Page 132 - Illustrated Pocket Dictionary of Chromatography

MONODISPERSE 131

being tested. The range of molecular weights that are analyzed is
determined by the total exclusion volume (in the above case com-
4
pounds that have a molecular weight of 10 or above are too large to
penetrate any pores in the packing material) to the total inclusion (or
2
permeation) volume (at a molecular weight of 10 or less, all pores
are accessible to the sample). It should be noted that not only the size
but also the shape of the sample molecule is critical to determining
the access to pores of various diameters (not just the molecular
weight).

molecular weight distribution For compounds that do not
have one defined molecular weight (e.g., benzene has a molecular
weight of 77g/mol), such as polymers, the molecular weight distribu-
tion defines the molecular weight range for the sample and the amount
of material present in the sample for each molecular weight segment.
Some of the ways polymer distributions are described are by the
average molecular weight, z-average molecular weight, z+1-average
molecular weight, etc.
moment Statistical moments are used to mathematically define the
elution profile (or distribution) and are in essence the concentration
(C) time (t) function of the profile.

Zeroth moment, peak area, A; A =ÚCdt
First moment, retention time, t r; t r = [1/A] ÚCtdt
2 2 2 2
Second moment, variance or peak width, s ; s = [1/A]ÚCt dt - t r
monochrometer A component in a spectrophotometer that is
used to produce a narrow range of radiation (e.g., a narrow spectral
wavelength band) that passes through the sample. For conventional
spectrophotometric detectors, the monochrometer is either a diffrac-
tion grating or a prism, is responsible for separating the source radi-
ation into spatially separated wavelengths, and is placed before the
sample cell. The exact bandwidth is determined by the placing and
width of the inlet. In a photodiode array detector there is no true
monochrometer because the separation of wavelengths occurs after
the radiation passes through the sample. The bandwidth is determined
by the angle subtended by an individual diode with respect to its posi-
tion and distance from the prism.

monodisperse Defines a sample that has a very narrow distribu-
tion of a property, such as molecular weight distribution for a polymer
or particle diameter for a support material.