Liquid chromatography is a technique used to separate a sample into its individual parts. This separation occurs based on the interactions of the sample with the mobile and stationary phases. Given there are several stationary/mobile phase combinations that can be used when separating a mixture, there are several different types of chromatography named for the physical states of those phases. Liquid-solid column chromatography features a liquid mobile phase which slowly filters down through the solid stationary phase, bringing the separated components with it. It’s the most popular chromatography technique.
The basic function of chromatography is to separate organic and inorganic samples to the particulate and single cell level for the purpose of measuring and identifying those particulates. The elements of the experiment are:
- The mobile phase is the catalyst material
- The stationary phase, which holds the sample and is passed over by the mobile phase. This can be liquid or solid
- The sample
When you use a piece of paper and pen-ink for chromatography (as might be used in a popular high school lab experiment demonstrating liquid chromatography), the ink is the sample, the piece of paper is the stationary phase, and the liquid solvent is the mobile phase which acts as the catalyst for the separation of the dyes. As the ink separates, you can observe the different colors of the dye components within the ink.
Today there are several types of liquid chromatography that are performed in a variety of industries. It is commonly used for environmental analysis, food analysis, quality control and cleanliness testing.
Present day liquid chromatography that generally utilizes very small packing particles and a relatively high pressure is referred to as high-performance liquid chromatography (HPLC).
In HPLC the sample is forced by a liquid at high pressure (the mobile phase) through a column that is packed with a stationary phase composed of irregularly or spherically shaped particles, a porous monolithic layer or a porous membrane. HPLC is historically divided into two different sub-classes based on the polarity of the mobile and stationary phases. Methods in which the stationary phase is more polar than the mobile phase (e.g., toluene as the mobile phase, silica as the stationary phase) are termed normal phase liquid chromatography (NPLC) and the opposite (e.g., water-methanol mixture as the mobile phase and C18 (octadecylsilyl) as the stationary phase) is termed reversed phase liquid chromatography (RPLC).
The first known chromatography is traditionally attributed to Russian botanist Mikhail Tswett who used columns of calcium carbonate to separate plant compounds during his research of chlorophyll. This happened in the 20th century (1901). Further development of chromatography occurred when the Nobel Prize was awarded to Archer John Porter Martin and Richard Laurence Millington Synge in 1952. They were able to establish the basics of partition chromatography and also develop Plate theory.
