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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Atomic absorption spectroscopy | 2/5 | https://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy | reference | science, encyclopedia | 2026-05-05T10:03:45.133964+00:00 | kb-cron |
==== Line Source AAS ==== In line source AAS (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. The monochromator has to resolve the analytical line from other radiation emitted by the lamp. This can usually be accomplished with a band pass between 0.2 and 2 nm, i.e., a medium-resolution monochromator. Another feature to make LS AAS element-specific is modulation of the primary radiation and the use of a selective amplifier that is tuned to the same modulation frequency, as already postulated by Alan Walsh. This way any (unmodulated) radiation emitted for example by the atomizer can be excluded, which is imperative for LS AAS. Simple monochromators of the Littrow or (better) the Czerny-Turner design are typically used for LS AAS. 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.
==== Continuum Source AAS ==== When a continuum radiation source is used for AAS, a high-resolution monochromator is required. The resolution must be equal to or better than the half-width of an atomic absorption line (about 2 pm) in order to avoid loss 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 (CCD) array with 200 pixels is used as the detector. The second monochromator does not have an exit slit; hence the spectral environment at both sides of the analytical line becomes visible at high resolution. As typically only 3–5 pixels are used to measure the atomic absorption, the other pixels are available for correction purposes. One of these corrections is for lamp flicker noise, which is independent of wavelength, resulting in measurements with very low noise level; other corrections are for background absorption, to be discussed further.
=== Atomizers === Atomizers are what allow for atomization in AAS. Atomization is the process in which the separation of molecules into individual atoms occurs. Specifically atomizers in AAS are able to perform this process by introducing the sample to a high temperature flame so free atoms can be produced.
==== Flame atomizers ==== The oldest and most commonly used atomizers in AAS are flames, principally the air-acetylene (C2H2) flame with a temperature of about 2300 °C, and the nitrous oxide (N2O)-acetylene flame with a temperature of about 2700 °C. The latter flame offers a more reducing environment, ideally suited for analytes with a high affinity to oxygen.
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 so that only the finest droplets (< 10 μm) enter the flame. This conditioning reduces interference, but causes only about 5% of the solution to reach 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 the long 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 flame processes include:
desolvation (drying), in which the solvent is evaporated leaving the dry sample nano-particles vaporization, in which the solid particles are converted into gaseous molecules 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 of the flame) atoms may be partially converted to gaseous ions Each of these stages includes the risk of interference if the degree of phase transfer is different for the analyte in the calibration standard and in the sample. Ionization is usually 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 while the sample is aspirated. This technique is typically used for determinations in the mg/L range and may be extended down to a few μg/L for some elements.
==== Electrothermal atomizers ====