Simultaneous Determination Of Concentrations Research Articles

Dissolution of a surfactant-water lamellar phase investigated by combining time-lapse polarized light microscopy and confocal Raman spectroscopy

HypothesisWhile the phase behavior of aqueous surfactant solutions is usually described in term of the equilibrium microstructures of lyotropic liquid crystals, the transformations which take place when a phase turns into another one, either by changing the concentration or the temperature, are still to be elucidated. A simultaneous determination of concentration and microstructure is at order to elucidate the phase behavior under changing conditions, such as in a dissolution experiment. ExperimentsConfocal Raman micro-spectroscopy and time-lapse polarized light microscopy are combined to study the phase transitions taking place in the dissolution of a common anionic surfactant (sodium laurylethersulfate) in water. FindingsBy comparing Raman concentration profiles and polarized light images, it is found that the aqueous solution, with initial surfactant concentration of 72 wt%, undergoes a sequence of complex microstructural transformations including distortion of the initial lamellar phase, formation of an intermediate striated texture, which can be considered as a precursor of a cubic phase, and a heterogeneous hexagonal phase going through a transition region before turning into a micellar phase. The effects of the sodium counter-ion and of water confinement are also investigated by analyzing the OH-stretching bands.

Simultaneous Determination of Concentration and Enantiomeric Composition of Amino Acids in Aqueous Solution by Using a Tetrabromobinaphthyl Dialdehyde Probe.

3,3'-Diformyl-1,1'-bi-2-naphthol or its methoxymethyl-protected derivative is found to undergo a highly selective reaction with excess bromine in CH2 Cl2 at reflux to give the novel 5,5',6,6'-tetrabrominated product (S)- or (R)-2. The observed electrophilic substitution at the 5,5'-positons of an optically active binaphthyl compound is unprecedented. Unlike unbrominated 3,3'-diformyl-1,1'-bi-2-naphthol, which is not suitable for fluorescent recognition in water, compound (S)-2, in combination with Zn2+ , exhibits a highly enantioselective fluorescent response toward amino acids in aqueous solution (HEPES buffer, pH 7.4). It is further found that the condensation product of (R)-2 with tryptophan, (R)-3, shows dual-responsive emissions toward amino acids; the short wavelength (λ1 =350 nm) emission is sensitive to the concentration of the substrate regardless of the chiral configuration and the long wavelength (λ2 >500 nm) emission is highly enantioselective. Thus, the use of (R)-3 allows the simultaneous determination of the concentration and enantiomeric composition of an amino acid sample from one fluorescence measurement.

Simultaneous Determination of Concentration and Enantiomeric Composition in Fluorescent Sensing.

Fluorescent sensors have found broad applications in determining the concentrations of diverse analytes by measuring specific sensor-analyte fluorescent responses. For a chiral substrate containing varying composition of two enantiomers, both the concentration and enantiomeric composition should greatly influence the fluorescent response of an enantioselective fluorescent sensor. Thus, multiple independent measurements are normally needed to determine both the concentration and enantiomeric composition of a chiral compound. In order to facilitate the application of the enantioselective fluorescent sensors, our laboratory has developed four strategies to simultaneously determine the concentration and enantiomeric composition of various chiral substrates by a single fluorescence measurement. A mixture of a chiral BINOL-based dialdehyde and an achiral compound salicylaldehyde in the presence of Zn2+ is used to interact with chiral diamines, amino alcohols, and amino acids. The fluorescence enhancement at λ1 = 447 nm due to the achiral sensor is mostly determined by the concentration of the substrates, and the fluorescence enhancement at λ2 > 500 nm due to the chiral sensor is highly enantioselective. A 3D graph combining the fluorescence intensities at λ1 and λ2 can be used to determine the enantiomeric composition. A chiral conjugated polymer containing the BINOL-dialdehyde units is shown to amplify the enantioselectivity of the small molecule sensor under the same conditions. Combination of the chiral polymer with salicylaldehyde allows simultaneous concentration and enantiomeric composition determination. In a pseudoenantiomeric sensor pair of the BINOL-based amino alcohols, one sensor shows greater fluorescence enhancement with one enantiomer of chiral α-hydroxy carboxylic acid at λ1 = 374 nm and another sensor shows greater fluorescence enhancement with another enantiomer at λ2 = 330 nm. Using a mixture of this sensor pair allows the determination of both concentration and enantiomeric composition with one fluorescence measurement. A BINOL-based trifluoromethyl ketone is found to exhibit dual emission responses toward a chiral diamine at λ1 = 370 nm and λ2 = 438 nm. The fluorescence enhancement at λ1 is mostly determined by the substrate concentration and that at λ2 is highly enantioselective. Thus, using one sensor with one measurement gives both parameters. A BINOL-naphthyl imine compound is designed to show two different fluorescent responses toward functional chiral amines in the presence of Zn2+. When the naphthylamine unit is displaced off the sensor by a chiral amine substrate via imine metathesis, the emission of naphthylamine is restored at λ1 = 427 nm, which allows determination of the substrate concentration. The fluorescence enhancement at λ2 > 500 nm due to the formation of the new chiral imine products is highly enantioselective. The work discussed here has provided convenient methods to obtain the two important parameters of a chiral molecule by a single fluorescence measurement. They should contribute to the development of analytical tools for the rapid assay of chiral compounds.

