Monolithic silica columns for high-efficiency chromatographic separations

A deconvolution methodology for overlapped chromatographic signals is proposed. Several single-wavelength chromatograms of binary mixtures, obtained in different runs at diverse concentration ratios of the individual components, were simultaneously processed (multi-batch approach), after being arranged as two-way data. The chromatograms were modelled as linear combinations of forced peak profiles according to a polynomially modified Gaussian equation. The fitting was performed with a previously reported hybrid genetic algorithm with local search, leaving all model parameters free. The approach yielded more accurate solutions than those found when each experimental chromatogram was fitted independently to the peak model (single-batch approach). The improvement was especially significant for those chromatograms where the peaks were severely affected by the tails of the preceding compounds. Peak shifts among chromatograms, which are a usual source of non-bilinearity, were modelled in a continuous domain instead of in a discrete way, which avoided some drawbacks associated with latent variable methods. An experimental design involving simulated chromatograms was applied to check the method performance. Five main factors affecting the deconvolution were examined: concentration pattern, chromatographic resolution, number of batches and replicates, and noise level, which were evaluated using first- and second-order figures of merit. The method was also tested on three real samples containing compounds showing different overlap. Four multi-batch deconvolution methods were considered differing in the nature of the processed information and kind of peak matching among chromatograms. In all cases, the multi-batch deconvolution yielded better performance than the single-batch approach.     

Gel-permeation chromatography. V. A study of “overloading effects”

In the calculation of molecular weight averages from GPC chromatograms and in the correction for zone broadening effects, it is assumed that the retention volumes of the individual species are not affected by the presence of the other components in the sample (linear fractionation process). This assumption was tested by the computer comparisons of the chromatograms of mixtures of narrow molecular weight distribution polystyrene standards and the envelopes of the individual chromatograms of the components of the mixtures. Disagreements between experimental and computer synthesized chromatograms were interpreted as non-linear fractionation effects. The investigation covered a range of molecular weights (1.8 × 10⁶ to 2100), molecular weight distributions (binary and seven component mixtures), and sample loadings (2 to 70 mg). Accurate normalizations of raw GPC chromatograms and calculations of molecular weight averages were made possible through an automated GPC apparatus using a high-speed computer for data acquisition and data reduction. It was found that some nonlinear fractionation effects (large disagreement between chromatograms of the mixture and the computed envelopes of the individual chromatograms of the components of the mixture) became more pronounced as sample loading was increased and as the molecular weight distribution of the sample narrows. The nonlinear effect was also found to be sensitive to the molecular weight of the sample.     

凝胶渗透色谱法峰加宽改正问题——Ⅱ.一种有效的解Tung方程的迭代法

本文运用一种病态线性方程组的迭代法解出 Tung方程。此法对定h及变h的 Gauss 核及 Gram-Charlier 核都是有效的,对窄分布样品的峰加宽改正也很适合。并与 Ishige 及 Smit等提出的迭代法作了比较。最后讨论了求积公式、归一化、对称化和GPC实验谱图的峰加宽改正问题。     

Application of headspace solid phase microextraction and gas chromatography to the screening of volatile compounds from some Brazilian aromatic plants

Summary An HS-SPME method was developed and applied for the isolation of volatile organic compounds from plants native or acclimatized to Brazil. Method optimization was performed using typical analytes from the target samples; fibers coated with 100 μm PDMS and 75 μm Carboxen/PDMS were tested. Using PDMS 100 μm fibers and GC-MS for separation and identification, up to 99.9% of the peak area in the chromatograms from plants were identified. The method was also applied to quantify the major volatile components of one of the samples (Aloysia gratissima) with results comparable to those from the conventional steam distillation method.     

Liquid adsorption chromatography of polyethers: experiments and simulation

The adsorption behavior of poly(ethylene glycol) (PEG) in reversed-phase chromatography is studied both experimentally and theoretically and a computer simulation of chromatograms is performed on the basis of these studies. The experimental conditions were: different reversed-phase adsorbents and a solvent methanol–water system as the mobile phase. At varying mobile phase compositions highly resolved chromatograms of PEG samples were obtained, in which all peaks could be identified, and the dependencies of the distribution coefficient on the degree of polymerization for PEG molecules were evaluated by processing these chromatograms. The data were interpreted by using a theory of homopolymers based on a continuum Gaussian chain model of flexible macromolecules and a slit-like model of pores of stationary phase. The theory proved to describe well the experimental data in the whole range of studied molecular masses, and the thermodynamic parameters characterizing interactions of ethylene oxide repeating units in PEG molecules with the adsorbent pore walls have been determined from the comparison of the theory with the experimental data. The dispersion of chromatographic peaks corresponding to individual oligomer molecules is also estimated. In the system studied the peak width occurred to be proportional to the distribution coefficient of corresponding macromolecule. The theory is used to develop a computer-assisted procedure for simulation of chromatograms for samples of linear homopolymers. Using the obtained data on the thermodynamic parameters and the estimates of peak dispersion, chromatograms are simulated for PEG samples at two different chromatographic conditions. These simulated chromatograms were in good quantitative agreement with the real chromatograms.     

