multi-channel spectrophotometer was employed for signal measurements. Periodic re-calibration as well as the standard addition method was implemented by using a single reference solution. Linear responses (r=0.999) were obtained for 0.500-10.0 mg l⁻¹ ascorbic acid, 2.00-50.0 mg l⁻¹ thiamine, 5.00-50.0 mg l⁻¹ riboflavine and 0.500-8.00 mg l⁻¹ pyridoxine. Detection limits were estimated as 0.08 mg l⁻¹ (0.5 μmol l⁻¹) ascorbic acid, 0.8 mg l⁻¹ (2 μmol l⁻¹) thiamine, 0.2 mg l⁻¹ (0.5 μmol l⁻¹) riboflavine and 0.1 mg l⁻¹ (0.9 μmol l⁻¹) pyridoxine at 99.7% confidence level. A mean sampling rate of 60 determinations per hour was achieved and coefficients of variation of 1% (n=20) were estimated for all species. The mean reagent consumption was 25-fold lower in relation to flow-based procedures with continuous reagent addition. Average recoveries between 95.6 and 100% were obtained for commercial pharmaceutical preparations. Results agreed with those obtained by reference methods at 95% confidence level. The flow system is suitable for application in quality control processes and in dissolution studies of vitamin tablets.     

Enantioselective, potentiometric membrane electrodes based on cyclodextrins: application for the determination of R-baclofen in its pharmaceutical formulation

Two enantioselective, potentiometric membrane electrodes based on α- and γ-cyclodextrins were proposed for the assay of R-baclofen. The slopes of the electrodes were 59.50 and 51.00 mV/pR-baclofen for α- and γ-cyclodextrin-based electrodes, respectively. The detection limits of the proposed electrodes were 7 × 10⁻⁹ mol l⁻¹ for α-cyclodextrin-based electrode and 1.44 × 10⁻¹⁰ mol l⁻¹ for γ-cyclodextrin-based electrode. The enantioselectivity was determined over S-baclofen. The proposed electrodes can be employed for the assay of R-baclofen raw materials and its pharmaceutical formulation, Norton-Baclofen^® tablets. The surfaces of the electrodes are stable and easily renewable by polishing on alumina paper.