Current Analytical Chemistry - Volume 13, Issue 2, 2017

Background: The clinical and therapeutic significance of dopamine, known as important hormone and neurotransmitter, motivated the development of the sensitive, rapid, and cost effective electrochemical methods for its determination. Methods: This review comments on the advances in the electrochemical sensing and biosensing of dopamine, covering the period 2005-2015. Results: The electrocatalytic properties and analytical performances of the dopamine sensors based on: bare electrodes, electrochemically pretreated electrodes, chemically modified electrodes (CNTs-, graphene-, NPs-, MIPs, polymers-, and miscellaneous inorganic and organic materials-modified electrodes), as well as of the biosensors based on tyrosinase carbon nanomaterials-modified electrodes and tyrosinase polymersmodified electrodes are compared and discussed. The presented data demonstrate that the electrochemical sensors and biosensors based on CNTs- and graphene- modified electrodes exhibit better analytical performances compared with the other groups for the sensitive and respectively for the simultaneous or selective determination of dopamine in the presence of the interfering species ascorbic acid and uric acid. Conclusion: The overview of the literature reveals that sensitivity enhancement and in vivo measurements still remain the challenges in the development of the electrochemical sensors and biosensors for dopamine determination.

Background: Oxidative stress plays a major role in the development of several diseases. A healthy food diet, supplying the organism with antioxidants, is generally believed to help prevent this stress. Methods: With the aim of screening several antioxidants commonly found in food, a method using β pancreatic cells and based on the reduction of a tetrazolium sodium salt into colored formazan in the presence of antioxidant compounds was used. In parallel, the same compounds were investigated using a chromatographic online antioxidant detection system (Coads) based on the detection of the reduced form of the radical cation 2,2-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^•+) which is first allowed to react online with the eluting compounds. Results: The obtained data suggested that most phenolic compounds possessed both antioxidant and prooxidant properties. Gallic acid, in particular, showed among the highest antioxidant activities with the Coads method (322±46 μM Trolox equivalent), but caused a decrease in cell viability (-25%). Conclusion: Moreover, when screening natural compounds for their antioxidant activity, combining both chemical and biological methods is a more targeted approach compared with the single use of either method.

Objective: The aim of this study was to prepare and certify a new Certified Reference Material (CRM) of quercetin for the quality control of quercetin and related functional foods and botanical supplements. Methods: Three different methods, including coloumetric titration (CT), differential scanning calorimetry (DSC) and mass balance (MB) methods were used in the purity determining process. Results: The purity results determined by the three methods were consistent, and the CT has the highest accuracy and the smallest uncertainty. Conclusion: CT and DSC methods were established for purity determination of quercetin for the first time. The purity of quercetin CRM, after validation and evaluation, was 99.3% ± 0.4% (k = 2, P = 0.95). The new CRM can be widely used to quantitatively analyse quercetin and other related functional foods and botanical supplements.

Background: Hypertension represents a widespread cardiovascular condition although usually asymptomatic, is a major risk factor for myocardial ischemia, renal failure and stroke. Amlodipine is considered a powerful dihydropyridine calcium channel blocker used as antihypertensive agent. Its mechanism of action depend on relaxing the smooth muscle of the artery wall and decrease peripheral resistance. Methods: An accurate, sensitive and robust chiral HPLC method was developed for separation and quantitation of S(-)- amlodipine simultaneously in the presence of its R(+)- isomer. Chiral separation was applied using Daicel CROWNPAK CR(+) (5μm, 4.0x150 mm) column which contains (3,3'-diphenyl-1,1'- binaphthyl)-20-crown-6-ether coated into a 5μm reversed phase silica support. The mobile phase system was aqueous acidic 70% HClO4 (pH 2.0) and methanol in the proportion of (95:5 v/v), filtered through 0.45 μm membrane and degassed by before use, pumped at a flow rate was 0.8 mL min-1 with UV detector adjusted at 238 nm. Results: The chromatographic HPLC method was validated with respect to ICH guidelines. Linear concentration range was 5-60 μg mL-1 with correlation coefficient (r) about 0.9998, the detection and quantitation limits was found 1.66 and 5.05 μg m-1; respectively. Accuracy was evaluated using standard addition technique with mean recovery about 98.40 % while precision was assessed at intraday and interday level. It was found that all %RSD values below 2%. Conclusion: A simple, enantioselective, chiral HPLC method was developed and validated for the quantitation of amlodipine [S-(+)- isomer] in racemic tablets using CROWNPAK CR(+) chiral stationary phase. The proposed method is specific, precise, accurate, and robust which can be successfully applied for the routine analysis of S-AML in bulk and pharmaceutical dosage formulations.

