Determination of melatonin and pyridoxine in pharmaceutical preparations for health-caring purposes by capillary electrophoresis with electrochemical detection

Ruthenium-Anchored Carbon Sphere-Customized Sensor for the Selective Amperometric Detection of Melatonin

Melatonin (MT), a pineal gland hormone, regulates the sleep/wake cycle and is a potential biomarker for neurodegenerative disorders, depression, hypertension, and several cancers, including prostate cancer and hepatocarcinoma. The amperometric detection of MT was achieved using a sensor customized with ruthenium-incorporated carbon spheres (Ru-CS), possessing C- and O-rich catalytically active Ru surfaces. The non-covalent interactions and ion-molecule adducts between Ru and CS favor the formation of heterojunctions at the sensor-analyte interface, thus accelerating the reactions towards MT. The Ru-CS/Screen-printed carbon electrode (SPCE) sensor demonstrated the outstanding electrocatalytic oxidation of MT owing to its high surface area and heterogeneous rate constants and afforded a lower detection limit (0.27 μM), high sensitivity (0.85 μA μM -1 cm-2), and excellent selectivity for MT with the co-existence of crucial neurotransmitters, including norepinephrine, epinephrine, dopamine, and serotonin. High concentrations of active biomolecules, such as ascorbic acid and tyrosine, did not interfere with MT detection. The practical feasibility of the sensor for MT detection in pharmaceutical samples was demonstrated, comparable to the data provided on the product labels. The developed amperometric sensor is highly suitable for the quality control of medicines because of its low cost, simplicity, small sample size, speed of analysis, and potential for automation.

Methylene blue molecularly imprinted polymer for melatonin determination in urine and saliva samples

A highly sensitive and rapid electrochemical sensor was developed for detecting melatonin using a molecularly imprinted polymer (MIP) with methylene blue as the functional monomer and melatonin as the template. The MIP was synthesized via a simple electropolymerization process that did not require an initiating reagent. The sensor demonstrated good selectivity for melatonin against common interferences such as lactate, cytosine, cytidine, urea, ascorbic acid, creatine, creatinine, serotonin, and tryptophan. Melatonin detection was achieved at a potential of 0.60 V vs. Ag/AgCl with a sensitivity of 138.8 ± 4.7 µA µM‒1 in the linear range 0.097 - 200 µM and a limit of detection of 29 nM (3SB/m). The sensor exhibited excellent reproducibility and repeatability for both within (intra) and between (inter) electrodes (%RSD < 3% for n = 3). The sensor was applied to authentic urine and saliva samples with recoveries of 103 ± 1% and 102 ± 1%, respectively.

Electrochemical sensor based on the synergy between Cucurbit[8]uril and 2D-MoS2 for enhanced melatonin quantification

We present the development of an electrochemical sensor towards melatonin determination based on the synergistic effect between MoS₂ nanosheets and cucurbit[8]uril. For the sensor construction cucurbit[8]uril suspensions were prepared in water, and MoS₂ nanosheets were obtained by liquid exfoliation in ethanol:water. The sensing platform was topographically characterized by Atomic Force Microscopy. Electrochemical Impedance Spectroscopy experiments allowed us to study the charge transfer process during melatonin oxidation. Moreover, stoichiometry of the resulting complex has also been determined. After the optimization of the sensor construction and the experimental variables involved in the Differential Pulse Voltammetric response of melatonin, detection limit of 3.80 × 10^-7 M, relative errors minor than 3.8% and relative standard deviation lower than 4.4% were obtained. The proposed sensor has been successfully applied to melatonin determination in pharmaceutical and biological samples as human urine and serum, with very good recoveries ranging from 90 to 102%.

Up-to-date studies regarding the determination of melatonin by chromatographic methods

Melatonin is an indolic compound that has been reported in the literature to exist in human-based samples, vertebrates, vegetables, fruits, and pharmaceutical products. Melatonin is considered a dietary supplement and can regulate circadian rhythms, although it has not been classified as a drug by the US Food and Drug Administration. Several analytical methods have been used for its detection. This study aimed to summarize the recent outcomes of the chromatographic methods such as electrophoretic methods, gas chromatography, and liquid chromatography, which have been used for the determination of melatonin in the last three decades, with the focus on gas chromatography and high-performance liquid chromatography methods.

