Chemically modified sorbents

Chiral stationary phases in tic have been primarily limited to phases based on normal or microcrystalline cellulose (44,45), triacetylceUulose sorbents or siHca-based sorbents that have been chemically modified (46) or physically coated to incorporate chiral selectors such as amino acids (47,48) or macrocyclic antibiotics (49) into the stationary phase. 

Polar compounds present the most problems because of their low breakthrough volumes with common sorbents. In the last few years, highly crosslinked polymers have become commercially available which involve higher retention capacities for the more polar analytes (37, 38). Polymers have also been chemically modified with polar groups in order to increase the retention of the compounds previously mentioned (35, 37). 

N. Masque, M. Galia, R. M. Marce and E. Boirull, Chemically modified polymeric resin used as sorbent in a solid-phase exti action process to determine phenolic compounds in water , J. Chromatogr. 771 55 - 61 (1997). 

As an alternative, stable high-coverage nonpolar RPC sorbents phases have been prepared by cross-linking hydrophobic polymers at the silica surface, either via free radical 143 or condensation 101 polymerization chemistry. In this case, the underlying silica becomes partly protected from hydrolytic degradation due to the presence of the hydrophobic polymer film coating that effectively shields the support material. Similar procedures have been employed to chemically modify the surface of other support materials, such as porous zirconia, titania, or alumina, to further impart resistance to degradation when alkaline mobile-phase conditions are employed. Porous polystyrene-divinylbenzene sorbents, be-... 

Natural polymers like cellulose and amylose comprise the Type IIIA CSPs, but the mechanical stability of these packings is not sufficiently adequate to be used as a chromatographic sorbent. More satisfactory sorbents have been obtained by chemically modifying them as ester or carbamate derivatives and then coating them onto large-pore silica (300 A) [276]. These CSPs are marketed under the trade names ChiralCel (cellulose) and ChiralPak (amylose). These packings have a wide scope of applications, good stability, and use on a preparative scale. 

Silica gel 60, the most versatile and most frequently used TLC sorbent, was taken as a basis. The mean particle size of this sorbent was optimized simultaneously, the particle size distribution was brought to within as narrow limits as possible (L- ) The sorbent material thus obtained was used t prepare HPTLC pre-coated plates silica gel 60, being followed subsequently by the development of other sorbents for processing into HPTLC pre-coated layers. The materials chosen were largely "reversed phase" sorbents, that is to say, chemically modified silica gels with a non-polar surface (, 8), as well as microcrystalline cellulose ( )""... 

A variety of sorbents have been used as the stationary phase in TLC, including silica gel, cellulose, alumina, polyamides, ion exchangers, chemically modified silica gel, and mixed layers of two or more materials, coated on a suitable support. Currently in the pharmaceutical industry, commercially precoated high-performance TLC (HPTLC) plates with fine particle layers are commonly used for fast, efficient, and reproducible separations. The choices of mobile phase range from single component solvent systems to multiple-component solvent systems with the latter being most common. The majority of TLC applications are normal phase, which is also a complementary feature to HPLC that uses mostly reverse-phase columns. 

On the other hand, adsorbents A and B modified with ODS and adsorbent C unmodified or modified with ODS as well as with ODS+HMDS exhibit higher adsorption capacity. Benzene shows similar adsorption behavior [68]. The above facts are due to the specific structure of adsorption sites which is formed after the chemical modification of the adsorbents. Adsorption energy sites constitute of spacially arranged CH, CH2 and CH3 groups of the octadecyl radical, chemically bonded to the surface of the silica gel and, in the case of carbosils of components of the carbon deposit (i.e. CH, CH2, CH3) planary distributed on the surface of the supporting material. There is an essential difference in the mechanism of the molecular adsorption on chemically modified and unmodified adsorbents [30]. The surface of chemically unmodified adsorbents can be considered as planar, i.e. two-dimensional, while that of the modified sorbents as three-dimensional. 

The book is divided into three sections consisting of chapters arranged in a consistent order, though some chapters could be put in the second or third section. On the other hand, a uniform treatment and style cannot be anticipated in the book that represents the efforts of many authors. Despite this, the presented works provide the comprehensive, high standard and modern study on the structure, investigations, preparation of inorganic sorbents, their numerous applications and deal with the adsorption on new and chemically modified inorganic solids. 

On the other hand, the modern sorbents are used more and more widely. The letter-number combinations such as RP-8 , RP-18 or NH2 indicate chemically modified layers all of which are based on a silica gel matrix. RP indicates reversed phase and the number represents the length of the hydrocarbon chain used for the modification. The material RP-2 is seen to be an exception, as this is a dimethylsUyl... 

In SPE, a small amount of organic solvent or surfactant is added to collected samples to prevent adsorption to sample containers. To increase recoveries of ng 1 levels of PAHs in SPE, ACN (40%) or surfactants above their CMC can be added to samples prior to preconcentration. Solid supports, chemically modified with copper phthalocyanine trisulfonic acid derivatives for selective sorption of PAHs, have been investigated. The selective interaction is thought to be with the tt electrons of the PAHs. Brij-35, a neutral polyoxyethylene lauryl ether surfactant was added above the CMC to water samples to prevent sorption on container walls. Before preconcentration by SPE, samples were diluted to reduce the surfactant concentration to just below the CMC. Recoveries of over 90% for SPE on solids containing copper phthalocyanine trisulfonic derivatives were obtained for spiked water samples at low ngl levels, except for NAP, ACE, and FLU. Experiments repeated using a C18 SPE preconcentration sorbent gave >90% recovery for all 16 EPA PAHs, except for ACY. Examples of the use of SDB as an SPE sorbent include the online analysis of seawater from the coast of Catalonia in Spain, where no PAHs above the low ng 1 level were detected, and the analysis of leachate from coal deposits. ... 

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