Why do chromatography?

Answer 1

Chromatography is the methods used to separate complex mixtures. The components to be separated are distributed between two phases, a stationary and mobile phase. There are many types of chromatography: Liquid Chromatography, Gas Chromatography, Thin-layer Chromatography, and Paper Chromatography. As the separation occurs, a mixture is separated into its components. As a result, the molecules of the components have different masses, and so they travel along a medium at different rates.

The process of chromatography can be used on a T-shirt. The mobile phase would be the alcohol and the stationary phase is the permanent markers. The solvent is the alcohol and the solutes are the marks made by the permanent markers. When the solvent, in this case, the alcohol, is distributed onto the shirt, some of the marks dissolve in the solvent. After a period of time, the marks on the medium, the T-shirt, end up spread out between the original spot and the point the solvent reaches. In designing the shirt, the alcohol spread the marker making concentric rings of different colors. This effect of concentric rings occurs since the different components of the permanents markers travel at different rates.

In any chemical or bio-processing industry, the need to separate and purify a product from a complex mixture is a necessary and important step in the production line. For example, pharmaceutical industry uses chromatography to isolate penicillin and other antibiotics. Proteins can even be separated into amino acids through the process of chromatography. Chromatography is also used in crime scene investigation for DNA and RNA sequencing as well as in many scientific studies to identify unknown organic and inorganic compounds. This separation of mixtures is useful to us in various ways.

Answer 2

Chromatography is a family of laboratory techniques for the separation of mixtures of chemicals into their individual components. The basic principle of chromatography is that different compounds adhere to a solid surface or dissolved in a liquid film to varying degrees. Chromatography is widely used in forensics, from the analysis of body fluids for the presence of illicit drugs, fiber analysis, blood analysis of a crime scene, and airports to detect residues of explosives.

When a gas or a liquid containing a mixture of different compounds is caused to flow over this surface, the molecules of the various compounds have a tendency to stick to the surface. If the viscosity is too high, a given molecule will be stuck and unstuck hundreds or thousands of times it is swept along the surface. This repetition exaggerating small differences in the various adhesion molecules, and they become distributed along the "track", because the sticky compounds move more slowly than those who are less sticky do. After a given time, the various compounds has reached different locations along the surface and are physically separated from one another. Or, they can all be brought to the end of the separation surface and be detected or measured one at a time as they appear.

Using variations of this basic phenomenon, chromatographic methods have become an extremely powerful and versatile tool for the separation and analysis of a wide variety of chemical compounds in amounts ranging from picograms (10 -12 gram) in tonnes .

Chromatographic methods all share certain characteristics, even if they differ in size, shape and configuration. Typically, a flow of liquid or gas (mobile phase) flows continuously through a tube (column) packed with a porous solid (stationary phase). A sample of the chemical mixture is injected into the mobile phase at one end of the column, and the compounds separated as they move along. The individual compounds can be separated one at a time removed upon release (or "elute") of the column.

Because it is generally not alter the molecular structure of compounds, chromatography can provide a non-destructive means to get pure chemicals from various sources. It works well on scales very large and very small; chromatographic methods are used both by scientists studying micrograms of a substance in the laboratory, and industrial chemists separating tons of material.

Chromatography technology has evolved rapidly in recent decades. It is now possible to obtain a separation of mixtures in which the components are similar if they differ only in the way the atoms are oriented in space, in other words, they are isomers of these compounds. It is also possible to obtain a separation of a few parts per million of a contaminant from a mixture of materials much more concentrated.

In gas chromatography (now called gas chromatography), which separates the material of the components is chemically bound to the solid support, thereby improving the stability of the temperature of the column packing. Gas chromatographs can be used at high temperatures, so that even large molecules may be vaporized and the progress through the column stationary phase without vaporization and bleeding off. In addition, since the mobile phase is a gas, the separated compounds are very pure, there is no solvent fluid to remove.

Forms chromatographic columns, vertical tubes at the origin of an inch or two (2 cm) in diameter, have become longer and thinner when it was found that this increase in separation efficiency. Finally, chemists have been using glass tubes coiled or fused silica capillaries less than a millimeter in diameter and many long projects. Hair can not be wrapped, but they are so narrow that the stationary phase may be simply a thin layer inside of the column.

A slightly different approach is the set of techniques known as "plan" or "thin layer" chromatography (TLC), in which a column is not used at all. The stationary phase film deposited on a glass plate or plastic. A point of the sample is placed on the plate, and the mobile phase migrates through the stationary phase by capillary action.

In the 1970s, interest in mobile phases for liquid chromatography column resurfaced when it was discovered that the separation efficiency can be greatly improved by pumping the liquid through a short column packed under pressure, rather than let it flow slowly to the vertical column by gravity. Liquid chromatography, also called liquid chromatography (HPLC) is now widely used in the industry. A variant is the HPLC supercritical fluid (SFC). Some gases (carbon dioxide, for example), when highly pressurized above a certain temperature, become a state of matter intermediate between liquid and gas. These "supercritical fluids" have unusual solubility properties, some of the advantages of both gases and liquids, and seem very promising for use in chromatography.

All chromatographs must have a detection device attached, and a kind of recorder to capture the output of detectorsually recorder or equivalent computer. In gas chromatography, various types of detectors have been developed, the most common are the thermal conductivity detector, the detector flame ionization, and the electron capture detector. For HPLC, the UV detector is normalized to the concentration of the separated compound. The sensitivity of the detector is of particular importance, and research has always focused on increasing the sensitivity, because chemists often need to detect and quantify extremely small amounts of material.

In recent decades, chromatographic instruments were attached to other types of analysis tools so that the components of the mixture can be identified and separated (which takes the concept of "detector" to its logical extreme). Most often, this second instrument is a mass spectrometer, which allows the identification of compounds based on the mass of molecular fragments that appear when the molecules of a compound are divided.

Adsorption chromatography (same type of chromatography) depends of physical forces such as dipole attraction to hold the molecules on the surface of the solid packing. By gas chromatography and HPLC, however, the solubility of the molecules of the mixture in the stationary phase coating determines which most progress through the column

After the contaminated feed was traced through the food chain in the Midwest in 1986, a scientist is preparing to test a tube of milk by gas chromatography to determine the amount of heptachlor in milk is sufficient to affect babies. © Bettmann / Corbis

slowly. The polarity can influence here too. In the gel filtration (also known as size exclusion or gel permeation) chromatography, the relative sizes of the molecules in the mixture to determine the output of which the first column. Large molecules flow throughout; ones are slow because they spend time trapped in the pores of the gel. Ion exchange chromatography depends on the relative strength with which the ions are held in an ionic resin. Ions are less strongly bound to the resin are displaced by more strongly bound ions. Hence the name ion exchange: a type of ion is exchanged against another. This is the principle on which water softeners work. Affinity chromatography uses a stationary phase consisting of materials which have been chemically modified. In this type of chromatography, the stationary phase is attached to a compound having a specific affinity for the desired molecules in the mobile phase. This process is similar to the ion-exchange chromatography, and is used principally for the recovery of biological compounds. Hydrophobic interaction chromatography is used for amino acids which carry a positive or negative charge. In this type of chromatography, the hydrophobic amino acids are attracted by the solid phase which is composed of materials containing hydrophobic groups.

Chemists choose the mobile and stationary phases with caution because it is on the interaction of compounds in the mixture together with these two phases determines the efficiency of the separation can be. If the compounds have no affinity for the stationary phase at all, they go right into the column without separating. If the compounds are too strongly attracted to the stationary phase, they may stick permanently within the column.

SEE ALSO analytical instrumentation, gas chromatograph-mass spectrometer.