Chromatography Glossary


Ion Exchange

A separation technique which includes groups covalently bonded to a solid support. Analyte ions are retained by displacing counterions associated with the bonded functional group. An anion exchange sorbent is bonded with a functional group capable of carrying a positive charge (e.g. quaternary ammonium ion). A cation exchange sorbent is bonded with a functional group such as a sulfonic acid ion which is capable of carrying a negative charge.

Ion Exchange Capacity

The number of accessible ino-exchange sites on a given ion-exchange material. It is given in terms of the material’s dry weight and typically ranges from 1 to 5 milliequivalents per gram (meq/g).

Ion Exchange Chromatography

Sometimes abbreviated to IEC or IEX. Refers to the separation of ionic materials on a stationary phase having fixed ionic sites. The sample ion of interest will exchange with ions already on the similarly charged site on the packing material. Retention is based on the affinity of different ions for the site and various other parameters, eg pH.

Ion Exchange Media

A solid or liquid, inorganic or organic substance containing ions exchangeable with others of the same charge, present in a solution in which the ion exchanger is considered to be insoluble.

Ion Exclusion

Refers to the process of ionized solutes being separated from partially ionized solutes using ion exchange resins. The separation is a result of Donnan membrane potential, ionic solutes exist at a higher concentration in solution than in the resin. On the other hand, non-ionic solutes are evenly distributed between the mobile phase and the resin. Because of this, ionic solutes will travel faster through the column than non-ionic solutes.

Ion Filtering

The removal of unwanted ions from the mass filter.

Ion Focus Lens

In mass spectrometry, part of the ion source that helps direct ions to the mass filter. A negative voltage is applied to the ion focus lens to control the shape or focus of the ion beam. The lens is positioned between the drawout plate and the entrance lens.

Ion Gauge

In mass spectrometry, a vacuum gauge used in the 10^-4 and lower range (high vacuum).

Ion Pair Chromatography

Refers to the addition of organic counterions to the eluent to separate ionic compounds on reversed phase columns.

Ion Pairing

A reversed phase chromatographic mode using an ion-pair reagent which possesses both a formal positive or negative charged portion and a non-polar portion (i.e., octyl sulfonate or t-butyl ammonium salts). The positively charged t-butyl ammonium ion forms a neutral ion pair complex with a negatively charged analyte. The neutral complex can then be retained on a reversed phase surface.

Ion Pairing Agent

An ionic organic compound added to the eluent in Ion Pair Chromatography which will ionically bind charged solutes. The agent usually (but not always) contains a hydrocarbon chain that makes the sample hydrophobic so that the ion pair can be retained on a reversed phase column. The agent is believed to form an ion pair with the sample ion via their respective ionizable groups. The resulting pair has a net charge, ie., the solute pair appears neutral, allowing for improved chromatography (eg., sharper peaks and better retention).

Ion Ratio

A term used in mass spectrometry for the ratio of two ions in a given spectra. The ion ratio may be used to aid in the identification of a spectrum. Alternatively, the ion ratio for two peaks in a spectrum may be used for diagnostic purposes.

Ion Source

In mass spectrometry, part of the mass spectrometer where ionization and ion focusing are achieved. The ion source also directs ions towards the mass filter.

Ion Suppression

Ion suppression refers to the use of an eluent pH which causes sample components to convert to their un-ionized form. This is a helpful technique for the improved peak shape of weak acids and bases in reversed phase chromatography.

Ion Trap

A mass-resonance analyzer that produces a three-dimensional rotationally symmetric quadrupole field capable of storing ions. By ejecting the stored ions selectively, mass-to-charge ratio may be determined and a mass spectrum produced.

Ion Volume

In Mass Spectrometry, the part of the ion source where ionization occurs.

Ionic Capacity

Measure of packing material's activity in ion chromatography. Usually defined by milliequivalents per millilitre (meq/ml) of resin, or other support matrix.

Ionic Strength

Sometimes referred to as ionic concentration. The quantity of ions in a liquid. Usually used when describing aqueous-based buffers, eg., 1M NaCl.


Utilized in mass spectrometry to fragment analyte molecules into smaller segments. These smaller mass segments are then separated and plotted to form a "mass spectrum" which is used to identify the parent molecule. Electron impact is one example of ionization used in mass spectrometry. Thermal and chemical ionization are two other methods.

