Normal-phase liquid chromatography NPLC

On-line coupling of normal-phase liquid chromatography (NPLC) and gas chromatography is today a well developed and robust procedure and has been regularly applied to environmental analysis. When a fraction of the NPLC sample is introduced in to the GC unit, a large-volume interface (LVI) is needed but, due to the volatility of the organic solvent used in NPLC, this does not present such a great problem. 

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 active sites (centers) of silica are mainly the free and bonded silanol groups as shown in Figure 1. Two vicinal silanol groups connected by hydrogen bonds form the bonded hydroxyl groups. In the case of normal-phase liquid chromatography (NPLC), these sites interact with the polar analyte functional group/s. Thus, silica is considered as a polar stationary phase. The properties of a specific silica depend on its surface area and the number of active sites. 

Normal-phase liquid chromatography (NPLC) is the oldest chromatographic mode, discovered by M.S. Tswett more than 100 years ago. It has been the predominant mode in thin-layer chromatography (TLC) and low-pressure dry-column liquid chromatography before the introduction of reversed-phase technique, which is a preferred mode in high-performance liquid chromatography (HPLC). 

In extraction techniques (LLC, SPE, etc.), normal-phase liquid chromatography (NPLC), and thin-layer chromatography, aliphatic hydrocarbons (e.g., -hexane, -heptane) are usually used. The elution strength of these solvents is often modified by addition of more polar solvents. The fundamental problem with the eluents in NPLC is dissolved water and trace amounts of olefins. These contaminations can induce a change in the wavelength cut-off values (UV detection, spectrophotometry), baseline perturbation, and poor reproducibility of retention data. Halogenated solvents such as dichloromethane can react with some organic solvents (e.g., acetonitrile) to form crystalline products. 

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