Jc Jetse Reijenga

Group-Type Identification of Oil Samples Using Comprehensive Two- Dimensional Gas Chromatography Coupled to a Time-of-Flight Mass Spectrometer (GC×GC-TOF)

In this work a comprehensive two-dimensional system (GC x GC) was coupled to a time-of-flight mass spectrometer (TOF/MS) for the analysis of oil samples. Group-types like the alkanes and saturated cyclic compounds (naphthenes), which are present in oil, are shown separately by selecting their unique masses. On selecting appropriate ion fragments, this method also permits the determination of sulfur-and oxygen-containing species in oil. Former results obtained by FID detection could be confirmed. After proper selection of unique ions in GC x GC-TOF both selectivity and sensitivity increase.

Effect of electroosmosis on detection in isotachophoresis

Abstract The effect of electroosmosis on the relative sharpness of the zone boundaries and hence the detection limit in isotachophoresis is discussed. A high concentration and a low pH of the leading electrolyte were found to be favourable. The magnitude and sign of the ζ-potential of the capillary wall was measured by streaming potential determinations as a function of concentration, pH and additives. Poly(vinyl alcohol), hydroxyethylcellulose and hydroxypropylmethylcellulose decrease the ζ-potential sufficiently, whereas cationic detergents reverse its sign and thus the direction of the electroosmotic flow. Both were found to be favourable in anionic separation. Fused-silica capillaries and the effect of methanol were also investigated.

Ionic strength and charge number correction for mobilities of multivalent organic anions in capillary electrophoresis

Abstract An empirical expression was derived for the actual mobility, μ, valid in an extended range of ionic strength, I , between 0.001 and 0.1 mol/l, using a three-parameter exponential fit for the ratio of actual and absolute mobilities ( μ 0 at I = 0), which consists of two variables: ionic strength of the buffer and charge number, z , of the solute. The charge numbers of the solute anions (21 aromatic sulfonates) ranged between 1 and 6, and the ionic strength was varied in the range mentioned using a sodium acetate buffer. The mobilities were determined at 25.0°C in a coated capillary with suppressed electroosmotic flow. The resulting fitting equation, valid under these conditions, is μ / μ 0 = exp(−0.77√ zI ). The difference between measured mobilities and mobilities calculated from a tenfold higher or lower ionic strength, using this equation, is within 5% for all experiments. For multivalent ions, μ values can be derived from μ 0 with comparable accuracy.

Computational simulation of migration and dispersion in free capillary zone electrophoresis. I: Description of the theoretical model

An instrument simulator was developed for high-performance capillary electrophoresis which allows for fast graphic illustration of the effect of a large number of variables on the shape of the electropherogram. The input data of the separands are values of pK and mobilities at 25°C and infinite dilution. The instrument parameters that can be varied include capillary material, lengths, inside diameter, wall thickness, zeta potential, cooling temperature, voltage, polarity and open or closed mode. Hydrostatic injection is simulated, using time and pressure as variables. The following properties of the buffer can be varied: pH, ionic strength and effective mobility of background electrolyte ions. The effective mobilities are corrected for temperature and concentration effects. Extra-column effects from injection and detection are taken into account, and also peak dispersion arising from electroosmosis, diffusion, electromigration and heat production.

Development of Methods for the Determination of pKa Values

The acid dissociation constant (pKa) is among the most frequently used physicochemical parameters, and its determination is of interest to a wide range of research fields. We present a brief introduction on the conceptual development of pKa as a physical parameter and its relationship to the concept of the pH of a solution. This is followed by a general summary of the historical development and current state of the techniques of pKa determination and an attempt to develop insight into future developments. Fourteen methods of determining the acid dissociation constant are placed in context and are critically evaluated to make a fair comparison and to determine their applications in modern chemistry. Additionally, we have studied these techniques in light of present trends in science and technology and attempt to determine how these trends might affect future developments in the field.

