The mathematical modelling of a production scale sequential continuous chromatographic refiner unit

Summary A plate to plate mathematical model of a sequential gas-liquid chromatographic separator used for the continuous separation of volatile organic mixtures has been extended to include theoretically determined temperature, pressure and concentration effects. A comparison has been made of predicted values with experimental results obtained on a 12 column (7.6 cm o.d. ×61 cm) sequential continuous chromatograph when separating an equivolume mixture of Arklone P-Genklene P at feed rates of up to 1400 cm³ hr^–1.     

Gas-liquid chromatographic determination of ethanol in “Alcohol-Free” beverages and fruit juices

Summary A rapid, sensitive and direct method is described for the detemination of ethanol in Alcohol-Free beverages and fruit juices. A known amount of internal standard is added to the sample and a 0.1 mm³ aliquot is injected directly into a gas chromatograph. Ethanol concentrations as low as 0.005% v/v can be detected and quantitatively determined. Five brands of beer, seven brands of grape juice and four brands of apple juice; all labelled Alcohol-Free were analyzed. Ethanol in concentrations ranging from 0.009% v/v to 0.385% v/v was found. These findings indicate that these products are not actually Alcohol-Free but alcohol-low or alcohol-poor.     

Development of a sequential injection system for trace mercury determination by cold vapour atomic absorption spectrometry utilizing an integrated gas-liquid separator/reactor

A simple and robust time-based on-line sequential injection system for trace mercury determination via cold vapour atomic absorption spectrometry (CVAAS), employing a new integrated gas-liquid separator (GLS), which in parallel operates as reactor, was developed. Sample and reductant are sequentially loaded into the GLS while an argon flow delivers the released mercury vapour through the atomic absorption cell. The proposed method is characterized by the ability of successfully managing variable sample volume up to 30 ml in order to achieve high sensitivity. For 20 ml sample volume, the sampling frequency is 25 h⁻¹. The calibration curve is linear over the concentration range 0.05-5.0 μg l⁻¹ of Hg(II), the detection limit is c_L = 0.02 μg l⁻¹, and the relative standard deviation is s_r = 2.6% at 1.0 μg l⁻¹ Hg(II) level. The performance of the proposed method was evaluated by analyzing certified reference material and applied to the analysis of natural waters and biological samples.     

Pd-catalyzed sequential reaction for the synthesis of fused tricyclic compounds containing a dihydrofuran or isobenzofuranone unit

A novel Pd-catalyzed sequential reaction for the synthesis of fused tricyclic compounds with a dihydrofuran or isobenzofur-anone unit was developed.     

An overview of sequential injection chromatography

New generation of sequential injection analysis (SIA) called sequential injection chromatography (SIC) has already been consolidated as a good alternative of high performance liquid chromatography (HPLC) for fast analysis of simple samples. Benefits of flow methods are automation, miniaturization and low sample and mobile phase consumption. Implementation of short monolithic chromatographic column into SIA opens new area—on-line chromatographic separation of multi-compound sample in low-pressure flow system, with the advantage of flow programming and possibility of sample manipulation. In the presented review the potential of SIC and its comparison with HPLC for determination of pharmaceutical mixtures is discussed and outlines past and recent trends focused on separation with SIC.     

An investigation of the changes in column efficiency accompanying the oxidation of apolar gas chromatographic stationary phases

Summary The effect of on-column oxidation upon the efficiency of apolar gas chromatographic columns is investigated. With an unsaturated hydrocarbon liquid phase and air as carrier gas dramatic changes of column efficiency are observed. Evidence is presented that suggests that this catastrophic deterioration of column performance is due to the oxidative crosslinking of the liquid phase molecules.     

Changes in peak width and concentration at the inlet and outlet of a chromatographic column

The original plate model of chromatography is extended to the sorption process occurring at the column inlet and the desorption process at the column exit. At the column inlet it is shown that sufficiently wide feed bands undergo no change in concentration but a fall in band width, i.e., the volume of mobile phase occupied by the solute band is reduced. The reduction factor is (1 + k) where k is the mass distribution ratio (capacity factor). Narrower bands suffer partial reduction in both band width and concentration. On desorption at the outlet, however, the change is always in band width and not concentration. A perfect detector registers the true concentration-time profile of the band in the column if the solute mass fraction in the stationary phase is below 10^?3 at the column outlet. The risks of stripping the stationary phase at high solute concentrations in analytical and preparative or production gas chromatography are compared.     

