Chromatography Glossary


Reagent Gas

In mass spectrometry,a gas used to detect, measure, or yield other substances.

Reciprocating Pump

Pumps with a single or multiple chamber, from which the mobile phase is displaced by reciprocating piston(s) or diaphragm(s).

Reconstructed Ion Chromatogram (RIC)

The mass spectral graphical representation of the separation achieved indicating total ion current versus retention time.


Is the amount of sample that elutes from a column relative to the amount that was injected.

Reduced Plate Height

Reduced plate height (h) is used to measure the efficiency of a column. A column with an 'h' value of = or <2 is considered to be well packed.

Reduced Velocity

Reduced velocity (v) is used to compare different columns, along with reduced plate height. It refers to the solute diffusion coefficient (Dm) in the mobile phase to the particle size of the column packing. (dp).v = dp/Dm.

Reference Chromatogram

The first chromatogram run, using a standard, on a new HPLC column. Sometimes referred to as a "Test Chromatogram".

Reference Standard

A substance normally of the highest metrological quality available at a given location, from which other measurements at that location are calibrated or derived.

Refractive Index (RI)

An index for measuring the change of direction of light when passing obliquely from one liquid medium to another. Refractive index values for typical HPLC solvents range from 1.3284 for methanol to 1.5717 for 1,2,4-trichlorobenzene.

Refractive Index Detector

This detector measures difference in refractive index between the flow cell and a static reference cell. The reference cell normally contains eluent. Hence a null balance is obtained when clean eluent passes through the flow cell. When a component from the analyte passes through the flow cell the difference in refractive index is detected. The main advantage of the RI detector is that it is universal, ie., it wil detect virtually any component. However its disadvantages include lack of sensitivity, plus the inability to use with gradient chromatography. It is also very sensitive to temperature changes and requires thermostating on all but its lowest sensitivity ranges.


The process of sequentially pumping a series of successively stronger solvents through a used column for the purpose of washing off strongly adsorbed compounds. Commercial regeneration services may also clean or replace the column end frits.

Relative Abundance

The abundance of an ion relative to the base peak in the mass spectrum. (That is, the m/z abundance values expressed in percent units relative to the base peak which is assigned a value of 100%).

Relative Recovery

Indirect determination of analyte capture and retrieval during the extraction process based on the recovery of an internal standard (which is a compound with a structure similar to the target analyte) that is added to the crude sample/matrix prior to the actual extraction procedure.

Relative Response Factor

A measure of the relative mass spectral response of an analyte compared to its internal standard. Relative Response Factors are determined by analysis of standards and are used in the calculation of concentrations of analytes in samples. Abbreviated RRF.

Relative Retention Values

The ratio of the adjusted or net retention volume (time) or retention factor of a component relative to that of a standard, obtained under identical conditions. Depending on the relative position of the peak corresponding to the standard compound in the chromatogram, the value of r may be smaller, larger or identical to unity.

Relative Stnadard Deviation (RSD)

The relative standard deviation is the standard deviation (SD) expressed as a percent of the mean. RSD = SD x 100 / mean. RSD is also called the coefficient of variation (CV).


The degree of agreement of a measured value, with replicate measurements recorded at the same time, or in the same place or on a similar instrument.


An electrode mounted inside the ion source to which a voltage is applied so as to push out the ions formed in the source.


The vessel which contains and preserves the integrity of the solvent used as a mobile phase.


The ability of a pump to produce the same flow rate each time a particular setting is made.

Residual Silanols

Refers to the silanol groups (-Si-OH) that remain on the surface of a bonded reversed phase packing material after the phase has been chemically bonded onto the surface. These remaining groups may not be accessible to the reacting large molecules of organosilanes like ODS, but may be accessible to smaller polar compounds. They can be removed by endcapping with a small organosilanes such as trimethylchlorosilane. (TMS).


Are solid packings used in ion exchange separation. The most commonly used resins are polystyrene - divinylbenzene copolymers.


The goal of chromatography. The difference in retention of adjacent peaks divided by their average band width. Sufficient resolution between peaks is required for proper quantification on unknown analytes.


