An alternative to UHPLC columns are monolithic columns, so-called because they are made from a single piece of porous material.
Because of the manufacturing process, Merck KGaA’s monolithic silica columns are surrounded by polymeric polyether ether ketone (PEEK), and cannot withstand the same kind of back pressures as traditional stainless steel particle packed columns can. But that doesn’t seem to be a problem since they possess a high permeability and are designed to yield fast, high efficiency separations using “conventional low pressure and cheap HPLC systems,” says Karin Cabrera, Ph.D., head of R&D for analytical chromatography at Merck Millipore.
In monolithic columns, it is the size of the through-pores (macropores) that determines permeability, while the mesopore size determines the surface area of the material and thus the separation efficiency. “We can independently design the macropore size and the mesopore size,” extolls Dr. Cabrera.
This is not possible with the classical particle packed column, where the interstitial volume corresponds to the macropore size, she continues. Smaller particles yield a smaller interstitial volume and require a correspondingly higher back pressure to operate. Yet separation efficiency is dependent on the particle size as well.
As there are no frits to clog, “dirty” samples can be injected directly on to monolith columns without the need to filter them. In addition, because the column is a single rigid piece, it has been shown to last much longer than packed columns, which are subject to shifting and cracking over time.
The new Chromolith HR columns, with a 1.1 µm macropore size, can operate at 3.5 mL/min at a back pressure of only 200 bar, with a measured efficiency surpassing that of a 3.0 µm packed particle column. “We started with 2 µm [macropore] and now with the second generation we have 1.1 µm; now of course we can go down further—to 0.9 µm, 0.8 µm , 0.7 µm, 0.5 µm, etc.,” Dr. Cabrera says, predicting that in the next five years “we will reach efficiencies that cannot be obtained any more with smaller particles.”