Page 80 - Illustrated Pocket Dictionary of Chromatography

76 FICK’S FIRST LAW OF DIFFUSION

Ferrules are typically one piece in HPLC; unions are made from stain-
less steel, PEEK, and other solvent-resistant polymers that can toler-
ate high pressure. Some high-pressure ferrules are two piece. In some
cases, the nut and ferrule are made together to form a component that
can be tightened by hand. Common materials of construction for GC
ferrules are graphite, vespel, and vespel-graphite blended material.
For gas unions, copper tubing with brass ferrules and fittings are used.

Fick’s first law of diffusion This law defines the variation of mass
flow through a surface as a function of the concentration gradient:

D
∂N ∂t =- ( ∂n ∂l)
where N is the number of molecules, t is the time, D is the diffusion
coefficient, n is the concentration of molecules, l is the diffusion
distance, and ∂n/∂l is the concentration gradient. Note that ∂N/∂t is
also called the flux or mass flow per unit time. The negative sign indi-
cates that the flux is in the direction of lower concentration.
Fick’s second law of diffusion This law describes the longitu-
dinal diffusion that occurs in a chromatographic column:

2
2
D
∂N ∂t = ( ∂ n ∂l )
Rearranged and solved for mass flux, n becomes:
n = [ N 2 (pD t) ] e -( l 2 4D t)
12
which describes a Gaussian-like curve where N is the number of mol-
ecules, t is the time, D is the diffusion coefficient, n is the concentra-
tion of molecules, and l is the diffusion distance. This law describes
the time rate of change of concentration with respect to the spatial
rate of change along the concentration gradient.
field-flow fractionation (FFF) A technique used for the sepa-
ration and characterization of high-molecular-weight and particulate
species. The separation is generated in an open flow channel across
which an applied field is applied. These external fields include gravi-
tational force fields, etc.
A measure of how much liquid stationary phase
film thickness, d f
is deposited on the support surface. Film thicknesses typically range
from 0.25 to 5mm. Thin films offer rapid phase transfer, minimize peak
broadening, and maximize resolution and peak capacity. Thick films
generate less resolution but offer much higher sample loading capacity.