Initial Analyte Concentration Research Articles

Solid-phase extraction and sample stacking–micellar electrokinetic capillary chromatography for the determination of multiresidues of herbicides and metabolites

Micellar electrokinetic capillary chromatography (MEKC) with diode array detection was used for the separation of 13 compounds (eight herbicides widely used in agriculture: metribuzin, lenacil, ethofumesate, atrazine, terbutryn, isoproturon, chlorotoluron and linuron, and five of their principal degradation products; namely, deethylatrazine, 2-hydroxyatrazine, deethyl-2-hydroxyatrazine, deisopropylatrazine and 3-chloro-4-methylphenylurea). Peak separation for the 13 analytes was not successful when a single surfactant system was employed, neither sodium dodecyl sulfate (SDS) nor dioctyl sulfosuccinate (DOSS) sodium salt. However, a mixture of these herbicides was successfully separated using a mixed micellar system involving SDS–DOSS in less than 14 min. An application study of an on-line concentration technique for MEKC was carried out to enhance sensitivity. The optimized on-line stacking procedure consisted simply of the addition of 50 m M of sodium chloride to the injection sample, the stacking effect being more intensive as analyte polarity increased. When this stacking procedure was combined with an off-line sample preconcentration step, based on solid-phase extraction, analytes could be detected in the ppb range. The whole method was applied to ultra-high-quality and natural waters. Linear relationships between the analytical signal and the initial analyte concentration were found to be independent of the type of water, except for the more polar analytes for which small differences were observed.

Ion-Exchange Voltammetry as a Solid-Phase Microextraction Analytical Method: Factors Influencing the Mass Transfer to Perfluorosulfonated Ionomer Film-Coated Electrodes and Some of Their Consequences on the Current Responses

A simple theoretical model of mass transfer kinetics at an electrode coated with an ion-exchange film polymer is proposed. The model takes into account the analyte depletion in solution and gives the relationship between the ion-exchange voltammetric (IEV) peak current and the initial analyte concentration in the sample matrix. The verification of the model is investigated at disposable Nafion film-coated screen-printed electrodes, using the redox cationic (ferrocenylmethyl)trimethylammonium salt. It is shown that the theoretical model and the experimental data fit satisfactorily insofar as the variation of the extraction and apparent diffusion coefficients of the salt with the film thickness are taken into account. Indeed, the film thickness plays a crucial role for the optimization of the IEV sensitivity, because the physicochemical properties of the recast Nafion polymer are dependent on the amount per unit area of Nafion deposited on the electrode surface.

Protein band spreading in capillary zone electrophoresis Effects of sample zone length and presence of polymer

It is commonly accepted that intra-column zone dispersion in CZE rests on multiple mechanisms, viz. diffusion, interaction of analyte with the capillary walls, Joule heat and conductivity differences between sample zone and the surrounding buffer. The most important extra-column contributor to bandwidth is thought to be the starting zone width. The present study shows that the length of the starting zone above 10 mm is linearly related to the bandwidth of R-phycoerythrin ( M r290·10 3). Below that length, bandwidth demonstrates a plateau preceded by a slight rise. Within the ‘plateau range’ the ratio of bandwidth to effective capillary length is close to constant while it is independent of electric field strenght in the range of 37 to 370 V cm −1 and of protein concentration in the range of 0.1 to 1000 μg ml −1. The experimental observations support the notion that the analyte-wall interaction is the determining source of intra-column zone dispersion. A slight rise observed at initial zone lengths of less than 2 mm was accounted for by a diffusion model taking into account a non-local initial concentration of analyte. The presence of polyethyleneglycol in the buffer within a concentration range up to 6% does not affect bandwidth. Above that concentration, the level of constant bandwidth is raised.

Measurement of Total and Bioactive Interleukin-2 in Tissue Samples by Immunoaffinity-Receptor Affinity Chromatography

The detection and measurement of cytokines is an important issue in the clinico-pathological diagnosis of several clinical entities, including organ transplant rejection. Existing techniques, although sensitive, measure only total cytokine concentrations and cannot measure bioactivity. A chromatographic system combining immunoaffinity chromatography with an immobilized receptor detection cartridge has been developed for measuring total and bioactive interleukin (IL)-2 concentrations in tissue extracts prepared from biopsy materials taken from renal transplant recipients during both rejection and drug-induced nephrotoxic episodes. The technique employs a short high-pressure chromatography column packed with antibody-coated glass beads for the initial analyte separation and concentration, followed by detection of bioactive molecules through their interactions with specific, immobilized receptors. This system compares favourably with both conventional bioassays and immunoassays for measuring IL-2 in tissue samples.

