Improved reversed-phase gradient retention modeling

Wide injection zone compression in gradient reversed-phase liquid chromatography

Chromatographic zone broadening is a common issue in microfluidic chromatography, where the sample volume introduced on column often exceeds the column void volume. To better understand the propagation of wide chromatographic zones on a separation device, a series of MS Excel spreadsheets were developed to simulate the process. To computationally simplify these simulations, we investigated the effects of injection related zone broadening and its gradient related zone compression by tracking only the movements of zone boundaries on column. The effects of sample volume, sample solvent, gradient slope, and column length on zone broadening were evaluated and compared to experiments performed on 0.32mm I.D. microfluidic columns. The repetitive injection method (RIM) was implemented to generate experimental chromatograms where large sample volume scenarios can be emulated by injecting two discrete small injection plugs spaced in time. A good match between predicted and experimental RIM chromatograms was observed. We discuss the performance of selected retention models on the accuracy of predictions and use the developed spreadsheets for illustration of gradient zone focusing for both small molecules and peptides.

Repetitive injection method: a tool for investigation of injection zone formation and its compression in microfluidic liquid chromatography

Sample introduction in microfluidic liquid chromatography often generates wide zones rather than peaks, especially when a large sample volume (relative to column volume) is injected. Formation of wide injection zones can be further amplified when the sample is dissolved in a strong eluent. In some cases sample breakthrough may occur, especially when the injection is performed into short trapping columns. To investigate the band formation and subsequent zone focusing under gradient elution in situations such as these, we developed the Repetitive Injection Method (RIM), based on the temporally resolved introduction of two discrete peaks to a column, mimicking both the leading and trailing edges of a larger, singly injected sample zone. Using titanium microfluidic 0.32 mm I.D. columns, the results of RIM experiments were practically identical to injection of a correspondingly larger single zone volume. It was also experimentally shown that zone width (spacing between two injected peaks) decreases during gradient elution. We utilized RIM experiments to investigate wide sample zones created by strong sample solvent, and subsequent gradient zone focusing for a series of compounds. This experimental work was compared with computationally simulated chromatograms. The success of sample focusing during injection and gradient elution depends not only on an analyte's absolute retention, but also on how rapidly the analyte's retention changes during the mobile phase gradient.

Recent progress in the fundamental understanding of hydrophilic interaction chromatography (HILIC)

With the exponential growth in the application of the HILIC technique, there has been a significant progress in understanding the fundamental aspects of hydrophilic interaction chromatography.

Use of individual retention modeling for gradient optimization in hydrophilic interaction chromatography: separation of nucleobases and nucleosides

In this study, the separation of twelve nucleobases and nucleosides was optimized via chromatogram simulation (i.e., prediction of individual retention times and estimation of the peak widths) with the use of an empirical (reversed-phase) non-linear model proposed by Neue and Kuss. Retention time prediction errors of less than 2% were observed for all compounds on different stationary phases. As a single HILIC column could not resolve all peaks, the modeling was extended to coupled-column systems (with different stationary phase chemistries) to increase the separation efficiency and selectivity. The analytical expressions for the gradient retention factor on a coupled column system were derived and accurate retention time predictions were obtained (<2% prediction errors in general). The optimized gradient (predicted by the optimization software) included coupling of an amide and an pentahydroxy functionalized silica stationary phases with a gradient profile from 95 to 85%ACN in 6 min and resulted in almost baseline separation of the twelve nucleobases and nucleosides in less than 7 min. The final separation was obtained in less than 4h of instrument time (including equilibration times) and was fully obtained via computer-based optimization. As such, this study provides an example of a case where individual retention modeling can be used as a way to optimize the gradient conditions in the HILIC mode using a non-linear model such as the Neue and Kuss model.

Retention and bandwidths prediction in fast gradient liquid chromatography. Part 2-Core-shell columns

Recently, we confirmed that the well-established theory of gradient elution can be employed for prediction of retention in gradient elution from the isocratic data, method development and optimization in fast gradient chromatography employing short packed fully porous and monolithic columns and gradient times in between 1 and 2min, or even less. In the present work, we extended this study to short core-shell reversed-phase columns. We investigated the effects of the specification of the stationary phase in the core-shell structure on the prediction of gradient retention data. Two simple retention models describing the effects of the mobile phase on the retention by two-parameter equations yield comparable accuracy and can be used for prediction of elution times. The log-log model provides improved prediction of gradient bandwidths, especially for less retained compounds. A more sophisticated three-parameter model did not offer significant improvement of prediction. We compared the efficiency, selectivity and peak capacity of fast gradient separations of alkylbenzenes, phenolic acids and flavones on seven core shell columns with different lengths and chemistry of bonded shell stationary phase. Within the limits dictated by a fixed short separation time, appropriate adjustment of the range of the composition of mobile phase during gradient elution is the most efficient means to optimize the gradient separation. The gradient range affects sample bandwidths equally or even more significantly than the column length. Both 5-cm and 3-cm core-shell columns may provide comparable peak capacity in a fixed short gradient time.