absolute error: the difference between a measured value and the true or accepted value.

absorbance: a measure of the amount of light absorbed by (usually) a solution. Equal to the negative logarithm of the ratio of initial and final light intensity.

absorption: the process by which a substance acquires energy by promotion to excited electronic, vibrational or rotational states.

absorption spectrum: the spectrum of radiation intensity vs. wavelength or frequency produced when atoms, ions or molecules are excited from low energy states to higher energy states.

accuracy: the agreement between a mean measured value and a true or accepted value as quantified by error. Related to determinate errors and quantified by bias.

Ag/AgCl electrode: standard electrode consisting of a Ag wire coated with AgCl and dipping into a solution of Cl ions (usually sat'd but must be specified). Produces a potential of about +0.222 V vs. SHE at 25 C.

analog data: can possess any value - it is continuously and infinitely variable.

analyte: species of interest in a determination.

anode: place where oxidation reactions occur in an electrochemical cell. Electrons flow away from the anode in a cell and the anode becomes positively charged.

band spectra: spectra (absorption or emission) showing broad peaks due to combination of electronic, vibrational and rotational transitions in molecules.

bandwidth: the width of a band. Units of nm etc. The effective bandwidth, is the width of the band at half-height.

base peak: the most intense m/z value in a mass spectrum. Assigned 100 % intensity.

blackbody radiation: a continuous emission spectrum produced by many small energy transitions (electronic and vibrational) in a solid when heated. Most emission is in the IR part of the EM spectrum but moves further into the visible (and UV) with increasing temperature.

blank: a blank contains all matrix components except the analyte and is used to determine the instrumental signal associated with non-analyte components, or with determinate or systematic errors.

bolometer: a sensitive resistance thermometer often used in IR instruments. Usually cooled with liquid nitrogen or helium to reduce dark currents.

bonded chromatographic column: stationary phase covalently attached to support material rather than by simple physical adsorption. Reduces stationary phase leaching.

boundary potential, E_b: potential developing across a permeable membrane due to different positions of surface equilibria when solutions of different activities are in contact with opposite sides of the membrane.

calomel electrode: standard electrode made of a piece of inert Pt wire dipping into a paste of liquid Hg, Hg₂Cl₂ (calomel) and KCl (concentration of KCl must be specified, often saturated to make a saturated calomel electrode, SCE). Produces a potenial of about 0.268 V vs. SHE at 25 C.

capacity factor k': term used to measure the migration of an analyte during chromatography. Equal to the (retention time minus the dead time) divided by the dead time.

carrier gas: mobile phase in gas chromatography. Usually helium, hydrogen or nitrogen.

cathode: place where reduction reactions occur in an electrochemical cell. Electrons flow towards from the cathode in a cell and the cathode becomes negatively charged.

chemiluminescence: an exoergic chemical reaction that produces a product molecule in an electronically excited state that relaxes via fluorescence or phosphorescence. The intensity of light produced is proportional to the rate of production of the excited state molecule. Chemiluminescent reactions are relatively rare but often involve strong oxidizers (e.g. O₃ or H₂O₂).

chromophore: an atom, ion or molecule that absorbs radiation. May refer to a particular part or functional group of a molecule (for example, the carbonyl group or halogen atom).

concomitants: all species in a matrix excluding the analyte.

cross-linked chromatographic column: individual stationary phase molecules polymerized (cross-linked) after attaching to support material. Reduces stationary phase leaching.

data domains: type of signal associated with a measurement. Can be non-electrical or electrical data. The electrical domains include current, voltage, charge, frequency, pulse width, phase, count, serial and parallel data.

dead time t_M: the time after injection for an unretained species to elute (be detected) in a chromatographic separation.

detection limit: the minimum detectable concentration of analyte that can be confidently measured. Defined as 3x the standard deviation of repeated measurements on a blank sample.

detector: any device which quantitatively responds to a change in its environment (e.g. temperature, pressure, light intensity).

deuterium lamp: a continuous source for UV part of the electromagnetic spectrum (180-340 nm). Some lines in visible.

digital data: can possess only discrete values. In the usual binary scheme these values are high/low, on/off or 1/0.

dipole moment (mu): a measure of the overall electric field associated with a molecule. Depends on (i) charges (or partial charges) and (ii) their separation. Units of debye (D). 1 D = 3.33E-30 Cm. A dipole moment of ~10 D corresponds to a +1 and -1 electron equivalent charge separated by 1 Angstrom. Typical molecular dipole moments range from zero to a few debye.

