Chromatography is also known component analysis. The chromatography applied in mud logging belongs to gas chromatography category. It is a analysis method using inert gas as the mobile phase (gas-solid chromatography) or a thin layer of high-boiling point organic liquid (gas-liquid chromatography) coated on the surface of inert solid material as the immobile phase for the separation of gaseous samples. The tubular container of certain length filled with immobile phase is called as chromatography column. When the mobile phase contains the separated gas sample goes through the column, the separated sample is subjected to several adsorption (gas-solid chromatography) or dissolving (gas-liquid chromatography). Owing to the difference of the absorption capacity of each component in the samples and the partition coefficient in the two phases, the components in the column have different flow conditions. If the column has a sufficient length, the component with the worst absorption or dissolving capacity will be first brought out by the carrier gas and the other components can further also be individually separated. The quality of the separation depends on various factors including column length, temperature and the type of the solid phase and carrier gas flow rate and so on.

Principle
The principle is based on the different partitions of chemical substances in two media (or other different behaviors such as adsorption, penetration, etc.) for the detection. One media belongs to solid phase, and may be solid or liquid; another is mobile phase and can be either a kind of liquid or a kind of gas. The substance to be separated can move along with the mobile phase while undergoing repeatedly multi-partitions in both phases. Different materials, due to their different partition coefficients in the two phases can move at different speeds, and finally separated from each other. This analysis method combing the separation and detection of various components is the so called chromatography analysis.

Brief History
chromatography was first proposed in 1903 by Russian botanist M. Tswett, he pour the petroleum ether extract of the plant pigment into a vertical glass tube containing calcium carbonate adsorbent (immobilize phase), and then further add3ed petroleum ether (mobile phase) to allow them to flow freely, resulting in the formation of bands of different colors, namely the separation of the different dyes of the solution in the tube, hence getting the term "chromatography,". Later, this method had been gradually used for separating colorless substance, but the term of "chromatography" has been inherited.

Classification
chromatography, from a different point of view, there are usually several kinds of classifications as follows: ① According to the status of the mobile phase, it can be classified into gas chromatography and liquid chromatography. Consider that the immobile phase also have two states, namely solid adsorbent and immobile phase with its solid carrier containing liquid; it can be further divided into gas-solid chromatography, gas liquid chromatography, liquid chromatography and liquid-solid liquid chromatography. According to this classification method, paper chromatography, thin layer chromatography, ion exchange chromatography, gel permeation chromatography can all be classified as liquid chromatography. ② According to the nature of the immobile phase, it can be further divided into column chromatography, paper chromatography and thin layer chromatography. Column chromatography can be divided into two categories: one has its immobile phase be mounted in a metal tube or in a glass tube called as chromatography column; the other has the immobile phase attached to the inner wall with the center being empty, called as capillary chromatography. ③ According to the basic principle of the separation process can be divided into adsorption chromatography, partition chromatography, ion exchange chromatography and exclusion chromatography. Adsorption chromatography take active adsorbent as the immobile phase, taking advantage of the different physical absorbing capability of the adsorbent surface on the different components for separation; partition chromatography takes advantage of the different partition coefficients of different components in the two phases; ion exchange chromatography takes ion exchange resin as immobile phase, taking advantage of the different ion exchange capacities of the different components for separation; exclusion chromatography takes molecular sieve gel as the immobile phase, taking advantage of the different molecular sizes of different components  for separation. Exclusion chromatography is also known as gel permeation chromatography. In addition, there are also complexing chromatography, electric chromatography, and heating chromatography.

Features
chromatography is characterized by high separation efficiency, excellent selectivity, high sensitivity, rapid speed and small sample size and other characteristics. We can choose different chromatographic techniques according to the nature and analysis requirement for the substance to be analyzed, and thereby having been widely used in agriculture and various other fields. The emergence of high-performance liquid chromatography (HPLC), ion chromatography and other new technologies, and the conjunction of chromatographic analysis with other methods such as chromatography - mass spectrometry, chromatography – spectroscopy make the application range of chromatography expanding. Chromatography, in addition of being used for qualitative and quantitative analysis, can also used for the preparation of high purity organic reagent and sample purification.