High-speed isocratic and gradient liquid-chromatography separations at 1500 bar

• Separations were conducted at 1500 bar using 1.5 μm core–shell particles.
• The effect of pressure and particle size on kinetic gain factors have been assessed.
• A 13-fold reduction in analysis time was achieved in gradient LC.

The need to improve either sample throughput on separation efficiency has spurred the development of ultra-high-pressure LC instrumentation, allowing to operate up to column pressures of 1500 bar. In the present study, the isocratic and gradient performance limits were assessed at UHPLC conditions applying columns packed with core–shell particles. First, the extra-column band broadening contributions were assessed and minimized. Using an optimized system configuration minimum reduced plate heights of 1.8 were recorded on 2.1 × 100 columns packed with 1.5 μm core–shell particles. Increasing the pressure limit from 500 to 1500 bar and at the same time reducing the particle size from 2.6 to 1.5 μm has allowed the analysis time to be decreased by a factor of 1.5 in isocratic mode, while maintaining separation efficiency (N = 54,000). The kinetic time-gain factor in isocratic mode was proportional to the ratio of the separation impedance of both columns multiplied with the pressure ratio applied. In addition, the effect of operating pressure on the time gain factor was assessed in gradient mode. Using optimized gradient steepness (tG/t0 = 12) and increasing the operating pressure from 500 to 1500 bar a time gain factor of almost 13 was achieved for the separation of a mixture of waste-water pollutants without compromising peak capacity.