direct probe: inlet systems in mass spectrometry allow insertion of a small heated vial, capillary or plate into the ionization chamber through an air-lock system. Heating of low vapor pressure analytes causes vaporization inside rhe instrument.

dispersion (1): change in refractive index with frequency. In normal dispersion, the refractive index increases with frequency. In anomalous dispersion, the refractive index decreases with frequency indicative of energy absorption.

dispersion (2): a measure of the angular spread of different wavelengths emerging from a prism or grating. More precisely, the reciprocal dispersion is the rate of change of wavelength with distance across a monochromator exit slit. It has units of nm/mm etc.

dispersion (3): measure of "dilution" of sample by solvent (usually in a tube or column) due to diffusion, migration and convection. Quantified as the sample concentration divided by the peak sample concentration measured at the detector. Influenced by many physical parameters including flow rate, temperature, tube inside diameter, tube length and sample volume.

dissociation: process of bond-breaking. For photodissociation, the absorption of a photon produces an excited state which is directly repulsive (for example an antibonding orbital) or in a sufficiently high vibrational state that the molecule is above the dissociation limit.

double-focusing mass analyzers: are widely used in high resolution mass spectrometry. They consist of an electrostatic defelction- and magnetic sector-type analyzer in series to simultaneously reduce ion energy deistribution and angular distribution aberrations.

dynamic dipole moment: a dipole moment whose magnitude varies with time, usually as a result of vibrational motion.

dynamic range: the range of concentrations (or other chemical/physical property) able to be quantified by a particular instrumental technique - typically 2 to 8 orders of magnitude or more. Defined as LOL divided by LOQ (see below).

dynode: an electrode between the photocathode and anode in a photomultiplier device. May be multiple dynodes acting to give an "electron avalanche" effect.

efficiency (chromatographic): a measure of the zone or band broadening in chromatography expressed as plate height or number of plates.

electromagnetic (EM) spectrum: range of wavelengths or frequencies of electromagnetic radiation ranging from cosmic and gamma rays (high energy/short wavelength) to radio waves (low energy/long wavelength). The UV part of the EM spectrum has wavelengths extending from about 180-350 nm, the visible from about 350 (violet)-700 (red) nm and the IR from about 700 to >3000 nm.

electron capture detector (ECD): gas chromatography detector based on measurement of current produced by capture of electrons from a radiaoctive source by carrier or analyte molecules. Essentially non-destructive, poor dynamic range, only moderate sensitivity for amny species but rugged.

electron impact (EI) ionization: is widely used in mass spectrometry to produce positive analyte ions by bombarding molecules with 70-100 eV electrons. EI excited spectra are characterized by multiple fragment ions.

electrospray ionization (ESI): is widely used in mass spectrometry to ionize large mw molecules. A solution of the analyte passes through a high-voltage capillary tube whereby individual droplets pick up multiple charges. As the solvent evaporates, the drops shrink and the charge density increases. At some critical size, charged analyte begin to desorb from the droplet.

eluate/eluent: the mobile phase in chromatography.

elution chromatography: the process of transporting material through a stationary phase by the continuous addition of solvent (mobile phase) causing separation.

elution order in chromatography: the order in which various components elute on a particular column can often be approximately determined by the following "rules of thumb": volatile components (low boiling point) elute before non-volatile components (ii) small molecules elute before large molecules (iii) in a polar mobile phase, polar molecules elute before non-polar molecules (iv) for a polar stationary phase, non-polar molecule elute before polar molecules. Exceptions to these rules frequently occur.

emission spectrum: the spectrum of radiation intensity vs. wavelength or frequency produced when atoms, ions or molecules relax from high energy states to lower energy states.

excitation: the process of raising the energy of an atom, ion or molecule from some low energy state to some higher energy state.

excited state: an atomic, ionic or molecular state with higher energy than the ground state. Can be electronically, vibrationally or rotationally excited.

external conversion: occurs when a molecule in an excited state relaxes non-radiatively by interactions (energy transfer) with neighboring molecules (usually solvent).

f number: a measure of the light gathering efficiency of an optical element (mirror, lens). Defined as the focal length, F, divided by the diameter of the optic, d. Small f numbers imply better light gathering efficiency.

Faradaic currents: are associated with redox reactions in electrochemical cells. In contrast, non-Faradaic currents are associated with simple capacitive charging in response to a changing potential.

