Preparation of a solid membrane lead(II) diethyldithiocarbamate electrode and its application for direct and indirect anion determination

A lead(II) selective electrode has been developed. This electrode is essentially formed by a lead wire, one end of which is coated with lead(II) diethyldithiocarbamate. The response is Nernstian for lead ions in a 10⁻¹ to 10⁻⁵M concentration range. The direct determination of this element and the indirect determination of several anions are possible.     

Multiresidue determination of chlorophenoxy acid herbicides in human urine samples by use of solid-phase extraction and capillary LC–UV detection

Chlorophenoxy acid herbicides are intensively applied to get rid of unwanted plants because of their low cost and selectivity. Due to their toxicity, which depends on their chemical form, the European Community has established legal directives to restrict their use and to control their maximum residue levels in several matrices. Determination of chlorophenoxy acids—2,4-dichlorophenoxyacetic acid (2,4-D), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2-(2,4-dichlorophenoxy)propanoic acid (2,4-DP), 2-(4-chloro-2-methylphenoxy)propanoic acid (MCPP), 4-(4-chloro-2-methylphenoxy)butanoic acid (MCPB) and 2-(2,4,5-trichlorophenoxy)propanoic acid (2,4,5-TP) in spiked human urine samples has been carried out by capillary LC, after solid-phase extraction on a column packed with silica C₁₈ restricted-access material. Chromatographic analysis was performed in gradient-elution mode at 25 °C, with injection of 20 μL low-organic-solvent composition herbicide solutions for focusing purposes on the head of the capillary column, and diode array detection at 232 nm. Urine samples collected during 24 h from healthy and unexposed volunteers were spiked in the concentration range 25–150 μg L⁻¹; recoveries obtained were between 66 and 100% (n = 6 for each spiked level) and RSDs (relative standard deviations) were between 1 and 5%. Detection limits in the urine samples from volunteers were between 3.5 and 6.0 μg L⁻¹. The developed methodology has allowed the clean-up and preconcentration of low volumes of untreated human urine without previous treatment, showing the effectiveness of the employed SPE sorbent for extracting the target analytes and ultimately resulting in the reduction of the sample-preparation time.     

New approach to optimize HPLC separations of acid–base compounds with elution order involved, by using combined three-band resolution maps

Some of the optimization methods in reversed phase high-performance liquid chromatography (RP-HPLC) are based on resolution of the critical band pair. Mobile phase composition is changed systematically to establish those conditions giving an acceptable resolution for such a critical band pair, but sometimes the critical pair may change with the separation conditions, which obliges to identify it for each of those conditions. In the case of ionizable compounds, more than two bands may be involved in resolution, showing—in some cases—changes in the elution order when the mobile phase composition was modified. In this paper, an alternative way that does not identify the critical pair after changing experimental conditions is proposed. The relative separation of the three bands involved in two alternating critical band pairs is evaluated as a sort of conjugate or combined resolution, represented as contour maps vs. two variables (content of organic modifier and pH). These maps are obtained from data of chromatograms made under different separation conditions; these conditions were generated by experimental design and data was mathematically processed with a computer program. Analytes of three families that have acid–base properties, triazines, phenoxyacids, and phenols, were used for this purpose. The chromatographic behavior when elution order reversion of ionizable compounds exists is studied.     

Chiral Determination of Salbutamol, Salmeterol and Atenolol by Two-Dimensional LC–LC: Application to Urine Samples

A heart-cut two-dimensional high-performance liquid chromatography method for enantiomeric determination of salbutamol, salmeterol and atenolol in urine is presented. It involves the use of two separations in a liquid chromatography?Cliquid chromatography achiral?Cchiral coupling. Target compounds were previously separated in a primary column (Kinetex? HILIC, 2.6???m, 150?×?2.1?mm I.D.) with a mixture of MeOH:ACN:ammonium acetate buffer (5?mM, pH 6) 90:5:5 (v/v/v) as mobile phase at a flow rate of 0.40?mL?min^?1. Enantiomeric separation was carried out by transferring peak of each compound through a switching valve to a vancomycin chiral column (Chirobiotic? V, 2.6???m, 150?×?2.1?mm I.D.) using MeOH:ammonium acetate buffer (2?mM, pH 4) 97:3 (v/v) as mobile phase at a flow rate of 0.50?mL?min^?1. Ultraviolet detection was done at 227?nm. The method was applied to determine target analytes in urine samples after enzymatic hydrolysis with ??-glucuronidase from Helix pomatia, followed by a solid-phase extraction procedure using Isolute^? HCX mixed-mode cartridges. Extraction recoveries ranged from 82 to 90?% in urine samples. Detection limits were 0.091?C0.095???g for each enantiomer of atenolol and between 0.058 and 0.076 and 0.18?C0.14???g for enantiomers of salbutamol and salmeterol, respectively (3?mL of urine). Linearity ranges were between 0.5 and 10???g?mL^?1. Intraday and interday reproducibilities of enantiomeric ratio and enantiomeric fraction, expressed as relative standard deviation, were between 1.9 and 9.0?%. The optimized method was successfully applied to the analysis of urine samples obtained from excretion studies in volunteers and in freeze-dried urine samples, containing urinary components with MW?<?10,000 and components with MW?>?10,000, spiked with different amounts of studied drugs.     

