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Atomic Absorption Spectroscopy, or AAS, is an analytical technique used to detect specific elements and their levels of concentration in a sample.
AAS is a type of atomic spectroscopy, which includes two other analytical methods: atomic emission and atomic fluorescence. The process of excitation and decay to the ground state is involved in all three methods. However, the difference between each technique is when the energy is measured. In atomic emission spectroscopy (AES), the energy emitted in the decay process is measured, while in AAS the energy absorbed during the excitation process is measured.
Atomic spectroscopy, and atomic absorption spectroscopy specifically, is commonly used in analytical chemistry, drug development, and industrial settings, and can perform both qualitative and quantitative analyses.
To obtain results, AAS uses the unique wavelengths that different atoms absorb to determine an element’s concentration in a liquid or solid sample, which is then compared to a standard, a previously recorded result of the same compound. It is a highly sensitive technique, and can measure concentration in parts per million (ppm) or parts per billion (ppb), enabling the detection of even trace elements in a sample.
AAS produces an absorption spectrum, which is recorded using an atomic absorption spectrometer. AA spectrometers typically include a flame burner—usually a hollow cathode lamp (HCL)—to atomize the sample, a monochromator, and a photon detector. AA Spectrometers that employ a flame burner are also referred to as flame atomic absorption spectrometers, employing flame atomic absorption spectrometry (FAAS or flame AAS) to obtain results. The detection limits of FAAS are of the order of 1–100 µg L−1, making it an excellent method of choice for determining minor and trace elements in liquid samples, or solid samples that have been converted to a liquid.
Depending on the model you’re interested in, some atomic absorption spectrometers come equipped with a turret or fixed lamp socket that holds multiple lamps to reduce downtime between samples or allow for sequential elemental analysis.
AAS can also be referred to as atomic absorption spectrometry, with the difference between the two being that spectroscopy is the study of how energy and materials interact, while spectrometry refers to how you apply spectroscopy as a measuring technique.
This device uses several components, but the three main pieces include: a light source, an atomizer, and a detection source. The main difference in AA spectrometer models is the atomizer used.
There are several types of light sources used in AA spectrometry, but perhaps the most common is a hollow-cathode lamp, or HCL.
In essence, an HCL is a lamp that contains a cathode, anode, and type of filament. To properly analyze a sample using AAS, an HCL also requires a certain amount of the element of interest in the sample to be present inside the lamp.
For example, if the element you want to identify is lead, then lead is placed within the hollow-cathode lamp as well. An electrical current is passed through the lamp, which the atoms of the element absorb and re-emit in the form of light, producing a unique wavelength specific to that atom.
The light passes through the atomizer of an AA spectrometer, and the free atoms present absorb the radiation, exciting the electrons within. As the electrons return from an excited-state to their ground-state, they in turn produce unique wavelengths. The signal passes through a prism, or monochromator, and into a photomultiplier tube.
Another light source used in AAS includes electrodeless discharge lamps (EDLs), which produce more intense radiant energy than HCLs, but can be less reliable due to the use of a radio frequency (RF) source to produce a field of radiation.
The most commonly used atomizers are also the two most well known techniques in atomic absorption spectroscopy. These two techniques are flame atomic absorption spectroscopy (FAAS), which uses a flame to atomize, and graphite furnace atomic absorption spectroscopy (GFAAS), which uses a graphite tube to atomize.
These atomizers, along with a nebulizer, are used to turn a sample from an aqueous solution into free, non ionized atoms in the gas-phase, ready to absorb electromagnetic radiation.
Relying on simple yet effective instrumentation, FAAS employs the use of a spectroscopic flame to atomize a sample that has been aspirated by a nebulizer. Liquid or dissolved samples are typical of FAAS.
The sample is prepared and injected into a nebulizer, which converts the sample into a mist or aerosol through evaporation. The mist is carried through to the flame which atomizes the sample. This separates the sample’s elements individually. As the atoms are separated using the flame, radiation from an HCL, which produces wavelengths unique to the element of interest, passes through.
Generally, the more absorption that occurs, the higher the levels of concentration of the target element present in the sample.
GFAAS, or graphite furnace atomic absorption spectroscopy, is similar to FAAS. It is also known as electrothermal atomic absorption spectroscopy, or ETAAS, and uses a graphite coated furnace to atomize a sample.
The sample is deposited into a small graphite or pyrolytic carbon coated tube, which is heated using a low-voltage, high-current power source. This process atomizes the sample, allowing for radiation to be absorbed, changing the state of the electrons present from ground-state to excited-state.
As the electrons decay, photons in the form of light are emitted, carrying a unique wavelength to the monochromator. Compared to FAAS, graphite furnace AA is more sensitive, measuring in parts-per-billion (ppb). It is also a less limited technique than FAAS due to its ability to block out more interference that occurs during analysis.
A type of prism, the monochromator is used to isolate the wavelength, or line, of interest. This device effectively separates the spectral line of interest from different wavelengths emitted by an HCL.
It uses either the phenomenon of optical dispersion which occurs in a prism, or diffraction that is performed using diffraction grating. In the case of an AA spectrometer, diffraction grating is used. This disperses the other wavelengths, allowing for one specific type to pass through to the detection system.
A photomultiplier tube is used as the detection system in an AA spectrometer. It operates similar to an amplifier. By amplifying the signal, the wavelength isolated by the monochromator is more easily detectable and measurable. This signal is then processed by a computer system which produces the output results.
Atomic emission spectroscopy, or AES, is another type of spectroscopy used for chemical analysis. Similar to AAS in design, AES obtains results from the emission spectrum produced by a sample, rather than the absorption spectrum.
Utilizing an atomizer in the same fashion, AES measures the unique wavelength emitted by a free atom in the gas-phase as it changes from ground-state to excited-state and then decays. This characteristic wavelength is then dispersed using a monochromator and detected for measurement.
This recording determines the level of concentration in a sample, which may be accomplished by using a flame, plasma, or electrical spark. This technique is performed using an atomic emission spectrometer or spectrophotometer.
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