AC Arc Plasma created by a power supply or function generator of alternating polarity. Typically, an arc excited at twice the frequency of the electrical grid (50 Hz except North America, which is 60 Hz). While not continuous, the period of low current as the polarity of the driving voltage commutes (switches sign) is brief compared to the overall experiment time.
Arc Current-carrying plasma. See also AC Arc, DC arc
Anode Electrode attractive to negative particles (electrons or anions). Thus, electrode at positive voltage.
Atomization Conversion of molecular or condensed forms of an analyte into free atoms.
Background Data value in a datastream in the absence of any analyte or interference.
Blackbody (or Planck) Radiation Intensity distribution of radition as a function of frequency in equilibrium with a body at a particular temperature.
Blaze Angle a) Tilt of an echellette with respect to the plane of a diffraction grating. b) Angle of incidence and diffraction at which the efficiency of a grating is maximum i.e. where light intensity diffracted/incident intensity is maximum.
Boltzmann Distribution Statistical behavior of particles of various energies in equilibrium. The probability that a particle will occupy an energy level E at a temperature T is proportional to e- E/kT, with k a constant, ~ 1.38 ×10-23 Joule Kelvin-1. If energy is due exclusively to kinetic energy of point-like particles so that E = 1/2 mv2, the distribution is dubbed a Maxwell-Boltzmann distribution.
Boxcar Integration Observation of a periodically-generated signal at a controlled time after each repetition is initiated. By averaging many repetitions at a given delay after event initiation, one obtains a more precise measurement of the behavior of the repetitive event's time course than could be obtained with a single observation.
Cathode Electrode attractive to positive particles (cations). Thus, electrode at negative voltage.
CCD, Charge-coupled Device, Charge-coupled Array Multi-pixel detector in which potential bias supplied to adjacent portions of a semiconductor create wells in which electrons may be stored during exposure and between which electrons may be transferred during readout.
Chemometrics The subset of statistical methods useful in extracting information from chemically-derived data.
DC Arc Plasma with current running in a single direction. Commonly used for analysis of powdered solids or metals. Cathode is primarily sampled electrode.
DCP or Direct Current Plasma A two or three electrode DC arc optimized for analysis of aerosols and sprayed solutions.
Detection Limit Smallest amount of a substance that generates a signal with magnitude at least 3 times the noise in the background.
Diffraction Grating Transmitting or reflecting optical element with regularly spaced scribes or grooves on its surface, designed to use phase-dependent constructive interference to separate light by dispersing it at wavelength-dependent angles.
Dispersion a) Change in refractive index with wavelength. b) Change in propagation direction or location of a light beam with wavelength.
Dynamic Range Ratio of the largest quantity reliably measured by a technique to the lowest quantity so measured. A method with detection limit 1 ppb and upper concentration of working curve linearity of 100 ppm has a dynamic range of 100 ppm / 0.001 ppm = 106
Echelle A diffraction grating designed to operate with incident and diffracted light at an angle greater than 45° from the grating normal.
Echellette A single scribed groove on a diffraction grating.
Electrode Metallic or semiconducting solid used to inject current into a plasma, to act as a sample specimen to be eroded by a plasma, or to probe ionic behavior within a plasma.
Flame Exothermic gas phase reaction, spatially confined, capable of emitting light or stimulating particles passing through it to emit light.
Glow Discharge Low-pressure (typically millitorr) discharge that expands to cover essentially the entire surface of an electrode rather than condensing into a narrow arc channel.
Grating See Diffraction Grating.
Grotrian diagram Graphical display of energy levels of an atom or molecule.
Inductively Coupled Plasma (ICP) A plasma receiving energy from an induction coil powered by a radio-frequency power supply (typically 27.12 MHz, but 13.56 MHz and 40.68 MHz are not uncommon). Because electrodes do not contact the plasma, contamination by elements in the power source and wiring is negligible. Common support gasses are Ar, He, or N2.
Interference Any substance or process other than the analyte or desired signal generation process that influences the detected signal other than by providing a constant offset.
