Mass Overload on Superficially Porous Particles

The benefits of superficially porous particles are without question.  Who wouldn’t want to perform faster separations without the need for expensive UHPLC instrumentation?  It sounds too good to be true.  There must be some drawback – right?

The solid, impermeable core present in Raptor particles increases column efficiency by decreasing the diffusion path at the cost of reduced surface area typically available in traditional fully porous material.  There is a potential concern that arises from a reduction in surface area: column loading ability.  Column overloading (mass overload) occurs when the amount of material injected onto the column exceeds the available active sites of the stationary phase.

Figure 1:  Classically observed peak shape due to column overload

The solid core in the Raptor 2.7 μm particle has a diameter of 1.7 μm with a porous layer that is 0.5 μm thick while the core in the Raptor 5 μm particle has a diameter of 3.4 μm with a porous layer that is 0.6 μm thick.  After doing some simple math, the volume of fully porous material is reduced by ~25% in the 2.7 μm particle and by ~40% in the 5 μm particle compared to fully porous particles of the same diameter (along with a proportional decrease in surface area).  Will this limit sample loading capabilities compared to fully porous silica?  Let’s check by looking at data collected by injecting a standard volume (1 μL) across a concentration range (0.01 – 100 μg on column) of a neutral probe (biphenyl).

Figure 2: Mass on Column vs. Peak Width (w_0.5) Normalized to w_0.5,min

The peak width at half height remains consistent until the mass on column approaches ~10 μg for both fully porous material and Raptor (Figure 2).  This corresponds to a 1 μL injection at 10 mg/mL.  For most modern detectors and assays, this is orders of magnitude higher than is typically injected.  How can we lose so much surface area, but still maintain loading ability at relevant concentrations?

The volume ratio of porous to non-porous material utilized in Raptor serves to reduce the diffusion pathway and maintain detection-appropriate loading abilities.  Reducing the size of the solid, impermeable core would increase the loading ability, but it would also decrease efficiency.  Likewise, increasing the size of the solid, impermeable core would increase your efficiency, but the loading ability would no longer be practical.  As manufactured, Raptor provides speed and efficiency without compromising the loading ability required for most modern methods.