Solid-phase adsorbants

In gas-solid extractions the sample is passed through a container packed with a solid adsorbent. One example of the application of gas-solid extraction is in the analysis of organic compounds for carbon and hydrogen. The sample is combusted in a flowing stream of O2, and the gaseous combustion products are passed through a series of solid-phase adsorbents that remove the CO2 and 1T20. 

When pushed to the limit by overriding human health concerns, residue chemists have achieved detection limits of Ippt (Ingkg ) or even into the low ppqr (1 pg kg ) range. An example at the 1 ppt level is provided by methods for 2,3,7,8-tetrachlorodibenzodioxin (TCDD) in milk and TCDD in adipose tissue. Eor relatively clean matrices such as water and air, preconcentration on solid-phase adsorbents followed by GC or gas chromatography/mass spectrometry (GC/MS) can provide detection limits of 1 ng m and less for air (examples in Majewski and Capel ) and 1 ngL and less for water (examples in Larson et A summary of units of weight and concentration used to express residue data is given in Table 1. 

Many other types of solid phase adsorbents, including those based on conventional and specialty materials like restricted access media (RAM), can increase analysis speed and improve assay performance. These types of materials, also known as internal reversed-phase packings, are especially useful for assaying target compounds in biological samples such as serum and plasma. They are chemically modified porous silicas that have hydrophilic external surfaces and restricted-access hydrophobic internal surfaces. The ratio of interior to external surface areas is large. Macromolecules such as proteins cannot enter the pores of the RAM (they are excluded from the hydrophobic internal surface) and they elute quickly through the column. However, the smaller analyte molecules that can enter the pores are retained via interactions with the hydrophobic bonded phase within... 

The adsorption of organics from the liquid to a solid phase is generally assumed to occur in three stages [50]. The brst is the movement of the contaminant (adsorbate or solute) through a blm surface surrounding the solid phase (adsorbant). The second is the diffusion of the adsorbate within the pores of the activated carbon. The bnal stage is the sorption of the material onto the surface of the sorbing medium. The overall rate of adsorption is controlled by the rate of diffusion of the solute molecules within the capillary pores of the carbon particles [27]. 

Adsorption (or desorption) is the process by which a net accumulation (or loss) of a substance occurs at an interface between two phases. In a typical experiment, two phases are mixed intimately to provoke a chemical reaction leading to adsorption or desorption, and then a physical separation is made, with one of the separates being a single phase and the other, a mixture of the two reacted phases. For example, a solid-phase adsorbent and an aqueous solution could be mixed, and then separated by centrifugation into an aqueous phase (supernatant solution) and a slurry that contains both the solid adsorbent and some aqueous solution. If is the moles of substance i in the reacted mixture and m is the molality of substance i in the separated aqueous phase, then the relative surface excess, np, of substance i, as compared to another substance j, is defined by1... 

Semivolatile analytes in vapor-phase samples are often associated with particulate matter or aerosols that are collected on glass or quartz fiber filters in a flowing air stream.1 However some semivolatile analytes that have higher VPs, for example, 3,3 ,5,5 -tetra-chlorobiphenyl in Fig. 8.2, can slowly vaporize from a particle trapped on a filter in the flowing air stream, and will be lost unless captured by an in-stream sampling device. Small glass or metal tubes containing polyurethane foam are often used to capture vaporized semivolatile analytes. Other-solid phase adsorbents, which are described in the next section, are also used to trap semivolatile analytes vaporized from particulate filters. 

There are two general approaches to sampling air, or vaporous emissions from stationary (stack) and mobile (automobile, truck, etc.) sources, for the laboratory determination of volatile analytes.1 Bulk vapor-phase samples can be taken in the field in various containers and transported to a remote or field laboratory for analysis. Containers used for bulk vapor-phase samples include flexible polyvinyl fluoride (Tedlar ) bags, evacuated glass or metal reservoirs, and thermally insulated cryogenic collection vessels. Alternatively, the volatile analytes can be separated from the main components of air in the field and just the analytes and their collection devices transported to the laboratory. The principal techniques used to separate volatile analytes from air in the field are cryogenic traps, impingers, and solid-phase adsorbents. 

Solid-phase adsorbents are simple and inexpensive devices used to separate volatile analytes from the principal components of air in the field.1 A porus solid-phase adsorbent is placed in a glass or metal tube which is taken to the field where air is drawn through the... 

