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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Atomic absorption spectroscopy | 3/5 | https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy | reference | science, encyclopedia | 2026-05-05T10:03:45.133964+00:00 | kb-cron |
Electrothermal AAS (ET AAS) using graphite tube atomizers was pioneered by Boris V. L'vov at the Saint Petersburg Polytechnical Institute, Russia, since the late 1950s, and investigated in parallel by Hans Massmann at the Institute of Spectrochemistry and Applied Spectroscopy (ISAS) in Dortmund, Germany. Although a wide variety of graphite tube designs have been used over the years, typical dimensions are 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 is introduced into the graphite tube and subject to a temperature program. This typically consists of stages of 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 – residues left in the graphite tube are removed at high temperature. 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, with the former method having a more homogeneous temperature distribution. The so-called stabilized temperature platform furnace (STPF), proposed by Walter Slavin, based on research of Boris L'vov, makes ET AAS essentially free from interference. 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 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−1 range (for a typical sample volume of 20 μL) and ng g−1 range (for a typical sample mass of 1 mg) can be carried out. It has a very high degree of freedom from interferences, so that ET AAS may be considered the most robust technique available for the determination of trace elements in complex matrices.
==== Specialized atomization techniques ==== While flame and electrothermal vaporizers are the most common atomization techniques, several other methods are available for specialized use.
===== Glow-discharge atomization ===== A glow-discharge device (GD) is 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 is a pair of electrodes applying a 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 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 used to analyze liquid samples as well as nonconducting materials by mixing them with a conductor (e.g. graphite).
===== Hydride atomization ===== Hydride generation techniques use specialized 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 ===== Cold-vapor atomization is a method limited to the determination of mercury due to it being the only metallic element with a high vapor pressure at ambient temperature. Because of this, it is important for determining organic mercury compounds in samples and their distribution in the environment. The method begins by converting mercury into Hg2+ by oxidation from nitric and sulfuric acids, followed by a reduction of Hg2+ with 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 nm. Detection limits for this technique are in the parts-per-billion range, making it an excellent mercury detection method.