In liquid chromatography liquid mobile phase flows through the column with stationary phase. The main principle of separation remains the same.

Compounds have different affinity to the stationary phase and are separated while flowing through the column. The compounds separated with liquid chromatography are disolved in the mobile phase. They have lower difussion coefficients than gaseous compound separeted with gas chromatography.

Except thin layer chromatography, majority of liquid chromatography is performed in high-pressure arrangement. The liquid is pushed through the column using high pressure pumps. In this case we are speaking about high pressure liquid chromatography (HPLC).

According the polarity of the mobile and/or stationary phase, the liquid chromatography separations can be divided in numerous methods for different kind of samples.

The gas chromatography is special type of chromatography, where the mobile phase is gas, such as helium or nitrogen. The stationary phase is usually solid support covered with liquid layer.

After the sample injection the mobile phase carries the sample compounds through the column. Usually, the temperature gradient is applied and compounds are then separated according theirs boiling points.

Gas chromatography instrumentation

The figure shows typical scheme of gas chromatograph. The gas (mobile phase) flows through the column placed in the oven with controlled temperature. After the separation is finished the individual compounds elute from the column and specific detector registers signal.

Applications

Gas chromatography is very useful for the analysis of small volatile compounds with boiling points lower than 300 °C. Gas chromatography is applied in chemistry industry (especially petrochemistry) to control the quality chemical products. GC can be also used for analysis of toxic compounds, environmental analysis and so on.

Chromatography is analytical chemistry method which is used (and useful) for the separation of complex mixtures of chemical compounds. The main mechanism of the separation is repeatable distribution of the tested compound in between two different phases.

Usually, one phase is solid, fixed in the separation device and the other is moving and flows through the unit. If gas is a second phase, we are referring to the gas chromatography, in case of liquid as a second phase the name is liquid chromatography.

The device where separation takes place is called chromatographic column. This cylindrical shape column is filled with the different kinds of materials – stationary phases. These materials are usually spherical silica particles with different, but well defined, surface chemistry.

The mobile phase flows through the column together with sample (mixture of compounds). Each compound has various affinity to the surface of stationary phase and therefore is separated form each other. In case of ideal state all compounds are eluted from the column in separated bands.

Various techniques are used to recognize these bands and transform them into the signal. In most of the cases the signal draws chromatographic peak – the “hill like” curve describing Gauss distribution.