Chrom and Spec Terms

atmospheric pressure chemical ionization

heat and a coaxial flow of N2 convert eluate into a fine mist from which solvent and analyte evaporate

base peak

The most intense peak in a mass spectrum. Intensities of other peaks are expressed as a percentage of the base peak intensity.

chemical ionization

produces less fragmentation than electron ionization. The ionization source is filled with a reagent gas such as methane, isobutane, or ammonia, at a pressure of 100 Pa

collisionally activated dissociation

Fragmentation intentionally increased in the region between the glass capillary and the skimmer cone

background gas for collisionally activated dissociation is

N2

desorption electrospray ionization DESI

micron-sized, charged droplets created by electrospray of analyte-free solvent are directed onto the surface of an object under study. Analyte on the surface dissolves in the droplets. Further bombardment knocks droplets into the air toward a mass spectrometer inlet.

double focusing mass spectrometer

attains high resolution by employing an electric sector with the magnetic sector to select ions with a narrow range of kinetic energy

electron transfer dissociation

Breaking of a chemical bond by the exothermic transfer of an electron from one species to another. This process is used in mass spectral sequencing of polypeptides because it cleaves peptide bonds without breaking other bonds in the molecule

electrospray ionization

A method for interfacing liquid chromatography to mass spectrometry. A high potential applied to the liquid at the column exit creates charged droplets in a fine aerosol. Gaseous ions are derived from ions that were already in the mobile phase on the column

extracted ion chromatogram

Chromatogram made by collecting consecutive full-range mass spectra, but selecting just one value of m/z for display.

ion mobility spectrometer

An instrument that measures the drift time of gaseous ions migrating in an electric field against a flow of gas

linear quadrupole ion-trap mass spectrometer

An instrument that separates gaseous ions by trapping them in stable trajectories inside a linear quadrupole by use of radio-frequency fields. Ions can be expelled from the trap in order of increasing m/z for mass spectrometry

magnetic sector mass spectrometer

A device that separates gaseous ions that have the same kinetic energy by passing them through a magnetic field perpendicular to their velocity.

matrix-assisted laser desorption/ionization (MALDI)

A gentle technique for introducing predominantly singly charged, intact macromolecular ions into the gas phase. An intimate solid mixture of analyte plus a large excess of a small, ultraviolet-absorbing molecule is irradiated by a pulse from an ultraviolet laser. The small molecule (the matrix) absorbs the radiation, becomes ionized, evaporates, and expands in a supersonic jet that carries analyte into the gas phase.

nitrogen rule

A compound with an odd number of nitrogen atoms—in addition to C, H, halogens, O, S, Si, and P—will have an odd nominal mass. A compound with an even number of nitrogen atoms (0, 2, 4, …) will have an even nominal mass

nominal mass

Integer mass of the species with the most abundant isotope of each of the constituent atoms. (For C, H, and Br, the most abundant isotopes are 12C, 1 H, and 79Br. Therefore, the nominal mass of C2H5Br is (2 12)  (5 1)  (1 79) 108)

orbitrap mass spectrometer

A device that traps ions in stable orbits around a central electrode. Ions oscillate from one end of the trap to the other, inducing image currents in the outer electrodes.

precursor ion

In tandem mass spectrometry (selected reaction monitoring), the ion selected by the first mass separator for fragmentation in the collision cell.

product ion

In tandem mass spectrometry (selected reaction monitoring), the fragment ion from the collision cell selected by the final mass separator for passage through to the detector.

reconstructed total ion chromatogram

In chromatography, a graph of the sum of intensities of all ions detected at all masses (above a selected cutoff) versus time

resolving power

Defined as m/m, where m is the separation of two peaks when the overlap at the base is 10% of the peak height and m is the smaller of the two m/z values. Can be taken as m/m1/2, where m1/2 is the width of a peak at half the maximum height

selected ion chromatogram

A graph of detector response versus time when a mass spectrometer monitors just one or a few species of selected mass-to-charge ratio, m/z, emerging from a chromatograph.

selected ion monitoring

Use of a mass spectrometer to monitor species with just one or a few mass-to-charge ratios, m/z

selected reaction monitoring

A technique in which the precursor ion selected by one mass separator passes through a collision cell in which the precursor breaks into several fragment ions (product ions). A second mass separator then selects one (or a few) of these ions for detection. Selected reaction monitoring improves chromatographic signal-to-noise ratio because it is insensitive to almost everything other than the intended analyte

three-dimensional quadrupole ion-trap mass spectrometer

An instrument that separates gaseous ions by trapping them in stable threedimensional trajectories inside a metal chamber to which a radiofrequency electric field is applied

time of flight mass spectrometer

Ions of different mass accelerated through the same electric field have different velocities: The lighter ions move faster than the heavier ions. It finds the mass-to-charge ratio by measuring the time that each group of ions requires to travel a fixed distance to the detector

transmission quadrupole mass spectrometer

A mass spectrometer that separates ions by passing them between four metallic cylinders to which are applied direct current and oscillating electric fields. Resonant ions with the right mass-to-charge ratio pass through the chamber to the detector while nonresonant ions are deflected into the cylinders and are lost.

