Theory of separation performance and peak width in gradient elution liquid chromatography: A tutorial

Separation performance in chromatography has been extensively studied since the dawn of the technique. Although the basic principles of band broadening and the resulting separation performance in isocratic elution are in general well known and understood, this is much less the case for gradient separations. In this tutorial, first the basic principles, concepts and parameters that determine separation performance, peak width and variance and analysis time in isocratic separations are reviewed. This is subsequently used to discuss the parameters that affect peak width in gradient elution, together with the concepts of plate count and plate height in this elution mode. In addition, the effect of peak compression in gradient elution is elaborated. Finally, the effect of extra-column dispersion on separation performance in gradient elution is discussed, and an overview of how these contributions can be experimentally evaluated is given.

Reduction of the extra-column band dispersion by a slow transport and splitting of a sample band in isocratic reversed-phase liquid chromatography

Sample introduction method was studied to reduce the extra-column effect in reversed-phase HPLC. Slow transport of a sample band (SToSB) in the pre-column space followed by the introduction of the band into the column at a near-optimum flow rate resulted in larger plate counts for a 1.0 mmID, 5 cm long column as much as 1.4-1.6 times for solutes with a retention factor (k) of 0.5-1.8 compared to a conventional elution method. Further reduction of the extra-column effect was possible by orthogonally splitting the sample band (SplSB) by flow switching during its slow transport followed by the introduction of the leading part of the band into the column. In this case, increased plate counts of up to 2-3 times for solutes with k of 0.5-1.8 were observed for a 1.0 mmID, 5 cm column. The sample introduction method, SToSB in the injector and the pre-column tube of a few μL, was found to reduce the extra-column band variance by 0.4-0.5 μL² for an UHPLC system with the extra-column volume (V_extra) of ca. 4.6 μL and the system variance (σ_extra²) of 1.1 μL² at flow rate of 100 μL/min, while SToSB and subsequent SplSB were found to be more effective, reducing σ_extra² by about 0.8 μL². With an UHPLC instrument with V_extra of about 10 μL and σ_extra² of ca. 3.6 μL² at flow rate of 300 μL/min, 1.4-2.1 times as many plate counts were observed with SToSB and SplSB compared to the normal elution method for early-eluting solutes with k=0.25-1.7 for a column, 2.1 mmID, 5 cm long. With this UHPLC instrument, SToSB and/or SplSB resulted in the reduction of σ_extra² by 1.2-2.2 μL².

Assessing effects of ultra-high-pressure liquid chromatography instrument configuration on dispersion, system pressure, and retention

This study involves the systematic assessment of the effects of system configuration on dispersion, pressure, and retention characteristics while operating a 1500 bar UHPLC system with 2.1 mm i.d. × 100 mm long columns packed with 1.5 µm core-shell particles in isocratic and gradient mode. Altering the system configuration by changing the i.d. of connection tubing and flow cells affects the elution time, dispersion characteristics, and the kinetic performance limits of the system. The gain in separation efficiency when decreasing tubing i.d. from 100 to 75 µm was found to contribute more to the decrease in separation impedance and the position of the kinetic performance curve than the loss in available column pressure induced by the narrower tubing. When applying steep gradients, characterized by gradient-to-column dead-time ratio < 7, optimizing instrument configuration leads to either a significant time gain factor of 3.9 without compromising peak capacity, or a gain in peak capacity with a gain factor of 1.3 while maintaining the analysis time constant. Due to the reduced fluidic volume of connection tubing of smaller i.d., a decrease in residence time is obtained. At the same time, an increase in k was observed due to a pressure-induced retention effect, and this effect is significant for late-eluting analytes.