Editing Atomic absorption spectroscopy (section)

=== Atomizers ===
The atomizers most commonly 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&nbsp;°C and the nitrous oxide<ref name="Koirtyohann1991" /> system (N<sub>2</sub>O)-acetylene flame with a temperature of about 2700&nbsp;°C. The latter flame, in addition, offers a more reducing environment, being ideally suited for analytes with a high affinity to oxygen.

[[File:FP8800GM.jpg|thumb|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]], transformed into an [[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&nbsp;μm) enter the flame. This conditioning process reduces interference, but only about 5% of the aerosolized solution reaches the flame because of it.

On top of the spray chamber is a burner head that produces a flame that is laterally long (usually 5–10&nbsp;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]] (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]] 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<sup>−1</sup> range and may be extended down to a few μg L<sup>−1</sup> for some elements.

==== Electrothermal atomizers ====
[[File:GFAA method development-courtesy jball-Analytik Jena USA.JPG|thumb|right |400 px|GFAA method development]]
[[File:Lvov furnace 2.JPG|thumb|right|Graphite tube]]

[[Graphite furnace atomic absorption|Electrothermal AAS]] (ET AAS) using graphite tube atomizers was pioneered by Boris V. L'vov at the [[Saint Petersburg Polytechnical University|Saint Petersburg Polytechnical Institute]], Russia,<ref>{{cite journal |last1=L'vov |first1=Boris |title=Recent advances in absolute analysis by graphite furnace atomic absorption spectrometry |date=1990 |journal=Spectrochimica Acta Part B: Atomic Spectroscopy |doi=10.1016/0584-8547(90)80046-L |volume=45 |issue=7 |pages=633–655 |url=https://pubs.rsc.org/en/Content/ArticleLanding/1988/JA/JA9880300009|bibcode=1990AcSpB..45..633L |url-access=subscription }}</ref> since the late 1950s, and investigated in parallel by Hans Massmann at the Institute of Spectrochemistry and Applied Spectroscopy (ISAS) in Dortmund, Germany.<ref>{{cite web|title=Analytical Methods for Graphite Tube Atomizers|url=http://hpst.cz/sites/default/files/attachments/gta-analytical-methods-0848.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://hpst.cz/sites/default/files/attachments/gta-analytical-methods-0848.pdf |archive-date=2022-10-09 |url-status=live|website=agilent.com|publisher=Agilent Technologies}}</ref>

Although a wide variety of graphite tube designs have been used over the years, the dimensions nowadays are typically 20–25&nbsp;mm in length and 5–6&nbsp;mm inner diameter. With this technique liquid/dissolved, solid, and gaseous samples may be analyzed directly. A measured volume (typically 10–50&nbsp;μL) or a weighed mass (typically around 1&nbsp;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]] – 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.<ref>{{Cite web|title=Atomic Spectroscopy - GF-AAS|url=https://sites.chem.utoronto.ca/chemistry/coursenotes/analsci/atomic/gfaas.html#:~:text=Graphite%20furnace%20atomization%20(also%20known,inside%20a%20hollow%20graphite%20tube.|access-date=2021-03-08|website=sites.chem.utoronto.ca}}</ref>

The graphite tubes are heated via their ohmic resistance using a low-voltage high-current power supply; the temperature in the individual stages can be controlled very closely, and temperature ramps between the individual stages facilitate the separation of sample components. Tubes may be heated transversely or longitudinally, where the former ones have the advantage of a more homogeneous temperature distribution over their length. The so-called stabilized temperature platform furnace (STPF) concept, proposed by Walter Slavin, based on research of Boris L'vov, makes ET AAS essentially free from interference.{{citation needed|date=July 2014}} The major components of this concept are atomization of the sample from a graphite platform inserted into the graphite tube (L'vov platform) instead of from the tube wall in order to delay atomization until the gas phase in the atomizer has reached a stable temperature; use of a chemical modifier in order to stabilize the analyte to a pyrolysis temperature that is sufficient to remove the majority of the matrix components; and integration of the absorbance over the time of the transient absorption signal instead of using peak height absorbance for quantification.

