Fast-scan a.c. voltammetry for better resolution of chromatographically overlapping peaks

A fast-scan phase-sensitive a.c. voltammetric detector for high-performance liquid chromatography is described. The mercury drop electrode is periodically polarized over a certain potential range and only the maximum current value of each scan is plotted. The technique makes it possible to resolve chromatographically overlapping peaks provided that the voltammetric peak potentials of the corresponding compounds differ enough. Practical examples of application of the technique to nitro compounds are reported, and limitations are discussed.     

Artificial neural networks for quantification in unresolved capillary electrophoresis peaks

The application of the combination of experimental design (ED) and artificial neural networks (ANNs) for the quantification of overlapped peaks in capillary zone electrophoresis is described. When the total separation cannot be achieved by separation techniques, the use of ED-ANN can be a suitable approach. The unstability of EOF causes peak shift that has to be corrected in order to apply ED-ANN methods. In this work, normalization procedure of electropherograms with consequent application of ANNs for quantification purpose was developed. Both, spectra and electropherograms can be used as multivariate data. In general, both kinds of data were found to be suitable for unresolved peaks quantification by ED-ANN approach.     

Simultaneous quantitative analysis of isobars by tandem mass spectrometry from unresolved chromatographic peaks

A method was developed for the simultaneous quantitation of isobars from unresolved chromatographic peaks. The method is based on differences in branching ratios of ion abundances in their tandem mass spectra and an assumption that the product ion mass spectra of a mixture can be considered as a linear combination of the spectra of individual constituents. We present analytical equations and a matrix-based approach for deconvoluting the concentration of individual components from the total peak intensity for two and three isobars and also a matrix-based generalization to any number of compounds. The feasibility of the simultaneous analysis of mixtures containing two compounds was assessed. The approach was evaluated for the analysis of structural isomers of methylmalonic and succinic acids in human plasma and urine samples for a group of 270 samples. The linear regression equation, standard error and correlation coefficient for the agreement with a traditional method utilizing chromatographic separation of the isomers were y = 0.999x - 0.005, 0.024 micro mol l(-1), and 0.985, respectively. The utility of a spectral contrast angle as a predictor of analysis feasibility was evaluated.     

Single- and multi-channel detection for generalized quantitative analysis in cases of unresolved chromatographic peaks

Computerized quantification of components under overlapping chromatographic peaks is done by calibration of chromatograms against component mixtures. For conventional (single-channel) detectors, the limitations of earlier methods based on ordinary multiple regression, can be circumvented by data reduction with the aid of principal component analysis with the partial least-squares approach. Simulation studies show that the method can be applied even when there is severe peak overlap, unstable baseline, noisy chromatograms or non-linear detector response. Advantages in the quantification of fused peaks by means of multichannel detectors are outlined. Present limitations on the quantitative evaluation of several overlapping component peaks from a single spectro-chromatogram by means of the partial least-squares method combined with multiple regression on the pure component spectra, are discussed with respect to practical high-performance liquid chromatography.     

Determination of S and Cl in crude oil by XRF method for unresolved peaks

An X-ray spectrometric method independent of the matrix composition has been developed for the determination of S and Cl in the case of overlapping peaks. The principle of the methods is based on the observation that the sum of the peak areas from S and Cl is independent of their concentration ratios, only the different sections of the peak depend on these values. Using this method the sulphur and chlorine contents have been determined for crude oil samples collected from different geographical regions. This work was supported in part by the Hungarian Academy of Sciences.     

Computerized detection and evaluation of peaks in survey spectra from photoelectron spectroscopy

The computer program described is written in Algol-60 and allows a fast data analysis of photoelectron peaks in survey spectra (0–1500 eV). Additional peak parameters such as area, background and peak width are also determined. The assumptions about peak shape are kept to a minimum. In spectra with good counting statistics, it is possible to detect peaks with peak/background ratio down to 0.01.     

A study of the practical limitations of principal components analysis and the labelling of unresolved HPLC peaks

Summary The method of iterative target transformation factor analysis (ITTFA) used in conjunction with second derivative peak finding has shown to be a practical method for the peak deconvolution and reconstruction of HPLC chromatograms and spectra. The second derivative method of peak finding is acceptable for resolutions above 0.5 for peaks of similar heights. Above 0.5 resolution the labelling gives correct results where the spectra are substantially different and also when reasonably similar. Below this value the peak labelling was still accurate where the spectra were different. Solvent effects on the spectra of the compounds studied are small and do not hinder the peak labelling process. Thus small “local” libraries are feasible. Presented at the 17th International Symposium on Chromatography, September 25–30, 1988. Vienna, Austria.     

Criteria for judging the quality of separation in chromatograms containing solvent peaks

Summary Formal procedures used for optimizing chromatographic selectivity require objective, numerical functions to assess the quality of each chromatogram obtained during the process. Many of such optimization criteria have been suggested for chromatograms in which all or a number of well-shaped peaks need to be separated. This paper describes a method by which resolution can be measured in chromatograms in which very large, highly non-symmetrical background peaks,e.g. typical solvent peaks, are present. Using this method, various optimization criteria can be calculated. The method is evaluated using a simulation program which constructs chromatograms from the recorded profiles of individual experimental peaks. It is also demonstrated for use in an experimental optimization procedure in reversed-phase liquid chromatography.     

