Chromatography Glossary



The number of openings per unit area in a sieve.

Meso Compound

A molecule which contains two or more chiral centers, but has a plane of symmetry and thus is optically inactive.

Metastable Ion

term used in Mass Spectrometry for an ion that decomposes during it's passage through the analyzer.

Method Detection Limit

Abbreviated MDL, the Method Detection Limit is the minimum concentration of a compound that can be measured and reported with a 99% confidence that the value is above zero.

Method Validation

Steps taken to prove that an analytical method is capable of producing high quality data. Usually indicates measurments of accuracy, precision, detection limit and spike recovery.

Micellar Chromatography

Is the addition of detergent (surfactant) micelles to the mobile phase in order to bring about a separation. The micelles act as displacing or partitioning agents to provide another parameter that can be used to change selectivity. Micellar chromatography has also been used for direct injection of biological fluids for analysis of eg., urine and serum samples, without fouling the column.

Micro LC

Is a collective reference to LC techniques in which the column used for the separation is of a smaller than usual internal diameter (id). Columns of less than 1.0mm are used.

Microbore Column

An analytical column with a small (< 1.0 mm) internal diameter. Microbore chromatography requires specialized equipment eg micro flow pump , or syringe pump, micro flow cell and micro injection valve.


Refers to small particles that are used as stationary phases in HPLC. Micoparticles are usually packings with a particle diameter of <10μm and are totally porous. Microparticulate may also refer to unwanted, insoluble materials found in buffers or which shed from system components such as pump seals and injectors due to wear and abrasion.

Microporous Resin

Also known as Microreticular resin. Is a crosslinked synthetic ion exchange resin which has pore openings that correspond to molecular sizes. Diffusion into the narrow pores can be weakened, with low exhange rates and poor performance occurring particularly for large molecules.

Midbore Column

An analytical column with a mid-range (2.0 to 3.5mm) internal diameter.

Minimum Detectability

The concentration or mass flow of a sample component in the mobile phase that gives a detector signal equal to twice the noise level. It can be calculated from the measured sensitivity (S) and noise (N):

D = 2N / S

where D is the minimum detectability, expressed either as concentration or mass-flow of the substance of interest in the mobile phase at the detector. Both sensitivity and minimum detectability must be determined for the same substance.

Minimum Detectable Quantity (MDQ)

The weight of material injected into a chromatograph that produces a peak height equal to three times the peak to peak height of the baseline noise. Also called MDL (minimum detection limit).

Minimum Plate Height

Is the term for the minimum height of the curve that is the result of the plot of plate height (H) versus linear velocity (µ). This value represents the most theoretical plates that can be obtained for a certain column and mobile phase system. The minimum plate height (optimum) usually occurs at lower flow rates.


The ability of two solvents to form a single phase when mixed in any ratio.


Solvents that are soluble in one another, and can be mixed in any ratio.


In HPLC, the mixer is that portion of the chromatograph, immediately downstream from the gradient former, that combines mobile phase components into a homogeneous mixture.

Mobile Phase

A general term for the moving phase in chromatography. In Gas chromatography, it is the carrier gas and in liquid chromatography, it is the eluent. In LC the mobile phase is a solvent mixture such as methanol and water for reversed phase LC or hexane and dichloromethane for normal phase chromatography.

Mobile Phase Strength

Determines how fast the sample moves through the column; a strong mobile phase results in sample bands coming out fast, and a weak mobile phase gives longer retention times for each band.


Chromatographic modes differ in the types of interactions that occur in the separation process. Liquid Chromatography modes include: adsorption, normal phase, reversed phase, ion pair, ion exchange and size exclusion. Chiral Chromatography is a specialized mode for separating enantiomers.


Modifier is an additive that changes the character of the mobile phase. For example, in reversed phase, water is the weak solvent; methanol, the strong solvent, is sometimes called the modifier.

Molecular Adduct Ion

In Mass Spectrometry, an ion formed (typically during CI) by the addition of an ionized species to a molecule. The resulting ion has a higher mass than the original molecule.

Molecular Diffusion Term

Refers to the B-term or second term of the van Deemter equation. It is also known as the longitudinal or axial diffusion term. This is important in bandspreading at very low flow rates below the minimum plate height, where the diffusion of individual solutes can occur in a longitudinal direction on the column.