Faraday: the charge carried by one mole of electrons or 96,485 C.

flame ionization detector (FID): gas chromatographys detector based on measurement of ion current in a flame. Sensitive and rugged but destructive to sample.

fluorescence: relaxation route producing radiation from short-lived (<10 microseconds) excited states.

fluorometer: an instrument for measuring luminescence (fluorescence, phosphorescence) spectra. Contains filters for excitation and emission but no wavelength scanning. Light is collected at 90 degrees from the incident light (excitation) direction.

focal length: for parallel incident rays, the distance between an optical element (lens, mirror) and the focal point where all rays converge.

Fourier transform (FT): a mathematical technique for converting time dependant data into frequency data. Most often used in Fourier transfrom infrared spectrometry (FTIR). The conversion from frequency to time is performed by the inverse Fourier transform function.

frequency: number of peaks of a wave passing a point per second. Units are /s or Hz (Hertz).

gas chromatography: usually refers to gas-liquid chromatography where the mobile phase is a gas (helium, hydrogen or nitrogen typically) and the stationary phase is an immobolized liquid bound to a support material.

general elution problem: compromise between resolution and experiment time. Optimized by gradient elution in liquid chromatography and temperature programming in gas chromatography.

glass electrode or pH electrode: electrode with porous non-crystalline glass membrane sensitive to changes in hydrogen ion activity (see boundary potential). Used with standard reference electrode (if in same housing called a combination electrode).

Globar: a heated SiC rod (~1500 K) used as a continuous source of IR radiation (pseudo-blackbody).

gradient elution in liquid chromatography: (also called solvent programming) controlled change of solvent (mobile phase) composition to reduce retention time of strongest retained species.

grating: a diffraction/refraction dispersion element used to separate radiation into component wavelengths. Consist of a series of grooves of lines "blazed" onto a piece of quartz or silicon. These lines are parallel and closely spaced (10-2000 per mm). Constructive interference at certain incidence angles produces monochromatic radiation at specific diffraction angles. Multiple order effects also produce radiation at one half, one third, one quarter... of the primary order (longest wavelength) radiation at the same diffraction angle.

ground state: the lowest energy electronic, vibrational or rotational state. A species may be in the ground electronic state but excited vibrational or rotational states or any other combination of ground and excited states.

hard ionization sources: deposit large quantities of energy into a molecule causing extensive subsequent fragmentation.

harmonic oscillator: a simple description of vibrational motion based on classical mechanics and producing a parabolic (E=1/2ky²) energy vs. displacement, y, curve.

Hooke's Law: relationship between restoring force, F, and displacement, d, for a harmonic oscillator: F=-kd. The proportionality constant k is called the spring constant.

interferent: potential or actual species whose presence in a matrix affects the quantitative measurement of an analyte.

internal conversion: occurs when a molecule in one electronic state non-radiatively converts to another state of same multiplicity (without spin change). Can lead to fluorescence or predissociation.

intersystem crossing (ISC): occurs when a molecule in one electronic state non-radiatively converts to another state of different multiplicity (with spin change, for example S₁ to T₁). Can lead to phosphorescence.

isocratic elution: the use of a single solvent composition in liquid chromatography.

isothermal elution: the use of a single column temperature in gas chromatography.

J (quantum number): total orbital angular momentum quantum number (simply) given by L+S.

junction potential, E_j: potential developing across any permeable material with different activity solutions on either side. Caused by different diffusion rates through the material for different ions.

limit of linearity (LOL): the maximum concentration of analyte that can be quantitatively determined using an instrumental technique.

line spectra: spectra (absorption or emission) showing narrow peaks due to electronic transitions in atoms.

liquid chromatography: usually refers to chromatography where the mobile phase is a liquid and the stationary phase is an immobilized liquid bound to a support material (partition) or a silica/alumina surface (adsorption). High performance liquid chromatography (HPLC) refers to the recent trend towards efficient separations using relatively high column inlet pressures, small particle sizes and immobilized liquid stationary phases.

level of quantitation (LOQ): the minimum detectable concentration of analyte at which quantitative measurements can be confidently conducted. Defined as 10x the standard deviation of repeated measurements on a blank sample.

luminescence methods: are based on the emission of radiation (fluorescence, phosphorescence and chemiluminescence).

magnetic sector: a mass spectrometric analyzer based on the deflection of charged particles (ions) by a magnetic field. The field lines are perpendicular to the ion direction.

mass-to-charge ratio (m/z): the quantity measured in mass spectrometry. Units are usually Daltons (Da) or atomic mass units.

matrix: all chemical components in a mixture to be analyzed including the analyte.

matrix-assisted laser desorption/ionization (MALDI): a recent desorption/ionization technique used in mass spectrometry. Analyte molecules are dissolved into a solution of solvent and UV-absorber. A drop of this solution is allowed to dry to form a solid crystalline matrix on a metallic pin and then inserted into the vacuum system. A pulsed UV laser is used to desorb ions from the solid matrix by a process not fully understood to date. Considered a soft ionization technique much used in large biomolecule characterization.