Determination of arsenic(III) and total inorganic arsenic in water samples using an on-line sequential insertion system and hydride generation atomic absorption spectrometry

A simple and robust on-line sequential insertion system coupled with hydride generation atomic absorption spectrometry (HG-AAS) was developed, for selective As(III) and total inorganic arsenic determination without pre-reduction step. The proposed manifold, which is employing an integrated reaction chamber/gas-liquid separator (RC-GLS), is characterized by the ability of the successful managing of variable sample volumes (up to 25 ml), in order to achieve high sensitivity. Arsine is able to be selectively generated either from inorganic As(III) or from total arsenic, using different concentrations of HCl and NaBH₄ solutions. For 8 ml sample volume consumption, the sampling frequency is 40 h⁻¹. The detection limit is c_L = 0.1 and 0.06 μg l⁻¹ for As(III) and total arsenic, respectively. The precision (relative standard deviation) at 2.0 μg l⁻¹ (n = 10) level is s_r = 2.9 and 3.1% for As(III) and total arsenic, respectively. The performance of the proposed method was evaluated by analyzing the certified reference material NIST CRM 1643d and spiked water samples with various concentration ratios of As(III) to As(V). The method was applied for arsenic speciation in natural waters samples.     

A theoretical plate model accounting for slow kinetics in chromatographic elution

The chromatographic elution has been studied from different perspectives. However, in spite of the simplicity and evident deficiencies of the plate model proposed by Martin and Synge, it has served as a basis for the characterization of columns up-to-date. This approach envisions the chromatographic column as an arbitrary number of theoretical plates, each of them consisting of identical repeating portions of mobile phase and stationary phase. Solutes partition between both phases, reaching the equilibrium. Mobile phase transference between the theoretical plates is assumed to be infinitesimally stepwise (or continuous), giving rise to the mixing of the solutions in adjacent plates. This yields an additional peak broadening, which is added to the dispersion associated to the equilibrium conditions. It is commonly assumed that when the solute concentration is sufficiently small, chromatographic elution is carried out under linear conditions, which is the case in almost all analytical applications. When the solute concentration increases above a value where the stationary phase approximates saturation (i.e. becomes overloaded), non-linear elution is obtained. In addition to overloading, another source of non-linearity can be a slow mass transfer. An extended Martin and Synge model is here proposed to include slow mass-transfer kinetics (with respect to flow rate) between the mobile phase and stationary phase. We show that there is a linear relationship between the variance and the ratio of the kinetic constants for the mass transfer in the flow direction (τ) and the mass transfer between the mobile phase and stationary phase (ν), which has been called the kinetic ratio (κ=τ/ν). The proposed model was validated with data obtained according to an approach that simulates the solute migration through the theoretical plates. An experimental approach to measure the deviation from the equilibrium conditions using the experimental peak variances and retention times at several flow rates is also proposed.     

Implementing stepwise solvent elution in sequential injection chromatography for fluorimetric determination of intracellular free amino acids in the microalgae Tetraselmis gracilis

The concept of sequential injection chromatography (SIC) was exploited to automate the fluorimetric determination of amino acids after pre-column derivatization with o-phthaldialdehyde (OPA) in presence of 2-mercaptoethanol (2MCE) using a reverse phase monolithic C₁₈ stationary phase. The method is low-priced and based on five steps of isocratic elutions. The first step employs the mixture methanol: tetrahydrofuran: 10 mmol L⁻¹ phosphate buffer (pH 7.2) at the volumetric ratio of 8:1:91; the other steps use methanol: 10 mmol L⁻¹ phosphate buffer (pH 7.2) at volumetric ratios of 20:80, 35:65, 50:50 and 65:35. At a flow rate of 10 μL s⁻¹ a 25 mm long-column was able to separate aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn), serine (Ser), glutamine (Gln), glycine (Gly), threonine (Thr), citruline (Ctr), arginine (Arg), alanine (Ala), tyrosine (Tyr), phenylalanine (Phe), ornithine (Orn) and lysine (Lys) with resolution >1.2 as well as methionine (Met) and valine (Val) with resolution of 0.6. Under these conditions isoleucine (Ile) and leucine (Leu) co-eluted. The entire cycle of amino acids derivatization, chromatographic separation and column conditioning at the end of separation lasted 25 min. At a flow rate of 40 μL s⁻¹ such time was reduced to 10 min at the cost of resolution worsening for the pairs Ctr/Arg and Orn/Lys. The detection limits varied from 0.092 μmol L⁻¹ for Tyr to 0.51 μmol L⁻¹ for Orn. The method was successfully applied to the determination of intracellular free amino acids in the green alga Tetraselmis gracilis during a period of seven days of cultivation. Samples spiked with known amounts of amino acids resulted in recoveries between 94 and 112%.     

