Page 137 - Illustrated Pocket Dictionary of Chromatography

136 NORMAL PHASE

normal phase The term came about after the development of
bonded phases that had a nonpolar character (e.g., octadecyl). A
simple way to remember this is that the original underivatized support
materials (e.g., silica, alumina) for LC had polar surfaces and nonpolar
(or polar-modified nonpolar mobile phase). These then were the usual
or normal-phase combinations used in a separation. Contrast that to
the nonpolar bonded phase (octadecyl) and the polar hydroorganic
mobile phases used to generate an elution and the term for these
became reversed phase. The present-day meaning has blurred some-
what with the advent of nonaqueous reversed-phase separations,
highly polar bonded phases, and other more specialized separations,
but in general the terminology and the distinction still exist for many
mainstream separations.

number-average molecular weight, M n Determined from the
elution profile generated in an SEC separation and mathematically
determined by:

M n = ( n 1 M 1 + n 2 M 2 + ... + i n M i ) ( n + n + ... + i n )
2
1
= Â ( i n M i ) Â i n
where n i represents the number of molecules having molecular weight
M i. Summation occurs over all possible M i molecular weights in the
sample.

nut A component, typically threaded, that along with the compres-
sion fitting and deformable ferrule, creates a leak-tight union around
tubing (e.g., connecting, column, etc).