Modifications

Differential CC

The distortions of the correlogram caused by imperfect injection and nonlinearities are proportional to the concentration differences between the compounds in the sample and the eluent. CC can be used in a differential mode. The differences mentioned can be made much smaller by making the eluent almost equivalent to the sample by adding the (known) main components, present in the sample, to the eluent. Also, another sample - possibly modified for optimum separation conditions - can be used as eluent; only differences between the samples are measured, resulting in positive and negative peaks in the correlogram. This can be very useful in environmental analysis and trace analysis of samples with a relatively complex matrix.

Simultaneous CC (SCC)

In principle, it is possible to determine simultaneously n different samples on the same column, each injected according to its own mutual uncorrelated random pattern, using multiplex techniques. Completely uncorrelated (pseudo) random patterns in one sequence are impossible. However, the use of only one long PRBS for each sample, with a time shift equal to an integral number of chromatogram duration different for each sample, allows simultaneous CC. The correlation time is equal to n + 1 chromatogram lengths. This technique can be used for high precision chromatography. Unknown sample and calibration standards can be processed under exactly the same conditions, resulting in a very accurate calibration. The determination of pure compounds simultaneously with a complex sample gives the exact place and peak shape of these compounds in the correlogram. This can be used for optimization purposes, for optimum intensity estimation, and for resolving strongly overlapped peaks.

Single-Sequence CC (SSCC)

Single-sequence correlation chromatography is an intermediate between single-injection chromatography and CC. The injection volume is significantly enlarged in comparison to single-injection chromatog-

Figure 2 Chemical modulation correlation chromatography system.

raphy. However, the rectangular input function is modulated with a fine structure; one sequence of a PRBS. A deconvolution procedure allows the preservation of good peak resolution for fast-elution narrow peaks, while the other more broadened peaks are processed in a conventional way. In both cases the signal-to-noise ratio is improved. Gradient elution liquid chromatography (or programmed temperature GC) is still applicable, in contrast to conventional correlation and multiplex techniques.

Special attention has to be given to deconvolution errors due to the transient nature of the signals. Some noise frequencies may be amplified enormously if the number of points used for deconvolution is not optimally chosen.

Chemical Concentration Modulation CC

Chemical concentration modulators can be used in CC instead of mechanical injection valves. They can be considered as a chemical switch positioned at the head of the column. The eluent, which is continuously pumped through the system, contains the sample. In 'off' position of the switch nothing happens and a constant sample concentration is introduced into the column. In the 'on' position a chemical reaction takes place in the cell, causing certain components to react, yielding reaction products with different chemical structures and different retention and detection characteristics. The chemical reaction may add selectivity to the method. Nonreacting components will not influence the correlogram, because of the different properties of CC. Figure 2 shows a set-up of a chemical modulation CC system.

Several modulators in gas CC, both destructive and nondestructive, are known. Examples, particularly suited for trace analysis in air, are the hot-wire modulator, the spark modulator, and the thermal desorption modulator. In correlation LC, electrochemical concentration modulation has been tried successfully.

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