Normal-phase chromatography silica

The most common technique used for agrochemicals is reversed-phase SPE. Here, the bonded stationary phase is silica gel derivatized with a long-chain hydrocarbon (e.g. C4-C18) or styrene-divinylbenzene copolymer. This technique operates in the reverse of normal-phase chromatography since the mobile phase is polar in nature (e.g., water or aqueous buffers serve as one of the solvents), while the stationary phase has nonpolar properties. 

The mechanism of reversed phase chromatography can be understood by contrast with normal phase chromatography. Normal phase liquid chromatography (NPLC) is usually performed on a polar silica stationary phase with a nonpolar mobile phase, while reversed phase chromatography is performed on a nonpolar stationary phase with a polar mobile phase. In RPLC, solute retention is mainly due to hydrophobic interactions between the solutes and the nonpolar hydrocarbon stationary surface. The nonpolar... 

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]. 

Hydrophilic interaction chromatography (HB IC) or aqueous normal-phase chromatography (ANP) refers to the use of polar stationary phases (e.g., bare silica, silica, or polymeric phases with bonded zwitterionic ligands, diol phases) in combination with a mobile phase rich in organic solvent but containing a significant amount of water (typically at least 3%). Bell [97] summarized the advantages of this technique as follows ... 

The addition of CO2 to mobile phases in normal phase chromatography using silica gel stationary phases was used as an adsorption-promoting solvent [56], Tetrahydrofiiran or chloroform with 3.5% ethanol was the organic components in... 

The stationary phase for normal-phase chromatography is silica gel that is more uniform in particle size (20-40 pm), has a pore size of 80-300 A, and spherical or irregular particle shape. In normal phase, peptides elute in order of decreased hydrophobicity, with the most hydrophilic peptide eluting last at the highest concentration of the hydrophilic solvent. This would be the aqueous or alcoholic component of the buffer system, which is also often acidic. 

Adsorption chromatography on bare silica is an example of normal-phase chromatography, in which we use a polar stationary phase and a less polar solvent. A more polar solvent has a higher eluent strength. Reversed-phase chromatography is the more common... 

Bonded-phase column packings for use in normal-phase chromatography are available in which the stationary phase is a polar functional group chemically bonded onto the silica surface. One... 

The carbamate pesticides can also be determined by normal-phase chromatography on different columns. Silica (113-116), amine (51), and cyanopropyl (112) columns have been used for the determination of carbamate residues. These columns were used with different solvent systems, with isopropanol-heptane being the mobile phase producing generally better results and methylene chloride-heptane the preferred solvent system when nonpolar carbamates were present in the sample. Another mobile phase used was iso-octane-dioxane (113). Although normal-phase mode was for the most part satisfactory, reversed-phase mode gave generally superior results. 

Normal Phase Chromatography—Separations mode run on nonbonded, anhydrous porous silica using a nonpolar mobile phase. See Adsorption Chromatography.)... 

This equation shows that we should ideally select a stationary phase with a polarity that is very different from that of the solute. Indeed, the recommendation to use normal phase chromatography (high 5,) for non-polar solutes (low 5() and reversed phase chromatography (low 8) for the separation of polar solutes (high 8 is not new. However, this rule of thumb is much too simple. A complication is caused by the availability of appropriate mobile phases. For instance, to satisfy eqn.(3.30) for the elution of non-polar solutes (5, 7) from a silica column (5, 16), a mobile phase with 8mx -2 would be required. 

In normal-phase chromatography, the retention is governed by the interaction of the polar parts of the stationary phase and solute. For retention to occur in normal phase, the packing must be more polar than the mobile phase with respect to the sample. Therefore, the stationary phase is usually silica and typical mobile phases for normal phase chromatography are hexane, methylene chloride, chloroform, diethyl ether, and mixtures of these. In reverse phase the packing is nonpolar and the solvent is polar with respect to the sample. Retention is the result of the interaction of the nonpolar components of the solutes and the nonpolar stationary phase. Typical stationary phases are nonpolar hydrocarbons, waxy liquids, or bonded hydrocarbons (such as Ci8, Q, etc.) and the solvents are polar aqueous-organic mixtures such as methanol-water or acetonitrile-water. In the strictest interpretation, normal and reverse phase are terms which only relate to the polarity of the column and mobile phase with respect to the sample as shown in Table 3-3 and drawn schematically in Figure 3-14. 

Bonded phases may be used in both normal and reverse phase chromatography. When normal phase chromatography is done on bonded phase packings, the packing is more polar than the mobile phase. Polar bonded phases such as the cyanopropyl and aminopropyl functionalities are popular for this use. These bonded phases are less subject to changing retention times of compounds because water is adsorbed from the mobile phase onto the stationary phase, a frequent concern when using bare silica packings for normal-phase separations. 

Steuer, W., Schidnler, M., and Emi, E. 1988. Gradient elution with normal phases on silica A comparison between high-performance liquid and supercritical fluid chromatography. Journal of Chromatography, 454 253-9. 

Cholestenone (derived from unreacted cholesterol) and progesterone were separated from deoxycorticosterone acetate (internal standard) by normal phase chromatography on a TSK-gel silica 150 column (4 mm x 250 mm)... 

The silanol group (Si-OH), present at least at low concentrations in all reversed-phase columns, is capable of adsorbing a layer of a polar solvent such as methanol or water. A column conditioning procedure based on this adsorption is an essential step in normal-phase chromatography on silica columns. This process is probably also important in reversed-phase... 

Unlike the more popular reversed-phase chromatographic mode, normal-phase chromatography employs polar stationary phases, and retention is modulated mainly with nonpolar eluents. The stationary phase is either (a) an inorganic adsorbent like silica or alumina or (b) a polar bonded phase containing cyano, diol, or amino functional groups on a silica support. Tlie mobile phase is usually a nonaqueous mixture of organic solvents. As the polarity of the mobile phase decreases, retention in normal-phase chromatography... 

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