Chemically modified sorbents chromatography

Cations, detection and identification of, 519-520 Cellulose, 108-109,925 detection of peptides and proteins on, 435-437 enantiomeric separations on triacetylcellulose thin-layer plates, 630-631 for separation of carbohydrates, 485-487 Centrifugal layer chromatography (CLC), 79,172 Cephalosporins, 452-457 Chemically modified sorbents, 110-119 hydrophillic modified phases, 110-112 hydrophillic modified precoated layers, 113-1 IS... 

Thin-tjayer Chromatography. Chiral stationary phases in tic have been primarily limited In phases based on normal nr inicrocrystalline cellulose, triaeelyleellulo.se sorbents or silica-based sorbents that have been chemically modified or physically coated to incorporate chiral selectors such as amino acids or macrocyclic antibiotics into the stationary phase. The cost of many chiral selectors, as well as the accessibility and success of chiral additives, may have inhibited widespread commercialization. 

Particularly in the beginning of thin-layer chromatography, but also today, sorbents without chemically modified surfaces are of most importance. In former times hand-made thin-layer plates were used nearly exclusively, but today the trend has moved in the direction of industrial precoated layers. In addition to glass plates, plastic and aluminium sheets are also offered as supports for the precoated layers. To stabilize the precoated layers mechanically, special binders are added that do not or nearly do not interfere with the chromatographic properties. To enlarge the possibilities of detection, added indicators can be mixed homogeneously with sorbents. Different types of silica gel are by far the most versatile and therefore the most frequently used stationary phases in the case of bulk sorbents as well as for application to precoated layers. 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

Modified celluloses Cellulose, described in Section II.D, was the first sorbent in thin-layer chromatography to be used for ion-exchange mechanisms after suitable modifications or impregnations (169). Functional groups used for chemical modification of celluloses are the following ... 

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