Experimental Study of Porous Silicon Shell Pillars under Retentive Conditions Wim De Malsche,* ,†,‡ Han Gardeniers, ‡ and Gert Desmet † Department of Chemical Engineering, Vrije Universiteit Brussel, Brussels, Belgium, and Research Programme Mesofluidics, MESA+ Institute for Nanotechnology, MESA+ Research Institute, Enschede, The Netherlands 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 anod- ization of the solid silicon pillars obtained using deep reactive ion etching. Using 10-μm-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 non- equilibrium 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 measure- ments 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. After Regnier and co-workers 1–3 introduced the pillar array concept around 1998 as a groundbreaking alternative for the traditionally employed packed bed columns, relatively little progress has been made to turn the concept in practically useful and superior performing separation columns. After a few theoreti- cal studies, 4,5 the first experimental van Deemter curves and chromatograms were only published in the past year. 6–11 It was found that reduced plate heights as small as h min ) 0.2 could be obtained under nonretained conditions and with pillars of 5 and 10 µm. Under retained conditions, reduced plate heights of the order of h min ) 1 could be obtained. In both cases, the band- broadening contribution originating from the sidewall region had to be omitted because the employed etching method could not prevent a machining tolerance on the distance between the closest row of pillars and the sidewall. Several potential solutions to this problem (among which the use of deep-UV technology) are currently under development. Other recent developments related to interfacing microfabri- cated chromatographic columns with on-chip UV-vis detection 12,13 and to the combination of microfluidic channels and electrospray tips for MS detection. 14,15 An important deficiency of the pillar array columns reported on in refs 6-11 is that they only consisted of nonporous pillars. The lack of retention surface of these columns is hence obvious. It was only very recently that the first dispersion measurements were made in an array of partially porous pillars. 16 These pillars were obtained via electrochemical anodization, 17,18 using the silicon pillars and the silicon channel substrate as the conductive medium for the anodization current. The feasibility of the integra- tion of porous silicon in chips has been clearly demonstrated by Tjerkstra et al. 19 The devices they fabricated included multiwalled microchannels useful for size exclusion separations and also 1-D open-tubular nanochannels having a porous bottom wall for use * To whom correspondence should be addressed. tel.: (+)32/2.629.37.81. Fax (+)32/2.629.32.48. E-mail: wdemalsc@vub.ac.be. † Vrije Universiteit Brussel. ‡ MESA+ Research Institute. (1) He, B.; Tait, N.; Regnier, F. E. Anal. Chem. 1998, 70, 3790–3797. (2) Regnier, F. E. J. High Resolut. Chromatogr. 2000, 23, 19–26. (3) Slentz, B. E.; Penner, N. A.; Regnier, F. J. Sep. Sci. 2002, 25, 1011–1018. (4) Gzil, P.; Vervoort, N.; Baron, G. V. Gert Desmet., Anal. Chem. 2004, 76, 6707–6718. (5) De Smet, J.; Gzil, P.; Vervoort, N.; Verelst, H.; Baron, G. V.; Desmet, G. Anal. Chem. 2004, 76, 3716–3726. (6) De Pra, M.; Kok, W.Th.; Gardeniers, J. G. E.; Desmet, G.; Eeltink, S.; van Nieuwekasteele, J. 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