The separation of overlapping peaks in cyclic voltammetry by means of semi-differential transformation

A method for extracting single peaks from complex linear sweep and cyclic voltamperograms is presented. Voltamperograms are transformed by means of semidifferentiation, then all undesired peaks are removed from the semiderivative curve and replaced by calculated baselines. The resulting curve is semiintegrated back, giving a voltamperogram with one peak only. Baselines in the semiderivative domain are determined by the least-squares curve-fitting of datapoints from peak border regions, using the equation that describes the semiderivative peak of a reversible electrode process. With this procedure peaks can be removed without assumptions about the mechanism of the underlying electrode reaction. Due to its design, the algorithm presented is suitable for the fully automatic processing of cyclic and linear sweep voltamperograms. Performance of the procedure was checked with generated reversible voltamperograms as well as in real experiments with both reversible and irreversible systems. The smallest distance between two peaks of equal height, for which the described method can yield correct results, has been found to be 110 mV for a reversible one-electron process at 298 K. This procedure can also be applied to the elimination of the cathodic current from the cyclic voltamperogram of a single component in order to get a pure anodic current value, free from cathodic contribution, or vice versa.     

Quantitative chiral analysis in capillary electrophoresis from unresolved peaks using derivative electropherograms,experimental design,and artificial neural networks

Quantitative capillary electrophoretic analysis of chiral compounds might be difficult or even impossible when baseline separation is not reached. In this work, the use of n-th derivative of the electropherogram was studied and examined on model and experimental data. The electropherograms should be first smoothed using Savitzky-Golay method and the quantitative analysis is then possible using either a graphical method or multivariate calibration applying a combination of experimental design (ED) and artificial neural networks (ANNs). The best results were obtained for the first derivative, higher derivatives are not suitable because of noise accumulation. The method was applied to real experimental data to quantify chiral amino acids from unresolved peaks, but it is applicable for quantitative analysis of any other chiral analytes from poorly resolved peaks. Precision of analysis from partially resolved peaks reached was about +/- 3.2% relative standard deviation.     

Mass analysis of overlapping ion kinetic energy peaks arising from charge separation in dications

A triple-sector instrument of reversed geometry, BEQQ, has been employed to resolve overlapping ion kinetic energy peaks arising from charge separation of metastable dications. Separation was achieved through mass resolution of the dication in the magnetic sector and of the singly charged product ion of greater mass in the quadrupole mass filter accompanied by energy resolution with the electric sector; the electric sector was scanned to cover the complete range of each charge separation peak. Two overlapping ion kinetic energy peaks arising from charge separation of the diphenylenimine dication and up to four overlapping ion kinetic peaks arising from charge separations of dications arising from benzene-1,3,5-d₃ have been resolved. The kinetic energy releases have been calculated in each case. Enhanced z-discrimination is observed with the final stage of mass analysis.     

System peaks and optimization of anion separation in capillary electrophoresis with non-reversed electroosmotic flow

We studied system peaks present in the electropherograms obtained in the separation of anions by capillary electrophoresis with indirect spectrophotometric detection and cathode electroosmotic flow (EOF) with a chromate background electrolyte. The system peaks correspond to the zones with changed concentration of the background electrolyte; they formed when the zones of each analyte passed through the outlet of the capillary and then moved towards the EOF detector. It has been revealed that the height and area of the system peaks linearly depends on the concentration of the corresponding anion and the areas of the system peaks can achieve 10% of the anion peak area. An algorithm has been proposed for the determination of the optimal conditions for anion separation using hydrodynamic pressure for the regulation of the EOF flow rate. This algorithm prevents the overlapping of the anion and system peaks.     

Computerized design of separation strategies by reversed-phase liquid chromatography: development of DryLab software

The development of DryLab software is a special achievement in analytical HPLC which took place in the last 16 years. This paper tries to collect some of the historical mile stones and concepts. DryLab, being always subject to change according to the needs of the user, never stopped being developed. Under the influence of an ever changing science market, the DryLab development team had to consider not just scientific improvements, but also new technological achievements, such as the introduction of Windows 1.0 and 3.1, and later Windows NT and 2000. The recent availability of new 32-bit programming tools allowed calculations of chromatograms to be completed more quickly so as to show peak movements which result for example from slight changes in eluent pH. DryLab is a great success of interdisciplinary and intercontinental cooperation by many scientists.     

Modeling of analytical peaks: peaks properties and basic peak functions

The general approach to the detailed characterizing of peak properties based on the use of characteristic points on a peak contour located at the certain levels is proposed. Three basic functions of peaks have been chosen which are submitted in normalized form on height and width. The ability to characterize a peaks shape of a certain analytical series is shown by an example of series of analytical peaks of thallium obtained by stripping voltammetry at linear and step potential sweep. Also the proposed shape parameters are studied for groups of stripping voltammetry and chromatographic peaks.