Reversed-phase solid sorbents

Solid-phase extraction techniques that are based mostly on reversed-phase (Cis) sorbents, have been also widely used for cleanup and concentration purposes (23, 25, 27, 31, 34, 37, 46, 51, 52, 55, 65). However, many applications have indicated that cleanup using these nonpolar materials may not be very effective in removing interfering substances from sample extracts. Hence, polar sorbents such as silica (23, 26, 29, 30, 32, 40, 42, 44, 52, 53) or Florisil (45) have been also suggested as more powerful alternatives for the isolation and/or cleanup of amphenicols. 

Cleanup and concentration of quinolones from coextracted matrix constituents can also be accomplished with solid-phase extraction columns that contain either nonpolar reversed-phase (Cis) sorbents (177, 197, 198), or polar sorbents such as alumina (189-191, 194), aminopropyl (182, 187), and propylsulfonic acid (188). Reversed-phase Cis material has also been employed as the sorbent in matrix solid-phase dispersion cleanup for the determination of oxolinic acid in catfish muscle (206). 

Agonists are particularly suited to reversed-phase solid-phase extraction due, in part, to their relatively nonpolar aliphatic moiety, which can interact with the hydrophobic octadecyl- and octyl-based sorbents of the cartridge (472, 473, 475, 480,486, 487). By adjusting the pH of the sample extracts at values greater than 10, optimum retention of the analytes can be achieved. Adsorption solid-phase extraction using a neutral alumina sorbent has also been described for improved cleanup of liver homogenates (482). 

Solid-phase extraction columns contain either nonpolar reversed-phase Cig sorbents, or polar sorbents such as alumina, aminopropyl, and propylsulfonic acidJ Matrix solid-phase dispersion cleanup, using reversed-phase Cig material, has also been employed for the determination of oxolinic acid in catfish muscleJ On-line dialysis and subsequent trace enrichment have been further described for the extraction/cleanup of flumequine residues from fish muscle,or oxolinic acid and flumequine from chicken liver and salmon muscleJ This process involves on-line use of a diphasic dialysis membrane, trapping of the analytes onto a preconcentration column filled with reversed-phase Cig or polymeric material, rinsing of the coextracted interfering compounds to waste, and, finally, flushing of the concentrated analytes onto the analytical column. 

Reversed-phase solid-phase extraction (SPE) involves the partitioning of organic solutes from a polar mobile phase, such as water, into a nonpolar solid phase, such as the C-18 sorbent (Fig. 4.1). Partitioning involves the interaction of the solute within the chains of the stationary phase, which may be a C-18 hydrocarbon, C-8 hydrocarbon, or the polymeric sorbents (such as styrene-divinylbenzene). The word hydrophobic mechanism is commonly... 

Structure of Reversed-Phase Sorbents Reversed Phase as a Partitioning Mechanism Chromatographic Plate Theory and Reversed-Phase Solid-Phase Extraction... 

Solid-phase extraction is also often used to remove interfering coextracted compounds. Solid-phase extraction columns contain either non-polar reversed-phase Cig sorbents or polar sorbents (such as alumina, aminopropyl acid, and propylsulfonic acid). Matrix solid-phase dispersion cleanup using reversed-phase Cig material has been also employed for the determination of oxohnic acid in catfish muscle.In-tube solid-phase microextraction (SPME) based on poly(methacrylic acid-ethylene glycol dimethacrylate) (MAA-EGDMA) monolith coupled to high-preformance liquid chromatography (HPLC) with ultraviolet (UV) and fluorescence detection (FED) was... 

Although SPE can be done in a batch equilibration similar to that used in LLE, it is much more common to use a small tube (minicolumn) or cartridge packed with the solid particles. SPE is often referred to as LSE, bonded phase or sorbent extraction SPE is a refinement of open-column chromatography. The mechanisms of retention include reversed phase, normal phase, and ion exchange. 

In matrix solid-phase dispersion (MSPD) the sample is mixed with a suitable powdered solid-phase until a homogeneous dry, free flowing powder is obtained with the sample dispersed over the entire material. A wide variety of solid-phase materials can be used, but for the non-ionic surfactants usually a reversed-phase C18 type of sorbent is applied. The mixture is subsequently (usually dry) packed into a glass column. Next, the analytes of interest are eluted with a suitable solvent or solvent mixture. The competition between reversed-phase hydrophobic chains in the dispersed solid-phase and the solvents results in separation of lipids from analytes. Separation of analytes and interfering substances can also be achieved if polarity differences are present. The MSPD technique has been proven to be successful for a variety of matrices and a wide range of compounds [43], thanks to its sequential extraction matrices analysed include fish tissues [44,45] as well as other diverse materials [46,47]. 

