Themes > Science > Chemistry > Analytical Chemistry > Methods and Instrumentation > Separations

Background -- A sample that requires analysis is often a mixture of many components in a complex matrix. For samples containing unknown compounds, the components must be separated from each other so that each individual component can be identified by other analytical methods. The separation properties of the components in a mixture are constant under constant conditions, and therefore once determined they can be used to identify and quantify each of the components. Such procedures are typical in chromatographic and electrophoretic analytical separations.

A mixture can be separated using the the differences in physical or chemical properties of the individual
components. As an example, dumping pinto beans and water in a colander separates the two components because the liquid water can run through the colander but the solid beans cannot. Some water will stick to the beans and some bean particles may go down the drain because the colander is not 100% efficient. An analogous example is the filtering of a solid precipitate to separate it from a solution. These separations are based on the states of matter of the two components, other physical properties that are useful for separations are density and size. Some useful chemical properties by which compounds can be separated are solubility, boiling point, and vapor pressure. Some separation procedures are listed in the following table.
Types of Separations

 Simple

 Crystallization
  Centrifugation  Distillation  Filtration  Extraction

 Instrumental

 Chromatography

 Electrophoresis

Chromatography is a separations method that relies on differences in partitioning behavior between a flowing mobile phase and a stationary phase to separate the the components in a mixture. A column holds the stationary phase and the mobile phase carries the sample through it. Sample components that partition strongly into the stationary phase spend a greater amount of time in the column and are separated from components that stay predominantly in the mobile phase and pass through the column faster.

Electrophoresis is a separations technique that is based on the the mobility of ions in an electric field. Positively charged ions migrate towards a negative electrode and negatively-charged ions migrate toward a positive electrode.  For safety reasons one electrode is usually at ground and the other is biased positively or negatively. Ions have different migration rates depending on their total charge, size, and shape, and can therefore be separated.


Information provided by: http://www.chemistry.nmsu.edu