Page 7 - BR20828_E Accucore 0713S

is shifted to a higher value and the curve
becomes flatter, which indicates that a wider
flow rate range can be used without losing
performance. For instance, for porous 5µm
particles the best linear velocity is less that
2mm/s and when the linear velocity increases
the efficiency drops quickly, whereas for
solid-core 2.6µm particles the linear velocity
that provides the best efficiency is around
3.5mm/s (which corresponds to 400µL/min
for a 2.1mm id column) and there is a wide
range of flow rates that can be used. The
highest efficiency and lowest rate of
efficiency loss with linear velocity is observed
for the solid-core material.
Figure 4 shows the column backpressure
measured for the same set of experiments.
Reducing the particle size increases the
observed back pressure and for the data
shown in Figure 4 it can be seen that for
chromatographic systems that have a
pressure limit of 400 bar this will reduce the
effective flow rate range that can be used on
a column. In this example the data was
generated on a 100 x 2.1mm column using a
mixture of acetonitrile and water, where the
optimum flow rate is approximately
400µL/min. Clearly the use of a sub 2µm
material will limit the use of many standard
HPLC systems where the maximum
operating pressure is 400 bar. However, the
solid-core material is able to operate at
800µL/min, double the flow rate before it
experiences the same issues.
This van Deemeter equation graphical
representation has limitations as it allows us
to understand the effect of band broadening
on the efficiency and how that varies with
linear velocity of the mobile phase but it
does not account for analysis time or
pressure restrictions of the chromatographic
system, or in other words, it does not
account for the flow resistance or the
permeability of the column. Kinetic plots [3]
are an alternative method of plotting the
same data (HETP and linear velocity values)
which takes into account the permeability of
the columns, which is a measure of column
length, mobile phase viscosity, and maximum
pressure drop across the column, and
therefore allow us to infer the kinetic
performance limits of the tested
chromatographic materials. The linear
velocity, conventionally plotted on the x-axis
in the van Deemeter plot, is transformed into
the pressure drop limited plate number.
Using a maximum pressure drop for the
system, any experimental set of data of
HETP- linear velocity obtained in a column
with arbitrary length and pressure drop can
be transformed into a projected efficiency
(N)-t0 representing the plate number and t0-
time, which could be obtained if the same
chromatographic support was used in a
column that was long enough to provide the
maximum allowed inlet pressure for the
given linear velocity.
The mathematics underlying the kinetic plot
method is very simple and is based on three
‘classical’ chromatographic equations
(Equations 3 to 5). Kinetic plots are ideally
suited to compare the performance of
differently shaped or sized LC supports.
Equation 3
L – column length
N – efficiency
H – HETP
Equation 4
µ - Linear velocity of mobile phase
t 0 – dead time of the chromatographic
system
Equation 5
ΔP – pressure drop
K v – column permeability
η - mobile phase viscosity
Impedance
Kinetic plots can take different forms, and
some of the simpler forms are displayed in
Figure 5 (a) and (b). These compare the
column efficiency per unit time (a) and
column efficiency per unit length (b), for the
fully porous 5, 3 and sub-2µm and solid-core
2.6µm particles. The Accucore 2.6µm
material is the most efficient per unit length
of column and the most efficient per unit
time, with the fully porous sub-2µm
performing similarly. Figures 5 (c) and (d)
L
µ
t 0 =
ΔPK v
ηL µ =
L= NH
Figure 3: Efficiency comparison using Van Deemter plots for Accucore 2.6µm and fully porous 5, 3 and sub-
2µm.
Figure 4: Comparison of column pressure for Accucore 2.6 µm and fully porous 5 and 3 and sub-2µm
(100x2.1 mm columns, mobile phase: water/acetonitrile (1:1), temperature 30°C).