Luciana Vera-Candioti

The EOF of polymer solutions

The EOF of polymer solutions is analysed in the framework of continuum fluid mechanics and the standard electrokinetic model. Two key aspects are taken into consideration: the non‐Newtonian character of the fluid and the polymer concentration near the interface, which greatly modify the fluid viscosity in the region where electroosmosis takes place. A satisfactory mathematical model is derived for the electroosmotic mobility of solutions that present polymer depletion at the wall. The case of solutions containing polymers that adsorb onto the wall is briefly reviewed, and a preliminary approach is discussed for the limit of strong polymer adsorption. In order to illustrate the theoretical discussions, experimental data obtained from aqueous solutions of carboxymethyl cellulose in fused‐silica capillaries are presented. Relevant results are observed, which are appropriately captured by the modelling proposed. The fundamental phenomena discussed in this work are of interest in microfluidics and electrophoresis.

Development of a novel strategy for preconcentration of antibiotic residues in milk and their quantitation by capillary electrophoresis.

A novel analytical method based on capillary zone electrophoresis coupled with diode array detection is developed and validated for the identification and simultaneous quantitation of four antibiotics in bovine raw milk. The studied antibiotics belong to different groups: beta-lactams, tetracyclines, quinolones, amphenicols and sulfonamides. An experimental design including both a factorial and a central composite design allowed a reduction in the number of optimization experiments. The multiple response criterion was successfully used to optimize the separation between chloramphenicol, ciprofloxacin, ampicillin, tetracycline and sulfamethoxazol, allowing the reduction of the analysis time with excellent peak resolutions and low capillary current. Different strategies for preconcentration and extraction of the studied antibiotics were applied, in order to remove potential interferences from the sample and to increase the sensitivity. Milk samples were prepared by a clean-up/extraction procedure based on protein precipitation with trichloroacetic acid followed by liquid-liquid extraction with dichloromethane combined with solid-phase extraction, and injection into the electrophoretic system hydrodynamically. The limits of detection and quantification (below 30 and 100 microg L(-1), respectively) were in all cases lower than the maximum residue limits tolerated for these compounds in milk. Accuracy was evaluated by computing recoveries for the target antibiotics which were between 93.08% and 102.89%.

Chemometric resolution of fully overlapped CE peaks: Quantitation of carbamazepine in human serum in the presence of several interferences

Drug monitoring in serum samples was performed using second‐order data generated by CE‐DAD, processed with a suitable chemometric strategy. Carbamazepine could be accurately quantitated in the presence of its main metabolite (carbamazepine epoxide), other therapeutic drugs (lamotrigine, phenobarbital, phenytoin, phenylephrine, ibuprofen, acetaminophen, theophylline, caffeine, acetyl salicylic acid), and additional serum endogenous components. The analytical strategy consisted of the following steps: (i) serum sample clean‐up to remove matrix interferences, (ii) data pre‐processing, in order to reduce the background and to correct for electrophoretic time shifts, and (iii) resolution of fully overlapped CE peaks (corresponding to carbamazepine, its metabolite, lamotrigine and unexpected serum components) by the well‐known multivariate curve resolution‐alternating least squares algorithm, which extracts quantitative information that can be uniquely ascribed to the analyte of interest. The analyte concentration in serum samples ranged from 2.00 to 8.00 mg/L. Mean recoveries were 102.6% (s=7.7) for binary samples, and 94.8% (s=13.5) for spiked serum samples, while CV (%)=4.0 was computed for five replicate, indicative of the acceptable accuracy and precision of the proposed method.

A microextraction procedure based on an ionic liquid as an ion-pairing agent optimized using a design of experiments for chromium species separation and determination in water samples

A microextraction methodology based on a room temperature ionic liquid (IL) as an ion-pairing reagent for determination of trace Cr(III) and Cr(VI) species is proposed. First, an ion-pair was formed between Cr(VI) species and the hydrophobic IL trihexyl(tetradecyl)phosphonium chloride (CYPHOS® IL 101). A simple and rapid microextraction procedure named ultrasound-assisted emulsification-microextraction (USAEME) was then developed for Cr species separation and preconcentration. Determination of Cr was performed by direct injection of the organic phase into an electrothermal atomic absorption spectrometer (ETAAS). Parameters that affect the efficiency of the microextraction step were investigated using a Plackett–Burman screening design. Then, the variables showing significant effects on the analytical response were considered within a further central composite design to optimize the experimental conditions. For 10 mL of water sample, the optimized USAEME procedure used 40 μL of tetrachloroethylene as extraction solvent, 5 min of extraction and 5 min of centrifugation at 1700 rpm. Selectivity among Cr species was obtained through pH selection. The concentration of Cr(III) species was calculated from the difference of total Cr and Cr(VI) concentrations. Under optimum conditions, the analyte extraction recovery was higher than 99% and yielded a preconcentration factor of 250. The limit of detection (LOD) obtained was 14.8 ng L−1 and the relative standard deviation for 10 replicate determinations at the 0.05 μg L−1 Cr(VI) level was 3.8%, calculated at peak areas. A correlation coefficient of 0.9983 was achieved. The method was successfully applied for Cr species determination in tap and river water samples.

