Normal phase chromatography separation mechanism

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]. 

HILIC is a variant of normal-phase chromatography that employs polar stationary phases and RPLC-type mobile phases. Because HILIC separations occur by a normal-phase mechanism, the organic component of the mobile phase... 

In normal-phase chromatography, polar stationary phases are employed and solutes become less retained as the polarity of the mobile-phase system increases. Retention in normal-phase chromatography is predominately based upon an adsorption mechanism. Planar surface interactions determine successful use of NPC in separation of isomers. The nonaqueous mobile-phase system used in NPC has found numerous applications for extremely hydrophobic molecules, analytes prone to hydrolysis, carbohydrates, and sat-urated/unsaturated compounds. In the future, with the advent of new stationary phases being developed, one should expect to see increasingly more interesting applications in the pharmaceutical industry. 

Combining different separation techniques governed by different mechanisms to a multidimensional method is suitable to increase the potential of the individual techniques by an order of magnitude (31,32). HPLC is one of the most powerful separation techniques available today for nonvolatile substances. For reasons mentioned above, HPLC most often employs the reverse phase separation mode. On-line coupling of HPLC with AMD using normal phase chromatography results in separation numbers around 500. 

In lc there are other sorption mechanisms that can cause separation, depending on whether we choose to use a liquid or a solid as the stationary phase, or what kind of solid we use. Liquid-liquid chromatography (11c) uses a liquid stationary phase coated onto a finely divided inert solid support. Separation here is due to differences in the partition coefficients of solutes between the stationary liquid and the liquid mobile phase. In normal phase 11c the stationary phase is relatively polar and the mobile phase relatively non-polar, whilst... 

In another study, the authors reported a comparative study of the enantiomeric resolution of miconazole and the other two chiral drugs by high performance liquid chromatography on various cellulose chiral columns in the normal phase mode [79], The chiral resolution of the three drugs on the columns containing different cellulose derivatives namely Chiralcel OD, OJ, OB, OK, OC, and OE in normal phase mode was described. The mobile phase used was hexane-isopropanol-diethylamine (425 74 1). The flow rates of the mobile phase used were 0.5, 1, and 1.5 mL/min. The values of the separation factor (a) of the resolved enantiomers of econazole, miconazole, and sulconazole on chiral phases were ranged from 1.07 to 2.5 while the values of resolution factors (Rs) varied from 0.17 to 3.9. The chiral recognition mechanisms between the analytes and the chiral selectors are discussed. 

To classify a separation technique by LC into these two types, it should be clear whether the sample copolymers adsorb on the surface of the stationary phase or precipitate on the top of the column (phase separation) when the sample copolymers are injected into a column. If the separation mechanism is not clearly understood or when the separation by the solubility difference of the sample copolymers between the stationary and the mobile phases can be considered, then the technique can be classified into normal-phase and reversed-phase chromatography as the third type (Table III). Initial and final mobile phases should be good solvents for the sample copolymers. The initial mobile phases in Table III are 15% THF in acetonitrile (AcCN) or 10% THF in cyclohexane (J5), 35% THF in n-hexane (J6), 20% CH2CI2 in AcCN (J7), and 10% CHCI3 in n-hexane (18). The final mobile phases are 65% THF in AcCN or 60% THF in cyclohexane (15), 85% THF in n-hexane (J6), 80% CH2CI2 in AcCN (17), and 40% CHCI3 in n-hexane (J8). 

Combining different separation methods, governed by different separation mechanisms, to multidimensional methods is suitable for multiplying the potential of the individual techniques. Reversed-phase chromatography high-performance columns (RP-HPLC) can be coupled with normal phase TLC [9,10]. 

There are a number of modes or mechanisms into which chromatography is divided. These include adsorption, normal-phase partition, reversed-phase partition, and ion exchange. Often, the term partition is deleted from the discussions of the differences and similarities of these modes. The word partition initially arose when supports had to be coated with a liquid phase (and the mobile phases saturated with them) to accomplish separations with these two modes. Today, bonded-phase versions of these liquid phases are available, making them easier to use with greater reproducibility. Perhaps it has been the use of these bonded supports that have enabled the name of the mode to be simplified. 

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