Determination of Initial Concentration of an Analyte by Kinetic Detection of the Intermediate Product in Consecutive First-Order Reactions Using an Extended Kalman Filter

Many analytical reagents react with analytes according to a rate law described by a consecutive first-order reaction mechanism, A [yields][sup k(1)] B [yields][sup k(2)] C, where a product formation step is followed by a product degradation step. In many cases, consecutive first-order reactions are used to determine the initial concentration of analyte A by kinetic detection of the intermediate product B. In this work, the factors that affect the determination of the initial concentration of analyte and the kinetic parameters for this class of reactions have been investigated by computer simulations using an extended Kalman filter. The flip-flop phenomenon exhibited by this kinetic model is discussed, and a unique approach for the determination of the initial concentration of analyte is proposed. Empirical and residual methods for obtaining initial estimates for the rate constants and the initial concentration from the kinetic data, based on the model for consecutive first-order reactions, the initial concentration of analyte, the mismatch between the measured time and the real time, the data density, the fitting range, and the initial estimate for the background signal have been evaluated by using synthetic data with Gaussian-distributed noise. 15 refs., 6 figs.

Characterization and mathematical correction of hemolysis interference in selected Hitachi 717 assays

The effect of hemolysis on several assays performed with the Hitachi 717 was quantified by relating the amount of error to the concentration of hemoglobin. Hemolysis interference was judged clinically significant when analyte concentration varied by > 10% from the initial value. Hemolysis interference was significant for alkaline phosphatase, aspartate aminotransferase, alpha-amylase, bilirubin, creatine kinase, gamma-glutamyltransferase, lactate dehydrogenase, lactate dehydrogenase-1, potassium, and theophylline assays. Error (expressed in absolute terms) was linearly dependent on hemoglobin concentration and independent of the initial analyte concentration in each case, except for bilirubin and theophylline, where multiple regression analysis was required to quantify the effect. Relative error was dependent on the initial analyte concentration in all cases. Correction formulas were calculated from linear regression of absolute error vs hemoglobin concentration. Clinical application of correction formulas and mechanisms of hemolysis interference for each assay are discussed.

New standards for the determination of geosmin and methylisoborneol in water by gas chromatography/mass spectrometry

Deuterium labelled geosmin and methylisoborneol (MIB) have been synthesized and evaluated as internal standards in the determination of geosmin and MIB in water by closed loop stripping followed by gas chromatography/mass spectrometry (GC/MS). The labelled standards were compared with chloroalkanes added as internal standards either at the time of sampling or immediately before closed loop stripping. When added at sampling time, the new standards enabled accurate determination of the geosmin and MIB present initially, even when the samples were analysed as much as 3 weeks later. The new standards gave better precision and accuracy than the chloroalkanes and overcame the underestimation of initial analyte concentration which usually results from losses of analyte through adsorption, volatilisation, biodegradation etc. during sample storage. Geosmin had a limit of detection of<0.1ng/l and 1 ng/lwas determined with a coefficient of variation (CV) of 1.2% ( n = 5). MIB was determined at 1 ng/l with a CV of 3.5% ( n = 5).

Kinetic treatment of unsegmented flow systems : Part 5. Comprehensive Treatment for Systems with Gradient Chamber

A variable-time kinetic model is used to derive a comprehensive set of equations for an unsegmented flow system with a gradient chamber for the situations in which the analyte, reactant or product is monitored. Equations are presented for single-channel systems with excess analyte and excess reactant. These response equations are used to develop relationships that permit one to calculate whether analyte or reactant is in excess, to understand the merits and limitations of dispersion coefficients and to relate measured time intervals to initial analyte concentration for both single- and dual-channel systems. Relationships between time intervals ( Δt) and the logarithm of initial analyte concentration ( C) are inherently non-linear and include terms for the reference-point concentrations between which time intervals are measured. Special cases for which Δt vs. In C relationships are linear and do not include the reference-point concentrations are discussed and the assumptions that lead to these simplified equations are identified. Computed response curves are included to illustrate behavior in different phases of the processes.