Simultaneous Determination of Micro Bromide and Iodide by Kinetic Spectrophotometric Method

ABSTRACT ABSTRACTA new kinetic spectrophotometric method for the simultaneous determination of concentrations of micro bromide and iodide has been proposed. It is based on their catalytic effects on the reaction of m-cresol purple oxidized by potassium periodate in hydrochloride acid medium. The reaction rate was monitored by measuring the decrease in absorbance at 528nm and the increase in absorbance at 455nm. The total difference in absorbance of the sum of bromide and iodide is identical with determination of bromide was carried out after Cr(VI) oxidized I− to I2, and I2 was removed by extraction with CCI4, and the amount of iodide was measured by subtracting the absorbance change of bromide from the total absorbance change in the presence of bromine and iodide. The optimum conditions influencing the reaction rate were studied. The linear range of determination is 0∼4.0μg/ml for Br− and 0∼3.0 μg/ml for I−. The detection limits are 0.032μg/ml for Br− and 0.059 μ/ml for I−. The method was successfully applied to the determination of micro amounts of bromide and iodide in food and life samples.

Spectroscopic and Potentiometric Investigation of a Diprotic Acid: An Experimental Approach to Understanding Alpha Functions

A laboratory experiment suitable for second- or higher-year chemistry students uses thymol blue, an organic dye, for investigating the fraction of each colored species present as a function of pH. Visible color changes in the vivid, highly absorbing dye permit students to “see” relative amounts of acid, base, and ampholyte species during the titration, while potentiometric measurement of pH and spectroscopic determination of concentrations permit quantitation of the species present. A standard Beer’s law calibration curve is constructed for known concentrations of the pure forms of each species at a pH where that species is present as nearly 100% of the total analytical concentration of the dye. Using the molar absorptivities of the three species computed from the calibration curves, simultaneous determination of concentration by species is performed on a set of solutions having constant total concentration of dye such that each solution represents one point in the range pH 0 to pH 13. A mathematical software application is used to solve the simultaneous equations and determine the fraction of each species present. Students then use a spreadsheet application to plot the fractions as functions of pH (the alpha functions). The significance of the isosbestic points in the spectra is also explored.

Seasonal and depth variability of reduced sulphur species and metal ions in mud-flat pore-waters of the Venice lagoon

An in-situ sampler was employed to monitor seasonal changes in the composition of pore-waters at different depths in a mud-flat located in the central part of the Lagoon of Venice, in proximity of the industrial area of Porto Marghera. The use of voltammetric methods of analysis allowed the simultaneous determination of concentrations of reduced S( − II), thiosulphate, Fe(II) and Mn(II) as well as zinc, lead, cadmium and copper ions. Other parameters such as the concentration of sulphate, Eh, pH and conductivity were also measured. Seasonal increases of S( − II) and Fe(II) concentrations in summer and autumn samples were observed, with the highest concentrations located at about 25 and 55 cm depth, respectively. An opposite seasonal dependence was observed for Mn(II) and S2O32− concentrations, which displayed higher values in winter: the highest concentrations were observed at a depth of 45 cm for Mn(II) and of 65 cm for thiosulphate. The concentrations of Cu, Cd, Pb and Zn ions showed a less evident seasonal variability. Top and bottom samples revealed high concentration values for zinc and copper ions, with the highest values in the 1,000–1,600 nM and 100–300 nM ranges, respectively. Cadmium and lead ions were present at lower concentrations, always less than 70 nM and 30 nM, respectively.

Simultaneous determination of concentrations and stable isotope enrichments of α‐ketoisocaproic acid, leucine, phenylalanine and tyrosine in blood plasma by gas chromatography/negative ion mass spectrometry

AbstractA simple procedure for the extraction and derivatization of α‐amino acids and α‐keto acids from human plasma is described. This stable isotope dilution method permits the quantification of isotopic enrichment and concentrations of several α‐keto and α‐amino acids simultaneously. The derivatization process results in methylformated, pentafluorobenzylated α‐amino acids and pentafluorobenzylated α‐keto acids. The derivatives proved to be very stable when stored at 5°C. Under electron capture chemical ionization (ECCI) conditions the derivatized amino acid produces abundant [M – PFB]− or [M – PFB – CH3OH]− anions; the α‐keto acid gives abundant [M – PFB]− anions. The method was applied to the analyses of 100 μl plasma samples in studies on protein metabolism. For the determination of concentrations [α‐ketoisocaproic acid (α‐KIC), Leu, Phe and Tyr] and enrichments ([1‐13C]‐α‐KIC, [1‐13C]‐Leu, D5‐Phe, D2‐Tyr, D4‐Tyr), resulting from administered tracers, the corresponding [M – PFB]− or [M – PFB – CH3OH]− anions were monitored. Higher labelled analogues were used as internal standards. The overall sensitivity was sufficient to study isotopic enrichments in the range 0.2–10 at.% excess.