Sorbent types

The most frequently used sorbent type for straight phase chromatographic systems in planar chromatography is porous, nonsurface-modified silica gel. 

The most common and diverse approach to cleanup (and extraction of water samples) in pesticide residue analysis is SPE. Over the last 20 years, improvements and diversifications in SPE formats, sorbent types, and apparatus have made SPE a widely used approach for a variety of applications, including the analysis of pesticide residues. SPE cartridges or disks can be likened to low-resolution HPLC columns in that similar stationary and mobile phases are used. A typical particle size in SPE is 40 pm, and the plastic cartridges are generally packed with 0.1-1 g of sorbent in plastic tubes. The choice of reversed-phase, normal-phase, and ion-exchange media in SPE is very diverse, and Table 2 lists some of the more popular SPE applications for the cleanup of pesticides. 

Organic pollutant type Sorbent type Predictive models of partition coefficient, koc and kow values Equation number... 

Analysis of data from the factorials indicates that pH has a consistently significant effect on compound recoveries. A summary of the effect of pH level on compounds used in the study is given in Table VI. There is also an interaction between pH and primary column sorbent type for some compounds. This interaction suggests that at low sample pH, a C18 column will produce the best extraction efficiencies for phenolic compounds. The effect of adding methanol to the sample before extraction clearly produced odd results when the recovery data from the 24 factorial was analyzed by using half-normal plots. This effect will be studied in future work. Additionally, different elution solvents will be examined as well as new sorbent phases as they become available. 

The optimum conditions needed to achieve the desired sulfur capture efficiency (90 percent) is dependent on sorbent type, coal type, and fluidized bed design. Representative conditions are a bed temperature of 1116 K, a sorbent particle of 1 mm, and a Ca/S ratio of about 3. The bed temperature selection is based on the admittedly shaky evidence that this is where the maximum sulfation peaks. The particle size selection is a compromise between a desire to use high gas velocities and the observed decrease in sulfation with increasing size (Figure 2). The Ca/S ratio corresponds to a maximum sulfation of about 0.3. For these conditions the mass feed rate of a limestone sorbent is 0.345 that of the coal feed and the flow rate of the partially sulfated spent sorbent is 0.285 the coal feed rate. 

Analyte Sample Sorbent type Elution solvent Determination method... 

Fig. 2. Influence of sorbents type and contents of compositional mixtures on type of DF purification from toxic admixtures 1, 2, S, 6, 8 - natur sorbents 3, 4 - two-component mixtures 7 - four-component mixture 9 - two-stage process of purification...

Surface Characteristics of Different Sorbent Types Figures 8-11 present reasonably probable representations for surface active sites associated with the three major adsorbent types used in fats and oils processing. 

The mechanism for the formation of metal hydroxide surface precipitates is not clearly understood. It is clear that the type of metal ion determines whether metal hydroxide surface precipitates form, and the type of surface precipitate formed (i.e., metal hydroxide or mixed metal hydroxide) is dependent on the sorbent type. The precipitation could be explained by the combination of several processes (Yamaguchi et al., 2001). First, the electric field of the mineral surface attracts metal ions (e.g., Ni) through adsorption, leading to a local supersaturation... 

The type of sorbent that will work for a given application will be chosen by consideration of these factors. Once the sorbent type is chosen, the particle size and shape need to be considered. These factors affect both the pressure drop in the sorbent bed and the mass transfer rate from the fluid to the sorbent surface. As general rules of thumb, the pressure drop through the bed increases as the particle size decreases. In contrast, the mass transfer rate usually increases as the particle size decreases. The factors do not vary to the same degree with particle size so some balance needs to be made in the chosen design [6]. 

Sorbent Type Diesel Light Crude Heavy Crude Bunker C Percent Oil ... 

In addition to the amino-sorbents, other sorbent types have also been tested to exploit possibilities to remove co-extractives not or marginally removed by PSA, such as lipids, waxes, chlorophyll. Acidic, neutral, and basic alumina sorbents were found to remove, although to a slightly smaller extent, similar co-extractives as amino-sorbents, however, with less influence on pH. These sorbents, being much cheaper than PSA, will thus be more intensively studied in future. 

