One column fits all was title of my poster presented at HPLC 2010 in Boston that described the preparation and characterization of hypercrosslinked monolithic stationary phases and their application in several chromatographic modes. We summarized poster’s results and submitted them as a paper in Journal of Chromatography A.
Today, I would like to describe my favorite chromatographic books: from one I bought even before I (really) knew what chromatography is to one which has chapter with my name on it.
Georges Guiochon pointed out in his excelent reivew about monolithic stationary phases four directions from which we can expect a serious improvement in (monolithic) columns performance.
High temperature chromatography, which causes a reduction in the viscosity of the mobile phase. So far, monolithic stationary phases have not yet been used at high temperatures but this is only a matter of time. High temperature liquid chromatography currently pioneered by Peter Carr and his group is going to be one of the major research areas in analytical chemistry for the next ten years. A significant reduction of analyses times by a factor between 3 and 4 is quite likely.
An increase in the maximum pressure available to the analyst. Most commercial instruments can operate at inlet pressures of up to 40 – 50 MPa. A few of them can reach inlet pressures of 100 – 120 MPa and pumps able to reach 900 MPa are available. The use of high pressures requires far more caution than chromatographers are used to apply. This may create new, some times unexpected, safety hazards against which analysts should be forewarned. One advantage of monolithic columns is that extremely efficient columns, able to generate one or even several millions of theoretical plates could be operated with conventional HPLC instruments if long enough columns could be prepared.
A decrease in the minimum value of the height equivalent to theoretical plate (HETP) of the columns used. This will come from a reduction of the heterogeneity of the radial distribution of the flow-through pore sizes, also from a reduction of the average size of the domains of the monolithic column used and from a reduction in the variance of the domain sizes.
We have to be able to control (and suppress) monolith heterogeneity. My small prediction: one who is able to prepare the (monolithic) stationary phase with no or limited heterogeneity will be able to achieve unimaginable efficiency and column performance. Like for example homogeneous pillars.
An increase in the column permeability. This requires an increase in the average flow-through pore size. Since this size is included in the domain size, this requirement is in conflict with the previous one. Both can be achieved only by decreasing the average size of the porons, which would increase the external and total column porosity at the expense of the internal column porosity and the total surface area of adsorbent in the column. There is no clear limit here but it does not seem that much can be gained. Most probably, a reduction in the variance of the domain size accompanied by an increase in the degree of radial homogeneity of the monoliths constitute the most promising avenues for the monolith designers and makers.
One of the possible ways how to connect these last two conflicting requirements can be preparation and optimization of hypercrosslinked monolithic stationary phases. The porous structure (flow through pores) can be optimized independently on the structure of the thin hypercrosslinked layer prepared on the surface of the monolith (micro- and mesopores). Firstly, the generic monolith is prepared (flow through pores) and then the surface of the stationary phase is modified with the hypercrosslinking reaction and thin layer of small pores is formed. Then, only the general models connecting the preparation and modification of the hypercrossllinked monoliths with their chromatographic properties have to be developed and understand.
What do you think about these suggestions?
PS: if you haven’t done yet – look at the review written by Georges Guiochon. There is all you need to know about monoliths but were afraid to ask.
Although the monolithic stationary phases suitable for separations were introduced in the 1990s [1,2,3], the idea of using a “continuous block of the porous gel structure” as stationary phase was discussed in Analyst by R. L. M. Synge, A. J. P. Martin, and A. Tiselius as longs ago as in 1952.
Both equilibrium and kinetic aspects of the molecular-sieve properties of zeolites have been studied in detail by Barrer, and it is clear that these equilibria could be used for the separation of small molecules on chromatographic columns of zeolites. Zeolites could not be used with larger molecules, as the spaces in them are too small. However, from dialysis and ultrafiltration studies enough is known of the properties of membranes and gel structures to suggest that these, though their pores could not be expected to possess the regularity of those of zeolites, could nevertheless be used for more refined separations than have hitherto proved possible. If used as powders in ordinary chromatograms, however, these substances would exhibit the disadvantages already discussed, namely that adsorption, increasing with molecular weight, would work in the opposite sense to molecular-sieve effects. An alternative possibility, suggested in discussions between Dr. A. J. P. Martin, Prof. A. Tiselius and one of us (R.L.M.S.), is to use electro-endosmosis to move a solution through a continuous block of porous gel structure. In this way the equivalent of movement of liquid through a very thick ultrafiltration membrane is attained without the necessity of great hydrostatic pressures, which would destroy the membrane structure. Here adsorption and molecular-sieve or frictional effects would all act in the same sense, tending to retard more the larger molecules.
And conclusions?
Because of lack of small pores it is difficult to separate small molecules with polymer monoliths in isocratic mode. We have prepared monolithic capillary columns and then hypercrosslinked them to afford a monolith containing an array of small pores [1].