Mass spectroscopy has become a staple in all analytical chemistry and is most often associated with/fitted to liquid chromatography or gas chromatography. Atomic absorption spectrometry (AAS) is akin to these methodologies in the sense that it uses the absorption and emission of light to quantify and assay. However, rather than analyzing and elucidating an entire structure or compound in an aqueous solution, the AAS atomizes the injected sample, surveying only one predetermined element.
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The basic hull of any atomic absorption spectroscopy device is an atomic emission lamp of choice, a flame for subsequent vaporization/atomization, a nebulizer, a test solution, a monochromator, a detector (traditionally a particle multiplier tube), and a data processor.
The Discovery of AAS
Sir Alan Walsh and his research division at the “Commonwealth Scientific and Industrial Research Organization” (CSIRO) were the first to develop AAS in the 1950s. After endeavoring for some time to measure small concentrations of ions/molecules with great accuracy, Sir Walsh turned to measuring absorption rather than emission. This was considered a differing notion to the common trend within his time.
The first application of this method used a flame akin to that of a Bunsen burner, while the typical photomultiplier tube had been replaced with a more archaic, yet selective photocell receiver. Using a sodium lamp Dr. Walsh was able to measure the concentration of sodium in a solid complex material, revolutionizing spectroscopy, and analytical chemistry.
How the Apparatus Functions in Respect to a Given Analyte
Firstly, the atomic emission lamp produces a specific wavelength of light. Within the lamp, there will rest an element that corresponds to the analyte of choice. For example, if one wished to identify and quantify the amount of cadmium in a given sample, a cadmium emission lamp should be in use. This apparatus functions on the principle that each molecule or particle will emit and absorb a particular wavelength of light.
What follows down the apparatus's path is the flame- adjacent to the injection port where the aqueous solution/sample is placed. The flame will vaporize the solvent, then heat the sample, and finally atomize the sample, encompassing analyte and noise. The electrons in an analyte will transition to a higher energy state and emit this characteristic light when relaxing to their ground state. AAS concerns itself with how these unexcited atoms at their ground state, absorb light that is attributed to their characteristic radiation.
By using this method, knowing how the analyte will absorb this light, we can tune out other noise (non-analytes). The emission lamp will shine its light through the flame, where it then reaches the monochromator farther along the apparatus's path. This monochromator will take the form of either a prism or a diffraction grating, each one harboring its own boons and cons. This monochromator will allow for the filtering out of one wavelength, that which the analyte absorbs.
This light will then hit the detector of choice, usually taking the form of a particle multiplier tube. The detector is made from a photocathode, which comes into direct contact with the diffracted light. This detector will amplify the radiated electrons by exposing this photocathode to light, turning the light into a signal which we can then interpret.
One reason this device is so sensitive is that we are only quantifying one wavelength, and therefore one signal, rather than many. Another is the fact that this machine measures the analyte's capacity to absorb light, rather than emit it. All atoms in their normal state are capable of absorbing light to reach a more excited energy level, meaning this technique is theoretically applicable to all stable isotopes. This absorption depends on the number of unexcited atoms present and hence, is independent of the flame temperature.
How the Flame Apparatus and Allows for Analysis
Much like traditional gas chromatography, the solvent is first evaporated, and particles with a higher boiling point will be left behind. Once ebullition has passed, the atoms (which are charged ions at this point) within the medium will be drawn towards the flame. These ions will absorb the kinetic energy in the form of heat and will excite to higher energy levels. Once this excitation has seized, the succeeding decrease in radiation will be measured. The decrease in transmitted light, following the natural law of entropy, is wholly related to the concentration of unexcited atoms, following the classic “beer lamberts law” A = εbc, where:
A = Absorbance
ε = Molar absorption coefficient (M-1cm-1)
b = optical path length (cm)
c = Molar concentration (M)
This theorem is what defines the positive correlation between the absorbance of a given species/solution, and the concentration.
Applications in Science
This technique is especially advantageous when quantifying trace metals in a given medium. In fact, using this device, metals are more easily quantifiable than any other species. This is because metals themselves, for the most part, have a narrow, range of emission and absorption wavelengths which can bring about a clearer signal. This has special usefulness to it, especially when considering that most known elements are metals. To quantify this, however, metal atoms must be segregated from biological contaminants, accomplished easily as AAS evaporates the solvent while atomizing the entire sample. This metal concentration is determined using a calibration graph derived from serial dilution standards.
This methodology covers a specific niche in chemistry. In doing so, AAS has made advancements in food and water analyses by detecting trace amounts of heavy metal toxins. Similarly, this practice has also been applied to the pharmaceutical industry, and clinical research.
Sources:
- Havezov I, (1996) Atomic absorption spectrometry (AAS) - a versatile and selective detector for trace element speciation. Anal Bioanal Chem. Jun;355(5-6):452-6
- Rai, A. K., Pati, J. K., Parigger, C. G., Dubey, S., Rai, A. K., Bhagabaty, B., Mazumdar, A. C., & Duorah, K. (2020). The Plasma Spectroscopic Study of Dergaon Meteorite, India. Molecules (Basel, Switzerland), 25(4), 984.
- Walsh, (1962) Atomic absorption spectroscopy, Proc. Int. Conf. Spectrosc., 10, 127-142.
- Walsh, Some recent advances in atomic absorption spectroscopy, Jl. N. Z. Inst. Chem., 30 (1966), 7-21.
- Walsh. (1968) Simultaneous multi-element analysis by atomic absorption spectroscopy, XIII Colloquium Spectroscopicum Internationale, Ottawa, 1967, pp. 257-268.
- J. W. Robinson, (1960) Atomic Absorption Spectroscopy Analytical Chemistry 32 (8), 17A-29A
- Walter Slavin (1982) Analytical Chemistry 54 (6), 685A-694A
Further Reading