Table 3.4 Comparison of solvated and dry surface areas Sorbent Type Surface area, m /g...

Liu, Y.,Li, Z., Xu, L. and Cai, N. (2012) Effect of sorbent type on the sorption enhanced water gas shift process in a fluidized bed reactor. Industrial Engineering Chemistry Research, 51, 11989-11997. 

Because other sections of this Handbook deal more specifically with cleanup methods, policies, and detailed spill procedures, this section only emphasizes some key points related to adsorption cleanup. The safety, health, and environment department must prepare formal written procedures for all types of chemical spillage. The specification of sorbent type must be based on an expert analysis of the MSDS (material safety data sheet) for every chemical onsite. The manufacturers technical data or MSD sheets provide information for the site-specific safety plan, such as flash point, ignition temperature, solubility, toxicology, density, reactivity, and chemical compatibility. The written procedures must cover all chemicals on site and all possible spill scenarios, particularly What is the worst incident that could happen Prevention techniques must obviously be in place and emergency plan procedures carefully prepared and practiced. 

As is revealed in Fig. 2, data for both a sorbent and a polymer are concentrated in some regions of o values. It is hypothesized that sorbents of the same type (for example silica gels) have different adsorptive properties and/or they were used at different temperatures. This causes variations in the critical compositions of the mobile phases and, thus, a dispersion of the o values corresponding to one sorbent type in Fig. 2. Taking this into account, it may be concluded that values of solubility parameters corresponding to a pair polymer-sorbent are, in the majority of cas-... 

Passive type personal exposure samples were collected Irom exposure assessment program participants identified and selected Irom specific SEGs. The passive type personal exposure samples consisted of a solid sorbent type sample media with an engineered diffusive membrane that allows for a specific air diffusion rate to occur during the sampling period. 

Once the sorbent type has been selected, extraction of the sample should be confirmed. The cartridge should be conditioned with 1-2 bed volumes of a solvent such as methanol, then prepared for sample introduction by passage of 10-20 bed volumes of the sample solvent. If an ion-exchange phase is used, it should first be converted to the proper counterion, then equilibrated at the appropriate pH. Because of the slow diffusion rates of macromolecules, samples should be applied at relatively low flow rates, preferably less than lOmL/min a cartridge syringe device can generate flow rates of 200 mL/min [3]. After sample application, the bed should be washed twice with 10-20 bed volumes of sample solvent followed by analysis of both washes by HPLC. Absence of sample components in the chromatogram indicates adequate extraction by the sorbent, whereas appearance of samples in the wash implies that a more retentive sorbent is needed. 

There is little literature available comparing plate-layer characteristics. To the benefit of the method, only two or three major manufacturers produce each sorbent type, so diversity with respect to a given sorbent is kept to a minimum. However, different manufacturers of the plates themselves employ different coating techniques, which in turn has an effect on the characteristics of sorbent that has been coated on the plates. Thus, even though two precoated-plate companies buy their sorbent from the same manufacturer, the resulting plates may not display identical separation characteristics. 

Another interesting sorbent type is the Amberlite XAD range of resins. These are cross-linked polystyrene resins, like many ion-exchange resins, but without the ionic functional groups. XAD-1 to XAD-4 differ in specific surface and pore size. XAD-7 has carboxylic ester groups. They have been used for the analysis of nonionics [12-14] and cationics... 

SPE disks are preferred over SPE cartridges when large sample volumes need to be processed or when the sample contains particles (which might clog the SPE cartridges). The disks are made of an inert fiber matrix in which the sorbent is incorporated. The sorbent type can vary from reversed phase (polystyrene divinyl-benzene, C8, and CIS) to ion exchange. When the disk is placed in the reusable holder, they are used as packed SPE cartridges. After use, the disk is thrown away. 

Solid-phase extraction is a relatively new sample preparation method based on chemically modified silica gel or other sorbent type packed into a small plastic disposable cartridge or column. The cartridges are usually coimected to the end of a syringe and samples passed through by pressure, and the columns and disks are normally used in a vacuum manifold. Sorbents with various functional groups can be obtained in different configurations and sizes from commercial suppliers, which can supply procedures for analyses and bibliographies of applications. 

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