The present disclosure relates to methodologies, systems and apparatus for controlling pressure in a CO.sub.2-based chromatography system. A first CO.sub.2 pump operates in constant flow mode and delivers CO.sub.2 to a chromatography column, and liquid modifier is introduced to the chromatography column according to a gradient. A second CO.sub.2 pump is disposed downstream of the column and operates in constant pressure mode to introduce CO.sub.2 into a flow stream at an output of the column. Liquid modifier is also introduced into the flow stream at the output of the column according to a reverse gradient compared to the gradient entering the chromatography column.

The invention relates to a method and an apparatus for determining a chromatogram. The method includes a first step where a sample is inserted in two separation columns (2.1, 2.2, 2.3), wherein for each separation column (2.1, 2.2, 2.3), a corresponding part of the sample is inserted in the respective separation column (2.1, 2.2, 2.3) with a corresponding insertion device (3.1, 3.2, 3.3) which is controlled by a corresponding modulation function for generating a corresponding modulated part of the sample in the respective separation column (2.1, 2.2, 2.3), wherein the modulation functions with which the parts of the sample are modulated in the separation columns (2.1, 2.2, 2.3) differ from each other. Furthermore, the method includes a second step where each modulated part of the sample is guided through the respective separation column (2.1, 2.2, 2.3), a third step where a signal of each modulated part of the sample is measured with a same detector (4) after having passed the respective separation column (2.1, 2.2, 2.3), and a fourth step where for each separation column (2.1, 2.2, 2.3), a correlation of the signal and the modulation function with which the corresponding part of the sample is modulated in the respective separation column (2.1, 2.2, 2.3) is calculated in order to determine the chromatogram of the respective separation column (2.1, 2.2, 2.3).

A method and system for continuous measurement chromatograph involves stochastically modulating a system variable. The sample can be introduced into a chromatography column. The sample introduction can be modulated stochastically. The sample output from the column can be detected and processed with the stochastic input to provide a sample analysis.

A gas chromatograph for detecting a component of an analytical mixture is proposed, including: a first chromatographic-detection column; a peak-modulator, which is coupled upstream to the first chromatographic-detection column and configured to be provided with the analytical mixture; and a first chromatograph-detector, which is coupled downstream to the first chromatographic-detection column, for detection of the component of the analytical mixture.