Megabore column: a fused-silica capillary chromatography column with a relatively large diameter of about 500 microns. Used to handle larger quantities of analyte than a conventional capillary column.

Michelson interferometer: a device consisting of a fixed mirror, a moving mirror and a beamsplitter. Used to convert the very high frequency time dependant electric field of EM radiation (particularly IR) into a much lower frequency time dependant electric field (measureable by a transducer). The ratio of incident to exiting light frequencies is approximately 10¹⁰.

mobile phase: a liquid or gas used in chromatography used to transport an analyte through a stationary phase.

molecular peak (M⁺): in mass spectrometry is the m/z value corresponding to the singly-charged molecular ion. Often (M-1)⁺ or (M+1)⁺ peaks are observed rather than the molecular peak depending upon the ionization method

multichannel transducer: a transducer sensitive to spatial position of incident stimulus. For example, a photodiode array consists of a series of p-n Si junctions arranged on a chip. If each junction is wired individually, current is produced only on those elements upon which light falls. As in all transducers, the magnitude of the current is proportional to the intensity of the radiation (up to saturation).

multiplicity (spin multiplicity): given by 2S+1 where S is the sum of individual electron spins, s. When 2S+1=1 (antiparallel spin electron pair) the state is called a singlet state, when 2S+1=2 (single unpaired electron) the state is called a doublet state and when 2S+1=3 (parallel spin electron pair) the state is called a triplet state.

Nernst glower: a heated rare earth oxide rod (1000-2000 K) used as a continuous source of IR radiation (pseudo-blackbody).

normal-phase liquid(-liquid) chromatography: is performed using a non-polar mobile phase (hexane... ) and a polar stationary phase (polyethylene glycol... ).

open tubular column: (also called capillary columns) have smaller internal diameters than regular packed columns but are more efficient. Often coated on the inside with stationary phase (wall-coated open tubular WCOT) or coated with stationary phase-coated small particles on the inside (support-coated open tubular SCOT). Usually made of fused silica.

packed column: (gas) chromatographic column filled with small particles to provide a porous phase with large surface area for partitioning. Usually made of stainless steel or glass.

partition ratio or partition coefficient K: the equilibrium constant describing the concentration distribution of analyte between stationary and mobile phases in chromatography.

parallel digital data: digital data encoding scheme using multiple data pathways (usually in binary representation in which each transmission line represents a different power of two).

period: (of a wave) time required for one wave oscillation (peak to adjacent peak for example) to pass a stationary point.

peristaltic pump: variable speed (flow rate) pump consisting of 8-12 rotating rollers pinching a flexible tube. Multiple channel pumps can pump several tubes at one time. Some pulsation. Used in flow injection and continuous flow techniques (amongst others).

phosphorescence: relaxation route producing radiation from a long-lived (triplet) state (>10 microseconds to hours).

photomultiplier tube (PMT): a radiation transducer consisting of a photoemissive cathode that produces electrons when irradiated by light of appropriate wavelength (usually visible), followed by a series of dynodes and an anode. The dynodes are biased increasingly positive. Electron collision with the first dynode produces a cascade of secondary electrons. These travel to the second dynode, producing more electrons and so on until the last dynode releases many electrons which travel to the anode. Current traveling between the anode and cathode is proportional to the light intensity falling on the cathode for weak light intensities. Gains of more than one million are easily possible (one incident photon producing one million electrons at the anode). Very sensitive but easily reaching saturation.

phototube: a radiation transducer consisting of a photocathode that produces electrons (photoelectric effect) when irradiated by light of appropriate wavelength (usually visible), and an anode. The current traveling between the cathode and anode is proportional to the light intensity for moderate intensities. Not as sensitive as a photomultiplier tube.

