Page 30 - BR20828_E Accucore 0713S

2 Equation 1
where ΔP – pressure drop across the column
a – constant (dependent on packing, normal
values in the range 150 -300 [1,2])
εi – interstitial porosity of the packed bed
F – flow rate through the column
L – length of the column
η – kinematic viscosity of the mobile phase
dp – particle diameter
dc – column internal diameter
The conventional approach to compare the
chromatographic performance of columns is to plot
a HETP - height equivalent to a theoretical plate
as a function of mobile phase flow rate or linear
velocity, often referred to as a van Deemter plot. This
approach does have limitations, since it does not
account for analysis time or pressure restrictions of
the chromatographic system. Kinetic plots [3] are an
alternative method of plotting the same experimental
data but allowing other parameters such as pressure
to be incorporated, and therefore allow us to infer the
these performance limits of the tested chromatographic
materials. There are a variety of ways in which this
data can be presented and all of these plots are referred
to as kinetic plots. In one of the most useful forms of
these plots a term called impedance is used. Impedance
(Equation 2) is a term that defines the resistance a
compound is subjected to as it moves down the column
relative to the performance of that column. This term
gives a true measure of the performance of the column
as it incorporates efficiency, time, and pressure, which
are critical practical considerations of a chromatographic
separation.
Equation 2
where E – impedance
ΔP – pressure drop across the column
η – kinematic viscosity of mobile phase
N – efficiency
t0 – column dead time
Pressure comparison
Figure 1 shows how the column backpressure of the
Accucore XL 4 μm column compares with that of the
Accucore 2.6 μm column. On average, across the flow
rate range tested, the pressure measured on the Accucore
2.6 μm column is 2.2 times higher. At 1 mL/min flow rate
the pressures measured are 94 and 202 bar for the 4 and
2.6 μm columns, respectively.
Figure 1: Comparison of column pressure for Accucore XL 4 µm and Accucore 2.6 µm columns
All columns 150 × 4.6 mm; test conditions: water / acetonitrile (50:50 v/v) mobile phase, 30° C column
temperature
Pressure (bar)
Flow rate (μL/min)
50
0
100
150
200
250
300
350
400
450
500 1000 1500 2000 2500
Accucore XL C18 4 μm Accucore C18 2.6 μm
ΔP = a
(1 – εi)2
εi
3
F L η
dc
2 dp
2 E =
ΔP t0
η N 2