Equilibrium between solution and adsorbed or sorbed phases is a condition commonly used to evaluate adsorption or sorption processes in soils or soil-clay minerals. As previously stated, equilibrium is defined as the point at which the rate of the forward reaction equals the rate of the reverse reaction. Two major techniques commonly used to model soil adsorption or sorption equilibrium processes are (1) the Freundlich approach and (2) the Langmuir approach. Both involve adsorption or sorption isotherms. A sorption isotherm describes the relationship between the dissolved concentration of a given chemical species (adsorbate) in units of micrograms per liter (pg L 1), milligrams per liter (mg L-1), microequivalents per liter (pequiv L-1), or millimoles per liter (mmol L-1), and the sorbed quantity of the same species by the solid phase (adsorbent) in units of adsorbate per unit mass of adsorbent (solid) (e.g., pg kg-1, mg kg-1, peq kg-1, or mmol kg 1) at equilibrium under constant pressure and temperature. Sorption isotherms have been classified into four types, depending on their general shape (Fig. 4.13) ... 

Maurer has reviewed the application of LC-MS and LC-MS/MS to the detection of alkaloids in human biofluids [14]. Extraction techniques include liquid-liquid extraction relying upon the ionization of alkaloids in aqueous acid, solid phase extraction (SPE) in which alkaloids are cleaned up and concentrated from the biomatrix by adsorption and subsequent elution from a small cartridge of solid phase adsorbent, and solid-phase microextraction (SPME), in which analytes are adsorbed directly from the matrix or the headspace above the heated matrix onto a fine fiber of adsorbent on fused silica. The latter process is more commonly used with GC-M S but is finding increasing use with LC-MS. 

Protein A, a structural component of certain strains of staphylococci, has been used successfully in the place of the second antibody in radioimmunoassay techniques, as well as the solid phase adsorbent of antibody (mentioned in the section Solid Phase Materials). This protein can be heavily labeled with and is, therefore, a reasonable alternative for the use of labeled antiglobulins in immunoassays. 

It consists of purging the sample (heated at a given temperature usually below 100° C) with an inert gas and passing the purging gas over a solid-phase adsorbent, such as Tenax. The solid-phase adsorbent is kept at a low temperature (such as 20° C). 

The solid-phase adsorbent is heated (at a temperature commonly between 200 C and 300° C) to desorb the analytes in a stream of carrier gas that is sent to a GC. 

Conte, E. D., Shen, C. Y., Perschbacher, P. W., and Miller, D. W. 1996. Determination of geosmin and methylisoborneol in catfish tissue (Ictalurus punctatus) by microwave-assisted distillation—solid phase adsorbent trapping, J. Agricul. Food Chem., 44 829-835. 

It is generally agreed among the practitioners of whole beer processing that product removal in the presence of cells is more difficult to attain, and requires that all physico-chemical parameters of the operation such as fermentation medium composition to be standardized. The purpose of this paper is to describe a new approach to achieve whole broth processing using immobilized solid phase adsorbents. 

One of the objectives of a biochemical separation system design is to minimize the number of steps (Figure 1) One way of accomplishing this is to perform the separation and concentration in situ that is directly with the whole fermentation broth using solids phase adsorbents. This type of separation requires the design of an affinity bead that provides for selective product removal from the fermentation broth. 

A novel approach to increase the overall extraction yield of a microbial product has been developed. The primary isolation steps of fermentation derived microbial products generally involve procedures including solid removal, wash and volume reduction using either solvent extraction or ion-exchange. These steps can be further simplified by the use of whole broth extraction. In the case of using solid phase adsorbents, extraction performance can be drastically affected by the size and the nature of the adsorbent used. If the adsorbent size is large, it exhibits internal diffusional resistance (pore diffusion) and the resultant mass transfer resistance reduces the overall rate of adsorption... 

Solid-phase adsorbants employed for the isolation of lectins... 

A second method of sample acquisition, based on use of a solid-phase adsorbent as an analyte trap, is widely used for low volatility species that are less suitable for sampling using canister methods because of the increased capacity of analyte condensation on the container walls. The adsorbent used in the trap is chosen in such a way to introduce an element of selectivity to the trapping mechanism, although in practice a trap-all approach is commonly used. 

Qi,m maximum solid phase adsorbed concentration for the species mole gads /rmole gads ... 

In SPE, choice of an appropriate solid phase is based on their possible molecular interactions with the targeted compounds. For historical reasons, reversed-phase type adsorbents have been largely utilized to extract compounds from aqueous samples, given the fact that many of them have been developed already for use in reversed-phase chromatography applications and are commercially available. Ion exchange adsorbents have been used to retain counter ions based on electrostatic interactions. Solid phase adsorbents combining hydrophobic and ionic interactions are also present, for example graphitized carbon black (GCB) obtained by... 

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