Sorbents normal phase

Gustavson, K.E. DeVita, W. Revis, A. Harkin, J.M. 2000, A novel use of a dual-zone restricted access sorbent Normal phase separations of methyl oleate and polynuclear aromatic hydrocarbons stemming from semipermeable membrane devices. J. Chromatogr. A 883 143-149. 

Reversed-phase sorbents, normal-phase sorbents mixed-mode for drug analysis. 

Other sorbents include mixed mode sorbents, normal-phase SPE sorbents or restricted access matrix sorbents. 

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. 

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. 

The TLC process is an off-line process. A number of samples are chromatographed simultaneously, side-by-side. HPTLC is fast (5 min), allows simultaneous separation and can be carried out with the same carrier materials as HPLC. Silica gel and chemically bonded silica gel sorbents are used predominantly in HPTLC other stationary phases are cellulose-based [393]. Separation mechanisms are either NPC (normal-phase chromatography), RPC (reversed-phase chromatography) or IEC (ion-exchange chromatography). RPC on hydrophobic layers is not as widely used in TLC as it is in column chromatography. The resolution capabilities of TLC using silica gel absorbent as compared to C S reversed-phase absorbent have been compared for 18 commercially available plasticisers, and 52 amine and 36 phenolic AOs [394]. 

Gustavson et al. (2000) developed a convenient and novel solid phase extraction (SPE) method for the removal of methyl oleate from SPMD dialysates containing PAHs. A small SPE column (1 g or 0.5 g) containing a dual-zone silica (normal phase)-based restricted-access sorbent (Diazem, Midland, MI, USA) is used for the separation. The capacity of this sorbent to remove methyl oleate is about 1.8% (lipid/sorbent wt wt ). The PAHs are eluted with 19 mL of hexane and methylene chloride (97 3 VV ) and recoveries of all PAHs are typically >72%. 

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. 

In liquid chromatography, reversed-phase materials such as Cig and Cg are the most commonly used sorbents (429, 430, 434, 438, 446, 447, 453, 454). Examples of baseline separations with reversed-phase columns of several groups of anabolics including stilbenes, resorcyclic acid lactones, and other, frequently used anabolics have been reported (463-466). In addition to reversed-phase separations normal-phase separations of anabolics using either Hypersil (467) and Brownlee (456) silica or diol-modified silica have been reported. Although not all analytes were completely separated, the latter column could be efficiently used to differentiate between estrogenic and androgenic compounds within a mixture of 15 anabolics and their metabolites (468). 

Relative elution solvent strength (or eluotropic strength) is depicted in solvent polarity charts (Figure 2.39). The relative elution strength for a solvent on a polar, normal-phase sorbent such as silica or alumina increases in reverse order to that measured on a nonpolar, reversed-phase sorbent. Ac-... 

CuUere, L., Aznar, M., Cacho, J., and Eerreira, V. (2003). Fast fractionation of complex organic extracts by normal-phase chromatography on a solid-phase extraction polymeric sorbent -Optimization of a method to fractionate wine flavor extracts. J. Chromatogr. A., 1017, 17-26. 

The TLC behavior of Co(dik)3, Cr(dik)3 and Ru(dik)3, in which acac ligands were successively replaced with dibzac ligands, was studied. The R values increased with nonaqueous and decreased with aqueous solvent systems. Unmodified silica gel can be used as a sorbent for both normal phase and RP-TLC of the metal complexes. The rule of a linear dependence between the Rm values of transition metal /3-diketonates and the number of substituted chelate ligands was extended. ... 

Two sorbent layers (e.g., silica gel and reversed phase) as adjacent zones with different selectivities can be used. The sorbent layer for the first development is a narrow strip (reversed phase), and for the second development, it is a large surface (normal phase). A suitable eluent system has to be used for each sorbent layer. 

Classical liquid chromatography is typically practiced in what is referred to as the normal-phase mode that is, the stationary phase is usually a polar sorbent such as silica and alumina and the mobile phase consists of a nonpolar constituent such as hexane modified with a somewhat more polar solvent such as chloroform or ethyl acetate. In this mode, the more polar compounds are preferentially retained. The reversed-phase (RP) mode utilizes the opposite approach for the separation of nonpolar analytes or compounds that have some hydrophobic character. In this case, the stationary phase must consist of sorbent that is nonpolar in nature and the mobile phase is composed of a primary polar solvent, usually water, that is modified by a more nonpolar constituent such as methanol, acetonitrile, or tetrahydrofuran. 

A variety of sorbents have been used as the stationary phase in TLC, including silica gel, cellulose, alumina, polyamides, ion exchangers, chemically modified silica gel, and mixed layers of two or more materials, coated on a suitable support. Currently in the pharmaceutical industry, commercially precoated high-performance TLC (HPTLC) plates with fine particle layers are commonly used for fast, efficient, and reproducible separations. The choices of mobile phase range from single component solvent systems to multiple-component solvent systems with the latter being most common. The majority of TLC applications are normal phase, which is also a complementary feature to HPLC that uses mostly reverse-phase columns. 

Solid-phase extraction (SPE) is an alternative to LLE. In SPE the analytes are partioned between a solid and a liquid [57, 58], Generally, interfering compounds are rinsed off the solid adsorbent and the analytes are then desorbed with an eluting solvent [58], A range (e.g., normal-phase, reversed-phase, ion exchange, restricted access) of sorbents and formats are available for SPE and the SPE systems are easy to automate [59, 60], In order to accomplish the isolation of the products from the fermentation matrix, both SPE and LLE were evaluated for use in papers I and II. 

For normal-phase SPE, cyanopropyl (CN), aminopropyl (NH2), and diol functional groups are chemically bonded to the silica gel. The loading on the cyano, amino, and diol colunms are sufficiently large ( 6-10% as carbon) that they may sometimes be used for reversed-phase applications, especially for the removal of hydrophobic solutes from water or other polar solvents. These hydrophobic solutes would otherwise sorb too strongly to a more hydrophobic C-8 or C-18 sorbent and would be difficult to elute. Straight silica gel also is... 

Normal-phase sorbents such as silica and Florisil are used to isolate low to moderate polarity species from nonaqueous solutions. Examples of applications include lipid classification, plant pigment separations, and separations of fat-soluble vitamins from lipid extracts, as well as the clean-up of organic solvent concentrates obtained from a previous SPE method or liquid-liquid extraction. Alumina is used to remove polar species from nonaqueous solutions. Examples include vitamins in feeds and food and antibiotics and other additives from feed. Normal-phase chromatography has been used for a number of years, and most applications for normal-phase column chromatography may be easily transferred over to normal-phase SPE. 

Finally, sorbents such as aminopropyl, cyanopropyl, and diol can be used for both reversed-phase and normal-phase separations. Many manufacturers supply their sorbents in variety packs that may be used for methods development. Also quality assurance reports are commonly available for the various sorbents, which is a good indication of their reproducibility. Later chapters will show specific applications, such as environmental, drugs and pharmaceuticals, and food and natural products. 

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