L6.12 ATOMIC ABSORPTION SPECTROSCOPY
Glossary

Below is a listing of terms commonly used when discussing Atomic Absorption Spectroscopy. Links in the text are to web pages that provide further explanations of the phenomena discussed or show pictures. Special thanks are due to the web site of Science Hypermedia, Inc., home of the on-line Encylopedia of Analytical Instrumentation, for some of the links used below.

Absorptivity: also known as the molar extinction coefficient in molecular spectroscopy, it is the wavelength-dependent absorption of an analyte as a function of concentration and pathlength and is expressed in units of concentration -1 * cm -1.
Analyte: a sample component whose concentration is being measured (i.e. analyzed). In atomic absorption spectroscopy (AA), this is an element.
Ashing: also referred to as charring, this is the step in a graphite furnace AA program that is designed to remove matrix constituents that might interference with the measurement of the analyte. Ashing temperatures vary from 200 to 1800 degrees C, depending on the matrix and analyte element.
Atomization: the process of producing atoms for the atomic absorption measurement. The atom-forming process usually requires a high temperature (except for cold-vapor Hg methods) which is produced by a flame or by electrical current flowing through a resistive medium.
Beer-Lambert Law: the law that defines a linear relationship between concentration and absorbance. It is often written as absorbance = absorptivity * pathlength * concentration or A = a*b*c.
Calibration: a quantitative procedure performed in order to relate the known concentration of standard solutions of the analyte element to the detector signal which is generated from the analyte in the unknown solutions. An extensive discussion of various methods of calibration including bracketing and standard addition can be found on this site in HTML, PDF, or MathCad formats.
Calibration curve: also known as a working curve, the relationship of instrument response (absorbance) as a function of concentration. Ideally, this should be a linear relationship in AA, under conditions that obey Beer's Law, where absorbance = (slope x concentration) + intercept. Minor curvature can be corrected by a curve-fitting algorithms.
Cold-Vapor: the method by which a cloud of atoms is produced from a solution containing Hg ions. Hg in the +2 state is reduced by the addition of stannous chloride and then swept by a flow of inert gas into a quartz-ended absorption cell kept at ~ 200 degrees C.
Detector: the part of the instrument that converts radiant energy from the light source to electricity. Typically a photomultiplier tube, but may also be a solid-state detector in more modern instrumentation.
Detection Limit: the minimum amount of an analyte that can be detected reliably. This is most often defined as three times the standard deviation of the blank measurement.
Deuterium Arc Background Correction: the first successful method to correct for background absorption in furnace AA, this method employs a continuum radiation source (the deuterium arc, a *white* light source) that is passed through the atomic vapor cell along with the HCL radiation. While the deuterium arc is not significantly absorbed by atoms of the analyte, it behaves similar to the HCL radiation with respect to molecular absorption and scatter, thus allowing an accurate background correction. However, it is not as accurate as Zeeman or Smith-Hieftje methods at high background absorbances that approach or exceed 2.0 absorbance units.
Double-beam optics: the optical design whereby a percentage of the radiation from the light source of an AA is diverted before it reaches the atomization cell and monitored to compensate for drift in light source intensity.
Drain trap: A hole at the bottom of the mixing chamber that leads though a plastic tube to a water filled trap, allowing waste sample solution to drain from the mixing chamber but not allowing combustion gases to escape.
Dynamic Range: sometimes known as linear dynamic range or linear range, the analyte concentration range over which response is a well defined (usually linear) function of the analyte concentration. The dynamic range can be increased by varying instrumental parameters, such as choice of analyte absorption line or decrease of absorption pathlength and sample volume.
Electrodeless Discharge Lamp (EDL): a more intense radiation source for AA than the HCL, it consists of a sealed quartz tube containing a small amount of the element of interest and an inert gas. The lamp is placed in a radiofrequency field, which exites the atoms to emit intense line radiation. EDL sources are less stable than HCL sources, but are far more intense and thus produce much better detection limits for elements such as As, Se, Hg, Sb, and Te.
Flame AA: The atomic absorption method that uses a flame as an atomization cell. Typical flames are air-acetylene (2400 degrees C) and nitrous oxide-acetylene (3000 degrees C).
Flowrate (solution): the volumetric flowrate (mL/min) of solution uptake into the nebulizer of a flame AA instrument. This is typically from 5 to 10 mL/min.
Flowrate (gas): the volumetric flowrate of combustible gases (L/min) into the mixing chamber of a flame AA instrument, or of inert gas used for graphite furnace and gas generation methods.
Flow Spoiler: This is a plastic, fan-shaped device placed in the mixing chamber of a flame AA to improve the mixing of combustion gases and analyte solution droplets and facilitate the removal of large droplets down the drain trap at the bottom of the mixing chamber.
Graphite tube: The atomization cell used in an electrothermal atomizer for AA. Typically made of pyrolytic graphite and bathed in an inert gas such as Ar to prevent decomposition. Can be heated up to 3000 degrees C. [photo]
Hollow Cathode Lamp (HCL): the most common radiation source for AA, consisting of a low-pressure inert-gas-filled tube containing an anode and a hollow cathode made from the element for which the lamp is to produce atomic line radiation. Current flowing through the lamp (3 to 30 mA) is carried by the inert gas and sputters atoms of the analyte element from the cathode, which are subsequently collisionally excited to produce radiation characteristic of the analyte element. [photo]