New insights on the spectrophotometric determination of melatonin pKa values and melatonin-βCD inclusion complex formation constant

Using UV-Vis spectrophotometry a stability study of melatonin at different pH values was done in aqueous media, finding that at acidic pH melatonin is unstable when interacting with the environment, however it becomes stable protecting it from light and oxygen. From the UV-Vis spectra and SQUAD software, melatonin pK_a values, in a completely protected aqueous medium, were estimated as 5.777±0.011 and 10.201±0.024. Using the same techniques, the melatonin and β-cyclodextrin inclusion complex formation constants were assessed at pH3, 7 and 11.5, giving the values of log β=(3.07±0.06), (2.94±0.01) and (3.07±0.06) M^-1, respectively. From the global acidity formation constants and the complexes' formation constants, the molar fractions were determined for each species of MT and MT-βCD, to build the molar fraction-[βCD]-pH 3D diagram and the molar fraction-pH 2D diagrams, where it was possible to observe the predominance of the MT species with and without βCD. A voltammetric study at pH3, allowed obtaining a value of log β=(3.15±0.01) M^-1, which corroborates that obtained through UV-Vis spectrophotometry, supporting strongly the rationale behind using simple, straightforward techniques.

Magnetic metal–organic framework nanocomposites for enrichment and direct detection of small molecules by negative-ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Fe₃O₄@ZIF-8 MNCs were proposed as an absorbent and a new matrix for negative ion MALDI-TOF MS, which exhibited interference-free background, good reproducibility and sensitivity for analysis of small molecules.

Spectrofluorimetric determination of melatonin in kernels of four different Pistacia varieties after ultrasound-assisted solid-liquid extraction

Melatonin is normally consumed to regulate the body's biological cycle. However it also has therapeutic properties, such as anti-tumor, anti-aging and protects the immune system. There are some reports on the presence of melatonin in edible kernels such as walnuts, but the extraction of melatonin from pistachio kernels is reported here for the first time. For this, the methanolic extract of pistachio kernels was exposed to gas chromatography/mass spectrometry analysis which confirmed the presence of melatonin. A fluorescence-based method was applied for the determination of melatonin in different extracts. When excited at λ=275 nm, the fluorescence emission intensity of melatonin was measured at λ=366 nm. Ultrasound-assisted solid-liquid extraction was used for the extraction of melatonin from pistachio kernels prior to fluorimetric determination. To achieve the highest extraction recovery, the main parameters affecting the extraction efficiency such as extracting solvent type and volume, temperature, sonication time and pH were evaluated. Under the optimized conditions, a linear dependence of fluorescence intensity on melatonin concentration was observed in the range of 0.0040-0.160 μg mL(-1), with a detection limit of 0.0036 μg mL(-1). This method was applied successfully for measuring and comparing the melatonin content in the kernels of four different varieties of Pistacia including Ahmad Aghaei, Akbari, Kalle Qouchi and Fandoghi. In addition, the results obtained were compared with those obtained using GC/MS. A good agreement was observed indicating the reliability of the proposed method.

Net analyte signal standard addition method (NASSAM) as a novel spectrofluorimetric and spectrophotometric technique for simultaneous determination, application to assay of melatonin and pyridoxine

In this work a new modification of the standard addition method called "net analyte signal standard addition method (NASSAM)" is presented for the simultaneous spectrofluorimetric and spectrophotometric analysis. The proposed method combines the advantages of standard addition method with those of net analyte signal concept. The method can be applied for the determination of analyte in the presence of known interferents. The accuracy of the predictions against H-point standard addition method is not dependent on the shape of the analyte and interferent spectra. The method was successfully applied to simultaneous spectrofluorimetric and spectrophotometric determination of pyridoxine (PY) and melatonin (MT) in synthetic mixtures and in a pharmaceutical formulation.

Comparison of analytical methods for the quality control of a new formulation containing soy extract and melatonin

Three analytical methods have been developed and compared for the quality control of a new formulation (Soymen GN(R) capsules) containing soy extract and melatonin for the treatment of menopausal symptoms. The first method is based on MEKC with diode-array detection, using a mixture of basic carbonate buffer (95%) and methanol (5%), containing 55 mM SDS, as the BGE. The second method is an HPLC method with UV detection at 260 nm. The third method is an HPLC method coupled to amperometric detection which is carried out at an oxidation potential of +0.8 V. In both HPLC systems, the chromatographic separation is obtained on an RP C18 column using a mixture of ACN and an acidic phosphate buffer (25:75 v/v) as the mobile phase. A feasible pretreatment procedure with a methanol/water mixture has been implemented to achieve the quantitative extraction of the main soy isoflavones and of melatonin from the capsules. The results obtained with the three methods are in good agreement with each other and satisfactory in terms of linearity (r(2) >0.9996), precision (RSD <5.4%) and accuracy (recovery >97%). Thus, each of the three analytical methods seems to be suitable for the simultaneous analysis of the main soy isoflavones and melatonin in the new commercial formulation.

