Glossary Of HPLC-LC Separation Terms – V

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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Vacancy chromatography: Technique in which a mobile-phase additive causes a positive detector signal output. When a solute is eluted from the column, it dilutes the signal and generates a negative peak or vacancy. The technique has been applied primarily to single-column ion chromatography in which mobile phases such as citrate and phthalate buffers absorb in the UV region. When a nonabsorbing anion is eluted, it dilutes the UV-absorbing background and causes a negative peak; the detector output leads usually are reversed so that the chromatogram looks normal. It also has been used in CE for detection.

van Deemter equation: An equation used to explain band broadening in chromatography. The equation represents the height of a theoretical plate (HETP) and has three terms. The A term describes eddy dispersion or diffusion that results from axial velocity heterogeneity. The B term is for the contribution of molecular diffusion or longitudinal diffusion of the solute while passing through the column. The C term is the contribution from interphase mass transfer, which allows for the finite rate of transfer of the solute between the stationary phase and mobile phase. In its simplest representation, h = A + B/ν + Cν. See also reduced plate height and reduced velocity.

V c See column volume.

V d See dead volume.

V e See interstitial volume.Velocity (u): Same as linear velocity.

v eo See electroosmotic flow.

V i See intraparticle volume.

Viscosity (η): Also called mobile-phase viscosity. The viscosity of the mobile phase varies with the temperature of the column. Low-viscosity mobile phases generally provide better efficiency than less-viscous ones because diffusion coefficients are inversely related to solvent viscosity. For example, column efficiency is higher in reversed-phase chromatography with acetonitrile as an organic modifier than with isopropanol, which is more viscous. Column back pressure is directly proportional to solvent viscosity.

V M See holdup volume. Also mobile-phase volume.

Void: The formation of a space or gap, usually at the head of the column, caused by a settling or dissolution of the column packing. A void in the column leads to decreased efficiency and loss of resolution. Even a small void can be disastrous for small-particle microparticulate columns. The void sometimes can be filled with glass beads or the same porous packing used in a column.

Void time (t 0 ): The elution time of an unretained peak; also called the dead time and the holdup time (t M). The void volume is determined by multiplying the void time and the flow rate.

Void volume (V M ): The total volume of mobile phase in the column; the remainder of the column is taken up by packing material. This volume can be determined by injecting an unretained substance. Also called dead volume. The symbol V 0 is often used to denote the void volume. This is valid only for a column packed with nonporous particles. V 0 is valid when used to denote the excluded volume (V e) in SEC.

V p See total permeation volume

V R See retention volume and elution volume.

V R 9See adjusted retention volume.

V t See total mobile-phase volume.

V o See exclusion volume.

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