Sorbents common

The quantity of a solute adsorbed can be given conveniently in terms of moles or volume (for adsorption) or ion-equivalents (for ion exchange) per unit mass or volume (dry or wet) of sorbent. Common units for adsorption are moV(m of fluid) for the fluid-phase concentration Cj and moV(kg of clean adsorbent) for adsorbed-phase concentration /ij. For gases, partial pressure may replace concentration. 

Controlled-access sorbents are intended to be either inclusive or exclusive of large molecules and macromolecules. Wide-pore, or large-pore, sorbents are designed intentionally to allow accessibility of macromolecules to the internal pore structure of the sorbent such that they will be retained. Conventional SPE sorbents commonly have pores of 60 A, whereas wide-pore SPE sorbents have pores of 275 to 300 A [75]. 

All forms of SPE generally follow the same four-step procedure. This process comprises (1) sorbent conditioning, (2) passage of the sample through the sorbent, commonly called sample loading, (3) removal of interferences, and (4) elution of the analyte of interest into a receiving vessel. The four steps that are involved in performing methods that require SPE as the principal means of sample preparation are now discussed ... 

Sohd sorbent materials have the abiUty to adsorb water vapor until an equiUbrium condition is attained. The total weight of water that can be adsorbed in a particular material is a function of the temperature of the material and of the relative humidity of the air (see Adsorption). To regenerate the sorbent, its temperature must be raised or the relative humidity lowered. The sohd sorbents most commonly used are siUca (qv), alumina (see Aluminum compounds), and molecular sieves (qv). 

Adsorption Processes. The processes based on adsorption of hydrogen sulfide onto a fixed bed of soHd material are among the oldest types of gas treating appHcations (4). Two common sorbent materials for low concentration gas streams are iron oxide and zinc oxide. 

Adsorption and ion exchange share so many common features in regard to apphcation in batch and fixed-bed processes that they can be grouped together as sorption for a unified treatment. These processes involve the transfer and resulting equilibrium distribution of one or more solutes between a fluid phase and particles. The partitioning of a single solute between fluid and sorbed phases or the selectivity of a sorbent towards multiple solutes makes it possible to separate solutes from a bulk fluid phase or from one another. 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. Miscellaneous gas-absorption equipment could include acid gas scrubbers that are commonly classified as either wet or diy. In wet scrubber systems, the absorption tower uses a hme-based sorbent liquor that reacts with the acid gases to form a wet/solid by-product. Diy scrubbers can be grouped into three catagories (1) spray diyers (2) circulating spray diyers and (3) dry injection. Each of these systems yields a diy product that can be captured with a fabric filter baghouse downstream and... 

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). 

Analysis of environmental samples is similar to that of biological samples. The most common methods of analyses are GC coupled to MS, ECD, a Hall s electrolytic conductivity detector (HECD), or a flame-ionization detector (FID). Preconcentration of samples is usually done by sorption on a solid sorbent for air and by the purge-and-trap method for liquid and solid matrices. Alternatively, headspace above liquid and... 

PT catalysts are often difficult to separate from the product, while it is also desirable that the catalyst should be reusable or recyclable. Distillation and extraction are the most common separation processes. The main disadvantage of lipophilic quats is their tendency to remain in the organic phase and consequently contaminate the product. Therefore, extraction in water often is not satisfactory. Furthermore, products in the fine chemicals industry often have high boiling points and/or are heat sensitive, which makes separation of the catalyst by distillation impossible. Often the only means to remove the catalyst in these cases is to adsorb it using a high surface area sorbent such as silica, Florisil or active carbon (Sasson, 1997). After filtration, the catalyst can then be recovered by elution. 

There are several types of sorbent materials in the market today, other than commonly used silica and alumina. Some of these sorbents are chemically modihed to a certain extent to improve the properties of the thin layer for a wide variety of compounds, as well as for better resolution. 

The number of stationary phases available is essentially unlimited, although relatively few are, in fact, in common use. Chapter 3 discusses sorbents and precoated layers in detail. Originally, PTL was undertaken on plates made in the laboratories where they were used, and this led to the use of a wide variety of stationary phases. 

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. 

Table 2 Common SPE sorbents used in analytical methods for pesticides...

Prior to the development of modern SPE formats, liquid-solid partitioning with charcoal, silica, Florisil, and/or alumina was common to aid in the removal of lipids in the determination of nonpolar pesticides, but these sorbents are less useful in the cleanup of semi-polar and polar pesticides owing to the large elution volumes needed. Applications of modern SPE are discussed in Section 3.2. 

Many types of matrices have been used in the past to measure the field stability of the test substance. Cotton gloves, cellulose patches, face wipe handkerchiefs and/or gauze face wipe matrices, long underwear (inner dosimeters), pants, shirts, coveralls (outer dosimeters), sorbent tubes, urine, and other matrices are common matrices that have been used for this purpose. 

Some more recent field techniques have focused on the location of the preparation of field fortification samples and have taken some of the responsibility for the preparation of the field fortification samples from the field personnel and placed them with the analytical laboratory. For example, it is becoming more common for the analytical laboratory to prepare air sample field fortifications in the analytical laboratory, freeze them, and ship them to the field for use in a frozen state. Whereas there may be some advantage to this technique in that the air tube fortification samples may possibly be fortified more accurately in the laboratory under controlled conditions than if done in the field, there are some inherent scientific problems with this method. First, one reason for the field fortification is to test the ruggedness of the field techniques of the researcher under extreme field conditions. Second, the act of freezing and thawing the sorbent matrix within the air mbe itself may have an impact on the recovery of the analyte from the air tube after exposing the sorbent to field conditions... 

The extent of fluorescence quenching often depends on the sorbent medium and is generally more severe for silica gel than for chemically bonded sorbents [183]. In many cases the emission signal can be enhanced by application of a viscous liquid to the layer before scanning the plate. Common fluorescence enhancing... 

The principal difference between analytical TLC and preparative TLC is one of scale and not of procedure or method. Scale up is achieved by increasing the thickness of the layer and the length of the edge of the plate to which the sample is. applied. Preparative TLC plates range in size from 20 x 20 cm to 20 X 100 cm and are coated with a sorbent layer 0.5 to 10.0 as thick. The most commonly used layer thicknesses are 1.0 and 2.0 mm. Analytical layers are suitable for micropreparative applications and when high resolution is required. In general, the loading capacity increases with the square root of the layer thickness but the resolution is usually less for thicker layers. 

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