Intrinsic advantages of packed capillaries over narrow-bore columns in very high-pressure gradient liquid chromatography

• 250 μm × 100 mm capillary columns were packed with sub-2 μm fully porous particles.
• The plot of their peak capacity vs. efficiency was measured at constant gradient steepness.
• Peak capacities of capillary columns are intrinsically higher than that of narrow-bore columns.
• This is partly explained by the complete elimination of radial temperature gradients.
• Also, transverse eluent mixing is more efficient in capillary than in narrow-bore columns.

250 μm × 100 mm fused silica glass capillaries were packed with 1.8 μm high-strength silica (HSS) fully porous particles. They were prepared without bulky stainless steel endfittings and metal frits, which both generate significant sample dispersion. The isocratic efficiencies and gradient peak capacities of these prototype capillary columns were measured for small molecules (n-alkanophenones) using a home-made ultra-low dispersive micro-HPLC instrument. Their resolution power was compared to that of standard 2.1 mm × 100 mm very high-pressure liquid chromatography (vHPLC) narrow-bore columns packed with the same particles. The results show that, for the same column efficiency (25 000 plates) and gradient steepness (0.04 min−1), the peak capacity of the 250 μm i.d. capillary columns is systematically 15–20% higher than that of the 2.1 mm i.d. narrow-bore columns. A validated model of gradient chromatography enabled one to predict accurately the observed peak capacities of the capillary columns for non-linear solvation strength retention behavior and under isothermal conditions. Thermodynamics applied to the eluent quantified the temperature difference for the thermal gradients in both capillary and narrow-bore columns. Experimental data revealed that the gradient peak capacity is more affected by viscous heating than the column efficiency. Unlike across 2.1 mm i.d. columns, the changes in eluent composition across the 250 μm i.d. columns during the gradient is rapidly relaxed by transverse dispersion. The combination of (1) the absence of viscous heating and (2) the high uniformity of the eluent composition across the diameter of capillary columns explains the intrinsic advantage of capillary over narrow-bore columns in gradient vHPLC.