Flow Injection analysis is mainly applied to automate or semiautomatic determination of some analytes. This is called flow injection analysis. However some reactions are slow and can’t be analysed via direct flow injection analysis. The aim of this work is to develop a method suits slow reactions. The method is based on the reaction of quinolones with excess Iron(III) nitrates. Excess amount of iron(III) added to quinolone forms an amber color. The excess iron is determined after complexing it using potassium thiocyanate in acidic medium to form red complex with higher absorptivity coefficient, which is then spectrophotometrically determined at 525nm. This technique may be called back flowinjection technique, as it is called in back titration technique with a slight modification. A modified Flow-injection spectrophotometric method (FIA) of analysis has been developed for the separate determination of ofloxacin, norfloxacin and ciprofloxacin antibiotics. The method is applied to the determination of quinolones in commercial tabulated forms available in the market. The manifold is designed so that constant flow rate of Iron(III), quinolone and sulfuric acid mixed and the baseline of the complex is set. Fixed amount of thiocyanate is then injected and the absorbance of the red colored complex formed between excess Iron(III) and thiocyanate is recorded. Lambert-Beers low was maintained in the range 0.1-90 ppm. The recovery was better than 99%. The detection limit reaches 20μg/l. The FIA method is fast (20/h), sensitive, precise, cheap to run and gave reasonable results. The method is suitable for slow rate reactions between analyte and complexing agents, which cannot be determined by direct flow-injection analysis.

Background: The implementation of the automation of enzyme based on the evaluation of enzyme activity or the determination of specific analytes has been allowed by the Sequential Injection Analysis (SIA) systems. Objective: Kinetic constants Km, Kcat and Kcat/Km. were evaluated in order to get the behavior of free (homogeneous media) and immobilized on controlled pore glass (heterogeneous media) using a SIA method. Method: Hydrodynamic and chemistry parameters were evaluated using a factorial design 25-1. The kinetics parameters were calculated at 40 °C and 60 °C and at pH 4 and 5. The results were compared with those obtained by a batch method. Results: By SIA, the optimum values for the enzyme free were at pH 4 and 40 °C (Km=1.33±0.05 mM; Kcat=6.80±0.08 s-1 and Kcat/Km=5117.43±132.03 M-1 s-1) and for the immobilized enzyme were at pH 5 and 60 °C (Km=4.85±0.80 mM; Kcat = 0.15±0.008; Kcat/Km =30.92±3.42) although at pH 5 and 40 °C Kcat was higher (0.56±0.024) indicating better exchange of substrate molecules at active site. 40 °C and pH 4 (0.23±2.2x10-4 mM) were the optimum conditions to get the higher efficiency by the batch method. Conclusion: The results show that by SIA it is possible to determine the catalytic and kinetic efficiencies guaranteed with an effective control of the reaction conditions as consequence of the well defined transport. The results also show that the affinity from the enzyme to the substrate as well as the kinetic reaction change when the enzyme is immobilized because, probably, the structure of the active sites is different.