Identification of herb Acanthopanax senticosus (Rupr. Et Maxim.) harms by capillary electrophoresis with electrochemical detection

In this paper, a rapid, high efficient, sensitive and inexpensive approach based on a combination of simple ultrasonic extract and capillary electrophoresis (CE) separation with electrochemical detection (ED), is described to identify herbs by comparing their CE-ED profiles (namely, CE-ED electropherograms). The proposed method takes advantage of ultra-small sample volume, low consumption of organic solvent, simple sample pretreatment and easy cleanup procedure. It was applied to analyze the CE-ED profiles of stems of herb Acanthopanax senticosus (Rupr. Et Maxim.) Harms from different sources and different parts (roots, rhizomes, stems and leaves) of this herb. By comparing peak number, peak height and peak height ratio, we found that the CE-ED profiles showed big differences for the herbs from the different sources and the different parts of this herb. In addition, the distribution of bioactive compounds (isofraxidin, rutin and chlorogenic acid) in the different parts of this herb and their content variations affected by the source were studied with the CE-ED method. Based on their own unique CE-ED profiles, these herbs from the different sources and the different parts of this herb could be easily distinguished. Therefore, the proposed approach could be used as a rapid, high efficient and sensitive method for the identification of herbal medicines.

Micellar electrokinetic capillary chromatography for fast separation and sensitive determination of melatonin and related indoleamines using end-column amperometric detection

MEKC was used in conjunction with end-column amperometric detection (AD) at a carbon disc electrode (0.3 mm diameter) for the selective and sensitive determination of melatonin and its five related indoleamines including its precursors and metabolites in the pineal gland. The introduction of a sample stacking technique in injection and the buffer additive SDS in the buffer solution system provided the rapid and sensitive analysis. Optimal buffer conditions (10 mmol/L phosphate containing 20 mmol/L SDS, pH 7.2), detection potential (+1.0 V vs. Ag/AgCl), and electrokinetic injection 10 s with the separation voltage of 24 kV were employed to achieve the baseline separation of six pineal hormones within 15 min. The peak currents and the analyte concentrations have a good linear relationship over the range of 6.0 x 10(-8) 6.0 x 10(-5 )mol/L. The detection limits for six pineal hormones by AD are 9.7 to 41.8 nmol/L (equal to 2.0 to 9.7 ng/mL) (S/N = 3), respectively. It is proved to provide about 30- to 250-fold improvement over UV, and be comparable with the sensitive fluorescence detection, which needs pre-column derivatization. The proposed method has been applied for analysis of melatonin and related indoleamines in rat pineal glands. A very simple sample pretreatment procedure, merely involving the homogenization step in perchloric acid, was enough to achieve recoveries in the range of 71 to 127% for all the analytes in the pineal gland.

Development and validation of a reversed-phase ion-pair liquid chromatography method for the determination of magnesium ascorbyl phosphate and melatonin in cosmetic creams

A reversed-phase ion-pair high-performance liquid chromatographic method was developed and validated for the determination of magnesium ascorbyl phosphate and melatonin in cosmetic creams. The determination was performed on a BDS C 18 analytical column (250 x 4.6 mm i.d., 5 microm particle size); the mobile phase consisted of 0.020 M tetrabutylammonium hydroxide and 0.025 M potassium dihydrogen phosphate (pH 6.8) mixed with acetonitrile in a ratio (77:23, v/v) and pumped at a flow rate 1.00 ml min(-1). The UV detector was operated at 260 nm. The retention times of the magnesium ascorbyl phosphate, melatonin and chlorthalidone that was used as internal standard, were 6.55, 9.18 and 11.07 min, respectively. Calibration graphs are linear (r better than 0.9990, n=6), in concentration range 1.00-10.00 microg ml(-1) for magnesium ascorbyl phosphate and 0.63-6.25 microg ml(-1) for melatonin. The intra- and inter-day R.S.D. values were less than 6.0%, while the relative percentage error E(r) was less than 3.5% (n=5). The quantitation limits were 0.69 and 0.47 microg ml(-1), for magnesium ascorbyl phosphate and melatonin, respectively. The method was applied to the analysis of a cosmetic cream and proved to be suitable for rapid and reliable quality control.

