Glossary of HPLC-LC Separation Terms – B

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.


β: See phase ratio.

B o See permeability.B solvent: Usually the stronger solvent in a binary eluent or gradient separation; typically the organic modifier or modifier-rich binary mixture with water in reversed-phase LC.

B term: The second term of the van Deemter equation. See also longitudinal diffusion and molecular diffusion term.

Backflushing: A column switching technique in which a valve is placed between the injector and the column. This valve allows for mobile phase flow in either direction. Backflushing is used to elute strongly held compounds at the head of the column. It can be used for analysis of these compounds or merely to remove them from the column.

Back pressure: Same as head pressure, column pressure.

Back pressure regulator: A device placed on-line after the detector to maintain a positive pressure on the flow cell minimizing solvent outgassing problems in the detector.

Band: Refers to the chromatographic peak as it moves down and is eluted from the column.

Band broadeningThe process of increasing width and concomitant diluting of the chromatographic band as it moves down the column. The peak is injected as a narrow slug and, ideally, each separated component would be eluted as a narrow slug of pure compound if not for the process of band broadening.

Bandwidth (t w ): The width of the chromatographic band during elution from the column. It usually is measured at the baseline by drawing tangents to the inflection points on the sides of the Gaussian curve that represents the peak. Small bandwidths usually represent efficient separations; also called peak width. See Figure 2.

Bar: A unit of pressure measurement in HPLC equal to 1 atm, ~15 lb/in.2, or 0.1 MPa.

Baseline: The baseline is the line drawn by the recording device representing the signal from the detector when only mobile phase is passing through. It also represents the point from which calculations are often made on peaks to determine peak area or peak height.

Baseline noise: Irregular variations (short term) in the chromatographic baseline due to electrical noise or temperature fluctuations, outgassing in the flow cell, or poorly mixed mobile phase solvents.

BET method: Developed by Bruner, Emmett, and Teller (BET), a method for measuring surface area that uses nitrogen adsorption–condensation in pores at liquid nitrogen temperature. Pore volume and pore size distribution also can be obtained from BET method calculations.

Bidentate silane: A specific type of bonded phase in which a short hydrocarbon bridge connects two silicon atoms in a silane that is bound to the surface through two siloxane groups.

Binary mobile phase: Mobile phase comprising two solvents or buffers.

Biocompatible: A term to indicate that the column or instrument component will not irreversibly or strongly adsorb or deactivate biomolecules such as proteins. Frequently means metal-free or ceramic surfaces and components.

Bonded-phase chromatography: The most popular mode in LC in which a phase chemically bonded to a support is used for separation. The most popular support for bonded-phase chromatography is microparticulate silica gel, and the most popular type of bonded phase is organo-silane such as octadecyl for reversed-phase chromatography. Approximately 70% of all HPLC applications are performed using chemically bonded phases.

Bonded-phase concentration: See coverage.Boxcar chromatography: See column switching.

Breakthrough volume: The volume at which a particular solute pumped continuously through a column will begin to be eluted. It is related to the column volume and the retention factor of the solute. It is useful to determine the total sample capacity of the column for a particular solute.

Buffer: A solution that maintains constant pH by resisting changes in pH from dilution or addition of small amounts of acids and bases.

Buffer capacity: A quantitative measure of the potential of a buffer solution (defined as the number of equivalents of strong acid or base to cause a one pH unit change in 1 L of a buffer solution) or simply the ability of a buffer to withstand injections of a buffered sample solution without changing mobile-phase pH; capacity determined by pH, buffer pK a , and buffer concentration.

Buffer strength: See buffer capacity.