Background: Arsenic is widely known as a toxic element and can cause adverse health effects. The determination of total arsenic does not indicate the level of their actual hazard. Therefore, the speciation analysis of arsenic is more significant than the determination of total arsenic concentration. We propose the use of trimethyl aminopropyl-silica-based SPE column which is commercially available and a simple chemical modification of this silica-based SPE column for the speciation analysis of arsenic (arsenite, arsenate and DMA). Methods: A two-column system including trimethyl aminopropyl-silica column (Si-Cl) and silver chloride- silica column (Si-AgCl) was used for the separation of arsenic species. Arsenic concentrations were analyzed by an ICP-OES spectrometer. The pH of solution, flow rate, eluent concentration and sample concentration were studied for assessing optimal conditions. Results: AsV could be adsorbed on the Si-Cl column at pH 7 and was consequently eluted by 1 mol/L HCl. Inorganic arsenic species (AsIII and AsV) were retained on the Si-AgCl column at pH 9, while DMA was passed through the column. The amount of AsIII was calculated from the subtraction of AsV and DMA from the total arsenic concentration. The method detection limits of AsIII, AsV and DMA were 2.30, 1.48, and 2.51 μg/L, respectively. The recoveries and relative standard deviations of AsIII, AsV and DMA were in the range of 97.2-104.3% and 0.72-2.56%, respectively. Conclusion: The solid phase extraction method of the separation inorganic arsenic (arsenite or AsIII and arsenate or AsV) and dimethyl arsenic acid (DMA) using strong anion exchange silica and silver chloride modified silica columns was successfully developed and validated. The proposed method was successfully applied for the speciation analysis of arsenic in real industrial wastewater.

Background: Microorganisms that can be potentially used as agents of bioterrorism need to be detected quickly and reliably. Most preferably, detection should take place directly on site suspected of contamination so that appropriate actions can be taken as soon as possible. An analytical method that could be applied for such purpose should be fast, accurate and possible to be used in portable devices. Due to its ability of detection in real-time, high sensitivity and miniaturization potential, surface plasmon resonance (SPR) appears to be an ideal method to be used in on-site detection of biological warfare. Methods: We used SPR technique for detection of biological warfare contamination. We demonstrated a prototype detecting device utilizing miniature TI Spreeta 2000 SPR chips. The device has been equipped with chips modified by covalent bonding of antibodies for specific detection of factors used as biological warfare - bacteria and spores. Results: The results show that our system is capable of detecting biological warfare in samples of concentrations starting from 50 cfu/ml and can be used for on-site detection or environmental monitoring. Conclusion: Binding selected antibodies onto surfaces of SPR chips resulted in construction of sensors capable of detecting biological warfare agents. The sensitivity of method, as well as detection limits vary depending on the type of agent being investigated but generally are fully acceptable for a device that would be used on site of the contamination for first scanning and examination of environmental samples. Presented work demonstrates that technique of surface plasmon resonance can be successfully used for detection of biological warfare.

Background: Salvianolic acid A (Sal A) is one of the major water-soluble compounds isolated from the root of Salvia miltiorrhiza, which is widely used as a kind of traditional Chinese medicine. Because of instability of Sal A, the degradation products of Sal A were studied to confirm the conversion of Sal A. Methods: To elucidate the structures of the degradation products, they were isolated by preparative high performance liquid chromatography (HPLC) and identified by nuclear magnetic resonance (NMR) and mass spectrometry (MS). To understand the formation pathways of the degradation products, stress testing and esterification of Sal A were performed and analyzed on HPLC. Results: Five products were identified as a pair of epimers (products 1 and 2), isosalvianolic acid C (3), salvianolic acid C (4) and salvianolic acid A methyl ester (5). Sal A in high humidity yielded 1 and 2 by electrophilic addition. Sal A in strong basic solution yielded 3 by a free radical intermediate. Sal A in high temperature or base or oxidant in water yielded 4 by an epoxide intermediate. Sal A in methanol yielded 5 by esterification catalyzed by acid. Eight other products were found in heated water solution. Conclusion: According to the formation mechanisms of the products, Sal A could be used in a solid form formulation and should be protected from humidity, oxidant and base when manufacturing, depositing and supplying.