Effect of temperature on the separation of chlorophenoxy acids and carbamates by capillary high-performance liquid chromatography and UV (or diode array) detection

In this work, the effect of temperature in isothermal and programmed modes on several chromatographic parameters such as retention factor, selectivity, resolution and plate number has been discussed. A critical comparison of isocratic/isothermal, gradient/isothermal and isocratic/program temperature modes has been made. Two representative families of pesticides have been selected for this study. One includes ionisable chlorophenoxy acids and two of their esters, some of which show similar polarities. The other one contains several weakly polar carbamates. Analysis was carried out using a reversed-phase capillary high-performance liquid chromatography (HPLC) system and focusing technique with UV or diode array detection (DAD).     

Preconcentration of triazine herbicides from water by an ion chromatography column and determination by gas chromatography-mass spectrometry

The efficiency of ion chromatography columns packed with styrene-divinylbenzene copolymer containing quaternary ammonium groups to preconcentrate triazine herbicides and their degradation products below μg/l levels has been established. Retention is studied for different types of water. Pure methanol was used in a one-step elution. Enrichment factors of at least 4000 are achieved. Determination was carried out by using gas chromatography-single-ion monitoring mass spectrometry. Recoveries for run-off agricultural water were between 67–100% and close to 100% for ground water. The maximum admissible concentration ion drinking water (0.1 μg/l) and the alert and alarm threshold values in surface water (1 and 3 μg/l, respectively) dictated by the European Union can be measured.     

Chemically modified polymeric sorbents for sample preconcentration

Solid-phase extraction is an attractive alternative in sample preparation because it overcomes many drawbacks of liquid-liquid extraction and makes on-line determination possible by hyphenation with chromatographic techniques. Driven by the need for more effective and more selective sorbents, advances in solid-phase extraction include the development of new materials. This paper describes different types of chemically modified sorbents for the solid-phase extraction of compounds from aqueous samples. Chemical introduction of different functional groups into a polymeric resin improves the efficiency of solid-phase extraction by providing better surface contact with the aqueous samples; also, these sorbents have a greater capacity than the typical solid-phase materials for polar compounds have. The most important new sorbents are the chemically modified resins based on styrene-divinylbenzene copolymers. Preparation of these new sorbents is described, and advantages and drawbacks of off-line procedures and on-line procedures are also discussed. Applications for off-line and on-line chromatographic determinations of polar compounds are presented.     

Multivariate analysis of chlorophenols by diode array spectrophotometry

A multivariate-multicomponent analysis method for the determination of chlorophenols by diode array derivative spectrophotometry is proposed. The method is based on extraction of chlorophenols directly or as ion pairs with tetrabutylammonium as the counterion in chloroform at pH 9.1. Prediction and quantitative determinations (within the concentration range 3.0–15.0 mg liter⁻¹) of two-, three-, and four component mixtures were possible using multivariate calibration of first-derivative spectra and a multicomponent analysis computer program based on least-squares regression.     

Chiral Determination of Salbutamol,Salmeterol and Atenolol by Two-Dimensional LC–LC: Application to Urine Samples

A heart-cut two-dimensional high-performance liquid chromatography method for enantiomeric determination of salbutamol, salmeterol and atenolol in urine is presented. It involves the use of two separations in a liquid chromatography–liquid chromatography achiral–chiral coupling. Target compounds were previously separated in a primary column (Kinetex™ HILIC, 2.6 μm, 150 × 2.1 mm I.D.) with a mixture of MeOH:ACN:ammonium acetate buffer (5 mM, pH 6) 90:5:5 (v/v/v) as mobile phase at a flow rate of 0.40 mL min⁻¹. Enantiomeric separation was carried out by transferring peak of each compound through a switching valve to a vancomycin chiral column (Chirobiotic™ V, 2.6 μm, 150 × 2.1 mm I.D.) using MeOH:ammonium acetate buffer (2 mM, pH 4) 97:3 (v/v) as mobile phase at a flow rate of 0.50 mL min⁻¹. Ultraviolet detection was done at 227 nm. The method was applied to determine target analytes in urine samples after enzymatic hydrolysis with β-glucuronidase from Helix pomatia, followed by a solid-phase extraction procedure using Isolute^® HCX mixed-mode cartridges. Extraction recoveries ranged from 82 to 90 % in urine samples. Detection limits were 0.091–0.095 μg for each enantiomer of atenolol and between 0.058 and 0.076 and 0.18–0.14 μg for enantiomers of salbutamol and salmeterol, respectively (3 mL of urine). Linearity ranges were between 0.5 and 10 μg mL⁻¹. Intraday and interday reproducibilities of enantiomeric ratio and enantiomeric fraction, expressed as relative standard deviation, were between 1.9 and 9.0 %. The optimized method was successfully applied to the analysis of urine samples obtained from excretion studies in volunteers and in freeze-dried urine samples, containing urinary components with MW < 10,000 and components with MW > 10,000, spiked with different amounts of studied drugs.

     

DEVELOPABLE SURFACE PATCHES BOUNDED BY NURBS CURVES

In this paper we construct developable surface patches which are bounded by two rational or NURBS curves, though the resulting patch is not a rational or NURBS surface in general. This is accomplished by reparameterizing one of the boundary curves. The reparameterization function is the solution of an algebraic equation. For the relevant case of cubic or cubic spline curves, this equation is quartic at most, quadratic if the curves are Bézier or splines and lie on parallel planes, and hence it may be solved either by standard analytical or numerical methods.