In ET AAS, a transient signal is generated, the area of which is directly proportional to the mass of analyte (not its concentration) introduced into the graphite tube. This technique has the advantage that any kind of sample, solid, liquid, or gaseous, can be analyzed directly. Its sensitivity is 2–3 orders of magnitude higher than that of flame AAS, so that determinations in the low μg L<sup>−1</sup> range (for a typical sample volume of 20&nbsp;μL) and ng g<sup>−1</sup> range (for a typical sample mass of 1&nbsp;mg) can be carried out. It shows a very high degree of freedom from interferences, so that ET AAS might be considered the most robust technique available nowadays for the determination of trace elements in complex matrices.{{citation needed|date=July 2014}}

====Specialized atomization techniques====
While flame and electrothermal vaporizers are the most common atomization techniques, several other atomization methods are utilized for specialized use.<ref>{{Cite web|url=https://chem.libretexts.org/Textbook_Maps/Analytical_Chemistry_Textbook_Maps/Map%3A_Analytical_Chemistry_2.0_(Harvey)/10_Spectroscopic_Methods/10.4%3A_Atomic_Absorption_Spectroscopy|title=Atomic Absorption Spectroscopy|last=Harvey|first=David|date=2016-05-25|website=chem.libretexts.org|access-date=2017-10-06|archive-url=https://web.archive.org/web/20171006113153/https://chem.libretexts.org/Textbook_Maps/Analytical_Chemistry_Textbook_Maps/Map%3A_Analytical_Chemistry_2.0_(Harvey)/10_Spectroscopic_Methods/10.4%3A_Atomic_Absorption_Spectroscopy|archive-date=2017-10-06|url-status=dead}}</ref><ref>{{Cite web|url=http://blogs.maryville.edu/aas/atomization-source/|title=Sample Atomization – Atomic Absorption Spectroscopy Learning Module|website=blogs.maryville.edu|language=en-US|access-date=2017-11-02}}</ref>

=====Glow-discharge atomization=====
A glow-discharge device (GD) serves as a versatile source, as it can simultaneously introduce and atomize the sample. The [[glow discharge]] occurs in a low-pressure argon gas atmosphere between 1 and 10 torr. In this atmosphere lies a pair of electrodes applying a [[Direct Current|DC]] voltage of 250 to 1000 V to break down the argon gas into positively charged ions and electrons. These ions, under the influence of the electric field, are accelerated into the cathode surface containing the sample, bombarding the sample and causing neutral sample atom ejection through the process known as [[sputtering]]. The atomic vapor produced by this discharge is composed of ions, ground state atoms, and a fraction of excited atoms. When the excited atoms relax back into their ground state, a low-intensity glow is emitted, giving the technique its name.

The requirement for samples of glow discharge atomizers is that they are electrical conductors. Consequently, atomizers are most commonly used in the analysis of metals and other conducting samples. However, with proper modifications, it can be utilized to analyze liquid samples as well as nonconducting materials by mixing them with a conductor (e.g. graphite).

=====Hydride atomization=====
Hydride generation techniques are specialized in solutions of specific elements. The technique provides a means of introducing samples containing arsenic, antimony, selenium, bismuth, and lead into an atomizer in the gas phase. With these elements, hydride atomization enhances detection limits by a factor of 10 to 100 compared to alternative methods. Hydride generation occurs by adding an acidified aqueous solution of the sample to a 1% aqueous solution of sodium borohydride, all of which is contained in a glass vessel. The volatile hydride generated by the reaction that occurs is swept into the atomization chamber by an inert gas, where it undergoes decomposition. This process forms an atomized form of the analyte, which can then be measured by absorption or emission spectrometry.

=====Cold-vapor atomization=====
The cold-vapor technique is an atomization method limited only for the determination of mercury due to it being the only metallic element to have a large vapor pressure at ambient temperature.{{citation needed|date=March 2016}} Because of this, it has an important use in determining organic mercury compounds in samples and their distribution in the environment. The method initiates by converting mercury into Hg<sup>2+</sup> by oxidation from nitric and sulfuric acids, followed by a reduction of Hg<sup>2+</sup> with [[SnCl2|tin(II) chloride]]. The mercury is then swept into a long-pass absorption tube by bubbling a stream of inert gas through the reaction mixture. The concentration is determined by measuring the absorbance of this gas at 253.7&nbsp;nm. Detection limits for this technique are in the parts-per-billion range, making it an excellent mercury detection atomization method.