H.C. Smit

Sample introduction in correlation liquid chromatography Application, properties and working conditions for a novel injection system

Abstract To apply correlation liquid chromatography in practice, a reliable, accurate and simple injection system is essential. The injection system described is suitable. The system is tested thoroughly for different sample concentrations. The effects of possible non-idealities and incorrect working conditions are studied. The resulting disturbances are listed, so that inappropriate operating conditions can be traced and eliminated. To demonstrate the performance of the system, a standard mixture of polynuclear aromatic hydrocarbons is analysed and the results are compared with those obtained by conventional chromatography. The injection system and some special software greatly facilitates the practical use of correlation chromatography.

Origin and correction of bias caused by sample injection and detection in capillary zone electrophoresis

It is well known that electrokinetic sample introduction leads to a bias in CZE due to differences in the electrophoretic mobility of the substances. Therefore, a correction by multiplying the peak area with the migration time is often deemed necessary. With on-column detection, the area of a peak in the electropherogram depends on the velocity of the zone. In CZE every component has a different velocity, leading to a bias in detection. For that reason another correction by dividing the peak area by the migration time can be used. It will be shown that the injection and detection bias are inversely proportional to each other. This means that no correction of peak area is necessary for these biases in the case of electrokinetic injection being used in combination with UV detection, in contrast with hydrodynamic injection.

Selective determination of trace levels of phenol in river water using electrochemical concentration modulation correlation chromatography.

Electrochemical concentration modulation (ECM) was used as a sample introduction technique in the correlation chromatographic (CC) trace determination of phenol in water. The linearity and sensitivity of the method were tested and detection limits were calculated. The selectivity of the technique was confirmed by comparison with loop injection experiments using several detection methods. In preliminary experiments is was found that ECM-CC, in combination with fluorescence detection, is selective and sensitive enough to be used for the monitoring of the phenol concentration in river water at the draining points for drinking water production.

Characteristics and application of an electrochemical concentration modulator in correlation chromatography Part 1. Characterization

Abstract An electrochemical concentration modulator is tested for its applicability as an injection device in correlation chromatography. The use of conventional signals to control the potential of the working electrode in the modulator cell produces ghost peaks in the correlogram. The effects of the modulation interval, the time scale of the control signal and the concentration of an electroactive model compound (hydroquinone) on the pattern of the ghost-peak intensities are described. A simple model, based on the combined action of a charging-current time constant and the relative position of the half-wave potential in the modulation interval, is used to describe the result on the concentration profile of the analyte. The mechanisms responsible for ghost peak generation are described.

Improvement of Signal-to-Noise Ratio of Electropherograms and Analysis Reproducibility with Digital Signal Processing and Multiple Injections

Poor signal-to-noise ratios and analytical reproducibility in capillary electrophoresis were improved by application of multiple injections of the same sample at (pseudo) randomly chosen time moments. One order of signal-to-noise ratio improvement was achieved in practice using an in-house developed computer-controlled pneumatic sampler. However, it is demonstrated here that better implementation of the capabilities of multiple input requires that the detector signal noise be “pre-whitened”, i.e. spikes, baseline drift, oscillations etc. must be removed from the detector signal before transforming it to the electropherogram in the common form. Such filtering of the detector signal was realized by implementing iterative, menu-driven software written in Matlab, a high-level programming language.

Evaluation and correction of signal model errors in a matched filter for the quantification of chromatographic data

Abstract The effect of errors in the signal model in a matched filter for use in the quantification of chromatographic data and the influence of the noise type on this effect are evaluated. The errors are modelled as incorrect parameter values in an otherwise correct model, and their effect is monitored in the signal-to-noise ratio in the output of the matched filter. Normalization of the matched filter on the variance of the noise in the output makes the matched filter output a direct measure of this signal-to-noise ratio. The normalized matched filter output as a function of the signal model parameters is derived mathematically for a Gaussian signal and first order band-limited white noise. This function has a maximum for the correct signal model, suggesting the use of the maximum in the output of the normalized matched filter as the criterion in a Simplex optimization of the signal model as a means of correcting for signal model errors. Computer simulations confirm the feasibility of this approach. The signal-to-noise ratio improvement of the Simplex optimized matched filter as a function of signal-to-noise ratio in the input and the time constant of the noise is discussed, both on the basis of the mathematically derived formulae and the results of the computer simulations.

Different approaches for improving the precision in chromatographic analysis of environmental samples by optimum signal processing and correlation techniques

Abstract An overview is given of the applicability of optimum signal processing and correlation techniques in chromatographic environmental analysis. Improving the precision and (sub)trace analysis is emphasized. The principles and problems of different approaches, particularly in the field of matched filtering and correlation (multiplex) chromatographic techniques, mainly based on previously published work, is discussed and illustrated with examples.

Expert system for the determination of the analytical strategy in a laboratory for elemental analysis: Part 1. Project definition

Abstract In an analytical laboratory where the expertise and the instrumentation to apply a variety of techniques for the determination of elemental compositions are present, establishing the strategy that has to be followed in order to solve a specific analytical problem is a crucial activity. To determine which techniques should be used and which procedures should be followed as well as their order to execution, requires the synthesis of the possibilities and requirements of the different techniques with respect to the properties of the material to be investigated and the clients demand. As the first step towards an expert system for the analytical technical aspects of this task, the structure of the knowledge domain has been investigated. It is concluded that an overall system can be built incrementally by constructing subsystems for the different techniques and putting these together in a general framework. The structure of the knowledge is found to be the same for all techniques and the developmet of one subsystem is therefore expected to ease the development of following subsystems. The development of such a subsystem for x-ray fluorescence spectrometry, is described in Part 2.

Single-sequence correlation chromatography. A novel technique to decrease detection limits

Abstract A new method, being an intermediate between single-injection chromatography and conventional correlation chromatography, is described. With respect to single-injection chromatography, the injection volume can be significantly enlarged. However, the rectangular input function is modulated with a finestructure. A deconvolution procedure makes it possible to preserve a good peak resolution for the fast eluting narrow peaks, while other more broadened peaks can be treated in a conventional way. In both cases the signal-to-noise ratio is improved, and the detection limit is decreased. The theoretical background is presented, and the feasibility of the method is confirmed by simulation studies and experiments. Potentially, gradient elution liquid chromatography or programmed-temperature gas chromatography is still applicable, in contrast to conventional correlation and multiplex techniques.

Correlation and digital signal processing techniques for the reduction of detection limits of bromate and bromide in model water samples by ion chromatography with direct ultraviolet detection

Bromate is a well known by-product produced by the ozonation (disinfection) of drinking water, of which the concentration is considered to be regulated to a low μg/l level. Successful ion chromatographic methods of determination involve the implementation of concentration columns. As an alternative to concentration, it is demonstrated that the low μg/l level can easily be attained using a simple and foolproof experimental arrangement of direct pseudo random injections of large quantities of the sample and ultraviolet (UV) detection at 204 nm with subsequent decorrelation of the detector signal. The approach, known as correlation chromatography, makes high demands to the reproducibility and other properties of the injection system. Although correlation chromatography is less sensible to spikes and baseline drift compared with normal chromatography, optimum results will be achieved by applying digital detector processing prior to decorrelation, combined with a proper modification of the eluent. This is demonstrated here.