Improving detection power in trace analysis using wavelet transform

Environmental analysis most often is trace analysis. Therefore, the concentrations are commonly in the lower working range near the limit of detection of the corresponding analytical method. However, whenever the instrument's analytical noise is too large, it dominates the signal curves and analytes cannot be detected anymore. Furthermore, the evaluation of peaks with defined baselines is hindered very much. One possibility for de-noising is wavelet transform which is presented in this work. Different wavelet functions are applied and Symlet4 is suggested as the most powerful for analytical peaks that resemble Gaussian distribution curves, as it improves limits of detection by factors 6 to 7. The comparison of different wavelet functions has been carried out for two modern analytical scopes. At first, chromatograms are de-noised for the speciation of four arsenic compounds via the coupling of HPLC and ICP-MS. Secondly, the determination of cadmium is shown by HR-CS AAS, which is one of the most recently developed devices in atomic absorption spectrometry and allows the registration of three-dimensional spectra in order to investigate the spectral vicinity of analytical lines. On the basis of these investigations, we recommend using wavelet transform with Symlet4 for all analytical techniques which are resulting in similar signal curves.     

Use of Gel Permeation Chromatography in the Crime Laboratory

Abstract

In this study, the ways in which GPC can be integrated into the crime laboratory's present analytical scheme for polymer products will be examined. Fingerprint chromatograms and chromatograms illustrating molecular weight determination of two types of commonly encountered physical evidence (fibers and tail light lense fragments) will be compared and discussed. The techniques involved in sample preparation and data interpretation will be given. Finally the potential advantages of GPC for use in the crime laboratory will be presented.     

Chromatogram simulation by area reproduction

A modified Poisson function has been developed for the simulation of chromatographic peaks. The proposed model is shown to have the property of exactly recreating the experimentally determined peak area. Model parameters are obtained directly from the experimental peak, and overlapping peaks are deconvoluted such that the area sum of overlapping peaks is kept unchanged. The method was applied to real, complex chromatograms.     

Application of Computer-Aided Spectral Deconvolution Technique in Validation of High Performance Liquid Chromatographic Peaks

Abstract

Reliable analysis with high performance liquid chromatography (HPLC) requires purity of the eluting peak. The present work has combined the advantages of the availability of full spectral data from HPLC photodiode array UV detector and computer algorithms to perform chromatographic peak purity check. A deconvolution technique based on multicomponent analysis has been applied to the UV spectra of co-eluting components. This method employs residual error (Relative-fit-error, RFE) between predicted spectrum and analyte's spectrum to detect presence of other component or contaminant. Typical RFE values for uncontaminated chromatographic peaks of norethisterone and ethynyloestradiol range between 1 and 3, while contaminated peaks have RFE values as large as 145. A systematic increase in ‘relative-fit error’ from 1.10 to 145 was observed for peaks of norethisterone when contaminated to varied extent with ethynyloestradiol. Extent of peak overlap in chromatogram was also mapped out with this technique. The co-prescribed oral contraceptive, norethisterone and ethynyloestradiol were used as model in this work. An advantage of the method is its applicability when the contaminant's spectrum is unavailable. The method, unlike several earlier techniques, is also applicable to chromatograms with concidental elution time for the components.     

Effect of glycols used as glycolysis agents on chemical structure and thermal stability of the produced glycolysates

In this study, the influence of glycols on chemical structure and thermal stability of glycolysates as polyurethane intermediates were investigated. The intermediates were obtained by the glycolysis process of waste polyurethane foams in the reaction with different glycols ranging from ethylene glycol to hexane-1,6-diol. The used glycols were not separated from the product after the glycolysis process has been terminated. The effects of different weight ratio of glycols to polyurethane (PU) foam on chemical structure and thermal stability were investigated by FTIR, GPC, and TG/DTG. FTIR analysis of the glycolysates revealed their similar chemical architecture as manifested by the similarity of absorption peaks within the entire wavenumber range of spectra. This may indicate that the glycol has no influence on the chemical composition of glycolysates. GPC analysis showed that the glycolysates were characterized by polydispersity smaller than 2 which is lower as compared to some commercial polyols used for PU synthesis. GPC chromatograms showed that the applied glycols and the conditions of PU glycolysis allowed recreation of the original polyol as documented on the chromatograms by a single, well-formed peak at the beginning of retention time. Based on TG thermograms, it was established that glycol used in transesterification of PUs affected the temperature at which the loss of glycolysate mass by 5 and 10?% occurs. It was also observed that glycol affected the temperature at which the decomposition rate of glycolysates was the highest.     