Planck's constant: proportionality constant, h, in the equation relating photon energy and frequency, nu. Energy=h x frequency. h=6.63E-34 Jás.

plate height (height of an equivalent theoretical plate) H: a term used to describe the efficiency of a chromatographic column and defined as [sigma]²/L where [sigma] is the standard deviation of the Gaussian-shaped peak and L the column length. More plates produce narrower bands and more efficient separation. The number of plates, N, in a column is given by L/H. It should be noted that physical "plates" do not exist in a column!

ppm and ppb: parts per million and parts per billion. Equivalent to micrograms per gram of solution and nanograms per gram of solution respectively.

polarity index P': number representing the average polarity of a molecule in a variety of solvents. Ranges from about -2 (very non-polar) to >10 (polar). n-Hexane has a value of 0.1 and water has a value of 10.2.

precision: the reproducibility of a measurement made multiple times in identical ways. Caused by indeterminate or random errors. Usually quantified by absolute or relative standard deviation.

predissociation: is caused by an internal conversion between a bound and dissociative state. Enhanced by vibrational overlap in the two states.

pressure drop (in a chromatography column): the difference in pressure between the inlet and outlet of any tube. The pressure drop represents the resistance of an open tubular or packed column to mobile phase flow. As a consequence, while the amount of material entering and exiting the column per unit time is constant, the flow rate u at the inlet is lower than at the outlet.

pyroelectric transducer: a device based on a piezoelectric material used to sense temperature changes. The piezoelectric material has a permanent dipole moment due to the arrangement of ions in the crystal structure. A deformation of the material (thermal or mechanical) changes the magnitude of this dipole moment which can be measured electronically as a small current flow.

quadrupole (mass spectrometer): a mass analyzer consisting of four parallel rods. DC and variable frequency RF voltages are applied to opposite pairs of the rods. Ions entering this field are forced into helical trajectories and for a particular set of DC potentials and RF frequencies, only ions of a narrow range of m/z values can exist in stable trajectories and reach the detector.

quantum mechanics: mathematical formalism developed in the first part of the 20th century based on treating atoms and molecules as possessing specific energy states. Discrete transitions are only allowed between two states.

quantum numbers: are used in quantum mechanics to describe electron wavefunction. Principle quantum number n (values: 1, 2, 3...), angular momentum quantum number l or L (values: 0 (=s), 1 (=p), 2 (=d)...), magnetic quantum number m or M, spin quantum number s or S (values: +1/2, -1/2). See also term symbols. A vibrational quantum number (nu) (values: 0, 1, 2, 3... ) is used in the description of the energy of molecular vibrational levels.

quartz tungsten halogen lamp (QTH): a high intensity tungsten lamp continuous (band) source producing ~200-3000 nm radiation. The quartz envelope (to resist high temperatures and pass UV) is filled with a small concentration of iodine vapor. This reacts with any W evaporating from the filament to produce gaseous tungsten iodide that is decomposed upon contact with the hot filament to redeposit metallic tungsten. Consequently, QTH lamps have a long lifetime.

reactor: small coil of tightly-wound tubing used in flow injection analysis used to thoroughly mix reactants.

recriprocating pump: piston pump used in HPLC for mobile phase pressurization up to 10,000 psi. Produces very pulsating flow.

refractive index: the ratio of the speed of EM radiation in a medium divided by the speed of EM radiation in a vacuum, c. c=2.99 million km/s (2.99E8 m/s). The refractive index varies with wavelength according to dispersion. Glass has a refractive index (eta) of about 1.50 at 590 nm, while air is about 1.00 at 590 nm (almost the same as in vacuum).

relaxation: return to lower energy state in atoms, ions or molecules accompanied by release of heat (non-radiative relaxation) or radiation (radiative relaxation - luminescence, emission).

resolution R_s (1): a measure of the separation of two chromatographic peaks defined as twice the separation of the peaks divided by the sum of the width of the peak bases. Values of less than 1.0 are considered unresolved peaks.

resolution R (2): a measure of the separation of two mass spectrometric peaks. The peaks are usually considered separated if the intensity between the two peaks is <10% of the intensity of the weakest peak. R is defined as the average m/z values of the two peaks divided by their separation. For example, if two peaks at m/z=400 and 401 are just separated, the resolution of the instrument is 400.5.

retention time t_R: the time after injection for a retained analyte to elute (be detected) in a chromatographic separation.

retention volume V_R: the volume of mobile phase eluted before the elution of an analyte in a gas chromatographic separation. Defined as the retention time t_R multiplied by the average mobile phase flow rate u.

reversed-phase liquid(-liquid) chromatography: is performed using a polar mobile phase (water, methanol, acetonitrile... ) and a non-polar or weakly polar stationary phase (dimethylsiloxane... ). Usual method for many molecules.

