Chromatography is an analytical technique used for separating a mixture of chemical substances into its components so that these can be identified or analyzed. Chromatography comes in many forms, e.g., paper chromatography, liquid chromatography, gas chromatography, ion-exchange chromatography, but all of these employ the same basic principles. Chromatography is extensively used in the semiconductor industry, especially in the identification of contaminants that cause yield, quality, and reliability problems.



Chromatography can separate a mixture into its components with great precision. In fact, it can be used to distinguish between two very similar components, such as proteins that may be different only by a single amino acid.  The conditions under which the separation process takes place are also not severe, allowing the use of chromatography on delicate products.  With the right materials and operating conditions, chromatography is capable of purifying any soluble or volatile substance.


In all types of chromatography,  the analyte, which is a sample of the mixture being analyzed, is applied and allowed to adhere to a stationary material known as the stationary phase, or adsorbent.  Another material, known as the mobile phase, carrier fluid, or eluent, is then made to flow through the adsorbent. 


Because the different components of the analyte exhibit varying degrees of strength of adhesion to the adsorbent, they also travel different distances through the adsorbent as the eluent flows through it, i.e., components that adhere more strongly to the adsorbent travel more slowly than those with weaker adhesion.  In effect, this process separates the various components of the analyte into individual samples that can be analyzed. 


Although the principle above works even in simple techniques such as paper chromatography,  modern chromatographic equipment today employ an analytical column where the actual separation process takes place.  The column is usually made of glass or metal tube that is capable of withstanding the range of pressure that may be applied to it. 


As discussed above, the column contains both a stationary phase and a mobile phase. A column is said to be a 'packed bed' column if its stationary phase is in granular form and is packed into the column, completely filling it as a homogeneous bed.   A column is 'open tubular' if it has a hollow center acting as a 'passageway,' with its stationary phase confined to the inner wall of the column as a film or layer.


In gas chromatography, the mobile phase is generally a chemically inert gas, such as nitrogen, helium, argon, and carbon dioxide.  The sample to be analyzed by gas chromatography is vaporized and then injected into the column.  It is then transported through the column by the flow of the mobile phase.


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In liquid chromatography, the mobile phase is usually a liquid of low viscosity that is flowing through the stationary phase bed. This bed usually consists of one of the following: 1) an immiscible liquid that coats a porous support; 2) a thin film of liquid bonded to the surface of a sorbent; or 3) a sorbent of controlled pore size.


In ion-exchange chromatography, the stationary phase packings of the column usually consist of ion-exchange resins bonded to inert polymeric particles of small diameter (typically 10 microns). For cation separation, the ion exchange resin is usually sulfonic or carboxylic acid, while for anion separation, the ion-exchange resin is usually a quaternary ammonium group. 


A basic chromatography process consists of the following steps: 1) feed injection, wherein the analyte is injected into the mobile phase or carrier fluid; 2) separation of the analyte in the column into its components as the mobile phase flows through the stationary phase, by virtue of the varying degrees by which these components are attracted to the stationary phase; 3) elution from the column, wherein the different components of the sample will emerge from the column at different times, with the component that's least bound to the stationary phase eluting first; and 4) detection, wherein the eluted components are collected and analyzed, usually by measuring certain properties of the components, such as the refractive index, uv absorbence, or solution conductivity.


The output of a chromatographic analysis is referred to as a 'chromatogram.'  It is a plot that consists of several different peaks representing the different components of the sample mixture.


Ion chromatography is a widely used ion-exchange chromatography technique in the semiconductor industry.  This is because it can provide quantitative analysis of anions in the ppb range, making it capable of detecting contaminants on the surface of a wafer, die, or package. Since ionic contamination is a major source of corrosion problems in the industry, ion chromatography is considered to be an indispensable tool when analyzing water samples suspected to be the cause of corrosion issues on the line.


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