Response is a detector characteristic which defines the types of compounds which are detectable by a particular detector. The amplitude of the response is commonly expressed in volts, amperes, or absorbance units.

Response Ratio

Refers to the ratio of absorbances at two different wavelengths. The response ratio gives qualitative information about the peak and can reveal hidden peaks.


The tendency of solutes to move through the column more slowly than the mobile phase. Retention is expressed either in terms of retention time or capacity factor, k'.

Retention Factor

The retention factor is a measure of the time the sample component resides in the stationary phase relative to the time it resides in the mobile phase. Mathematically, it is the ratio of the adjusted retention volume (time) and the hold-up volume (time):
k = V'R / VM = t'R / tM

Retention Mechanism

process involved in the partitioning of a solute between the mobile phase and the stationary phase.

Retention Parameters

Measured in terms of chart distances or times, as well as mobile phase volumes; e.g., t'R (time) is analogous to V'R (volume). If recorder speed is constant, the chart distances are directly proportional to the times; similarly if the flow rate is constant, the volumes are directly proportional to the times.

Retention Time

The time between the injection of a sample to the appearance of the peak at maximum height at the detector. Each compound has a characteristic retention time on a specific column under a given set of conditions. Therefore this information is used to qualitatively identify the compounds in the sample.

Retention Volume

Retention volume (Vr) is the volume of mobile phase that is required to elute a substance from a column.


The process of repeating a validation usually made necessary by introducing change to a validated method or validated system.

Reversed Phase

A chromatographic mode in which nonpolar to moderately polar analytes are extracted from a polar solution using a nonpolar sorbent.

Reversed Phase Chromatography (RPC)

Refers to a nonpolar stationary phase and a polar mobile phase and is the most common separation mode for HPLC. Reversed Phase Chromatography (RPC) uses hydrophobic packings such as octadecyl- or octylsilane phases bonded to silica or neutral polymeric beads. Mobile phase is usually water and a water-miscible organic solvent such as methanol or acetonitrile.

RF Modulator

In mass spectrometry for part of the quadrupole electronics that modulates the signal.

Rotary-Vane Pump

This low vacuum pump takes large volumes of low pressure gas and compresses it into smaller volumes by the action of a rotating device. the highly compressed molecules are then discharged.


A device used in the rotary-vane pump to move (rotate) the vane.

Rotor Blades

A term used in mass spectrometry for a rotating set of blades used in the turbomolecular pump to direct molecules through the system.

Rough Pump

In mass spectrometry, the low vacuum pump used to evacuate the diffusion pump. A common type of rough pump is a rotary-vane pump also known as a mechanical pump.

Rough Vacuum

In mass spectrometry, the vacuum usually provided by a rotary pump, typically in the range 1300 to 0.13 Pa (approximately 1x10^-3 Torr). Sometimes referred to as backing pressure from the use of a rotary pump to back a diffusion pump.

Roughing Valve

In mass spectrometry, a valve used with isolation valves to bring the source and analyzer manifold to the rough vacuum before opening the isolation (butterfly) valves.

Run Time

The time it takes to complete a chromatographic run, ie., elute all the components of interest, including any re-equilibration time required if gradient elution was used.

Salting Out

The addition of salt to an aqueous solution prior to extraction. Hydration of the salt ions decreases the amount of water available for interaction with the analyte causing the analyte to become less soluble.


The mixture consisting of a number of components.

Sample Capacity

Refers to the amount of sample that can be injected onto a column without overloading it. Sometimes referred to as sample loading, it is measured in terms of grams of sample per gram of packing.

Sample Components

The chemically pure constituents of the sample. They may be unretained (i.e., not delayed) by the stationary phase, partially retained (i.e., eluted at different times) or retained permanently.

Sample Injection Module

A device used to improve the loading of low-solubility samples. The sample is dissolved in a strong solvent, mixed up with loose silica, and dried under vacuum. The sample-laden silica is then poured into a new sample injection barrel and placed in-line with the flash system. Mobile phase elutes the sample onto the column in a tighter band than would otherwise be possible.

Sample Loop

The tube in the injector that holds the sample before injection. Typical injection loop volumes range from 5 to 100 µL for analytical HPLC, and 1 to 10mL for semi-prep and preparative HPLC.