Atomic absorption spectroscopy (AAS) is an elemental analysis method for determining the concentration of metals in a given sample.
The principle of AAS relies on the vaporization of metals within a sample when introduced to a flame. Every ground state metal absorbs light radiation (and excites) at a different wavelength. This uniqueness allows each metallic element to have its own absorption spectrum that corresponds to its identity. The total absorbed radiation at a specific wavelength by an element in the sample is proportional to the density of atoms of the element. The quantification of this relationship is used to determine the concentration of specific metals in the sample.
History The modern form of AAS was largely developed during the 1950s by a team of Australian chemists led by Sir Alan Walsh at the Commonwealth Scientific and Industrial Research Organisation, Division of Chemical Physics, in Melbourne, Australia.
Alan Walsh first described AAS in an article titled "The Application of Atomic Absorption Spectra to Chemical Analysis" published in 1955 by the journal Spectrachemica Acta. In this article, Walsh emphasizes the importance of establishing a new technique that can provide an absolute method that can produce reliable chemical standards, which was not available at the time. He posits that instead of using emissive spectroscopy methods, an absorptive spectroscopic method can be used to achieve precise results. In 1960, James W. Robinson emphasizes that the main advantage of AAS is its ability to not be effected by environmental factors like other elements present in the experimental space. Before AAS, flame photometry was commonly used to determine the concentration of metal ions which can produce a wide array of results due to its sensitivity to aspects such as elements present in the air, flame temperature, and solvents. AAS circumvents these issues almost completely due to its reliance on the physical properties and interactions of atoms which are majority present in the ground state compared to the majority excited state atoms in flame photometry. Such comparisons highlight the utility of AAS as a novel technique at the time.
In the early 2000s, scientists turned toward high resolution line continuum AAS (HS LC AAS) which was considered revolutionary in the field since the invention of AAS as HS LC AAS was able to overcome previous limitations. Such limitations include issues like accurate background measurement and correction. Around this time, the first commercial instrument for HS LC AAS also became available.
Instrumentation
An atomic absorption spectrometer contains many components such as the radiation source, atomizer, focusing lenses, monochromator, detector, amplifier, signal processor, and finally, the sample. However, the most crucial parts of this instrument are the radiation source and atomizer.
Radiation sources Radiation sources in spectrometers are what excite the atoms in the provided sample. They provide two outputs: continuum and line sources. Continuum sources are able to emit electromagnetic radiation in a wide range of wavelengths. Line sources emit electromagnetic radiation at specific wavelengths.
Hollow cathode lamps Hollow cathode lamps (HCL) are a common radiation source used in AAS. The HCL is filled with an inert gas at low pressure. Inside, there is a hollow cup, the cathode, that contains the sample. The anode is a tungsten wire. Once a high voltage is applied across the anode and cathode, the gas begins to ionize. The gas ions accelerate towards the cathode, collide with the metal, and sputter atoms from the material. These metal ions are excited and emit specific wavelengths of radiation that allow for element identification.
Often, single element lamps are used where the cathode consists predominantly of compounds with the target element. These single element lamps provide precision with specific and stable emission lines that are element specific. Multi-element lamps are available with combinations of compounds containing the target elements as the cathode but are less accurate. Multi-element lamps have slightly less sensitivity than single element lamps, so the combinations must be selected carefully to avoid spectral interference. Atomic absorption spectrometers can feature as few as 1-2 hollow cathode lamp positions or, in automated multi-element spectrometers, 8-12 lamp positions may be available. Usually, separate single element lamps are used for different elements.
Electrodeless discharge lamps Electrodeless discharge lamps (EDL) are another radiation source used in AAS. EDL is frequently used instead of HCL when the desired sample consists of volatile metals (e.g. Arsenic) or lower sensitivity metals (e.g. Antimony). A small quantity of the metallic sample is sealed in an evacuated quartz tube filled with a low-pressure inert gas, most commonly Argon. The sealed tube is then placed into a microwave discharge cavity which allows for the gas to transform into a plasma state. The plasma state gas excites the metal atoms. The excited metal ions emit wavelengths that are then detected and organized into a spectrum.
EDLs need a separate power supply and might need a longer time to stabilize.
Deuterium lamps Deuterium HCL, hydrogen HCL, and deuterium discharge lamps are used in line source AAS for background correction. The radiation intensity emitted by these lamps decreases significantly with increasing wavelength, so that they can be only used in the wavelength range between 190 and about 320 nm.
Continuum sources When a continuum radiation source is used for AAS, it is necessary to use a high-resolution monochromator. The lamp emits radiation of an intensity at least an order of magnitude above that of a typical HCL: wavelengths ranging from 190 nm to 900 nm. A special high-pressure xenon short arc lamp, operating in a hot-spot mode, has been developed to be used as the continuum radiation source.
Source Type AAS AAS can be performed with either line source emissions or continuum source emissions.
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
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.