Atomic Absorption Spectroscopy (AAS) PDF

**Atomic absorption spectroscopy** (**AAS**) is a spectroanalytical procedure for the quantitative determination of [chemical elements](https://en.wikipedia.org/wiki/Chemical_elements) by free atoms in the gaseous state. Atomic absorption spectroscopy is based on absorption of light by free metallic...

**Atomic absorption spectroscopy** (**AAS**) is a spectroanalytical procedure for the quantitative determination of [chemical elements](https://en.wikipedia.org/wiki/Chemical_elements) by free atoms in the gaseous state. Atomic absorption spectroscopy is based on absorption of light by free metallic ions. In analytical chemistry the technique is used for determining the concentration of a particular element (the analyte) in a sample to be analyzed. AAS can be used to determine over 70 different elements in solution, or directly in solid samples via electrothermal vaporization,[^\[1\]^](https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy#cite_note-1) and is used in [pharmacology](https://en.wikipedia.org/wiki/Pharmacology), [biophysics](https://en.wikipedia.org/wiki/Biophysics), [archaeology](https://en.wikipedia.org/wiki/Archaeology) and [toxicology](https://en.wikipedia.org/wiki/Toxicology) research. PRINCIPLES\-\-- The technique makes use of the atomic absorption spectrum of a sample in order to assess the concentration of specific analytes within it. It requires standards with known analyte content to establish the relation between the measured absorbance and the analyte concentration and relies therefore on the \[Beer--Lambert law\]. A common and practical expression of the Beer--Lambert law relates the optical attenuation of a physical material containing a single attenuating species of uniform concentration to the [[optical path length]](https://en.wikipedia.org/wiki/Optical_path_length) through the sample and [[absorptivity]](https://en.wikipedia.org/wiki/Molar_absorptivity) of the species. This expression is: A = ε ℓ c A= **εcl** Where - A A=is the [[absorbance]](https://en.wikipedia.org/wiki/Absorbance) - ε **ε=**is the [[molar attenuation coefficient]](https://en.wikipedia.org/wiki/Molar_attenuation_coefficient) or [[absorptivity]](https://en.wikipedia.org/wiki/Molar_absorptivity) of the attenuating species - ℓ l=is the optical path length - c c=is the [[concentration]](https://en.wikipedia.org/wiki/Molar_concentration) of the attenuating species **Instrumentation** Atomic absorption spectrometer block diagram In order to analyze a sample for its atomic constituents, it has to be atomized. The atomizers most commonly used nowadays are flames and electrothermal ([[graphite]](https://en.wikipedia.org/wiki/Graphite) tube) atomizers. The atoms should then be irradiated by optical radiation, and the radiation source could be an element-specific line radiation source or a continuum radiation source. The radiation then passes through a [[monochromator]](https://en.wikipedia.org/wiki/Monochromator) in order to separate the element-specific radiation from any other radiation emitted by the radiation source, which is finally measured by a detector. ### Atomizers The used nowadays are spectroscopic flames and electrothermal atomizers. Other atomizers, such as glow-discharge atomization, hydride atomization, or cold-vapor atomization, might be used for special purposes. #### Flame atomizers The oldest and most commonly used atomizers in AAS are flames, principally the air-acetylene flame with a temperature of about 2300 °C and the nitrous oxide[^\[4\]^](https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy#cite_note-Koirtyohann1991-4) system (N~2~O)-acetylene flame with a temperature of about 2700 °C.  A laboratory flame photometer that uses a propane operated flame atomizer Liquid or dissolved samples are typically used with flame atomizers. The sample solution is aspirated by a pneumatic [analytical nebulizer](https://en.wikipedia.org/wiki/Analytical_nebulizer), transformed into an [aerosol](https://en.wikipedia.org/wiki/Aerosol), which is introduced into a spray chamber, where it is mixed with the flame gases and conditioned in a way that only the finest aerosol droplets (\< 10 μm) enter the flame. On top of the spray chamber is a burner head that produces a flame that is laterally long (usually 5--10 cm) and only a few mm deep. The radiation beam passes through this flame at its longest axis, and the flame gas flow-rates may be adjusted to produce the highest concentration of free atoms. The burner height may also be adjusted, so that the radiation beam passes through the zone of highest atom cloud density in the flame, resulting in the highest sensitivity. The processes in a flame include the stages of desolvation (drying) in which the solvent is evaporated and the