Choice of Adsorbent

Solid-Phase Extractions In a solid-phase extraction the sample is passed through a cartridge containing solid particulates that serve as the adsorbent material. For liquid samples the solid adsorbent is isolated in either a disk cartridge or a column (Figure 7.17). The choice of adsorbent is determined by the properties of the species being retained and the matrix in which it is found. Representative solid adsorbents... 

The search for a suitable adsorbent is generally the first step in the development of an adsorption process. A practical adsorbent has four primary requirements selectivity, capacity, mass transfer rate, and long-term stabiUty. The requirement for adequate adsorptive capacity restricts the choice of adsorbents to microporous soUds with pore diameters ranging from a few tenths to a few tens of nanometers. 

The shape of an isotherm depends on the choice of adsorbate, substrate, temperature T and, in a solution-phase system, the solvent. 

As stated earlier the procedure for this analysis is based largely on the methods developed by Hangartner.(4) Figure 1 outlines the scheme utilised for sample processing and analysis. In addition to the detection system already discussed the only other significant difference in this work is the choice of adsorbent which is Carbotrap D-l a graphitised carbon black (GCB). The use of GCB s in environmental analysis is well documented in the literature both as column materials and adsorbants. (7, 8) Initial work within Severn Trent confirmed the claimed superiority of GCB s compared with adsorbents based on porous polymers such as Tenax GC. No evaluation of the relative merits of GCB s and activated carbons have been made at this laboratory but tests with the latter are likely in the future. 

The Parex and MX Sorbex processes are quite similar with regard to the adsorbent section mechanics so all of the discussion about the functional zones in the section for the Parex process applies to the MX Sorbex process also. The MX Sorbex process produces m-xylene at 99.5-99.8% purity at a recovery in excess of 95%. The major differences between the two technologies are the choice of adsorbent and desorbent... 

The Sir Galahad has been found to provide rapid, rehahle analysis, often in minutes, while other techniques rely on a long sample-collection timescale. Recent developments in the choice of adsorber have provided additional benefits, mainly related to reduced back pressure and less thermal lag. These allow the analyses to be completed in shorter timescales. 

Adsorbent choice. The choice of adsorbent material depends on the volatile compounds in the food. Of the synthetic porous polymers, the most widely used and best overall adsorbent is Tenax TA (poly-2,6-diphenyl-p-phenylene oxide) 60 to 80 mesh. While Tenax does not show an adsorption capacity for all volatiles, especially very small polar compounds such as acetaldehyde, it has good thermal stability and desorption capabilities. It also traps little water and generates very few artifacts. Table G1.2.2 shows a few limitations and advantages of various adsorbents, all of which can be purchased from chromatography suppliers. If very small volatiles are the goal, various Carbosieves could be used, or traps containing several adsorbents in series. Traps with mixed adsorbents should be desorbed immediately, before transfer between phases occurs. 

The formal similarity between adsorption and complexation reactions can be exploited to incorporate adsorbed species into the equilibrium speciation calculations described in Sections 2.4 and 3.1. To do this, a choice of adsorbent species components (SR r in Eq. 4.3) must be made and equilibrium constants for reactions with aqueous ions must be available. A model for computing adsorbed species activity coefficients must also be selected.8 Once these choices are made and the thermodynamic data are compiled, a speciation calculation proceeds by adding adsorbent species and adsorbed species (SR Mp(OH)yHxLq in Eq. 4.3) to the mole-balance equations for metals and ligands, and then following the steps described in Section 2.4 for aqueous species. For compatibility of the units of concentration, njw) in Eq. 4.2 is converted to an aqueous-phase concentration through division by the volume of aqueous solution. 

The choice of adsorbate and the methodology depends on the nature of the adsorbent. In principle, the following categories should be distinguished as follows... 

The pressure range used for the determination of the adsorption isotherm depends on the nature of the metal and on the choice of adsorbate. For example, with hydrogen on Ni the pressure range is usually from about lOkPa to about 50kPa, whereas for Pt, pressures an order of magnitude lower may be used. 

Quantitative structure-retention relationships studies are widely investigated in high-performance liquid chromatography (HPLC), gas chromatography (GC), and thin-layer chromatography (TLC). Recently, QSRR studies in TLC have attracted more and more researchers [1], It is known that TLC has some advantages It is rapid, relatively simple, low cost, and easy to operation, there is a wide choice of adsorbents and solvents, and very small amounts of substance are needed. In this entry, the establishment and apphca-tion of QSRR studies are reviewed. 

Referring to this differentiation in the nature and the polarity of suitable solvents a first classification concerning the adsorbent and thus the phase system can be made. Figure 4.10 describes how the solvent and the nature of the separation problem influence the choice of adsorbent. Separation of enantiomers needs chiral stationary phases (CSP), which will be discussed in Section 4.3.4. 

The adsorbents that have been used are numerous and varied in nature They include charcoal, cellulose. Fuller s earth, silica gel, and alumina. The choice of adsorbent is based on the polarity and characteristics of the compounds to be separated, but in most instances an appropriate grade of activated alumina gives excellent results. In general, the polarity of the adsorbent should be significantly less than that of the substances to be separated. 

The requirement for adequate adsorptive capacity restricts the choice of adsorbents for practical separation processes to microporous adsorbents with pore diameters ranging from a few Angstroms to a few tens of Aflgstroms. This includes both the traditional microporous adsorbents such as silica gel, activated alumina, and activated carbon as well as the more recently developed crystalline aluminosilicates or zeolites. There is however a fundamental difference between these materials. In the traditional adsorbents there is a distribution of micropore size, and both the mean micropore diameter and the width of the distribution about this mean are controlled by the manufacturing process. By contrast, the micropore size of a zeolitic adsorbent is controlled by the crystal structure and there is virtually no distribution of pore size. This... 

In Table 19.1, various kinds of adsorbents (solid phases) used in column chromatography are listed. The choice of adsorbent often depends on the types of compounds to be separated. Cellulose, starch, and sugars are used for polyfunctional plant and animal materials (natural products) that are very sensihve to acid-base interactions. Magnesium silicate is often used for separating acetylated sugars, steroids, and essential oils. Silica gel and Florisil are relatively mild toward most compounds and... 

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