Glossary Of HPLC-LC Separation Terms – M

The listing should be helpful to those just starting in HPLC but it also can serve as a refresher for long-time users in the field.

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μ: See electrophoretic mobility.

Macroporous resin (macroreticular): Cross-linked ion-exchange resins that have molecular-scale micropores and also macropores of several hundred angstroms. These highly porous resins have large internal surface areas that are accessible to large molecules.

Mass transfer (interphase): The process of solute movement between the moving and stationary zones. The C term of the van Deemter equation is called the interphase mass transfer term. The faster the mass transfer process, the better the column efficiency. In HPLC, slow mass transfer is the most important factor affecting column efficiency. Its rate can be increased by using small-particle packings, thin stationary-phase layers, low-viscosity mobile phases, and high temperatures.

Mean pore diameter: The average diameter of the pore of a porous packing. It most commonly is determined by the BET method and is reported as fourfold the specific pore volume divided by the specific surface area (4V/A) based on the assumption of uniform cylindrical pores. The pore diameter is important in that it must allow free diffusion of solute molecules into and out of the pore so that the solute can interact with the stationary phase. Additionally, the pores must be well-connected, with a minimum of dead ends, so many paths can allow a molecule to access any part of the pore space. In SEC, the packings have different pore diameters; therefore, molecules of different sizes can be separated. For a typical substrate such as silica gel, 60- and 100-Å pore diameters are most popular. Pore diameters greater than 300 Å are used for the separation of biomolecules. Pores usually are classified as micro (<20 Å), meso (20–500 Å), and macro (>500 Å).

MECC: See micellar electrokinetic capillary chromatography.

Megapores: See perfusion chromatography.

MEKC: See micellar electrokinetic capillary chromatography.

Metal-affinity chromatography: A special form of ligand-exchange chromatography used to separate biopolymers with a particular affinity for a specific metal cation, typically copper(II), zinc(II), and iron(II).

Metalophile: A compound that has high affinity for active acidic silanol groups on silicas surface. Usually a strongly basic amine or multifunctional carboxylate or phenol.

Method development: A process for optimizing the separation, including the sample pretreatment, to obtain a reproducible and robust separation. Usually, it emphasizes the search for the stationary phase, eluent, and column temperature combination that provides an adequate, if not optimum, separation.

Method validation: A process of testing a method to show that it performs to the desired limits of precision and accuracy in retention, resolution, and quantitation of the sample components of interest.

Micellar chromatography: Adding micelles to the mobile phase to cause separation. The micelles may act as displacing or partitioning agents and provide another parameter to change selectivity. Surfactants at concentrations greater than their critical micelle concentration are used in micellar chromatography and in MEKC.

Micro-LC: Refers collectively to techniques in which a column of smaller than conventional inner diameter is used for separation. The term micro-LC most often is used for HPLC in columns with inner diameters smaller than 0.5 mm; micro-LC is used in high-sensitivity analysis when the sample amount is limited and with certain ionization techniques in LC–MS in which the volume of solvent flowing into the ionization source must be minimized.

Microbore: Refers to the use of smaller-than-usual inner diameter columns in HPLC. Columns of 2 mm and less are considered to be microbore sizes. Inner diameters of 0.5 mm and smaller are considered micro-LC columns.

Microchip devices: Microdevices based on silicon, glass, and other types of microfabricated chips in which experiments can be miniaturized into single- or multichannel microfluidic circuits. These devices can be used for CE and CEC. They should be low cost and disposable. Using microdevices for separation currently is in its infancy, and applications should expand with time.Microparticulate: Refers to the small particles used in HPLC. Generally packings with a particle diameter of less than 10 μm that are totally porous are considered microparticles.

Microporous resin: Same as microreticular resin.

Microreticular resin: Cross-linked, synthetic ion-exchange resins that have pores with openings that correspond to molecular sizes. Diffusion into the narrow pores can be impaired, and low exchange rates and poor performance can occur, especially for large molecules.

Migration rate: See electrophoretic mobility.

Migration time (t m ): The time it takes for a charged molecule to move from the point of injection to the point of detection in a CE capillary. Distinct from holdup time (t M).

Minimum plate height: The minimum of the van Deemter curve that results from a plot of H versus ν. This value represents the most theoretical plates that can be obtained for a certain column and mobile-phase system. Usually occurs at excessively low flow rates. Also known as the optimum plate height. It typically is two- to threefold the particle diameter of well-packed columns.

Mixed-bed column: Combination of two or more stationary phases in the same column, used most often in IEC (mixed anion and cation resins) and SEC (mixture of different pore size packings). Its advantage in IEC is the total removal of both cationic and anionic compounds. Useful in SEC because a wider molecular weight range can be accommodated by the same column.

Mixed-mode separation: A separation that occurs in a single column caused by the retention and selectivity provided by a dual-retention mechanism. For example, a reversed-phase column with residual silanols at intermediate-to-high pH values can separate by hydrophobic interaction and ionic interaction by the ionized silanols. Sometimes mixed-mode separations can be quite beneficial to the selectivity (band spacing), but they can cause peak asymmetry, and the precise balance of interactions may be difficult to reproduce with subsequent packing batches.

Mobile phase: The solvent that moves the solute through the column. In LC, the mobile phase interacts with both the solute and the stationary phase and, therefore, can have a powerful influence on the separation.

Mobile-phase strength: See solvent strength.

Mobile-phase velocity (u M ): The velocity at which the mobile phase percolates through the bed of particles; u M = L/t M, where L is column length and t M is holdup time. See also adjusted retention volume, holdup volume, and dead volume.

Mobility: See electrophoretic mobility.

Modifier: An additive that changes the character of the mobile phase. For example, methanol is the strong solvent in reversed phase and sometimes is called the modifier (water is the weak solvent); sometimes other additives — competing bases such as triethylamine or ion-pairing reagents — are referred to as modifiers, but they more correctly should be called additives. See also additives.

Molecular diffusion term (B term): Refers to the B term (second term) of the van Deemter equation. Also called longitudinal or axial diffusion term. It dominates band broadening only at very low flow rates below the minimum plate height at which the diffusion of individual solutes can occur in a longitudinal (lengthwise) direction on the column. The contribution to the B term arises from diffusion in the mobile phase and is 2γDM, where γ is the obstruction factor (typically 0.6–0.8) and DM is the diffusion coefficient. See also van Deemter equation.

Molecular weight distribution: The distribution of molecular weight of molecules in a polymer sample. Distribution can be defined as weight average and number average.

Molecularly imprinted phases: See imprinted phases.

Monodisperse particles: Particles that fall into a narrow range of diameters. See also polydisperse particles.

Monomeric phase: Refers to a bonded phase in which single molecules are bonded to a support. For silica gel, monomeric phases are prepared by the reaction of an alkyl- or aryl- monochloro- or alkoxysilane. Polymeric phases generally are prepared from a di- or trichlorosilane or an alkoxysilane reactant in the presence of water.

Moving zone: To be distinguished from the mobile phase, this zone is the fraction of the mobile phase in the column that occupies the interstitial spaces. See also stationary phase.

Multidimensional chromatography: The use of two or more columns or chromatographic techniques to generate a better separation. It is useful for sample cleanup, increased resolution, increased throughput, and increased peak capacity. It can be used off-line by collecting fractions and reinjecting them onto a second column or on-line by using a switching valve. Also called coupled column chromatography, column switching, multicolumn chromatography, and boxcar chromatography.

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