Atomic Absorption Spectrometry (AAS)
|Atomic Absorption Spectrometry (AAS)|
 General Description
AAS is an analysis that test for the quantities of certain chemical elements present in a sample by measuring the absorbed radiation by the chemical element of interest. The sample can be in a liquid or solid state.
Atomic absorption methods measure the amount of energy in the form of photons of light that are absorbed by the sample. A detector measures the wavelengths of light transmitted by the sample and compares them to the wavelengths which originally passed through the sample. Atomic absorption is very sensitive, it being able to measure down to parts per billion of a gram in a sample.
It occurs when a ground state atom absorbs energy in the form of light of a specific wavelength and is elevated to an excited state. In other words, converting the light signal to an electric signal. The increase of the amount of absorbed light energy signals that the number of the atoms belonging to the specific element increased.
AAS can be used to determine over 70 different elements (all metals, metalloids and some non metals such as B, Si, P) in solution or directly in solid samples used in pharmacology, biophysics and toxicology research. The samples can be in various forms, as a solution in diluted mineral acids such as HNO3, HCL, diluted in biological fluids (plasma, blood, etc.) or as suspensions in solid samples (surries). The results are displayed via a calibration curve. This is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration.
The functional characteristics of this analysis includes:
- converting the light signal to electric signal - the detector (photomultiplier tube or PMT) is an electronic tube that is capable of converting a photon current into an electrical signal and of amplifying this signal. A PMT consists of a photo cathode and a secondary electron multiplier;
- quantification of the absorption of ground state atoms in the gaseous state;
- assessing the concentration of an analyte in a sample - done by the computer attached to the spectrometer via a software specific to the type of machine used.
The conventional spectrometer has the following parts:
- hollow cathode lamp (HCL) - primary light source, specific to each element; provides an intense and constant analytical light line for the element of interest;
- nebulizer - sucks the sample at a controlled rate transforming it into a fine aerosol, then mixing it with the fuel and oxidant for introduction into the flame;
- continuum radiation source - D2 lamp;
- atomizer (flame, graphite furnace) - destroys the analyte ions and breaks down complexes creating atoms of the element of interest);
- background corrector - corrects interferences which can occur during the analysis;
- monochromator - isolates the analytical lines of the photons which are passing through the flame, removing the scattered light of other wavelengths from the flame;
- detector for the light (PMT) - determines the intensity of the photons of the analytical line which exits the monochromator;
- computer - processes and displays the data.
The flames require fuel and oxidant. As fuel Acetylene is mostly used, but also Hydrogen and Argon. As oxidant, the options are air, oxygen and nitrous oxide. Certain samples can release toxic fumes when going through the process, therefore an exhaust vent is required which has a minimum flow rate of 7000 L/ min is necessary. Also, the high temperature output requires a water cooling system. The water supply should be free of sediment with 6.5 - 7.5 pH and a maximum flow of 2.5L/min.
There are various types of atomic absorption, such as:
- Flame based (FAAS) for determination of higher concentrations (tenths to tens μg/ml);
- Graphite Furnace based (GFAA) for determination of trace and ultra trace concentrations (tens pg/ml to tens ng/ml);
- Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES);
- Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
The main difference between these four types of AAS is the sensitivity and costs, both of them rising from 1 through 4. This page will mainly focus on the first type, Flame Atomic Absorption Spectroscopy (FAAS), since it is widely used, simple, effective and relatively low cost. It is ideal for laboratories analyzing large numbers of samples for a limited number of elements and for the determination of major constituents and higher concentration analysis.
In FAAS, the liquid sample is aspirated into a nebulizer at a rate of 4-7 ml/min. This creates an aerosol which is introduced into flame, where it goes through the process of desolvation, evaporation and then analyte atomization. After, a steady-state absorbency signal is measured.
