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31


Figure 2. Scanning electron micrograph of sphere-on sphere particles.


of silica nanoparticles of size 10-16 nm is added and the process is repeated many times until the desired shell thickness is achieved. An alternative procedure is to apply a polymer to the cores that can absorb several layers of sol particles, such that the porous shell grows from 5-10 layers at a time. This process has been modifi ed into a 1-step coacervation procedure shown in Figure 1. In each approach, the polymer is fi nally removed by burning, and the particles are sintered to improve their mechanical properties. Another method involving a one pot synthesis, is the sphere on sphere process [3] where silica microspheres are coated with a single layer of nanospheres (Figure 2). The surface of this material has very small pores with diameter < 2 nm but its signifi cant porosity, allowing use for the separation of large proteins, results from the spaces between the surface nanospheres.


Effi ciency. It is often proposed that a 2.7 µm shell column has ‘the same performance as a sub-2 µm porous column but at around half the back pressure’. This statement is rather confusing because column porosity does not affect its back pressure: the pores are too small to allow fl ow through them, which takes place instead principally around the particles. Thus shell and porous columns of same particle diameter (dp


) generate similar


back pressure. The lower back pressure of a 2.7 µm shell particle is merely due to its larger dp


. The effect arises instead due to


the smaller minimum reduced plate height (h) of shell particles (as low as 1.2-1.5) compared with totally porous particles (typically 1.9-2.1). This improved effi ciency could be due to:


(i) The narrower particle size distribution of shell particles (rsd 5% compared with 20% for totally porous particles).


(ii) More likely it is due principally to the superior packing of shell particles (reduction in the A term of the van Deemter equation); to a lesser extent, reduction in the B term contributes due to there being less mobile


Figure 3. Dimensions of a Halo superficially porous particle.


phase in column; there may also be some reduction in the C term especially for large molecules due to the reduced diffusion distance, although this effect is considered small for small molecules with higher diffusivity.


(iii) Shell columns have fewer problems with frictional heating due to the greater thermal conductivity of the solid core compared with the mobile phase it replaces in porous silica. This property allows the use in some circumstances of larger 4.6 mm i.d. columns of ~ 2.5 µm shell particles in place of 2.1 mm columns of sub- 2 micron particles. The use of larger diameter shell columns may even be advantageous, as wider bore columns are generally easier to pack, and they are also more tolerant of instrument band spreading.


Sub 2µm shell particle columns are increasingly used to obtain higher effi ciencies than totally porous particle columns at similar pressures.


Possibility of overloading effects. Figure 3 shows the structure of a typical shell particle (Halo, Advanced Materials Technology) with dp


= 2.7 µm and shell thickness 0.5 µm. While the typical two dimensional representation of the structure of these particles is misleading, a true spherical three-dimensional consideration reveals that they can be considered to have a rather thick shell. Indeed, the non-porous fractional volume of the particle can be calculated using simple geometry as occupying only 25% of the total, implying that the porous volume is 75% of that of a totally porous particle of the same diameter. These dimensions are in stark contrast to those of the early ‘pellicular’ particles which contained only a very small fraction of porous material [1]. Thus, there is no particular reason to suspect that modern shell particles should be prone to overloading. Of course, particles with thinner porous shells have been developed especially for the analysis of large molecules such as proteins, where slow diffusion in and out of a thick shell may compromise


column effi ciency. These particles may have somewhat reduced loadability. Another factor to consider is that the surface area of the porous material in a shell column may not be the same as that in a totally porous column, due (at least) to their different methods of manufacture. Figure 4 shows a practical comparison of the loading capacity of a totally porous Zorbax C18 and a Poroshell column both from the same manufacturer (Agilent). The basic probe nortriptyline has been shown to overload readily on C18 columns at low pH (e.g. ammonium formate buffer pH 3.0 as used here) [4]. The overloading effect is shown by reduction in effi ciency as the concentration of injected solute increases. As this reduction in effi ciency varies with the retention factor k of the solute, particularly at low values of k, a constant high value (k =10) was used in these experiments by adjusting the concentration of organic modifi er (acetonitrile) in the mobile phase. For both columns, overloading is reduced as the concentration of buffer in the mobile phase increases. The summary Table below Figure 4 shows the concentration of solute necessary to reduce the small sample concentration effi ciency (N0 (C0.5


) by one half


) for the totally porous column (dp µm) and three types of shell column (dp


mass. C0.5


= 1.8 ~


2.5 µm). All columns gave good peak shape (asymmetry factor As


~ 1) for small sample


is shown to be approximately the same for all these columns, indicating that there are no major differences in loading capacity. Similar results were obtained also for other solutes [4].


Effects of Instrumental Bandspreading


The effects of instrumental bandspreading are much greater for small volume peaks generated by short, narrow, high effi ciency columns. This can be seen from a consideration of Equation 1.


σ2experimental = σ2column + σ2extracolumn . (1)


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