The effect of the frictional heat on retention and efficiency in thermostated or insulated chromatographic columns packed with sub-2-μm particles

Retention prediction of monoamine neurotransmitters in gradient liquid chromatography

Retention prediction of monoamine neurotransmitters has been compared for the generally applied linear solvent-strength model and quadratic polynomial three-parameter model. The design of experiments protocol has been applied to plan linear gradients within the experimental space with altered gradient time, mobile phase flowrate, and column temperature. Relative prediction errors increased at elevated temperature, which is more significant for the linear solvent-strength model when compared to the polynomial model. On the other hand, the predefined design of experiments space controls the retention time errors, as predictions for LC conditions that are outside of the plan are much less accurate and should be avoided. The final part of the work deals with the effect of extracolumn band dispersion on the peak capacity of linear gradients at various gradient times, mobile phase flowrates, and column temperature. The peak capacity determined for corrected experimental data were consistent with the published results dealing with the optimization of peak capacity in gradient elution. This article is protected by copyright. All rights reserved.

The influence of injection volume on efficiency of microbore liquid chromatography columns for gradient and isocratic elution

The injection volume and the associated column volume overload is one of the most common issues in miniaturized chromatography. The injection volume should not exceed 10% of the effective column volume. A further reduction of the injection volume leads to an increase in chromatographic efficiency. However, the signal intensity must be above a certain threshold to generate a chromatographic peak that can be detected. Therefore, the injection volume has to be optimized to reach the ideal balance between chromatographic efficiency and sensitivity. This study examined the general influence of the injection volume for both isocratic and gradient elution, depending on the retention factor and peak standard deviation. For this purpose, substances of different polarity were selected to represent a broad elution spectrum. Besides the model analyte naphthalene, these were mainly pharmaceuticals. For all measurements a microbore column with an ID of 300 µm and packed with 1.9 μm fully porous particles was used. For isocratic elution, the injection volume was varied between 4 and 16% of the effective column volume. The retention factors were adjusted between 2 and 10. For gradient elution, the injection volume was varied between 4 and 160% of the effective column volume. The observed effects were further investigated using the gradient kinetic plot theory. In isocratic elution, a loss in plate height up to 50% was observed for components that elute near the void time. A significant reduction of the chromatographic efficiency was noticed up to a retention factor of 4. In gradient elution, a reduction in peak capacity could only be observed if the injection volume exceeded 40% of the effective column volume. For some substances, only a slight loss in peak capacity was noticed even with a volume overload of 160%.

Liquid chromatography above 20,000 PSI

Continued improvements in HPLC have led to faster and more efficient separations than previously possible. One important aspect of these improvements has been the increase in instrument operating pressure and the advent of ultrahigh pressure LC (UHPLC). Commercial instrumentation is now capable of up to ~20 kpsi, allowing fast and efficient separations with 5-15 cm columns packed with sub-2 μm particles. Home-built instruments have demonstrated the benefits of even further increases in instrument pressure. The focus of this review is on recent advancements and applications in liquid chromatography above 20 kpsi. We outline the theory and advantages of higher pressure and discuss instrument hardware and design capable of withstanding 20 kpsi or greater. We also overview column packing procedures and stationary phase considerations for HPLC above 20 kpsi, and lastly highlight a few recent applicatioob pressure instruments for the analysis of complex mixtures.