General Principles of Chemical Separations

Almost all the separatory and clean-up methods relevant to this book are ultimately based on differences in partition coefficients of various compounds between two phases. The concept of a phase in the present context (see also Section 4.3.2e) is that of a portion of matter that is separated from others by a clearly defined boundary thus, solid, liquid and gas phases satisfy this definition, but it is possible to have two distinguishable liquid phases (e.g., water and carbon tetrachloride). A simple example of an application of the latter, that is familiar to anyone who has taken an organic chemistry laboratory course, is liquid-liquid extraction a first step in purification of a desired synthesized compound is often to place the crude 

The equilibrium distribution of compound A between two phases (I and II) is usually described by the partition coefficient  

There are two general types of GC column packed and capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with a viscous liquid stationary phase. Most packed columns are 1.5-lOm in length and have an internal diameter of 2-4mm. The original glass capillary columns had an internal diameter of a few tenths of a millimeter and were also one of two types, wall-coated open tubular (WCOT) and support-coated open tubular (SCOT). WCOT columns consist of a capillary tube whose walls are coated with liquid stationary phase. In SCOT columns the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed. Both of these types of capillary column provide greater separation efficiency than packed columns, but since 1979 they have 

Many of the clean-up methods used in trace analysis exploit HPLC mobile and stationary phases in a somewhat different way. For example, a crude sample extract can be loaded onto a small cartridge packed with a suitable adsorbent under conditions such that the analyte (and its internal standard if present) are strongly retained. Washing the loaded cartridge removes many of the coextracted compounds and the analyte plus SIS can then be eluted from the cartridge by a suitable change of solvent in effect this is a kind of binary chromatography, i.e., a compound is either retained on the stationary phase or it is not. This approach (solid phase extraction 

Types of Retention Mechanism Expioited in Anaiyticai Chromatography Adsorption 

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