Correlation Chromatography Research Articles

Separation, optimization, miniaturization and signal space. Optimum measurability factors in process analytical chromatography

AbstractThe theory concerning optimum separation conditions with respect to the precision and minimum peak estimation error is extended with the derivation of the conditions for optimum measurability in case of chemical process control, resulting in extremely short columns to be used. High‐speed correlation chromatography (CC) with a special input device is suggested as a solution for the practical problems of these required columns. An example of a one‐second correlogram with a spark modulation input device is shown. An alternative is monitoring chemical processes by multiple input chromatography, applying a polynomial fit procedure. Copyright © 2009 John Wiley & Sons, Ltd.

Monitoring of randomly varying chemical processes by correlation chromatography

Correlation chromatography (CC) is a well known method for reducing detection limits in chromatographic analysis. Its usefulness for the monitoring of varying substance flows is however more ambiguous. Although generally believed as a useful tool for this purpose, very little work has been done to prove it. In this paper CC and common, single injection chromatography (SIC) were compared with respect to their restoration capabilities of a stochastic first order process, which is assumed to be the varying concentration input function of a chromatograph. It is demonstrated that CC can restore the input function with a smaller error than SIC when the process rate and input sequence length are related by a certain condition. The effect was found to be more profound when measurement noise is present due to the multiplex advantage of the CC method.

Monitoring time-varying concentrations in sample streams by multiple input chromatography

The possibility of monitoring time-varying concentrations with conventional chromatography or correlation chromatography (CC) is restricted. The highers frequency that can be monitored is, due to Shannon, limited for conventional chromatography to (2 x chromatogram length) −1 , and for CC, due to a moving average effect, to (sequence lenght) − . A method is presented in which considerable higher frequencies can be monitored. Theoretically the power spectral density of the chromatographical peak shape limits the upper frequency that can be monitored. The method originates from CC. A pseudo-random binary sequence (PRBS) is used as injection pattern. The detector signal however is not correlated or deconvoluted with a PRBS as in CC. The time-varying concentrations of the components are described as functions of time and a number of parameters, e.g. parameters of a Fourier series. These parameters then are optimised in a non-linear fitting procedure. A detector signal is calculated using the parameters, the known PRBS and known peak shapes; and the squared differences with the real detector signal are minimised. The results from simulated experiments and practical experiments are shown.

Monitoring time-varying concentrations in sample streams by multiple input chromatography

The possibility of monitoring time-varying concentrations with conventional chromatography or correlation chromatography (CC) is restricted. The highers frequency that can be monitored is, due to Shannon, limited for conventional chromatography to (2 x chromatogram length) −1, and for CC, due to a moving average effect, to (sequence lenght) −. A method is presented in which considerable higher frequencies can be monitored. Theoretically the power spectral density of the chromatographical peak shape limits the upper frequency that can be monitored. The method originates from CC. A pseudo-random binary sequence (PRBS) is used as injection pattern. The detector signal however is not correlated or deconvoluted with a PRBS as in CC. The time-varying concentrations of the components are described as functions of time and a number of parameters, e.g. parameters of a Fourier series. These parameters then are optimised in a non-linear fitting procedure. A detector signal is calculated using the parameters, the known PRBS and known peak shapes; and the squared differences with the real detector signal are minimised. The results from simulated experiments and practical experiments are shown.

Feedback shift register sequences versus uniformly distributed random sequences for correlation chromatography

Two alternative input sequences are commonly employed in correlation chromatography (CC). They are sequences derived according to the algorithm of the feedback shift register (i.e., pseudo random binary sequences (PRBS)) and sequences derived by using the uniform random binary sequences (URBS). These two sequences are compared. By applying the “cleaning” data processing technique to the correlograms that result from these sequences, we show that when the PRBS is used the S/ N of the correlogram is much higher than the one resulting from using URBS.

Comparison of sample concentration methods and correlation chromatography for polymer dynamic head-space studies

Gas chromatography has been used to study the kinetics of evolution of volatiles from polypropylene samples (at 70°C). Correlation chromatography (CC) and the trapping of volatiles on Tenax sorbents and activated charcoal were employed to improve the detector signal. As a reference, the kinetics of gas evolution was studied by making a series of direct single injections of polymer head-space gas on a GC column. The results of trapping differed from those obtained by single injection. However, the data from single injections and CC were in good agreement. The CC method was found to offer far more operator convenience for dynamic head-space analysis than sample trapping, being less subject to operator errors.

Improvement of the performance of single-column ion chromatography by computerized detector signal processing and correlation chromatography

The performance of medium-quality ion chromatographic equipment was improved by using pseudo-random sample intoduction correlation chromatography (CC). The importance of the digital preprocessing of the detector signal prior to decorrelation was demonstrated for realization of the multiplex advantage of the CC method. The detection limits obtained for single-column ion chromatography using conductivity were 0.1, 0.12, and 0.17 ppm for Cl −, NO 3 − and SO 4 2− ions, respectively. Using the same apparatus without CC the detection limits are in the range 1–3 ppm.

Characteristics of a hot-wire thermal decomposition modulator for correlation gas chromatography

A chemical modulation device for correlation chromatography (CC) is proposed and tested. The modulation process is a chemical reaction on the surface of a hot metal wire in an air atmosphere at a slightly elevated pressure. The influence of several experimental parameters on the modulation efficiency is discussed and modulation efficiency curves are presented for some organic components and a few modulator configurations. Coiled-wire modulators are found to be preferable to straight-wire types. A 100% modulation efficiency can be accomplished resulting in the normal correlation chromatographic gain in signal-to-noise ratio. By using carefully chosen operating parameters the modulator can more or less selectively modulate certain categories of compounds. The calculated correlograms contain ghost peaks. The influence of experimental parameters on the amplitude and sign of the ghost peaks is investigated. A well known correction method is applied and is found to work satisfactorily. A linear response curve was found for ethane—air mixtures. A duty-cycle injection control signal was used to obtain ghost peak-free correlograms. The use of this non-mechanical injection device is limited by the lifetime of the wire.