dry sample nano-particles remain, [vaporization](https://en.wikipedia.org/wiki/Vaporization) (transfer to the gaseous phase) in which the solid particles are converted into gaseous molecule, atomization in which the molecules are dissociated into free atoms, and [ionization](https://en.wikipedia.org/wiki/Ionization) where (depending on the ionization potential of the analyte atoms and the energy available in a particular flame) atoms may be in part converted to gaseous ions. Each of these stages includes the risk of interference in case the degree of phase transfer is different for the analyte in the calibration standard and in the sample. Ionization is generally undesirable, as it reduces the number of atoms that are available for measurement, i.e., the sensitivity. In flame AAS a steady-state signal is generated during the time period when the sample is aspirated. This technique is typically used for determinations in the mg L^−1^ range, and may be extended down to a few μg L^−1^ for some elements. #### Electrothermal atomizers GFAA method development Graphite tube [Electrothermal AAS](https://en.wikipedia.org/wiki/Graphite_furnace_atomic_absorption) (ET AAS) using graphite tube atomizers was pioneered by Boris V. Although a wide variety of graphite tube designs have been used over the years, the dimensions nowadays are typically 20--25 mm in length and 5--6 mm inner diameter. With this technique liquid/dissolved, solid and gaseous samples may be analyzed directly. A measured volume (typically 10--50 μL) or a weighed mass (typically around 1 mg) of a solid sample are introduced into the graphite tube and subject to a temperature program. This typically consists of stages, such as drying -- the solvent is evaporated; [pyrolysis](https://en.wikipedia.org/wiki/Pyrolysis) -- the majority of the matrix constituents are removed; atomization -- the analyte element is released to the gaseous phase; and cleaning -- eventual residues in the graphite tube are removed at high temperature.[^\[7\]^](https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy#cite_note-7) RADIATION SOURCES:\-- 1\) line source AAS (LS AAS) 2\) continuum source AAS (CS AAS). We have to distinguish between line source AAS (LS AAS) and continuum source AAS (CS AAS). In classical LS AAS, as it has been proposed by Alan Walsh,[^\[10\]^](https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy#cite_note-10) the high spectral resolution required for AAS measurements is provided by the radiation source itself that emits the spectrum of the analyte in the form of lines that are narrower than the absorption lines. Continuum sources, such as deuterium lamps, are only used for background correction purposes. The advantage of this technique is that only a medium-resolution monochromator is necessary for measuring AAS; however, it has the disadvantage that usually a separate lamp is required for each element that has to be determined. In CS AAS, in contrast, a single lamp, emitting a continuum spectrum over the entire spectral range of interest is used for all elements. The lamps used for line sources are:\-- #### Hollow cathode lamps #### Electrodeless discharge lamps #### Deuterium lamps #### Continuum sources:- When a continuum radiation source is used for AAS, it is necessary to use a high-resolution monochromator, #### Spectrometers for LS AAS In LS AAS the high resolution that is required for the measurement of atomic absorption is provided by the narrow line emission of the radiation source, and the monochromator simply has to resolve the analytical line from other radiation emitted by the lamp.^\[[*citation\ needed*](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)\]^ This can usually be accomplished with a band pass between 0.2 and 2 nm, i.e., a medium-resolution monochromator. Photomultiplier tubes are the most frequently used detectors in LS AAS, although solid state detectors might be preferred because of their better [signal-to-noise ratio](https://en.wikipedia.org/wiki/Signal-to-noise_ratio). #### Spectrometers for CS AAS When a continuum radiation source is used for AAS measurement it is indispensable to work with a high-resolution monochromator. The resolution has to be equal to or better than the half-width of an atomic absorption line (about 2 pm) in order to avoid losses of sensitivity and linearity of the calibration graph. These spectrometers use a compact double monochromator with a prism pre-monochromator and an echelle grating monochromator for high resolution. A linear [charge-coupled device](https://en.wikipedia.org/wiki/Charge-coupled_device) (CCD) array with 200 pixels is used as the detector. The second does not have an exit slit; hence the spectral environment at both sides of the analytical line becomes visible at high resolution.