Background: Imipenem, first member of the carbapenem class of β-lactams, is an antibiotic produced by Streptomyces cattleya. It is an N-formimidoyl derivative of thienamycin, co-administered with cilastatin, a specific and highly active dehydropeptidase I (DHP-I) inhibitor. Imipenem plays a key role in the treatment of infections not readily treated with other antibiotics, because of its high resistance to the β-lactamase enzymes produced by drug-resistant Gram-negative bacteria. Methods: The elaborated method of micellar electrokinetic chromatography (MEKC) followed by UV absorption detection at 210 nm was used to separate imipenem and cilastatin from its related substances. The best results were obtained with borate buffer (25 mM) pH 9.2 and sodium dodecyl sulphate (75 mM). Uncoated capillary (60/50cm; 75μm I.D.) with normal polarity and voltage values of 25 kV, were used throughout the investigation. The optimised method of imipenem and cilastatin determination was validated in terms of linearity, accuracy and precision, and provides a detection limit of 0.4 μg/mL and 0.2 μg/mL at S/N=3 for imipenem and cilastatin, respectively. Results: The repeatability of the method, expressed by relative standard deviations (RSD) in the migration times, reached 0.60% and 0.78% for imipenem and cilastatin, respectively, whereas for the corrected peak areas RSD were 1.14% and 1.02%, accordingly. Satisfactory separation of analyzed compounds was achieved within 7 minutes of electrophoresis. Conclusion: A comparison of newly proposed MEKC with existing HPLC method was performed and demonstrated an improvement in analysis time, an increase in theoretical plate number and a lower amount of both sample and background electrolyte (BGE) required.

Background: Copper is one of the important elements for human health and requires for normal metabolic processes thus detection of copper ions at micro molar level or less is great importance. The aim of this paper is to introduce a rapid, sensitive and selective method based on AgNPs for the determination of trace amounts of Cu(II) by Uv-vis spectrophotometry. Methods: A colorimetric sensor have been developed using AgNPs for the detection of Cu(II). The 2,2'- thiodiacetic acid (TDA) was selected to modify the surface of AgNPs and to improve the selectivity of AgNPs for the detection of Cu(II). Results: In order to optimize determination of copper, various parameters such as TDA concentration, pH and reaction time on aggregation of TDA-AgNPs in the presence of Cu(II) were investigated. Under optimum conditions, the limit of detection was 17 nM. A linear correlation was obtained between the absorbance ratio of A570/A393 and the concentration of Cu(II) in the 0.3 to 6.0 μM range. Conclusion: A new selective and economical probe was developed for the detection of copper ions. The preparation of the probe is performed by reduction of AgNO3 in the presence of TDA. The aggregation of AgNPs are based on the interaction of donating groups of TDA with Cu(II). This method successfully applied to determine copper in real water and urine samples and gave recoveries that ranged from 97.5 to 101.1% with RSD values 0.9-3.4%.

In this research application of thiol functionalized mesoporous silica coated magnetite nanoparticles was studied as an adsorbent in solid phase extraction for the determination of Cr³⁺, Ag⁺, Co²⁺, Pb²⁺, Zn²⁺, and Cu²⁺ metal ions from water samples using flame atomic absorption spectrometry (FAAS). Magnetic nanoparticles (MNPs) carrying target metals were easily separated with applying an external magnetic field from the aqueous solution. After extraction and collection of MNPs, the metal ions were desorbed with 3 mol L^-1 of the HCl solution containing 2% (w/v) thiourea. Effects of pH, adsorbent amount, contact time, sediment time, type and concentration of the eluent, volume of sample, stability and reusability of adsorbent and effect of interfering ions on the quantitative recovery of the metal ions were checked and optimized. In the improved conditions, the limits of detection for Cr³⁺, Ag⁺, Co²⁺, Pb²⁺, Zn²⁺, and Cu²⁺ metal ions were 7.4, 4.1, 3.7, 7.0, 1.2 and 7.0 μg L^-1, respectively. The enrichment factor of proposed method for the target metal ions obtained in the range 111-125 and the relative standard deviations (RSD, C= 0.02 mg L-1, n= 3) were lower than 5.0%. Accuracy of the method was evaluated by recovery measurements on the spiked samples, and the recoveries achieved in the range of 91 to 106%.