Gas chromatographic analysis of xylidine isomers on heteroaromatic compounds

Summary Nicotinic acid, picolinic acid, isonicotinic acid, histidine and other amphoteric heteroaromatic compounds, uracil, thymine, isatin and other heteroaromatic compounds having imino and carbonyl groups as well as IH-benzotriazole, benzoguanamine and guanidine phosphate effectively resolved all the xylidine (dimethylaniline) isomers. Furthermore, the mixed phases of nicotinamide + p-amino-benzoic acid and melamine+barbituric acid also provided an appreciable resolution. These analyses were carried out using a support which were not treated with an alkali.     

Sequential injection chromatographic determination of ambroxol hydrochloride and doxycycline in pharmaceutical preparations

A new separation method based on a novel reversed-phase sequential injection chromatography (SIC) technique was used for simultaneous determination of ambroxol hydrochloride and doxycycline in pharmaceutical preparations in this contribution.The coupling of short monolith with SIA system results in an implementation of separation step to until no-separation low-pressure method.A Chromolith^® Flash RP-18e, 25-4.6 mm column (Merck, Germany) and a FIAlab^® 3000 system (USA) with a six-port selection valve and 5 ml syringe were used for sequential injection chromatographic separations in our study. The mobile phase used was acetonitrile-water (20:90, v/v), pH 2.5 adjusted with 98% phosphoric acid, flow rate 0.48 ml min⁻¹, UV detection was at 213 nm.The validation parameters have shown good results: linearity of determination for both compounds including internal standard (ethylparaben) >0.999; repeatability of determination (R.S.D.) in the range 0.5-5.4% at three different concentration levels, detection limits in the range 0.5-2.0 μg ml⁻¹, and recovery from the pharmaceutical preparation in the range 99.3-99.9%. The chromatographic resolution between peak compounds was >5.0 and analysis time was <9 min under the optimal conditions. The method was found to be applicable for routine analysis of the active compounds ambroxol hydrochloride and doxycycline in various pharmaceutical preparations.     

Comparison of the elution curves of pure components and their mixture and prediction of elution curves by a theoretical plate model in preparative gas-liquid chromatography

Summary Two close-boiling materials, diethyl ether (DEE) and dichloromethane (DCM), were separated to investigate the effects of the pure components and the mixture on elution in preparative gas-liquid chromatography.Nitrogen was used as the carrier gas, and the chromatographic column (1 cm I.D. and 0.75m length) was packed with Chromosorb A with different quantities of dinonyl phthalate stationary phase and particle sizes. Below ca. 7% (by wt.) of feed concentration, the experimental elution curves of pure DEE and DCM were almost the same as those of the mixture, and the theoretical plate model can be used successfully to predict the elution curves.     

Optimisation of chromatographic separations by use of a chromatographic response function, empirical modelling and multivariate analysis

Summary The chiral separation of the drug substance R,S-oxybutynin chloride on a reversed phase HPLC system has been optimised by use of empirical modelling and multivariate analysis. The separation was characterised by a new chromatographic response function developed to modulate both quality of separation and retention time. The study includes a comparison between three different multivariate techniques (multi-layer feed-for-ward neural networks, multiple linear regression and partial least squares regression) of their capabilities to model the new chromatographic response function and predict its value for new experiments. It was indicated that the most accurate models were achieved with neural networks, although partial least squares regression could also be used to solve the problem since it gives the major directions for the optimal settings of the variables.     

Possibilities and limitations of the sequential injection chromatography technique for the determination of anticoccidial agents in water, pharmaceutical formulations and feed

This paper explores the potential of applying reversed-phase sequential injection chromatography (SIC) to determine the anticoccidial agents Lasalocid and Toltrazuril in various matrices including ground water, pharmaceutical formulations and feed. SIC was performed by connecting a 25 × 4.6 mm monolithic C18 column to a 2 m long pathlength capillary flow cell, where the usage of a flow cell lowers the detection limit compared to a conventional short-distance flow cell, providing a simple detection system for these two compounds which are initially poorly UV absorbents.The proposed set-up provides a high injection throughput of 12 h^{− 1}, as well as a limit of detection of 0.019 and 0.010 mg/L for Toltrazuril and Lasalocid, respectively. The repeatabilities obtained (n = 10) were lower than 2% and 4% for Toltrazuril and Lasalocid, respectively.     

Molecular modelling studies and the chromatographic behaviour of oxiracetam and some closely related molecules

Summary Modelling studies have been carried out on the cellulose-based chiral stationary phase used to separate the enantiomers of three simple lactams. These studies have helped in understanding differences in the chromatographic behaviour of these molecules.