Monolithic columns pore size

High performance monolithic columns were prepared from styrene and divinyl-benzene (PSDVB, 200 pm i.d.) (Oberacher et al., 2004). The monoliths possess 5-300 nm pores with porosity of ca. 50% and 20% for external and internal pores, respectively, with specific surface areas of 30-40 m2/g. The column showed permeability K= 3.5 x 10 15m2 in water and slightly less in acetonitrile. The pore size... 

Chromatographic use of monolithic silica columns has been attracting considerable attention because they can potentially provide higher overall performance than particle-packed columns based on the variable external porosity and through-pore size/skeleton size ratios. These subjects have been recently reviewed with particular interests in fundamental properties, applications, or chemical modifications (Tanaka et al., 2001 Siouffi, 2003 Cabrera, 2004 Eeltink et al., 2004 Rieux et al., 2005). Commercially available monolithic silica columns at this time include conventional size columns (4.6 mm i.d., 1-10 cm), capillary columns (50-200 pm i.d., 15-30 cm), and preparative scale columns (25 mm i.d., 10 cm). 

Motokawa, M., Kobayashi, H., Ishizuka, N., Minakuchi, H., Nakanishi, K., Jinnai, H., Hosoya, K., Ikegami, T., Tanaka, N. (2002). Monolithic silica columns with various skeleton size and through-pore size for capillary liquid chromatography. J. Chromatogr. A 961, 53-63. 

Monolithic column — The trend to use shorter columns in liquid chromatography means that the resultant lower separation efficiency is of concern. One way to improve HPLC separation efficiency on a shorter column is to reduce the size of the packing material, but at the cost of increased backpressure. Another approach to improve performance is increasing permeability with a monolithic column. Such a column consists of one solid piece with interconnected skeletons and flow paths. The single silica rod has abimodal pore structure with macropores for through-pore flow and mesopores for nanopores within a silica rod8182 (Figure 12.1). 

FIGURE 12.1 SEM photographs of monolithic silica columns (A) monolithic silica prepared from TMSO in a test tube (B) 50 /mi inner diameter silica skeleton, size 2 /(m, through-pore size 4.5 (ini. (Source From Ikegami, T. and Tanaka, N., Curr. Opin. Chem. Biol., 2004, 8, 527. With permission from Elsevier Scientific Publishing.)... 

Commercially available monolithic columns are based either on silica or organic polymer and are generally characterized as a polymeric skeleton with macropores, with a diameter of approximately 2 pm, and mesopores, with a diameter of approximately 13 nm. The role of the macropores (through-pores) is to provide channels with high compounds permeability, which permits the use of higher flow rates with respect to columns based on conventional particle size, and an extended surface area, which is comparable to conventional columns packed with 3 pm particles. 

The secondary structure, the mesopores, is similar to the internal structure of standard HPLC particles. This secondary structure provides the surface for retention. The standard pore size is in the order of 13 nm, resulting in a specific surface area of about 300 mVg. Due to the lower ratio of retentive structure to interstitial space, the retentivity of monoliths and the preparative loadability tends to be significantly lower than the retentivity and loadability of packed beds of 10-nm particles. Since the monolithic columns described here are made from silica, they can be derivatized in the same way and with the same technology as silica-based particles. Also, the useful pH range is the same as for silica-based particles. 

The absolute permeability (8 x 10 ° cm ) of currently available silica monoliths (Chromolith, Merck) is similar to that of columns packed with 9 jm particles [213], To increase efficiency, columns are usually packed with particles of 2, 3, and 5 jm, which possess absolute permeabilities of approximately 4x10 ", 9x10 ", and 2.5xl0" °cm, respectively. If we try to compare columns packed with 5 pm particles and monolithic columns with through-pores of an average size of 1.5 pm, we observe that the permeability of the monolithic columns is around three to four times larger than that of packed columns. 

A study of the influence of the average sizes of the through-pores on the performance of a series of monolithic columns was carried out by Motokawa et al. [214]. This group found permeabilities between 8 x 10 ° and 1.3 x 10 cm. ... 

The ratio between the through-pore size (ca. 8 pm) and the skeleton size (ca. 2.2 pm) shown in Fig. 5.2a is much greater than in a packed bed of a particle-filled column. Figure 5.3 shows the plots of skeleton size against the through-pore size in a column for a silica monolith prepared in a capillary or in a mould, as well as in a particle-packed column. The through-pore size/skeleton size ratios observed with the... 

Fig. 5.3. Plots of the skeleton size against the through-pore size of the continuous monolithic silica prepared in a capillary (O), and the larger-sized silica rod columns (7 mm x 83 mm) having constant through-pore size/skeleton size ratio ( ) [15]. Also plotted are the particle size (vertical axis) against the size of interstitial voids (25-40% of dp as indicated by the bars) found with a conventional particle-packed column.

Zhang s group in China developed monolithic poly(styrene-co-divinylbenzene) CEC column in which EOF is supported by carboxyl groups of polymerized methacrylic acid units (Xiong etal. [51]). In a typical procedure, vinylized 75 mm i.d. capillaries were filled with a mixture of 5% styrene 21, 10% divinylbenzene 22, 5% methacrylic acid 1, and 80% toluene containing 1% azobisisobutyronitrile (in respect to monomers) and polymerized at 70°C for 24 h. The pore volume of 0.098 mL/g and mean pore size of 40 nm determined for this monolith appear to be rather small and do not correspond with the published SEM pictures that reveal existence of large pores, and the chromatographic performance of the columns in CEC mode. 

Fig. 6.21. Electrochromatographic separation of benzene derivatives on monolithic capillary column prepared by UV initiated polymerization. Conditions capillary column, 100 pm i.d. x 25 cm active length stationary phase poly(butyl methacrylate-co-ethylene dimethaciylate) with 0.3 wt. % 2-acrylamido-2-methyl-l-propanesulfonic acid pore size, 296 nm mobile phase, 75 25 vol./vol mixture of acetonitrile and 5 mmol/L phosphate buffer pH 7 UV detection at 215 nm 25 kV pressure in vials, 0.2 MPa injection, 5 kV for 3 s. Peaks thiourea (1), benzyl alcohol (2), benzaldehyde (3), benzene (4), toluene (5), ethylbenzene (6), propylbenzene (7), butylbenzene (8), and amylbenzene (9).

Fig. 6.22. Electrochromatographic separation of Gly-Tyr (1), Val-Tyr-Val (2), methionine enkephalin (3), and leucine enkephalin (4) on monolithic methacrylate capillary column with a pore size of 492 nm. (Reprinted with permission from [55]. Copyright 1999 Wiley-VCH). Conditions Mobile phase 10% of aqueous 10 mmol/L sodium 1-octanesulfonate and 90% of a 2 8 mixture of 5 mmol/L phosphate buffer pH=7.0 and acetonitrile. UV detection at 215 nm. Total sample concentration 1 mg/mL.

Fig. 6.26. Differential pore size distribution profiles of porous polymeric monolithic capillary columns with mode pore diameters of 255 (curve 1), 465 (2), 690 (3), and 1000 nm (4) (Reprinted with permission from [64]. Copyright 1997 American Chemical Society).

The major advantage of CEC compared to classical HPLC is that much higher column efficiencies can be achieved using identical separation media. For columns packed with beads, the column efficiency of both of these methods is particle size dependent, and increases as the size of the packing decreases [1], Since the monolithic columns are molded rather than packed, issues of particles size become irrelevant, and instead, the size of the pores within the monolithic material is the variable most... 

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