Liquid chromatography combines the Separation principles of liquid-liquid extraction and chromatography. As in extraction, a biphasic liquid system is used and the basis for the separation is that the differing partitioning behaviour of the mix solutes between the two phases. As in chromatography, one of the two phases involved in the separation is kept stationary. Despite these similarities, several substantial differences put LLC apart from traditional chromatography with strong stationary phases. In LLC the user adopts both stages, the mobile phase and the stationary phase, by mixing two or more solvents that form biphasic system. The liquid stationary phase is held in position during operation by application of centrifugal force and typically occupies 60-80% of the column volume. High sample loading is possible because the whole quantity of the liquid stationary phase is available to the solutes.
Irreversible adsorption cannot happen, allowing for complete sample retrieval. Since its early years, the isolation and Purification of organic products has been the top area of LLC software. The flexibility of hplc testing is clearly demonstrated by the huge assortment of printed applications, such as separation of synthetic drugs, herbicides, pesticides, vitamins, amino acids, peptides, proteins, dyes and inorganic components. In LLC, the column is a specially-designed housing mounted on the axis of a centrifuge. This assembly is normally known as a system and replaces the classical cylindrical column used in HPLC. Commercially-available machines can be grouped into two types based on their structure and the resulting centrifugal area: hydrodynamic and hydrostatic. In hydrodynamic machines, a constant piece of Hollow tube wound around a bobbin creates the column.
The bobbin rotates around its own axis while concurrently revolving around the axis of the centrifuge leading to a varying rotational force throughout the length of the tube. Mixing and settling zones are alternatingly dispersed along the whole length of the column. LLC performed with hydrodynamic machines is typically known as Counter Current Chromatography. In hydrostatic machines, the column is made of Several identical metallic disks laid one over the other with annular Teflon plates between. Each disc has a series of circumferentially engraved cells, which are interconnected by ducts fig. 2c. The last cell of a single disc is connected to the first cell of the next by openings in the Teflon plates. The column is set on a rotor of a centrifuge and has one axis of rotation, leading to a continuous rotational force profile.
