Page 168 - Illustrated Pocket Dictionary of Chromatography

R f VALUE 169

retention time window A predefined elution time range that is
established for a unique analyte and specific operating conditions in
order to assign peak identification. Instrument operational stability
(e.g., pump, injector, column, detector, integrator) can also readily be
assessed in this fashion.

retention volume, V r The retention time, t r, multiplied by the
flow rate F:

V r = t r ¥ F
reversed phase Term derived from the fact that original LC sepa-
rations were conducted on polar supports (e.g., silica) using nonpo-
lar to slightly polar mobile phases (e.g., hexane or 99/1 v/v hexane/
methylene chloride); these conditions were “normal.” Reversed
phases have nonpolar supports (e.g., octadecyl) and polar hydroor-
ganic mobile phases (e.g., 60/40 v/v water/methanol). Reversed-phase
supports, in part because of the wide range available, are by far the
most commonly used in LC separations. It should be noted that with
the advent of polar bonded phases (e.g., diol) in conjunction with
hydroorganic mobile phases the clear distinction between normal and
reversed phase has blurred.

Reynolds’ number, Re A dimensionless parameter that esti-
mates the relative internal and viscous strengths in a flowing liquid:

Re =rvd h

where r is the fluid density, is the fluid velocity, d is the tube diam-
eter, and h is the fluid viscosity. The Reynolds’ number is used to
predict whether flow is laminar or turbulent. A low number signifies
laminar flow.

R f value The thin-layer chromatography parameter that defines the
position of the analyte band on the plate:
( distance of the analyte band from initial spot)
R f =
( distance to the solvent front from the initial spot )

Note that it is important that the solvent from never reach the top of
the plate. If this happens, then an R f value cannot be calculated.