UHPLC AND HPLC COLUMN PERFORMANCE continued
Figure 2 – Plots (h-u) for two different solutes on a 2.7-µm Fused-Core C8 silica, 50 × 4.6 mm column showing solute-dependent velocity optima. Samples: naphthalene (MW 128.2); mobile phase: 60% acetonitrile/40% water; lorazepam (MW 321.2); mobile phase: 30% acetonitrile/70% 20 mM sodium phosphate, pH 3.5; temperature: 24 °C. Reproduced with permission from Ref. 4.
be selected for high-speed methods. Critical pairs with similar structures are less likely to be a problem than solutes having structural differences. Although solute-dependent kinetic behavior was shown by Kirkland for Fused-Core columns, it has been observed for all HPLC and UHPLC silica-based columns, especially in reversed-phase mode. Giddings has described this behavior in terms of adsorption-desorption kinetics and diffusion-controlled kinetics.6
Results and discussion
Potential advantages for monodisperse, porous silica Early theory by Giddings strongly supported the idea that columns with uni- form particles should be more efficient and consistent.6 Knox7,8
In 1999 and 2002, revisited the van Deemter relationship and proposed that significant
performance gains could still be made by improving HPLC column-bed uniformity. This suggested that some of the performance advantages for core-type particles might be explained by its unique, narrow PSD of only 10% RSD; porous particles typically vary in PSD between 20 and 30% RSD or more. Earlier experiments9
with porous particles had not conclusively
established performance advantages for narrow PSD silica; however, the success of core-type particles renewed interest in narrow PSD10 means to improve performance of porous silica columns.
as a possible
The first commercial porous silica to feature a narrow PSD that was equal to Fused-Core silica was described in 2012 for 1.9-µm, Titan C18 particles.11
Figure 3 – Kinetic plots (h-u) for 1.9-µm Titan porous and 2.7-µm Fused- Core C18 columns demonstrating comparable, high performance for the two monodisperse particles; a broader-distribution 1.7-µm C18 column with lower performance is shown for comparison. Sample: uracil, diaz- epam, toluene, naphthalene, biphenyl; columns: 50 × 3.0 mm; mobile phase: 60% acetonitrile/40% water; temp: 35 °C, flow rate: variable; detection: 254 nm.
a
b
Figure 3 demonstrates reduced plate heights below 2 (h =
1.7) at the optimum velocity for monodisperse, totally-porous silica C18 columns. The van Deemter plots for small molecules are shown to nearly overlap for the two columns with monodisperse particles, indicating that monodispersity may be an important performance factor; a column with broader distribution porous particles is shown for comparison. Figure 4 compares particle distributions for monodisperse Titan and Fused-Core silica to commercial porous silicas; particle samples were removed from new commercial columns for testing.
Figure 4 – Particle size distribution (A) for different silicas with an SEM photograph (B) showing monodisperse Titan silica.
AMERICAN LABORATORY • 10 • AUGUST 2015
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