Normal phase alumina

Recently, Janjic et al. published some papers [33-36] on the influence of the stationary and mobile phase composition on the solvent strength parameter e° and SP, the system parameter (SP = log xjx, where and denote the mole fractions of the modiher in the stationary and the mobile phase, respectively) in normal phase and reversed-phase column chromatography. They established a linear dependence between SP and the Snyder s solvent strength parameters e° by performing experiments with binary solvent mixtures on silica and alumina layers. 

Liquid chromatographic clean up [441,443,450] has been used either in normal phase flow using alumina, silica, or florisil [22,189,403,481,484] or with reverse-phase (RP) columns [409,452,480]. In most cases these techniques are well established and are used in an off-line mode, primarily to remove the bulk of co-extracted materials prior to a more refined clean-up prior to the final determination. These columns may be prepared in the laboratory [22,403 -405] or commercial solid phase extraction (SPE) cartridges can be used [409,452, 463,470,485,486]. In both cases, the normal phase cartridges and column materials are disposable since many of the polar co-extractants bind firmly to the substrate surface and are difficult to remove. This has been overcome to some... 

Example 2 Chromatography of nitroaniline isomers. The elution order of the nitroaniline isomers was ortho, meta, and para in normal-phase liquid chromatography using H-butanol-w-hexane mixtures as the eluent, when the stationary phase material was either silica gel, alumina, an ion-exchanger, polystyrene gel, or octadecyl-bonded silica gel. The results indicate that the separation of these compounds can be performed on a range of different types of stationary phase materials if the correct eluent is selected. The best separation will be achieved by the right combination of stationary phase material and eluent.68... 

Figure 4.19 Solvent strength of combination of n-pentane and more polar solvents in normal-phase liquid chromatography using alumina. Symbols. A, methyl acetate, 0> acetone, , chloroform, and O, benzene.

Normal phase (NP) was the original mode of high-performance liquid chromatography (HPLC) reported. This uses a polar stationary phase (e.g. sihca, alumina) and non-polar mobile phase. It can also be referred to as adsorption... 

Alumina has been widely used in normal phase chromatography. Its surface exhibits amphoteric properties (available as basic, acidic, and neutral alumina) and can react as a cation or an anion exchanger. The surface area is in the usual range around lOOm /g, however, the pore structure is not very favorable as a relatively large portion of narrow pores (<2nm) is present. 

Normal-phase adsorption on silica gel with a relatively less polar mobile phase is the most widely used mode in conventional TLC. To improve separations, silica gel may be impregnated with various solvents, buffers, and selective reagents. Other commercial precoated layers include alumina, florisil, polyamide, cellulose, and ion exchangers. 

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. 

Normal phase refers to systems in which the stationary phase is more polar than the mobile phase. Historically, it was the first mode of operation, and the development of theory on the retention mechanism has been under way for several decades. Solid supports of this type are silicagel, alumina, and zirconia. 

In normal"-phase LC systems, die solid phase is a polar solid such as silica gel (most common) or alumina and die liquid is generally an organic solvent of low polarity. In such a case, polar compounds bind more strongly to die polar silica gel surface and thus travel more slowly along the surface, whereas... 

The decision about which HPLC column to choose is really controlled by the separation you are trying to make and how much material you are trying to separate and/or recover. I did a rather informal survey of the literature and my customers 15 years ago to see which columns they used. I found 80% of all separations were done on some type of reverse-phase column (80% of those were done on C18), 10% were size separation runs (most of these on polymers and proteins), 8% were ion-exchange separations, and 2% were normal-phase separation on silica and other unmodified media, such as zirconium and alumina. The percentage of size- and ion-exchange separations has increased recently because of the importance of protein purification in pro-teomics laboratories and the growing use in industry of ion exchange on pressure-resistant polymeric and zirconium supports. 

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-... 

The typical stationary phases employed in normal-phase or adsorption chromatography are common porous adsorbents, such as silica and alumina,... 

For the samples that will be subjected to other (so-called interactive) LC techniques, the next question involves the nature of the solvent in which the sample has been or can be dissolved. If this is a non-polar solvent, such as n-hexane, then the sample solution is compatible with Normal Phase LC (NPLC), in which mobile phases with a relatively low polarity are used in combination with more polar stationary phases (see section 3.2.3). In this form of chromatography solid adsorbents (such as silica or alumina) may be used as stationary phases (LSC). Alternatively, polar chemically bonded stationary phases may be used (see section 3.2.2). 

Chromatographic Mode Normal Phase (Silica, Florisil , Alumina, Diol, NH2, etc.) Reverse Phase (C18, CN, etc.) Ion Exchange (NH2, Anion Exchange, Cation Exchange, etc.)... 

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... 

---