Molecular Formula

A term used in mass spectrometry for a shorthand notation indicating the type and number of atomic elements in a molecule. The molecular formula conveys no information about the molecule's structure.

Molecular Ion (Ion Source)

In mass spectrometry, in the ion source, a singly ionized molecule. No molecular fragmentation has occurred. No adduct ion has been formed.

Molecular Weight

A measure of the mass of a compound (the sum of atomic weights of all atoms in a molecule). Common unit of measure is in daltons, molecular weights are used widely in biochromatography to characterize proteins, peptides and other biomolecules.

Molecular Weight Distribution

Refers to the distribtion molecular weight of the molecules in a polymer sample. The distribution can be defined as the average weight and average number.

Moleular Ion

In mass spectrometry, in the spectrum, the molecular ion may not be present. If present, the molecular ion is the fragment of highest mass (excluding other, associated ions due to isotopes of the molecular ion).

Momentum Exchange Principle

In mass spectrometry, a principle used in some high vacuum pumps (including diffusion and turbomolecular pumps) where a large momentum item (diffusion pump fluid molecule or turbo pump blade) interacts with a low momentum item (sample molecule, air, water,...). The higher momentum entity forces the lower momentum entity in a specific direction.


Monochromator is a device for isolating one narrow wavelength region from a wide wavelength range of radiation. Used in variable wavelength UV-visible detectors and fluorescence detectors for isolating and directing incident and emitted light.

Monolithic Quadrupole

A term used in Mass Spectrometry for a quadrupole with four hyperbolic surfaces formed with a single piece of material.

Monomeric Phase

Monomeric phase refers to bonding of HPLC stationary phases using monofunctional silylating reagents, yielding an unbranched typed bonded phase rather than polymeric type.

Mother Liquor

The residual saturated liquid that remains after the crystallisation of a liquid. A mother liquor may contain unrecovered products (i.e., unreacted starting materials, intermediates, trace levels of the API (active pharmaceutical ingredient) and/or impurities).


MSDS is a compilation of information under the OSHA Communication Standard on the identity of hazardous chemicals and their associated health and physical hazards, exposure limits and precautions.


Methyl tert-butyl ether. MW 88.14, Boiling Point 55.2 C, Polarity Index 2.51, Solubility in water 4.8% at 20 C, UV cutoff 210nm. Common solvent used in normal phase chromatography.

Multidimensional Chromatography

Is the use of two or more columns or techniques to produce a better separation. This is helpful for sample cleanups and increased resolution. It can be used both on-line, by using a switching valve, or off-line by collecting fractions and reinjecting onto a second column.

Multilinear Gradient

A gradient which is made of several linear segments of different slope. Often used to stimulate a curved gradient.

Narrow Bore Column

Refers to the internal diameter of the HPLC Column. Narrow bore columns fit in the range between microbore and mid-bore, ie 1.0mm - 2.0mm internal diameter.


A device that is used in Mass Spectrometry to disperse a liquid stream into small droplets. This accelerates the vaporization of the liquid. Nebulizers are commonly used in LC/MS interfaces.


Abbreviation for effective plate number.

Negative Peak

Negative peaks are caused when sample bands passing through the detector are less responsive than the background (baseline) signal of the mobile phase. For example, mobile phase solvents or additives which absorb strongly in the UV-visible range may be employed. Zeroing the detector after column equilibration will result in a negative or "vacancy peak" when lower-absorbing analytes pass through the detector. Under these conditions, detection is referred to as "Indirect UV". In ion chromatography, the addition of eg., hydroxybenzoic acid, a strongly UV-absorbing compound, to the mobile phase results in vacancy peaks for the analysis of anions.

Neutral Species

A term used in mass spectrometry for non-ionized sample molecules.

NH2 Phase

Amino (NH2) bonded phases can be employed as reversed phase, normal phase, or a weak anion exchange material. In reversed phase useful for separating carbohydrates. In normal phase has different selectivity to silica and is not dectivated by small amounts of water. Acts as a weak anion exchanger in the presence of buffers, and can separate anions and organic acids.


NIOSH is an abbreviation of National Institute of Occupational Safety and Health.


The small piece of tubing that protrudes beyond the ferrule in a tube fitting. The length of the nipple must be long enough to eliminate a gap within the fitting and must not be so long that it prevents the ferrule form sealing.

Nitro Phase

NO2, or nitro bonded phase is used in normal phase separations. Used for separating aromatic compounds and compounds containing double bonds.