Page 8 - BR20828_E Accucore 0713S

show the relationship between impedance
and mobile phase linear velocity or
impedance and efficiency. Impedance
(Equation 6) 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. Impedance is often plotted in a
reversed N – axis to mimic the van Deemter
plot (Figure 5d). From the four materials in
this study, the solid-core 2.6µm HPLC
columns provide the highest efficiency with
the lowest impedance. Of particular interest
is the comparison of the impedance of sub-
2µm fully porous and the solid-core
materials. These show similar values of
efficiency (as demonstrated in Figure 3),
however the impedance is directly
proportional to the pressure drop across the
column. Solid-core particle packed columns
show a pressure drop that can be half or
even less that of a fully porous sub-2µm
particle packed column, of the same length
and internal diameter, when run under
identical conditions. Therefore, solid-core
particles are favoured because for the same
nominal pressure they provide the highest
efficiency. The kinetic plots in Figure 5 were
plotted using a template downloaded from
reference [4].
Equation 6
E – impedance
ΔP – pressure drop
η - kinematic viscosity of mobile phase
N - efficiency
Peak capacity, resolution and sensitivity
In isocratic separations efficiency, measured as
plate height or number of theoretical plates, is
used as a measure of chromatographic
performance. However, this concept is not
applicable to gradient elution. An alternative
measure of separation efficiency is peak
capacity, which is a concept first introduced by
Giddings [5]. Peak capacity (Pc) represents
the maximum number of components that
can be chromatographically separated with
unit resolution (Rs ~1) within a given time
window (Δt) under a given set of experimental
conditions. For a linear solvent strength
gradient, the peak capacity, Pc, is given by
Equation 7 [6].
Equation 7
Δt – analysis time for which the peak capacity
is calculated
σ t – standard deviation of a peak
R s – resolution
Equation 8
t g – gradient time
W – peak width at baseline
ΔPt
ηN2 E =
Pc = Δt
4 σ t R s 1 +
Pc =
t g
w 1 +
Figure 5: Performance comparison of Accucore 2.6µm and fully porous 5, 3 and sub-2µm using kinetic plots:
(a) efficiency per unit time, (b) efficiency per column unit length, (c) impedance relative to mobile phase linear
velocity, (d) impedance relative to efficiency.
Figure 6: Effect of column loading on chromatographic parameters. (a) Comparison of loading 2µ g on solid-core 2.6µ m and fully porous sub-2µ m; (b) Effect of loading
on peak asymmetry (As), retention time (Tr) and efficiency (N) on the solid-core column.