As well as typical sample preparation methods such as filtration and liquid-liquid extraction, newer developments are now extensively used. The first of these is solid-phase extraction (SPE). This is a rapid, economical, and sensitive technique that uses several different types of cartridges and disks, with a variety of sorbents. Sample preparation and concentration can be achieved in a single step. Interfering sugars can be eluted with aqueous methanol on reversed-phase columns prior to elution of flavonoids with methanol. 

Solid-phase extraction seems to be more suitable for multiresidue cleanup. This procedure has become the method of choice for isolation and/or cleanup of -lactam antibiotics from biological matrices, because it requires low solvent consumption, it is generally less time-consuming and labor-intensive, and offers a variety of alternative approaches that allow better extraction of the more hydrophilic -lactam antibiotics such as ampicillin. It is usually performed using reversed-phase Ci8 (69-71, 80-83, 90, 92-94, 99, 107, 112-116, 121-125) or Cg (103), anion-exchange (95, 124), and polar diol (95) or alumina (101, 113) sorbents. In recent applications, some workers demonstrated the potential of online solid-phase extraction in the determination of monobasic penicillins in bovine muscle tissues using a reversed-phase Cis 5 m cartridge and an automated... 

Cleanup of macrolides and lincosamides from coextracted material can also be accomplished with solid-phase extraction columns. Nonpolar sorbents such as XAD-2 resin (148) or reversed-phase sorbents (133, 134, 137, 141, 142) are usually employed in solid-phase extraction. In the latter case, ion-pairing with pentanesulfonic acid can also be applied for enhancing retention onto the hydro-phobic Ci8 material (154). However, these sorbents are not always effective for efficient cleanup of liver and kidney extracts. The basic character of macrolides and lincosamides suggests that cation-exchange sorbents such as aromatic-sulfonic acid (145,147), or polar sorbents such as silica (144,152,153), aminopropyl (139), or diol (149-151), can be powerful alternative approaches for isolation and/or cleanup of these compounds. 

Cleanup of nitro furans from coextracted substances and concentration of the extracts can also be accomplished with solid-phase extraction columns. Nonpolar sorbents such as reversed-phase (Cis) (37, 159-161, 170, 173, 177) or XAD-2 (178) materials are usually employed, since they provide high recovery of the analytes. However, in many cases, cleanup on these nonpolar sorbents is not effective in removing interfering substances from the extracts. Therefore, polar sorbents such as silica (29, 162), alumina (160, 179, 180), or aminopropyl (175, 176) materials are also frequendy employed as a more powerful alternative for extract cleanup. 

Cleanup by solid-phase extraction has also been widely employed since it is a simple, fairly inexpensive, and easy-to-perform procedure for purification of the crude extract. The use of disposable solid-phase extraction columns is currently part of most, if not all, modern analytical methods for the determination of anthelminthics in biological matrices at residue levels. Both normal-phase columns based on silica (333-335, 340, 367, 372), alumina (346, 373-375), or aminopropyl (339, 365, 370) materials, and reversed-phase columns based on Ci8 (319, 323, 324, 328, 344, 346, 347, 349-351, 357-359, 364, 367) and cyclohexyl (329, 332, 360) sorbents have been described in analytical applications. 

Ion exchange is a process in which cations or anions in a liquid are exchanged with cations or anions on a solid sorbent. Cations are interchanged with other cations, anions are exchanged with other anions, and electroneutrality is maintained in both the liquid and solid phases. The process is reversible, which allows extended use of the sorbent resin before replacement is necessary. 

Dissolved organic sulfur was determined in aqueous solutions after isolation by solid-phase extraction on macroporous resins and reversed-phase sorbents.156 The sulfur in the extracts was determined by pyrohydrogenolysis of the extract in a heated quartz tube (1100°C) in a hydrogen atmosphere followed by flame photometric detection. 

On the other hand, the lack of internal pore structure with micropellicular sorbents is of distinct advantage in the analytical HPLC of biological macromolecules because undesirable steric effects can significantly reduce the efficiency of columns packed with porous sorbents and also result in poor recovery. Furthermore, the micropellicular stationary phases which have a solid, fluid-impervious core, are generally more stable at elevated temperature than conventional porous supports. At elevated column temperature the viscosity of the mobile phase decreases with concomitant increase in solute diffusivity and improvement of sorption kinetics. From these considerations, it follows that columns packed with micropellicular stationary phases offer the possibility of significant improvements in the speed and column efficiency in the analysis of proteins, peptides and other biopolymers over those obtained with conventional porous stationary phases. In this paper, we describe selected examples for the use of micropellicular reversed phase... 

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