Buffer capacity, ionic strength and heat dissipation in capillary electrophoresis

The composition of background electrolyte (BGE) in capillary electrophoresis (CE) is important for both selectivity and efficiency. Although the importance of pH and ionic strength predominates, attention should also be paid to specific conductivity, heat dissipation and buffering capacity. At a given pH/ionic strength combination, several buffers can in principle be used. Preference should be given to the so called Good type buffers because of the combination of higher buffering capacity and lower specific conductivity. Heat dissipation, leading to disturbances in migration and efficiency can thus be minimised. Buffer performance was calculated using a recently developed buffer selection and evaluation computer program.

Computational simulation of migration and dispersion in free capillary zone electrophoresis, II: Results of simulation and comparison with measurements

The results of the simulation of electropherograms for CZE obtained by the instrumental simulator described in Part I are presented as functions of various experimental parameters. The electropherograms, demonstrating the effects of the injection zone length and sample composition on peak broadening, of the ζ potential on migration and efficiency, of minute changes in pH on the resolution, of the ionic strength of the buffer on the separation selectivity and of the co-ion of the background electrolyte on peak shape, are simulated within a few seconds. A comparison was made between simulated electropherograms and those obtained by measurements with real equipment.

Thermodynamics of chiral selectivity in capillary electrophoresis : separation of ibuprofen enantiomers with β-cyclodextrin

The effect of temperature on the electrophoretic chiral separation of ibuprofen with β-CD was investigated. Background electrolytes with sodium acetate or formate were chosen because of their constant pK within 0.03 units in the temperature range 25–50°C. Ibuprofen has a temperature independent pK value of 4.36, and a mobility of 23.3·10−9 m2/Vs at 25°C. The mobility has a temperature coefficient of 2.0%/°C. At that same temperature, formation constants K1 for the uncharged enantiomers are 9955 and 10294 M−1 respectively. The formation constant K2 for the charged form is 5256 M−1 for both isomers. For these chiral formation constants, ΔH values are around −50 kJ/mol, whereas ΔS values are around −90 J/mol/K.

Transient modelling of capillary electrophoresis, isotachophoresis

Different algorithms for transient modelling of capillary electrophoresis have been described in several papers. Programs based on such algorithms were applied to various modes of CE. Surprisingly, simulations of capillary isotachophoresis (cITP) at realistic current densities (>1 kA/m2) were not reported. Using these programs for practical cITP conditions resulted in either severe oscillations, mass-balance violation or unexpected program termination. This paper addresses several numerical paths available for modelling one-dimensional capillary electrophoretic behaviour. Tests for determining the validity of the presented solutions with respect to cITP were mass balance checks, zone boundary thickness and the Kohlrausch regulating function. Six different numerical schemes fulfilled these requirements, yet only few could be used for simulating practical current density situations without causing the aforementioned problems. Attention was paid to space discretization (central difference and quadratic upwind) and time integration (implicit, explicit). A single computer program comprising these strategies was developed. Special features for studying transient state phenomena were visualization of concentrations, velocities, Peclet and Courant numbers, electric field strength, conductivity, pH, buffering capacity and charge excess. All parameters could be displayed in both the space domain (profile) as well as in the time domain (electropherogram).

Pherogram normalization in capillary electrophoresis and micellar electrokinetic chromatography analyses in cases of sample matrix-induced migration time shifts.

When analyzing bio-matrix samples using capillary electrophoresis (CE) or micellar electrokinetic chromatography (MEKC), unwanted shifts in the time axis are often observed, both between samples and standards and between samples, thus hampering identification. These shifts are caused by either or both of two sample matrix-induced effects: variations in stacking conditions (effective field strength or migration length) and variations in electroosmotic flow. Based on elementary CE principles and provided that any two peaks in the pherograms can be linked, these variations can be separately accounted and quantitatively corrected for, so that perfectly overlapping pherograms of standards and samples can be obtained after normalization. The method was validated using samples of a DNA ladder, separated in a sieving polymer. In addition, a number of data files from CE and MEKC analyses (steroids, opioids, beta-blockers, amines, and inorganic anions) previously published by other authors were successfully normalized. A freeware computer programme, CEqualizer, for normalizing ASCII files of detector signals using the method described, is available to the CE community from http: //www.ceyork.f2s.com.