Hydride-generation: the method by which hydride-forming elements, such as As, Se, Sb, and Te are released from solutions of their ions using sodium borohydride. The released hydrides are swept from solution by an inert gas and decomposed to atoms in an absorption cell at a temperature of approximately 1000 degrees C.
Integration: a process for identifying and calculating the amount of a component by measuring the area greater than the baseline defined by the instrument blank over a specific time period. In flame AA, integration times of three to ten seconds are most commonly used, since continuous signals are measured. In furnace and gas-generation methods that produce transient signals, the "peak" produced by the analyte is integrated from baseline to baseline.
Matrix Modifier: an element or compound that is added to the sample in a graphite furnace AA measurement in order to increase the volatility of the matrix (and thus remove it during the ashing stage of the temperature program), or to decrease the volatility of the analyte element so that it can be atomized at high temperatures. Some typical matrix modifiers are palladium, nickel and ammonium phosphate.
Microwave Digestion: the preferred method for dissolving most samples in acid for analysis by AA. The method uses a closed Teflon container into which 5 to 10 mL of acid and approximately 0.5 grams of sample are subjected to an increasing microwave field for periods up to an hour. The high pressure and temperature inside the container rapidly dissolves most samples and no volatile analytes are lost since the container is sealed.
Mixing chamber: the heart of the sample introduction system for a flame AA, this is a plastic chamber in which combustible gases are mixed with the solution droplets from the nebulizer and then transported to the flame. Larger droplets (approximately 95% of the sample) are removed from the mixing chamber through the drain trap. [photo]
Modulation: the periodic variation of the radiation from the light source, either electronically or mechanically with a chopper, at frequencies between 30 and 200 Hz. Modulation of the light source allows the instrument to discriminate against other sources of radiation that might reach the detector and bias the absorbance measurement.
Monochromator - a wavelength selection device used in AA spectrometers to isolate the absorbable radiation from the light source from other extraneous radiation, both from the source (non or weakly absorbing lines) and the atomizer (flame or furnace emission).
Nebulizer: the component of a flame AA sample introduction system that draws aqueous solution into the mixing chamber and converts it to a fine mist of small droplets that are swept into the flame. It is typically a "pneumatic" nebulizer that operates on the principle of the Bernoulli effect, where the low pressure produced by air flowing rapidly into the mixing chamber through the nebulizer pulls the analyte-containing solution through a capillary tube. [photo]
Noise: the variation in the signal produced by the instrument. Noise is caused by short and long-term variations in different instrument components.
Peak: the transient increase in atomic absorption whose area represents the concentration of analyte element in a sample.
Peak Area: the area enclosed between the peak and the peak base.
Photomultiplier: the most often used detector in an AA instrument. It consists of a vacuum tube containing an alkali-element photocathode that produces electrons when struck by photons of sufficient energy (the photoelectric effect). Each photoelectron is then multiplied by collisions with a series of dynodes so that the electrical signal produced by each photon is greatly amplified. [photo]
Platform Atomization: also known as the L'vov platform, it is a small platform onto which a sample is placed inside the graphite furnace tube rather than placing the sample on the tube wall. This delays sample atomization until the gas temperature inside the furnace is higher than it would be for wall atomization, which reduces some interference effects. A good discussion of graphite furnace history and the design of modern instruments can be found at this site at the University of Umea.
Qualitative Analysis: the determination of the identity of the components in the sample.
Quantitative Analysis: the determination of the amount or concentration of the components in the sample.
Resolution: a measurement of how well two spectral lines are separated from each other. In AA, this is of significance primarily in the spectrum of the light source.
Smith-Hieftje Background Correction: a method to correct for background absorption in furnace AA that pulses the HCL at low and then at high current. During the high current pulse, a large cloud of atoms is formed in front of the lamp cathode. This cloud essentially prevents absorbable radiation from reaching the analyte in the atomic vapor cell and thus allows discrimination of atomic absorption from other sources of absorption.
Standard Addition: a method of calibration that compensates for matrix-induced enhancement or supression of analyte signals. A known concentration of analyte element is added to the sample and the instrument response of the known concentration of added element is used to calibrate the instrument response for the sample.
Sensitivity: the relationship of analyte concentration to instrument response. Mathematically, this is the slope of the linear plot of "instrument response vs. analyte concentration". Traditionally, in AA, the sensitivity is defined as the concentration of analyte that produces an instrument response of 0.0044 absorbance units (1% absorption).
Zeeman Background Correction: a method to correct for background absorption in furnace AA that uses a magnetic field around the atomizer. The field splits the energy levels of the absorbing atoms and allows discrimination of atomic absorption from other sources of absorption.

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