Page 36 - BR20828_E Accucore 0713S

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Figure 1: Comparison of column pressure for Accucore XL 4 µm HPLC column and fully porous 5 µm and 3 µm columns.
All columns: 150 x 4.6 mm; test conditions: mobile water / acetonitrile (50:50 v/v); column temperature: 30 °C.
Equation 1
where
ΔP = pressure drop across the column
a = constant (dependent on packing, normal
values in the range 150-225)
εi = interstitial porosity of the packed bed
F = flow rate through the column
L = length of the column
η = 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
normalized efficiency (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 [1] are an
alternative method of plotting the same experimental
data but allow other parameters, such as pressure, to
be incorporated. Therefore, we can infer the kinetic
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 kinetic
plots, a term called impedance is used. Impedance
(Equation 2) 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
t = dead time of chromatographic system
ΔP= pressure drop
η = kinematic viscosity of mobile phase
N = efficiency
In kinetic plots, 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)-t 0. This represents the plate number and
t 0-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.
Pressure Comparison
Figure 1 shows how the column backpressure of the
Accucore XL 4 μm HPLC column compares with that
of the fully porous 5 μm and 3 μm columns tested.
On average (across the flow rate range tested), the
Accucore XL 4 µm HPLC column gives 42% higher
pressure than fully porous 5 µm and 13% lower
pressure than fully porous 3 µm HPLC columns.
Even when running the 150 x 4.6 mm Accucore XL
4 µm HPLC columns at a flow rate of 2 mL/min, the
backpressure is only 200 bar.
Pressure (bar)
Flow rate (µL/min)
400 200
50
0
100
150
200
250
600 800 1000 1200 1400 1600 1800 2000
Accucore XL 4 µm
Fully porous 5 µm
Fully porous 3 µm
∆P = a η F L (1 - ) 2
d 2
c d 2
p 33
i
3i E =
∆P t
N η 2
∆P = a η F L (1 - ) 2
d 2
c d 2
p 33
i
3
i