Interferometer Any apparatus to measure the phase shift of light traversing a variable path compared to a fixed reference path. Typically, a device to measure light of multiple wavelengths simultaneously using a moving mirror to produce phase shifts of fixed distance for all wavelengths. The fixed distance corresponds to variable phase shift since 2pd/l is different for each wavelength.
Internal Standard A substance, added to a specimen, presumed not otherwise to be present in the specimen, which after addition acts similarly to but does not interfere with the sought-for substance. By monitoring a signal due to the internal standard simultaneously with that of the analyte, signal changes due to incomplete sample uptake or due to variations in sample transport can be compensated.
Lockin Amplification Extraction of information of a particular frequency from a datastream by multiplying a periodic waveform of the desired frequency times the raw datastream, followed by signal averaging.
Matrix All components of a sample except the sought-for species. If one is attempting to analyze all components of a sample, then every component is both an analyte and (for all the other components) part of the matrix.
Microwave Plasma Plasma created by radiation with a wavelength less than 10 cm. Typically 2.45 GHz, the same frequency as used in microwave ovens (l ~ 1.22 cm).
Nebulize Convert a bulk liquid into a spray of small drops.
Noise Random fluctuations in an observable datastream, typically time-varying, that obscure elements of the datastream related to the sought-for substance or quantity. Noise reduction typically lowers the detection limit.
Photomultiplier Tube Vacuum tube with a negatively-biased photocathode that, with useful quantum efficiency, transduces light to a ballistically-launched electron. The electron ricochets to a series of dynodes at less negative potentials, impacting the dynodes and generating an avalanche of electrons. The electrons are collected at an anode so that each detected photon produces a pulse of electrons a few nanoseconds after the initial photon hit the photocathode.
Plasma Partially or totally ionized gas i.e. gas atoms or molecules are dissociated into cations and either electrons or anions, allowing facile conduction of electric current.
Quantum Efficiency Ratio of the number of events of a desired type to the number of stimulae for that type of event. Typically, the ratio of electrons produced by a transducer to the number of incident photons, though it may also mean the ratio of the number of photons fluoresced to the number absorbed or the number of photochemical reactions initiated per absorbed photon.
Refraction Change of direction of light as it passes between dissimilar materials.
Resolution Separation of signals that should, under some circumstance, be distinguishable. Relative resolution (unitless) is the value of the distinguishable parameter (say, wavelength) divided by the smallest value difference that an experiment can actually distinguish. Absolute resolution (has units) is the smallest difference that can be discerned in a measured quantity.
Rydberg a) Energy unit: the energy required to remove a ground-state electron orbiting an infinitely massive, gravitationless point positive charge to the edge of the universe with zero residual kinetic energy. 109737.316 cm-1. b) A Rydberg state is a highly excited state of an atom or molecule such that the electron, for all practical purposes, behaves as if it is orbiting a hydrogen atom.
Self-absorption Line broadening due to saturation of emission in the center of an emission line as the light output reaches the maximum output for a black body at the temperature and wavelength in question.
Self-reversal Diminution of light intensity in the center of an emission line due to atomic absorption by cold atoms of a given element surrounding the hot plasma producing the original emission.
Signal-to-noise Ratio Magnitude of the observed datastream related to the sum of sought-for and interfering substances plus background, and fluctuations unrelated to the sought-for measurements.
Solar-blind Characteristic of a detector (typically a photomultiplier tube) that responds only to ultraviolet light, not visible or infrared radiation.
Spark Transient plasma triggered by imposing so high a voltage across a small gap between conducting electrodes that the intervening gas ionizes.
Spectrograph Instrument to display light of various wavelengths at separated physical positions simultaneously.
Spectrometer Any instrument used to study light as a function of wavelength.
Spectrophotometer An instrument to quantitatively measure the intensity of light as a function of wavelength, typically for making absorbance measurements.
Term Symbol Summary of the momentum, parity, and spin for an atomic or molecular energy level.
Time Gating Observing only a portion of a datastream by selecting time intervals, subsets of the overall datastream, for recording.