P


PACKING: the particles that fill the column; also called COLUMN PACKING or STATIONARY PHASE.	Generic Column End Fitting Cutaway

PARTIAL-LOOP INJECTION : injection of a sample volume which is smaller than the volume of the loop of the injector. See also FILLED LOOP INJECTION	Partial-Loop Injection

PARTICULATES: usually refers to small solid particles in the sample or mobile phase; these particles can plug the column or other parts of the HPLC system.
PEAK: The deviation from baseline in a chromatogram caused by the properties of the analyte as it transits the detector cell. In practice, the terms peak and band are often used interchangeably; in principle, a band is the distribution of analyte molecules in the chromatographic system, while a peak is the trace detector response as the band passes through the detector.

PEAK AREA or HEIGHT : used to measure the size of the peak for use in quantitative analysis.	In most cases, either area or height measurements can be used for quantitation

PEAK ASYMMETRY: see ASYMMETRY FACTOR, TAILING FACTOR.
PEAK VALLEY: the lowest point between two adjacent peaks; the height of the valley from the baseline (as a percent of the peak heights) is a measure of how good the separation is.

PHENYL: a kind of bonded phase used for reversed-phase chromatography.
PHOTOMETRIC DETECTOR: the most commonly used LC detector, based on the passage of UV light through a flow cell containing column effluent. The light passing through the flow cell is converted into a plot of sample concentration versus time (the chromatogram).
PHOTODIODE ARRAY DETECTOR: a photometric detector that allows the selection of many simultaneous wavelengths for detection (usually in the range from 190 to 360 nm).
PICOGRAM (pg): a unit of mass, equal to a trillionth of a gram. See Section 4B for the relation to other units of mass.
PLATE NUMBER (N): a number that describes how good a column is in keeping sample bands narrow. Columns with large plate numbers give narrow bands; long columns packed with small particles give the highest plate numbers.	For a perfectly symmetrical peak, both of these calculations should yield the same value for N

POLAR: compounds, solvents or bonded phases that dissolve in water or prefer water to nonpolar organic solvents such as hexane.
POLAR-BONDED-PHASE CHROMATOGRAPHY: another HPLC method, different from reversed-phase chromatography. Mobile phases that do not contain water are used with this method. Most polar-bonded phase separations are classed as NORMAL-PHASE CHROMATOGRAPHY.
PORE: the particles of column packing are in most cases porous, meaning that each particle has small tunnels or "pores" that criss-cross or "honeycomb" the particle; sample molecules that are retained by the particle or stationary phase enter these pores and interact with the inside of the particle.	Most HPLC packings are porous. Most of the stationary phase surface area is on the inside of the particles.

PRECISION : the extent to which results from LC analysis cluster about the same value, so that repetitive analysis of the same sample gives similar results. Precise values are not necessarily accurate or "correct". See Accuracy.	Accuracy and Precision are not the same thing!

PRECOLUMN: a column (usually packed with silica) that is connected between the pump and the sample injector. The precolumn is normally intended to precondition the mobile phase in order to minimize chemical attack by the mobile phase on the column packing.
PREPARATIVE COLUMN: a larger-diameter column used for recovering purified materials by chromatographic separation. Can be contrasted with "analytical columns" that are used mainly for analysis, rather than recovery of fractions.
PRESSURE: the reading given by a gauge, meter, or other read-out device for an LC system; this is the pressure measured between the pump and column, and usually measures the pressure drop across the column.

PUMP : the LC module that delivers mobile phase to the injector and column.	Cross-sectional diagram of a "generic" single-piston HPLC pump.