Stationary Phases for Normal-Phase Chromatography

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

Chapter 2 Separations in High-Performance Liquid Chromatography 

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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. 

Despite the limitations of unmodified silica as a stationary phase for normal phase chromatography, it remains the most common adsorbent in this field, although there has been a recent renewal of interest in the use of alumina. 

Diol phases arc about as retentive as silica or aminopropyl bonded phases (17). Compared to silica and alumina, diol phases are much less sensitive to the water content of the mobile phase. They arc also less reactive than aminopropyl phases. Furthermore, they can be washed with water without difficulty. Thus they are the best choice among the stationary phases for normal-phase chromatography. Although the stationary phase is readUy available from many sources, it is most commonly found as a packing for aqueous size-exclusion dromatography. 

Table 8.1 Typical stationary and mobile phases for normal and reverse phase chromatography ...

It is important for the analyst to be able to select the best stationary phase to use for a particular chromatographic analysis. Silica gel can be used in two modes of chromatographic separations as a stationary phase in normal elution development or as a stationary phase in exclusion chromatography. 

Compared with liquid column chromatography, in PLC there is a certain limitation with respect to the composition of the mobile phase in the case of reversed-phase chromatography. In planar chromatography the flow of the mobile phase is normally induced by capillary forces. A prerequisite for this mechanism is that the surface of the stationary phase be wetted by the mobile phase. This, however, results in a Umitation in the maximum possible amount of water applicable in the mobile phase, is dependent on the hydrophobic character of the stationary RP phase. To... 

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. 

Restricted access phases are another approach to exploiting the differences in characteristics of analytes. Large analytes are excluded from an internal surface on which an adsorptive stationary phase is present. A herbicide analysis for Metsulfuron methyl, Bentazone, Bromoxynil, methylchlorophenoxy acid, and Mecoprop in the presence of humic acid was performed on restricted access reversed phase media.52 The cytostatic compound epirubicin and its metabolites were separated from plasma using a Pinkerton GFF II column.53 Gradient separations of polymers on reversed phase and on normal phase represent an alternative to gel permeation chromatography. Polyesters of noncrystalline materials were separated on a variety of such phases.54... 

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]. 

Jandera, R, Urbanek, J., Prokes, B., Churacek, J. (1990). Comparison of various stationary phases for normal phase high perfomance liquid chromatography of ethoxylated alkylphe-nols. J. Chromatogr. 504, 297-318. 

Aboul-Enein and Ali [78] compared the chiral resolution of miconazole and two other azole compounds by high performance liquid chromatography using normal-phase amylose chiral stationary phases. The resolution of the enantiomers of ( )-econazole, ( )-miconazole, and (i)-sulconazole was achieved on different normal-phase chiral amylose columns, Chiralpak AD, AS, and AR. The mobile phase used was hexane-isopropanol-diethylamine (400 99 1). The flow rates of the mobile phase used were 0.50 and 1 mL/min. The separation factor (a) values for the resolved enantiomers of econazole, miconazole, and sulconazole in the chiral phases were in the range 1.63-1.04 the resolution factors Rs values varied from 5.68 to 0.32. 

In ion-exchange chromatography (IEC), the mobile phase modulator is typically a salt in aqueous solution, and the stationary phase is an ion-exchanger. For dilute conditions, the solute retention factor is commonly found to be a power-law function of the salt normality [cf. Eq. (16-27) for ion-exchange equilibrium]. 

A novel development for HPLC is something called bonded reversed-phase columns, where the stationary phase is a nonpolar hydrocarbon, chemically bonded to a solid support. You can use these with aqueous eluents, usually alcohol-water mixtures. So you have a polar eluent and a nonpolar stationary phase, something that does not usually occur for ordinary wet-column chromatography. One advantage is that you don t need to use anhydrous eluents (very small amounts of water can change the character of normal phase columns) with reversed-phase columns. 

The stationary phases available for HPLC are as numerous as those available for GC. As mentioned previously, however, adsorption, partition, ion exchange, and size exclusion are all liquid chromatography methods. We can therefore classify the stationary phases according to which of these four types of chromatography they represent. Additionally, partition HPLC, which is the most common, is further classified as normal phase HPLC or reverse phase HPLC. Both of these are bonded phase chromatography, which was described in Chapter 11. Let us begin with these. 

Answer the following with normal phase or reverse phase. For which type of liquid chromatography is a C18 column used Which is similar to adsorption chromatography in terms of the polarity of the stationary phase ... 

The most popular and versatile bonded phase is octadecylsilane (ODS), n-C18H37, a grouping that is non-polar and used for reverse phase separations. Octylsilane, with its shorter chain length, permits faster diffusion of solutes and this results in improved peak symmetry. Other groups are attached to provide polar phases and hence perform normal phase separations. These include cyano, ether, amine and diol groups, which offer a wide range of polarities. When bonded stationary phases are used, the clear distinction between adsorption and partition chromatography is lost and the principles of separation are far more complex. 

Gas chromatography (GC) employs a gaseous mobile phase, known as the carrier gas. In gas-liquid chromatography (GLC) the stationary phase is a liquid held on the surface and in the pores of a nominally inert solid support. By far the most commonly used support is diatomaceous silica, in the form of pink crushed firebrick, white diatomite filter aids or proprietary variants. Typical surface areas of 0.5-4 m2/g give an equivalent film thickness of 0.05-1 pm for normal liquid/support loadings of 5-50 per cent by mass. 

Spherical porous silica gel is the easiest stationary phase material to handle however, although it is physically strong it is chemically unstable. Surface modification can expand its capability for different modes of chromatography, such as normal-phase, reversed-phase, size-exclusion, and ion-exchange liquid chromatography. These stable modifications are performed by chemical deriva-tization of the surface silanol groups. 

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