Ionization Chamber

In mass spectrometry, the area of the ion source between the filament and target and the repeller and lenses. The volume of the ion source where molecules are transformed to ions.


The opening in the front plate of an electron multiplier detector in mass spectrometry.

Irregular Packing

Refers to the shape of the silica gel based packing. The packing is made by grinding silica gel into small particles and sizing them into narrow fractions.

Irregular Packing Material

Packing material made from a milling-sieving process resulting in particles of irregular and random shape.

Irreversible Adsorption

Refers to a chemical reaction between a sample and the surface of the adsorbent. The sample is so strongly adsorbed that it cannot be eluted from the column.


Refers to holding the eluent composition constant throughout a chromatographic run.

Isocratic Analysis

The procedure in which the composition of the mobile phase remains constant during the elution process.

Isocratic Elution

An HPLC separating technique using a constant solvent composition as opposed to gradient elution in which the composition will vary. Isocratic LC systems only require one pump with premixed solvents. Alternatively multiple or gradient pumps may be used to deliver constant proptions for an isocratic separation.

Isoelectric Point

This is the point of electric neutrality; the pH value at which a substance (protein, etc.) is neutral. At a lower or higher pH value it acts as either a base or an acid, respectively. The Isoelectric Point is denoted by the symbol "pI".

Isolated Silanol

A free silanol group. The OH group on the silca surface is free to interact with analytes. Many reversed phase columns are endcapped to minimize free silanol effects.

Isolation Valves

Valves at the top of the diffusion pumps to allow for continued diffusion pump operation during source/analyzer maintenance.


Isomers are generally classified into two types:

1. Constitutional (positional isomers, which are optically inactive

2. Stereoisomers, which may (enantiomers) or may not (geometric) be optically active.

Isothermal Chromatography

A procedure in which the temperature of the column is kept constant during the separation.

Isotope Ratio

The ratio of two peaks that are the result of naturally occuring isotopes in a mass spectrum. For example, when onr chlorine is present, the 35Cl to 37Cl ratio is 3:1.


Different forms of the same atom that differ only in atomic mass. Isotopes differ in the number of neutrons in the nucleus. This difference results in the isotopes of a given element having different weights (for example, 35Cl and 37CL.)

Isotopic Signature

Characteristic pattern in a mass spectrum that suggests the presence of specific combination of isotopes.For example, a molecule that contain one bromine will have an isotopic signature of two large peaks of about the same intensitry, two mass units apart.


IUPAC is an abbreviation of International Union of Pure and Applied Chemistry. The Analytical Chemistry Division of IUPAC has established two commissions for recommending Nomenclature for Chromatography. They are the Commission on Chromatography and other Analytical Separations and the Commission on Analytical Nomenclature.

Jet Separator

A separator interface in which lighter sample molecules are removed by effusion ouy of a gas jet and pumped away by the vacuum system.Heavy sample molecules pass through a separate jet into the ion source.


Symbol for the retention factor. In terms of measured parameters, k' = (tr - t0) / t0.


Symbol for the Distribution Coefficient. The ratio of the equilibrium concentration of solute in the stationary phase to the concentration in the mobile phase.


Is a diatomaceous earth used in both column chromatography and also as a sample cleanup medium for the extraction of polar compounds. It is used as a support in liquid chromatography and is a weak adsorptive.


Abbreviation for kilopascal.


Used as symbol for wavelength.

Lambda Max

Lambda Max is the wavelength absorbance maximum for a given molecule determined by spectral analysis. May also refer to the optimum wavelength used for fluorescence excitation or emission.

Laminar Flow

In mass spectrometry, the flow of gas in which the motion of each gas molecule is considered to be independent of it's neighbors and there are infrequent collisions.

Late Peak

Peak(s) that elutes after the peak(s) of interest.


All vacuum systems leak, it is just a matter of degree. In practice a leak is considered to be any extra leakage that tends to increase the normal "leak-free" pressure. These leaks should be detectable by a mass spectrometer if a solvent is applied to the leaking area, but in practice this is not always as simple a test as it seems.


A plate in the ion source with a slit or hole through which the ion beam passes. Voltages applied to lenses control the shape (focus) of the ion beam.