adsorption chromatography

A solid stationary phase and a liquid or gaseous mobile phase are used. Solute is adsorbed on the surface of the solid particles. The more strongly a solute is adsorbed, the slower it travels through the column

affinity chromatography

This most selective kind of chromatography employs specific interactions between one kind of solute molecule and a second molecule that is covalently attached (immobilized) to the stationary phase. For example, the immobilized molecule might be an antibody to a particular protein. When a mixture containing a thousand proteins is passed through the column, only the one protein that reacts with the antibody binds to the column. After all other solutes have been washed from the column, the desired protein is dislodged by changing the pH or ionic strength

diffusion coefficient

Defined by Fick’s first law of diffusion: J –D(dc/dx), where J is the rate at which molecules diffuse across a plane of unit area and dc/dx is the concentration gradient in the direction of diffusion

distribution coefficient

For a solute partitioned between two phases, is the total concentration of all forms of solute in phase 2 divided by the total concentration in phase 1

gel filtration chromatography

A technique in which the stationary phase has a porous structure into which small molecules can enter but large molecules cannot. Molecules are separated by size, with larger molecules moving faster than smaller ones

ion-exchange chromatography

A technique in which solute ions are retained by oppositely charged sites in the stationary phase.

linear flow rate

In chromatography, the distance per unit time traveled by the mobile phase.

longitudinal diffusion

Diffusion of solute molecules parallel to the direction of travel through a chromatography column.

open tubular column

In chromatography, a capillary column whose walls are coated with stationary phase.

partition chromatography

A technique in which separation is achieved by equilibration of solute between two phases.

plate height

Length of a chromatography column divided by the number of theoretical plates in the column. Calculated as the variance, 2 , of the analyte band divided by the distance, x, it has traveled: H 2 /x.

retention factor

In chromatography, the adjusted retention time for a peak divided by the time for the mobile phase to travel through the column. Retention factor is also equal to the ratio of the time spent by the solute in the stationary phase to the time spent in the mobile phase.

retention volume

The volume of solvent needed to elute a solute from a chromatography column.

silanization

Treatment of a chromatographic solid support or glass column with hydrophobic silicon compounds that bind to the most reactive Si—OH groups. It reduces irreversible adsorption and tailing of polar solutes

von Deemter equation

Describes the dependence of chromatographic plate height, H, on linear flow rate, ux: H A  B/ux  Cux. The constant A depends on band-broadening processes such as multiple flow paths that are independent of flow rate. B depends on the rate of diffusion of solute in the mobile phase. C depends on the rate of mass transfer between the stationary and mobile phases.

volume flow rate

In chromatography, the volume of mobile phase per unit time eluted from the column.

cold trapping

Splitless gas chromatography injection technique in which solute is condensed far below its boiling point in a narrow band at the start of the column.

electron capture detector

Gas chromatography detector that is particularly sensitive to compounds with halogen atoms, nitro groups, and other groups with high electron affinity. Makeup gas (N2 or 5% CH4 in Ar) is ionized by -rays from 63Ni to liberate electrons that produce a small, steady current. High-electron-affinity analytes capture some of the electrons and reduce the detector current.

flame ionization detector

A gas chromatography detector in which solute is burned in an H2-air flame to produce CHO ions. The current carried through the flame by these ions is proportional to the concentration of susceptible species in the eluate.

guard column

In high-performance liquid chromatography, a short column packed with the same material as the main column and placed between the injector and the main column. It removes impurities that might irreversibly bind to the main column and degrade it.

molecular sieve

A solid particle with pores the size of small molecules. Zeolites (sodium aluminosilicates) are a common type.

purge and trap

A method for removing volatile analytes from liquids or solids, concentrating the analytes, and introducing them into a gas chromatograph. A carrier gas bubbled through a liquid or solid extracts volatile analytes, which are then trapped in a tube containing adsorbent. After analyte has been collected, the adsorbent tube is heated and purged to desorb the analytes, which are collected by cold trapping at the start of a gas chromatography column.

retention gap

In gas chromatography, a 3- to 10-m length of empty, silanized capillary ahead of the chromatography column. The retention gap improves the peak shape of solutes that elute close to solvent when large volumes of solvent are injected or when the solvent has a very different polarity from that of the stationary phase.

retention index

A logarithmic scale that relates the retention time of a compound to those of linear alkanes. Pentane would be given an index of 500, hexane 600, heptane 700, and so on.

septum

A disk, usually made of silicone rubber, covering the injection port of a gas chromatograph. The sample is injected by syringe through the septum.

solid-phase microextraction

Extraction of compounds from liquids or gases into a coated fiber dispensed from a syringe needle. After extraction, the fiber is withdrawn into the needle and the needle is inserted through the septum of a chromatograph. The fiber is extended inside the injection port and adsorbed solutes are desorbed by heating (for gas chromatography) or solvent (for liquid chromatography).