Dispersive liquid–liquid microextraction of quinolones in porcine blood: Optimization of extraction procedure and CE separation using experimental design

A dispersive liquid–liquid microextraction procedure was developed to extract nine fluoroquinolones in porcine blood, six of which were quantified using a univariate calibration method. Extraction parameters including type and volume of extraction and dispersive solvent and pH, were optimized using a full factorial and a central composite designs. The optimum extraction parameters were a mixture of 250 μL dichloromethane (extract solvent) and 1250 μL ACN (dispersive solvent) in 500 μL of porcine blood reached to pH 6.80. After shaking and centrifugation, the upper phase was transferred in a glass tube and evaporated under N2 steam. The residue was resuspended into 50 μL of water–ACN (70:30, v/v) and determined by CE method with DAD, under optimum separation conditions. Consequently, a tenfold enrichment factor can potentially be reached with the pretreatment, taking into account the relationship between initial sample volume and final extract volume. Optimum separation conditions were as follows: BGE solution containing equal amounts of sodium borate (Na2B4O7) and di‐sodium hydrogen phosphate (Na2HPO4) with a final concentration of 23 mmol/L containing 0.2% of poly (diallyldimethylammonium chloride) and adjusted to pH 7.80. Separation was performed applying a negative potential of 25 kV, the cartridge was maintained at 25.0°C and the electropherograms were recorded at 275 nm during 4 min. The hydrodynamic injection was performed in the cathode by applying a pressure of 50 mbar for 10 s.

Second-order capillary electrophoresis diode array detector data modeled with the Tucker3 algorithm: A novel strategy for Argentinean white wine discrimination respect to grape variety.

Data obtained by capillary electrophoresis with diode array detection (CE‐DAD) were modeled with the purpose to discriminate Argentinean white wines samples produced from three grape varieties (Torrontés, Chardonnay, and Sauvignon blanc). Thirty‐eight samples of commercial white wine from four wine‐producing provinces of Argentina (Mendoza, San Juan, Salta, and Rio Negro) were analyzed. CE‐DAD matrices with dimensions of 421 elution times (from 1.17 to 7.39 minutes) × 71 wavelengths (from 227 to 367 nm) were joined in a three way data array and decomposed by Tucker3 method under non‐negativity constraint, employing 18, 18 and six factors in the modes 1, 2 and 3, respectively. Using the scores of Tucker model, it was possible to discriminate samples of Argentinean white wine by linear discriminant analysis and Kernel linear discriminant analysis. Core element analysis of the Tucker3 model allows identifying the loading profiles in spectral mode related to Argentinean white wine samples.

Dispersive liquid–liquid microextraction of quinolones in porcine blood: Validation of a CE method using univariate calibration or multivariate curve resolution-alternating least squares for overlapped peaks

In the previously published part of this study, we detailed a novel strategy based on dispersive liquid–liquid microextraction to extract and preconcentrate nine fluoroquinolones in porcine blood. Moreover, we presented the optimized experimental conditions to obtain complete CE separation between target analytes. Consequently, this second part reports the validation of the developed method to determine flumenique, difloxacin, enrofloxacin, marbofloxacin, ofloxacin, ciprofloxacin, through univariate calibration, and enoxacin, danofloxacin, and gatifloxacin through multivariate curve resolution analysis. The validation was performed according to FDA guidelines for bioanalytical assay procedures and the European Directive 2002/657 to demonstrate that the results are reliable. The method was applied for the determination of fluoroquinolones in real samples. Results indicated a high selectivity and excellent precision characteristics, with RSD less than 11.9% in the concentrations, in intra‐ and interassay precision studies. Linearity was proved for a range from 4.00 to 30.00 mg/L and the recovery has been investigated at four different fortification levels, from 89 to 113%. Several approaches found in the literature were used to determinate the LODs and LOQs. Though all strategies used were appropriate, we obtained different values when using different methods. Estimating the S/N ratio with the mean noise level in the migration time of each fluoroquinolones turned out as the best studied method for evaluating the LODs and LOQs, and the values were in a range of 1.55 to 4.55 mg/L and 5.17 to 9.62 mg/L, respectively.