On-line preconcentration techniques in determination of melatonin and its precursors/metabolites using micellar electrokinetic chromatography

Determination of melatonin (MT) (N-acetyl-5-methoxytryptamine) and related indole compounds using standard capillary electrophoresis (CE) system with UV detection was investigated. Satisfactory separations of six analytes i.e. l-tryptophan (l-TRP), 5-methoxyindoleacetic acid (5-MIAA), 6-hydroxymelatonin (6-HMT), MT, serotonin (SER) and 5-methoxytryptamine (5-MTRA) were performed employing micellar electrokinetic chromatography (MEKC). The optimal background electrolytes (BGE) used for separations were 20mM tetraborate buffer (pH 9.2) and 20mM phosphate buffer (pH 3.3) when employing techniques with normal and reverse migration of micelles, respectively. Fifty millimolar sodium dodecyl sulfate (SDS) was employed as the pseudostationary phase and voltage of +/-20kV was used throughout the investigation. On-line preconcentration techniques, stacking and sweeping, were applied in order to overcome high detection limits that are a serious drawback of CE with UV detection. A comparison of used techniques, concerning enhancement factors and limits of detection (LOD), is presented. Obtained results show that the use of stacking with reverse migrating micelles (SRMM) as one of preconcentration techniques allows obtaining the lowest estimated LODs for MT at the level of 30ng/mL with injection time of 99s at 0.5psi. Estimated LODs for other analytes in these conditions were, 21, 26 and 100ng/mL for l-TRP, 5-MIAA and 6-HMT, respectively. Signals of 5-MTRA and SER obtainable only with 10s injection allowed reaching estimated LODs of 62.5 and 130ng/mL, respectively. Analysis of spiked, diluted human serum was carried out as a preliminary application illustration of developed procedure.

Determination of melatonin and its precursors and metabolites using capillary electrophoresis with UV and fluorometric detection

A micellar electrokinetic chromatography method has been developed for simultaneous determination of melatonin and its precursors and metabolites. A 20 mM borate buffer pH 9.5 with 50 mM SDS served as the electrolyte. Tryptophan, 5-methoxyindoleacetic acid, 6-hydroxymelatonin, melatonin, serotonin, and 5-methoxytryptamine were baseline separated in less than 13 min. The limits of detection for UV detection and fluorometric detection based on native fluorescence of analytes were at the sub-ppm level. The proposed method with UV detection was applied to melatonin content control in pharmaceutical tablets with a precision expressed as RSD (n = 7) = 1.6%. For biological samples extraction with chloroform and ethyl acetate was examined. With ethyl acetate and chloroform recoveries of 87.2% and 82.1% melatonin, respectively, were obtained from plasma samples. The recovery of melatonin from spiked urine samples was 80.0% for ethyl acetate and 82.5% for chloroform. Fluorometric detection provides about two-fold improvement over UV in the detection of melatonin and minor improvements for three other analytes, but is much poorer than UV for tryptophan and 6-hydroxymelatonin in applied conditions.

Determination of antioxidants in cosmetics by micellar electrokinetic capillary chromatography with electrochemical detection

A new and efficient method for the determination of antioxidants [Propyl gallate (PG), tert-butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA), and butylated hydroxytoluene (BHT)] in cosmetics has been established by using micellar electrokinetic capillary chromatography with electrochemical detection (MECC-ED). Under the optimum conditions of the method, all analytes were successfully separated within 13 min at the separation voltage of 18 kV in a 20 mmol/L borate running buffer (pH 7.4) containing 25 mmol/L sodium dodecyl sulfate. The excellent linearity was obtained in the concentration range from 5.0 x 10(-4) to 2.0 x 10(-6) mol/L and the detection limits (S/N = 3) of PG, TBHQ, BHA, and BHT range from 3 x 10(-7) to 3 x 10(-6) mol/L.

Simultaneous spectrofluorimetric and spectrophotometric determination of melatonin and pyridoxine in pharmaceutical preparations by multivariate calibration methods

Partial least-squares (PLS) calibration and principal component regression (PCR) methods were utilized for the simultaneous spectrofluorimetric and spectrophotometric determination of pyridoxine (PY) and melatonin (MT). Since emission and adsorption spectra of these drugs overlap, PY and MT cannot be directly determined by fluorimetric nor by spectrophotometric methods. Full-spectrum multivariate calibration PLS and PCR methods were developed for both fluorimetry and spectrophotometry. The conditions were optimized for fluorimetric as well as for spectrophotometric determination of both drugs. The simultaneous determination of PY and MT was carried out in mixtures by recording the emission fluorescence spectrum between 324 and 500 nm (lambda(ex) 285 nm) for fluorimetry, and by recording the absorption spectrum between 250 and 350 nm for spectrophotometry (lambda(max(PY)) 310 nm, lambda(max(MT)) 278 nm). The experimental calibration matrixes were designed orthogonally. At the optimum conditions, dynamic ranges were 0.04-1.3 and 0.1-4 microg ml(-1) for fluorimetry and 1-22 and 1-24 microg ml(-1) for spectrophotometry for MT and PY, respectively. The calibration concentrations were prepared in the dynamic ranges. The parameters of the chemometrics procedure for the simultaneous determination of MT and PY were optimized, and the proposed methods were validated with prediction set. Finally the procedures were successfully applied to simultaneous spectrofluorimetric and spectrophotometric determination of PY and MT in synthetic mixtures and in a pharmaceutical formulation.