solvent trapping

Splitless gas chromatography injection technique in which solvent is condensed near its boiling point at the start of the column. Solutes dissolve in a narrow band in the condensed solvent.

stir-bar sorptive extraction

Sample preparation method similar to solid-phase microextraction, except the sorptive layer is coated on the outside of a stirring bar. Coating volume is greater than the fiber volume in solid-phase microextraction, so it provides 102 times greater sensitivity for traces of analyte. Analyte is removed from the coating by thermal desorption for chromatography.

thermal conductivity detector

A device that detects substances eluted from a gas chromatography column by measuring changes in the thermal conductivity of the gas stream.

thermal desorption

A sample preparation technique used in gas chromatography to release volatile substances from a solid sample by heating.

bonded stationary phase

In high-performance liquid chromatography, a stationary liquid phase covalently attached to the solid support.

charged aerosol detector

Sensitive, nearly universal detector for liquid chromatography in which solvent is evaporated from eluate to leave an aerosol of fine particles of nonvolatile solute. Aerosol particles are charged by adsorption of ions and flow to a collector that measures total charge reaching the detector versus time.

dwell volume

In chromatography, volume between the point of mixing solvents and the start of the column.

electrochemical detector

Liquid chromatography detector that measures current when an electroactive solute emerges from the column and passes over a working electrode held at a fixed potential with respect to a reference electrode

eluent strength

A measure of the ability of a solvent to elute solutes from a chromatography column. A measure of the adsorption energy of a solvent on the stationary phase in chromatography.

evaporative light-scattering detector

A liquid chromatography detector that makes a fine mist of eluate and evaporates solvent from the mist in a heated zone. The remaining particles of liquid or solid solute flow past a laser beam and are detected by their ability to scatter the light

gradient elution

Chromatography in which the composition of the mobile phase is progressively changed to increase the eluent strength of the solvent.

hydrophilic interaction chromatography

Chromatographic separation of polar solutes with a hydrophilic stationary phase using mixed organicaqueous eluent. Eluent strength increases with decreasing organic solvent. Commonly called HILIC

isocratic elution

Chromatography using a single solvent for the mobile phase.

microporous particles

Chromatographic stationary phase consisting of porous particles 1.5–10 m in diameter, with high efficiency and high capacity for solute

normal-phase chromatography

A chromatographic separation utilizing a polar stationary phase and a less polar mobile phase

refractive index detector

Liquid chromatography detector that measures the change in refractive index of eluate as solutes emerge from the column.

reversed-phase chromatography

A technique in which the stationary phase is less polar than the mobile phase.

superficially porous particle

A stationary phase particle for liquid chromatography containing a thin, porous outer layer and a dense, nonporous core. Mass transfer is faster in the superfically porous particle than in a fully porous particle of the same diameter.

anion exchanger

An ion exchanger with positively charged groups covalently attached to the support. It can reversibly bind anions

capillary electrochromatography

A version of high-performance liquid chromatography in which mobile phase is driven by electroosmosis instead of a pressure gradient. Solutes are separated by partitioning between the mobile and stationary phases.

capillary electrophoresis

Separation of a mixture into its components by a strong electric field imposed between the two ends of a narrow capillary tube filled with electrolyte solution. Unlike chromatography, there is no stationary phase in electrophoresis. Solutes are separated by differences in mobility

capillary gel electrophoresis

A form of capillary electrophoresis in which the tube is filled with a polymer gel that serves as a sieve for macromolecules. The largest molecules have the slowest migration through the gel.

capillary zone electrophoresis

Ionic solutes are separated because of differences in their electrophoretic mobility.

dialysis

A technique in which solutions are placed on either side of a semipermeable membrane that allows small molecules, but not large molecules, to cross. Small molecules in the two solutions diffuse across and equilibrate between the two sides. Large molecules are retained on their original side

donnan equilibrium

Ions of the same charge as those fixed on an ionexchange resin are repelled from the resin. Thus, anions do not readily penetrate a cation-exchange resin, and cations are repelled from an anionexchange resin.

electrokinetic injection

In capillary electrophoresis, the use of an electric field to inject sample into the capillary. Because different species have different mobilities, the injected sample does not have the same composition as the original sample

electroosmosis

Bulk flow of fluid in a capillary tube induced by an electric field. Mobile ions in the diffuse part of the double layer at the wall of the capillary serve as the “pump.”

electrophoresis

Migration of ions in solution in an electric field. Cations move toward the cathode and anions move toward the anode. Ions can be separated from one another by their differing rates of migration in a strong electric field.

hydrodynamic injection

In capillary electrophoresis, the use of a pressure difference between the two ends of the capillary to inject sample into the capillary. Injection is achieved by applying pressure on one end, by applying suction on one end, or by siphoning.

micellar electrokinetic chromatography

A form of capillary electrophoresis in which a micelle-forming surfactant is present. Migration times of solutes depend on the fraction of time spent in the micelles.

molecularly imprinted polymer

A polymer synthesized in the presence of a template molecule. After the template is removed, the polymer has a void with the right shape to hold the template, and polymer functional groups are positioned correctly to bind to template functional groups.