In honor of LCGC's celebration of 30 years covering the latest developments in separation science, we asked a panel of experts (listed in the sidebar) to assess the current state of the art of liquid chromatography (LC) column technology, and to try to predict how the technology will develop in the future. This article is part of a special group of articles covering the state of the art in sample preparation, gas chromatography (GC) columns, GC instrumentation, LC columns, and LC instrumentation.
The Size of Superficially Porous Particles
The first question we asked our expert panel was whether they foresee any limits to the decrease in size of superficially porous particles (SPPs).There should be no limits in terms of synthesizing the particles themselves, points out Ron Majors, a senior scientist at Agilent Technologies and the longtime editor of LCGC's "Column Watch" and "Sample Prep Perspectives" columns. It has already been demonstrated, he notes, that both smaller and larger SPP particles can be prepared with various shell thicknesses and pore sizes, and that they are robust, can be derivatized with the popular bonded phases, and can be successfully packed into most column configurations (albeit with a bit of work).
The question, then, is whether smaller particle sizes for SPPs are desirable, given the higher pressures generated, the real possibility of frictional heating, and the instrumental considerations of extracolumn effects. "Perhaps a new generation of low dispersion instruments may be required for smaller SPPs," he considers.
And of course, as several experts point out, making these particles smaller would eliminate the advantage of being able to use them on regular LC equipment to achieve efficiencies close to those of ultrahigh-pressure liquid chromatography (UHPLC). Thus, the panel almost unanimously agreed that the current size of SPPs, with most commercial SPPs at 2.7 µm and experimental SPPs between 1 and 2 µm, is likely to remain where it is.
And as Jack Kirkland of Advanced Materials Technology notes, particles smaller than 1 µm likely are not practical, and particles with diameters of <2 µm already are problematic, requiring very high pressures that tax instruments, decrease reliability, and increase operating costs. "Such small particles are only of value for fast separations of samples with limited components," he says. "How many users require <10-s separations rather than 30 s? Complex samples require larger particles, longer columns."
Joe Glajch of Momenta Pharmaceuticals also notes, "Additional gain in efficiency is probably not the major direction for column particles in the future; selectivity will become more important."
And there is indeed a lower limit to the size of SPPs, says Georges Guiochon of the University of Tennessee. The pressure required to achieve the optimum flow rate, he says, increases as the square of the inverse of the particle size at constant efficiency, for a given compound on a given system.
This, he says, becomes practically impossible for the analysis of low-molecular-weight compounds (such as pharmaceuticals and peptides) on columns packed with particles finer than 1 µm. However, Guiochon continues, the analysis of large biochemicals on columns packed with 0.5-µm particles would be possible if particles that size (but not smaller), with large mesopores could be manufactured and packed in efficient columns.
David Hage, of the University of Nebraska, agrees. "Pore sizes of 100 Å or less work well for small molecules, but pore sizes of 300–500 Å or even larger are needed to provide suitable access if the same kinds of supports are to be used for biomacromolecules," he says.
"The limit might also depend on the column diameter desired," adds Guiochon, "The efficiencies of wide-bore columns (>4.6 mm) and of capillary columns (<0.1 mm) tend to exceed that of narrow-bore (0.5–2.1 mm) ones."