saturation (of a transducer): produces a constant (high) signal level regardless of the intensity of stimulus (also see limit of linearity, LOL). For a radiation transducer based on an electron avalanche (e.g. PMT), the effect is due to the finite recovery time of each area of the photocathode and/or dynode following release of a secondary electron.

selectivity coefficient, h_x,y: ratio of selectivity of membrane material to interfering ions. A selectivity coefficient of 0.00 means the membrane reacts only with the ion of interest x. A selectivity coefficient of 1.00 means that the membrane is as selective to interfering ion y as ion x. A selectivity coefficient of much greater than 1.00 implies the electrode is selective for species y and might be useful as an ion selective electrode (ISE) for species y.

selectivity factor [alpha]: the partition ratio K_B of species B divided by the partition ratio K_A of species A. A measure of the relative interactions strengths of species A and B with a stationary phase material in chromatography.

serial digital data: digital data encoding scheme using one data pathway (usually in binary representation in which each successive bit represents an increasing power of two). More efficient than simple counting.

sensor: a device able to quantitatively and reversibly detect chemical species and produce an electrical signal.

Snell's Law: relationship between the sines of incident and refracted angles for a beam passing between two media of different refractive indices.

soft ionization sources: deposit small excess quantities of energy into a molecule to ionize it and cause relatively little subsequent fragmentation.

spectrofluorometer: an instrument for measuring luminescence (fluorescence, phosphorescence) spectra. Contains two monochromators: one for varying excitation and one for varying emission wavelengths. Only one monochromator is scanned during spectral acquisition (excitation fluorescence spectra are collected with fixed emission wavelength and emission fluorescence spectra are collected with fixed excitation wavelength). Light is collected at 90 degrees from the incident light (excitation) direction.

spring constant, k: a measure of the "stiffness" of a spring. The proportionality constant in Hooke's Law. Units of N/m. Typical effective molecular spring constants are 1-10 N per meter.

standard hydrogen electrode, SHE: standard reference electrode assigned a value of 0.000 V. Made by dipping an inert Pt wire into a acid solution with hydrogen ion activity of 1.000 M and bubbling H₂ gas around the wire at a partial pressure of 1.000 atm. Difficult to operate so alternatives are often used (calomel or Ag/AgCl for example).

stationary phase: a solid or immobilized liquid used in chromatography that has some interaction with an analyte in a mobile phase, and so can partition the analyte between phases. Common stationary phase materials for gas chromatography include polysiloxanes and polyethylene glycol (PEG or Carbowax). Stationary phase polarity is an important component affecting capacity factors and selectivity factors.

Stokes shift: red-shift occurring between molecular excitation and emission (fluorescence or phosphorescence).

support-coated open tubular (SCOT) column: a capillary chromatography column of about 150-400 micron diameter with the inside wall coated with a thin layer (10-50 microns) of solid particles each coated with a thin layer (1-10 microns) of stationary phase.

temperature programming in gas chromatography: controlled change of column temperature during elution to reduce retention time of strongest retained species.

term symbol: a shorthand notation for describing the electronic structure of a species written as ^2S+1L_J.

thermal conductivity detector (TCD): gas chromatography detector based on measurement of a heated filament temperature when surrounded by gas of a given thermal conductivity. Sensitive and non-destructive but variable sensitivity

thermocouple a pair of junctions of two fine wires (dissimilar materials) that produces a small voltage proportional to the difference in temperature between the two junctions. A number of thermocouples arranged in series form a thermopile.

transducer: a device able to quantitatively convert non-electrical domain data into electrical domain data (e.g. photocell)

ultraviolet (UV): electromagnetic radiation between about 190 and 350 nm.

van Deemter equation: H=A+B/u+Cu. Expression relating the mobile phase flow rate u, multipath (eddy) diffusion A, longitudinal diffusion B and mass transport C (to and from the stationary phase) to the theoretical equivalent plate height H in chromatography. A plot of H versus u is called a van Deemter plot.

vibrational relaxation: can occur as a vibrationally excited molecule non-radiatively loses energy by internal or external conversion processes. Rapid - 10^-13 to 10^-11 s.

wall-coated open tubular (WCOT) column: a capillary chromatography column of about 150-400 micron diameter with the inside wall coated with a thin layer of stationary phase.

wavelength: distance between two adjacent peaks (or troughs) of a wave.

wavenumber: reciprocal of wavelength measured in cm. Units of per cm.

wave velocity: the number of peaks on a wave passing per second (frequency) multiplied by the distance between two adjacent peaks (wavelength).

zone or band: the resolved Gaussian-shaped concentration profile of analyte emerging from a chromatographic column.