Experimental study of porous silicon shell pillars under retentive conditions

Experimental measurements of the retention capacity and the band broadening in perfectly ordered porous shell pillar array columns are presented for a wide range of retention conditions and layer thicknesses. The porous silicon shells were obtained using electrochemical anodization of the solid silicon pillars obtained using deep reactive ion etching. Using 10-microm-wide pillars, minimal reduced plate height values of the order of h(min) = 0.4-0.5 were obtained under nonretained conditions, even in cases where the outer shell made up 20% of the total diameter. Under retained conditions, minimal plate heights around h(min) = 0.9 were obtained, even at retention factors up to k' = 12. Using a model based on Giddings nonequilibrium theory, and using a newly calculated value for the stationary zone configuration factor for the case of porous shell cylinders, a plate height model describing the band broadening in porous shell pillar arrays has been established. The validity of this model is demonstrated by showing that the geometrical parameters appearing in the model and fitted using band-broadening measurements under nonretained conditions can be used to relatively accurately predict the band broadening under retained component conditions. Using this model, some speculations on the ultimate performance of porous pillar array columns could be made.