Determination of uric acid in human saliva by capillary electrophoresis with electrochemical detection: potential application in fast diagnosis of gout

The clinical manifestations of gout result from the formation and deposition of uric acid (UA) crystals. The monitoring of UA level in less invasive biological samples such as saliva is suggested for diagnosis and therapy of gout, hyperuricemia and the Lesch-Nyhan syndrome. In order to investigate the correlation between trace amounts of UA in human saliva and urine and explore the potential application in fast diagnosis of gout, capillary electrophoresis with electrochemical detection (CE-ED) was applied for the determination of UA in human saliva and urine in this work. Under the optimum conditions, UA and three coexisting analytes could be well separated within 14 min at the separation voltage of 14 kV in 80 mmol L(-1) borax running buffer (pH 7.8). A good linear relationship was established between peak current and concentration of analytes over two orders of magnitude with detection limits (S/N = 3) ranging from 1.09 x 10(-7) to 5.0 x 10(-7) mol L(-1) for all analytes. This proposed method has been successfully applied for study of the correlation between the UA content of human saliva and urine, providing an alternative and convenient method for rapid diagnosis of gout.

Voltammetric determination of pyridoxine (Vitamin B6) by use of a chemically-modified glassy carbon electrode

A novel carbon nanotube-modified glassy carbon electrode was described for the direct determination of pyridoxine. The electrochemical behavior of pyridoxine was investigated, and a well-defined oxidation peak with high sensitivity was observed at the modified electrode. Owing to the unique structure and extraordinary properties of multi-wall carbon nanotube (MWNT), the MWNT-modified glassy carbon electrode shows obvious electrocatalytic activity to the oxidation of pyridoxine, since it greatly enhances the oxidation peak current of pyridoxine as well as lowers its oxidation overpotential. Based on this, a very sensitive and simple voltammetric method was developed for the measurement of pyridoxine. A sensitive linear voltammetric response for pyridoxine was obtained in the concentration range of 5 x 10(-7)-1 x 10(-4)mol/L, and the detection limit is 2 x 10(-7)mol/L using differential pulse voltammetry. Compared with other voltammetric methods, this proposed method possesses many advantages such as very low detection limit, fast response, low cost and simplicity. The practical application of this new analytical method was demonstrated with pyridoxine drugs.

Voltammetric and flow amperometric methods for the determination of melatonin in pharmaceuticals

Melatonin can be sensitively detected in pharmaceuticals by two different and simple electrochemical methods: cyclic voltammetry (CV) and amperometric detection in a flow injection analysis system (FIA-ED). An adequate pre-treatment of the carbon paste electrode in the first case and the employ of a high flow rate in the second one were the key for obtaining a very good reproducibility (R.S.D. values of 1.5 (n=10) and 1.3% (n=20), respectively). Low limits of detection were achieved and with the coupling of a flow system a linear dynamic range of three orders of magnitude (from 10(-8) to 10(-5) M) was obtained. Both methods were applied to the determination of melatonin in pharmaceuticals. In order to best validate these methodologies a fluorescent procedure was developed to contrast the results. As no interferences from the matrix were found the employ of a separation technique is not necessary. In this way the procedure is fastened and simplified. Moreover, the low price, ease of handling, possibility of automation and high sample throughput are important advantages that convert the flow methodology in an attractive alternative for quality control of pharmaceuticals.

Determination of histamine by capillary zone electrophoresis with amperometric detection

Capillary zone electrophoresis was employed for the determination of histamine using end-column amperometric detection with a carbon fiber microelectrode, at a constant potential. The optimum conditions of separation and detection were 10 mmol/L phosphate buffer, pH 5.6 for the buffer solution, 15 kV for the separation voltage, and 1.35 V (versus SCE) for the detection potential. The linear range was from 6.3 x 10(-7) to 1.5 x 1(-5) mol/L with the regression coefficient of 0.9997, and the detection limit was 4.0 x 10(-7) mol/L (S/N = 3). The proposed method was successfully applied to the direct determination of histamine in the beer samples without any sample clean-up procedures.