An algorithm for overlapped chromatogram separation

Non-negative matrix factorization (NMF) is a recently developed method for real time data analysis. In the past it has been used for facial recognition and spectral data analysis. Most of the NMF algorithms do not converge to a stable limit point and uniqueness in results is also a problem in NMF. To improve the convergence, a new NMF algorithm with modified multiplicative update (ML-NMFmse) has been proposed in this work for strongly overlapped and embedded chromatograms separation. To get same results for all the runs, instead of random initialization, three different initialization methods have been used namely, ALS–NMF (robust initialization), NNDSVD based initialization and EFA based initializations. The proposed ML-NMFmse algorithm is applied on the simulated and experimental overlapped chromatograms obtained for acetone and acrolein mixture, using Gas Chromatography–Flame Ionization Detector. Before applying NMF, Principal Component Analysis (PCA) was applied to determine number of components in the mixture taken. The result of proposed ML-NMFmse is compared with that of existing Multivariate Curve Resolution-Alternating Least Squares method in optimal conditions for both the algorithms. In the case of embedded chromatogram, the proposed ML-NMFmse with Robust method (ALS-NMF) of initialization performs better than all other methods. For a resolution of severely overlapped chromatograms, the proposed ML-NMFmse with NNDSVD method of initialization outperforms all other methods.     

Effects of lithium bromide on the gel-permeation chromatography of polyester-based polyurethanes in dimethylformamide

The hydrodynamic volume changes of the polyester-based polyurethanes, as dimethylformamide solutions with and without lithium bromide, were studied. Gel-permeation chromatographic (GPC) and intrinsic viscosity techniques were employed as methods for characterization. Decreases in intrinsic viscosity as well as dramatic changes in the GPC chromatograms were observed. These observations are explained by the polarity of these polyester-based polyurethane molecules and the resulting characteristics of molecular expansion, association, and polymer–column interaction normally associated with polyelectrolytes.     

Calibration of chromatographic systems for quantitative prediction of chromatography of homopolymers

A method is proposed for obtaining from the experimental data the main parameters, which determine the chromatographic behavior of homopolymers: a dependence of the adsorption interaction parameter on the mobile phase composition, and the pore volume and pore size values. The interaction parameter can be determined by this method in both adsorption, critical, and SEC-type regimes of chromatography. The method is tested in ‘mathematical experiments’ with theoretical data that mimic experiments, and is applied to calibrate real chromatographic systems. With the obtained calibrations quantitative modeling of chromatograms of PEG samples on two typical ODS columns with different pore size in methanol/water and acetone/water mixed solvents is performed; the simulation results are compared with real chromatograms.     

On the Use of the Knox Equation. II. the Efficiency Measurement Problem

Abstract

The Knox equation, which relates the reduced plate height, h, to the reduced linear velocity, v, has a great importance in column and stationary phase testing. Significant A, B, and C terms of the Knox equation cannot be obtain with erroneous plate heights, h. Four methods for efficiency determination, the inflection (0.6H) method, the height/area method, an asymmetry based (0.1H) method and the moment method, were critically compared using 200 real chromatograms. Efficiency obtained using Gaussian assuming methods were highly overestimated, while the asymmetry based method gave quite acceptable results. Using the exponentially modified Gaussian model, it is demonstrated that the b/a ratio (the sum b + a is the peak-width at 10% of the peak height) is not an empirical figure of merit but can be related to a “peak skew” expression.     

Concentration and clean-up of trace hydrocarbons from atmospheric particulate matter

Summary The aim of this work is to establish the best conditions for concentration and purification steps in the trace analysis of aliphatic and polycyclic aromatic hydrocarbons (PAH) from atmospheric particulate matter by gas chromatography-flame ionisation detection (GC-FID) and high performance liquid chromatography with ultraviolet and fluorescence detection (HPLC-UV-FL). The best results for the more volatile compound were obtained with a combination of rotary evaporation and a stream of nitrogen (near to 100% for aliphatic hydrocarbons and from 70 to 105% for PAH). Two types of solid phase extraction (SPE)cartridges (Supelclean ^tm LC-Silica SPE tubes and Sep-Pak^? Plus silica cartridges) and glass column were examined for the purification and fractionation step. Blank chromatograms of both types of cartridges analysed by GC-FID made this study difficult, because a PSS (programmed split-splitless) injector was employed thereby increasing the sensitivity. This problem was not observed in the HPLC-UV-FL blank chromatograms of these cartridges. Glass columns filled with silica and alumina were chosen because no interference was found in the GC-FID blank chromatograms and the best recoveries in the fractionation of both aliphatic hydrocarbons and PAH were achieved. This is especially important when aliphatic hydrocarbons concentrations are lower than 1 μg mL⁻¹. Finally, the selected conditions were applied to the analysis of hydrocarbons in real atmospheric particulate samples.     

