Since 2001 when they became available in North America, monolithic silica HPLC columns have become valuable analytical tools in the R&D and methods development laboratories.
Since 2001 when they became available in North America, monolithic silica HPLC columns have become valuable analytical tools in the R&D and methods development laboratories. Recently the range of monolithic silica columns has been extended with the introduction of Chromolith® RP18e columns with 4.6 mm and 3 mm i.d. and their increased sensitivity for detecting low level impurities.
Using Chromolith® columns of any diameter solves the particulate packed column problem where backpressure increases beyond the capability of the pumping system. Many publications show the utility of the Chromolith® RP18e chemistry for doing fast HPLC since flow rates can be increased with no problems with backpressure for routine separations using standard HPLC systems without the need for a special UHPLC system.
Although most chromatographers apply rigid sample and mobile phase preparations to protect their columns, sub-2 μm particulate columns will more readily clog with impurities to generate even higher backpressures, and worse, lead to peak broadening that allows more samples to be applied with little clogging or increasing backpressure even with hundreds of samples. With increased column longevity, the need to replace columns in the middle of the sample study is reduced.
Figure 1 shows the ability to increase the flow rate for the fast gradient separation of a mixture of alkylbenzenes on the Chromolith® RP18e, 2 × 50 mm column. Normally when converting from a 4.6 to a 2 mm i.d. column, the flow of 1 ml/min on the 4.6 mm i.d. column is reduced to 0.2 ml/min to make the linear velocity (and efficiency) equal from one i.d. column to the other. Using 0.2 ml/min as a starting point, the initial separation (top chromatogram) is completed in 4 min, with only 250 psi of backpressure. Increasing flow to 5 times that, or 1 ml/min, only 1200 psi is generated, and the separation is completed in 0.8 min. At any flow rate, less than 1 ml of solvent has been used.
Figure 2 illustrates another fast gradient for the separation of 10 antihistamines, with acetonitrile/water with 0.1% TFA in each, with a flow rate of 1 ml/min. The components are separated in approximately 2.2 min, and the backpressure was 1740 psi, still consumed and the volumes are ideal for next steps with LC–MS. More importantly, increasing the flow rates give shorter, fast chromatographic runs, but with relatively low backpressures, so traditional LC systems produce the fast HPLC desired by laboratories everywhere. within the capabilities of any standard HPLC system.
These are just two examples of using 2 mm i.d. monolithic columns for excellent chromatographic results. Less solvent has been consumed and the volumes are ideal for next steps with LC–MS. More importantly, increasing the flow rates give shorter, fast chromatographic runs, but with relatively low backpressures, so traditional LC systems product the fast HPLC desired by laboratories everywhere.
EMD Chemicals, Inc., An affiliate of Merck KGaA, Darmstadt, Germany
480 South Democrat Road, Gibbstown, NJ 08027