Monolithic columns limitations

Monolithic columns with the chiral anion exchange-type selectors incorporated into the polymer matrix obtained through in situ copolymerization process of a chiral monomer (in situ approach) [80-83,85] or attached to the surface of a reactive monolith in a subsequent derivatization step (postmodification strategy) [84], both turned out to be viable routes to enantioselective macroporous monolithic columns devoid of the limitations of packed columns mentioned earlier. 

In the mid 1990s, the first reports on the performance of monolithic columns created much excitement in the scientific community. High-performance separations at low backpressure and a short analysis time were the promise. Nevertheless, it was several years before the first commercial products became available,and made it possible to obtain a proper judgement on the ability and the limitations of the technology. 

Zarghi et al. [76] developed an HPLC method, using a monolithic column, for quantification of omeprazole in plasma. The method is specific and sensitive with a quantification limit of 10 ng/ml. Sample preparation involves simple, one-step extraction procedure, and analytical recovery was complete. The separation was carried out in reversed-phase conditions using a Chromolith Performance (RP-18e, 100 x 4.6 mm) column with an isocratic mobile phase consisting of 0.01 mol/1 disodium hydrogen phosphate buffer-acetonitrile (73 27) adjusted to pH 7.1. The wavelength was set at 302 nm. The calibration curve was linear over the concentration range 20-1500 ng/ml. The coefficients of variation for intra- and interday assay were found to be less than 7%. 

Separations of complex steroid mixtures were achieved recently by Que et al. [76] using both isocratic and gradient elution. Mass spectrometric detection gave femto-mole detection limits while laser-induced fluorescence of dansylated ketosteroids ranged in attomole levels (Fig. 10.16). Monolithic column packings were used with a 35 cm (25 cm packed bed) x 100 pm i.d. capillary packed with a polymer prepared from 5% T (total monomer concentration), 60% C (total crosslinker concentration), 3% polyethylene glycol, 10% vinylsulfonic acid and 15% lauryl acrylate. Details of the monolithic column preparation can be found in refs. 36,76, and 193. Similar monolithic columns can be used for the separation of bile acids [194],... 

Monolithic columns do have disadvantages. Although very high flow rates are used to speed up separations, this generates a considerable amount of solvent waste for >4.6-mm-bore columns. The number of phases and column sizes is very limited at present, as is the number of commercial manufacturers. Also, the technology of particle-packed columns is not static, but continues to improve as well. Monolithic columns have not yet demonstrated the performance capabilities exhibited by sub-2-pm particles and UHPLC. However, advances in monolithic column technology in the years to come promise to bridge that gap. 

With the use of new monolithic columns with narrow diameters for IC, the detection limits for ions have been decreased even further. The silica gel-based solid in the monolith is functionalised with the ion exchange material and it is highly porous with a high capacity. These columns have low back pressure so high fiow rates, fast separations and well defined peaks mean better sensitivity (sub ppb). 

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