Resolution of Overlapping Chromatographic Peaks Using a Genetic Algorithm

ABSTRACT

A genetic algorithm for resolution of overlapping chromatographic peaks (GAROCP) using real-number coding, non-uniform mutation and arithmetical crossover methods is described in this paper. It was applied to resolution of highly overlapped multicomponent high-performance liquid chromatographic peaks by fitting experimental chromatogram to the exponentially modified Gaussian (EMG) model. The genetic algorithm was used to find the minimum of fitting error to optimize the parameters in the EMG functions which determine the shape and area of each peak. The applicability of the method was investigated with both simulated signals calculated by EMG functions and experimental multicomponent overlapping chromatograms.     

以六氟异丙醇为溶剂的聚甲醛凝胶渗透色谱研究

研究了均聚和共聚甲醛在六氟异内醇中的凝胶渗透色谱（ＧＰＣ）。对共聚甲醛出现的缔合现象从主链结构作了解释，并找到了避免缔合的方法；对共聚甲醛ＧＰＣ淋洗曲线上的异常小峰从聚合机理角度进行了探讨，推测得到小峰所对应的物质。确定了以六氟异丙醇为溶剂的聚甲醛的凝胶色谱表征方法，并探索采用凝胶色谱－粘度计联用法得到分子量和分子量分布结果。     

Determination of theoretical plates in gel permeation chromatography by using polydisperse materials (polymers)

The need for an improved method of expressing performance of GPC columns and operating conditions is discussed. It is shown that theoretical plates can be calculated from chromatograms of high molecular weight materials (polymers) if the curve widths are normalized for polydispersity. The resulting value N, where N = 16d²(v/W)², is a useful measure of the real plate count afforded by the columns to polymers and is useful for comparison purposes. Since N does not provide absolute data on separation, it is better to use formulas for resolution and fractionation when such data are needed.     

Towards obtaining more information from gas chromatography–mass spectrometric data of essential oils: An overview of mean field independent component analysis

Mean field independent component analysis (MF-ICA) along with other chemometric techniques was proposed for obtaining more information from multi-component gas chromatographic–mass spectrometric (GC–MS) signals of essential oils (mandarin and lemon as examples). Using these techniques, some fundamental problems during the GC–MS analysis of essential oils such as varying baseline, presence of different types of noise and co-elution have been solved. The parameters affecting MF-ICA algorithm were screened using a 2⁵ factorial design. The optimum conditions for MF-ICA algorithm were followed by deconvolution of complex GC–MS peak clusters. The number of independent components (ICs) (chemical constituents) in each peak cluster was estimated using morphological score method. Eigenvalue profiles of evolving factor analysis (EFA) and pure variables from orthogonal projection approach (OPA) were used as initial mixing matrix (chromatograms) in iterative process. The resolved mass spectra were satisfactorily identified using NIST mass spectral search system. Finally, the results of optimized MF-ICA were compared with those obtained using multivariate curve resolution-alternating least square (MCR-ALS), multivariate curve resolution-objective function minimization (MCR-FMIN) and heuristic evolving latent projection (HELP) methods. It is demonstrated that MF-ICA can be used as an alternative method for a quick and accurate analysis of real multi-component problematic systems such as essential oils.     

Optimization of GPC Experiments as an Effective Means of Significantly Enhancing the Resolution of Multicolumn Sets to be Used for Analyzing Specific Polymer Systems

Abstract

Problems concerning GPC band spreading effects and their elimination were discussed, reference being also made to particular requirements to be met in the case of copolymer studies. GPC analysis optimization experiments were performed in order to minimize instrumental spreading for a multicolumn set operated in carbon tetrachloride. Results of experiments carried out at mobile phase flow rates of 1.0, 0.3 and 0.1 cm³/min for narrow MWD solutes were used to compute the values of the degree of polydispersity and other MWD obtainable parameters determining the extent of spreading minimization attained. The GPC data generated for runs conducted at the lowest flow rate employed were shown to have been rendered sufficiently accurate for the raw chromatograms to represent the polymers being analyzed without the need of performing mathematical corrections for imperfect GPC resolution.