Glossary Of HPLC-LC Separation Terms – N

The listing should be helpful to those just starting in HPLC but it also can serve as a refresher for long-time users in the field.

N

n: See peak capacity.

N: The number of theoretical plates; N = 16(t _R/w _b)², where t _R is retention time and w _b is the base width of the peak. A measure of the efficiency of a column. Sometimes measured as N = 5.54(t _R/w _h)², where w _h (or w¼) is the peak width at half height. See also efficiency and theoretical plate.ν: See reduced velocity.

NanoLC: LC practiced with columns less than 100-um in internal diameter; usually requires specialized instrumentation; often used in proteomic studies where sample is limited and sensitivity is required.

Narrow-bore column: Columns of less than 2-mm i.d. used in HPLC. Also called microbore.

N _eff : See effective theoretical plates.

Noise: See Baseline Noise

Nonaqueous reversed-phase chromatography: Refers to reversed-phase chromatography performed without water as a component of the eluent on a reversed-phase packing. Used for very nonpolar compounds that cannot be eluted or are difficult to elute from a reversed-phase column with 100% methanol or acetonitrile. In these cases, solvent A should be acetonitrile, and solvent B should be a stronger solvent such as tetrahydrofuran. Reversed-phase rules apply to nonaqueous reversed-phase chromatography; that is, the more nonpolar the analyte, the greater the retention.

Nonpolar: A nonpolar molecule is one that the electrons are distributed more symmetrically and thus does not have an abundance of charges at the opposite sides. The charges all cancel out each other. Nonpolar compounds, solvents or bonded phases readily dissolve in organic solvents, such as hexane, or prefer such solvents in place of water. Nonpolar substances do not readily dissolve in water.

Nonporous packing: Particles similar to porous-layer bead but with particle diameters in the sub-5-μm range; particles often are in the sub-2-μm d _p range. Used for high-speed separations in short columns. Common column abbreviations include NPS, which refers to nonporous silica; NPR, which refers to nonporous resins; and NPZ, which refers to nonporous zirconia.

Nonporous particle: Refers to a solid particle used as a support for a porous coated or bonded phase; pellicular particles are nonporous particles of large particle diameter (~40 μm). Nonporous silicas and resins with small particle diameters of less than 3 μm usually are microbeads with thin porous outer coatings of silica gel, bonded silica gel, or polymeric phase.

Normal-phase chromatography: A mode of chromatography performed when the stationary phase is more polar than the mobile phase. A typical normal-phase system would be adsorption chromatography on silica gel or alumina using mixtures of less polar eluents such as hexane–diethethyl ether as a mobile phase. Also refers to the use of polar bonded phases such as cyano and alumina. Sometimes called straight-phase chromatography.

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