Sorbents silica gel

The TLC process is an off-line process. A number of samples are chromatographed simultaneously, side-by-side. HPTLC is fast (5 min), allows simultaneous separation and can be carried out with the same carrier materials as HPLC. Silica gel and chemically bonded silica gel sorbents are used predominantly in HPTLC other stationary phases are cellulose-based [393]. Separation mechanisms are either NPC (normal-phase chromatography), RPC (reversed-phase chromatography) or IEC (ion-exchange chromatography). RPC on hydrophobic layers is not as widely used in TLC as it is in column chromatography. The resolution capabilities of TLC using silica gel absorbent as compared to C S reversed-phase absorbent have been compared for 18 commercially available plasticisers, and 52 amine and 36 phenolic AOs [394]. 

This list includes several aliphatic and aromatic amines that are soluble in water and therefore are not significantly affected by retention of moisture on the silica gel sorbent tube. In some cases, such as with chloroacetaldehyde, collection of moisture may even improve the collector. Chloroacetaldehyde forms a very stable hydrate when collected from humid atmospheres with silica gel, besides being efficiently collected from a dry environment. 

The commercial alumina and silica gel sorbents are mesoporous, i.e., with pores mostly larger than 20 A (see Fig. 1). Activated alumina is produced by thermal dehydration or activation of aluminum trihydroxide, A1 (OH)3 (Yang, 1997), and is crystalline. Commercially, silica is prepared by mixing a sodium silicate solution with a mineral acid such as sulfuric or hydrochloric acid. The reaction produces a concentrated dispersion of finely divided particles of hydrated Si02, known as silica hydrosol or silicic acid ... 

Sperling, M., Yan, X., Welz, B. Electrothermal atomic absorption spectrometric determination of lead in high-purity reagents with flow-injection on-line microcolumn preconcentration and separation using a macrocycle immobilized silica gel sorbent. Spectrochim. Acta Part B 51, 1875-1889 (1996)... 

Hexane was used to extract milk and juice samples and sonication was used to mix the samples. The hexane layer was separated and passed through a silica-gel column. Bread and vegetable samples were pulverized and extracted with hexane then cleaned in a silica-gel sorbent. 

Khan et al. (11) used a Chromatotron CPTLC apparatus in the separation of some unusual clerodane diterpenes (e.g.. Compound 5) in a chemotaxonomic study of the flacourtiaceous species Zuelania guidonia. Silica gel sorbent and a... 

About 10-30 L of air at a flow rate of 20-200 mL/min is passed over a silica gel sorbent tnbe, desorbed with 95% ethanol for 1 honr in an ultrasonic bath, and injected into a GC eqnipped with an FID. Chromosorb 103 (80/100 mesh) is snitable for the pnrpose. A nitrogen-specific GC detector may be used instead of an FID. Anisidines in air (25-300 L at a flow rate of 500-1000 mL/min) may be collected over a XAD-2 solid sorbent tube, desorbed with 5 mL of methanol, and analyzed by HPLC, UV detection at 254 nm (NIOSH 1984, Method 2514). A stainless steel-packed colnmn containing Bondapak C or eqnivalent is snitable for the pnrpose. o-Tolidine and o-dianisidine in air may be analyzed by an HPLC-UV technique similar to benzidine by NIOSH (1984) Method 5013 (see Section 9.14). 

Bagheri H., Gholami A. and Najafi A. (2000) Simultaneous preconcentration and speciation of iron(II) and ironflfl) in water samples by 2-mercaptobenzimidazole-silica gel sorbent and flow injection analysis system. Analytica Chimica Acta, 424, 233-242. 

Moreover, because the additive concentration in this extract is not sufficient for the detection of legal limits, it is necessary to preconcentrate the analytes. LLE is commonly used to separate additives from their coextracts. Nevertheless, the recent trend is to replace LLE with solid-phase extraction (SPE). It is possible to use SPE as a single cleanup/preconcen-tration step or as an additional step after LLE. The use of SPE has become popular because of the great variety of polar and nonpolar sorbents and ionic exchange resins that exist. The sorbents most often used to preconcentrate food additives are RP-Cig, silica gel, sorbents with amino groups, XAD-2 polymeric sorbent, polyamide and wool. 

As with silica gel sorbent, neutral lipids will be poorly retained as compared to polar lipids such as phospholipids... 

The commercial silica gel sorbents are mesoporous, that is, with pores mostly larger than 20 A. Silica gel can be formed by two routes (1) polymerization... 

Jacob (1992) has reviewed studies on the TLC of porphyrins and noted that this technique is a versatile one for separating these compounds. The sorbent most often used is silica gel. TLC allows for the separation of most of the common classes of porphyrins—i.e., the porphyrin carboxylic acids and their esters, alkyl porphyrins, and porphyrin-metal complexes. TLC is also useful for the separation of the diagnostically important porphyrins from clinical specimens such as urine, feces, and blood. Routine methods for the separation by TLC of porphyrins from human specimens are now available. With various combinations of the mobile-phase benzene-ethyl acetate-methanol and silica gel sorbent, porphyrin methyl esters can be separated according to the number of carboxyl groups, with the values being inversely proportional to that number (Jacob, 1992). 

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