The flames used are usually the ones bellow:
- Air - Acetylene: for readily atomizable metals (Mg, Pb, Ca, Cu, Cd, Zn, Mn, Fe, alkali metals, Cr);
- N2O - Acetylene: for hardly atomizable elements (Sr, Si, V, B, Mo, Al, Ba, Cr etc.).
|Fuel||Oxidant||Unit||Fuel Price per Unit||Oxidant Price per Unit||Temperature range (°C)|
|Acetylene||Compressed Air||1 L each||19.12 €||12 €||2100 - 2400|
|Hydrogen||Compressed Air||1 L each||7 €||12 €||2000 - 2100|
|Acetylene||Nitrous Oxide||1 L each||19.12 €||11 €||2600 - 2800|
|Acetylene||Oxygen||1 L each||19.12 €||3 €||3050 - 3150|
The factors that influence the sensitivity of the FAAS:
- selection of analytical line;
- current of the lamp;
- efficiency of sample aerosol formation;
- length of atomization zone.
|FAAS|| Very easy-to-use
Extensive application information available
| Low sensitivity
Single-element analytical capability
Cannot be left unattended (flammable gas)
Partial ionization of some elements (alkali metals, alkali earth metals) in the flame
Inaccurate results in the presence unknown amounts of other easily ionizable elements
Chemical interferences induced by the sample matrix resulting in the incomplete atomization of the analyte.
 AAS: Determining the presence of Cr in water
Atomic absorption can be used to determine the total contents of elements such as Cr, Mg, Mn, K, Zn, etc. in water. For this a blank solution of deionized water is needed, which can be easily prepared in the laboratory beforehand. The analysis proceeds according to a set of Standard Conditions owned by the laboratory which performs the determination.
The standard solutions used in this analysis are prepared as stated by the Standard Conditions. Before the analysis, the samples have to be prepared. That means extracting the desired element into a compound which can be detected by the spectrometer. For Cr(III), this compound is methyl isobutyl ketone (MIBK). The pH is adjusted to 6-7 after which the sample is refluxed with acetylacetone in a flask. For this, a finger condenser and a magnetic stirring bar are used. After cooling the sample, hydrochloric acid is added to acidify the solution after which the MIBK compound is extracted by shaking the sample for a short period of time in a separator funnel. The organic extract is then adjusted to a desired weight and analyzed at the spectrometer.
The results are displayed via a calibration curve.
 Description Framework
AAS is an analysis that test for the quantities of certain chemical elements present in a sample by measuring the absorbed radiation by the chemical element of interest. It can be used to determine different elements in solution or directly in solid samples used in pharmacology, biophysics and toxicology research. The technique makes use of absorption spectrometry to assess the concentration of an analyte in a sample. It requires standards with known analyte content to establish the relation between the measured absorbance and the analyte concentration and relies therefore on the Beer-Lambert Law.
|For more information, see also the Wikipedia page|
|Temperature|| 22-251.15 °C |
|Operating environment||(not specified)|
 Information system
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- ^ Calibration curve, http://en.wikipedia.org/w/index.php?title=Calibration_curve&oldid=659861979
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- ^ Burrows, Jennifer (text), Step 2. Flame Atomiser, http://www.ltt.com.au/simulab/5/PMLTEST506/42step02.htm
- ^ Import Analysis and trends of of acetylene 99 6 gas. https://www.zauba.com/importanalysis-acetylene+99+6+gas-report.html.
- ^ Siminiati, Dubravka (2011). "Price reduction on compressed air". Engineering Review 31 (1): 63–67. http://hrcak.srce.hr/index.php?show=clanak&id_clanak_jezik=104821.
- ^ a b Prices of elements and their compounds. http://en.wikipedia.org/w/index.php?title=Prices_of_elements_and_their_compounds&oldid=664173114.
- ^ Nitrous oxide ≥99.998%. http://www.sigmaaldrich.com/catalog/product/fluka/00583?lang=fr®ion=FR.
- ^ Chau, Yiu-Kee; Sim, Soo-San; Wong, Yu-Hou (1968), "Determination of chromium by atomic absorption spectrophotometry of chromium acetylacetonate : Determination of chromium in sea water", Analytica Chimica Acta 43: 13–18, doi:10.1016/S0003-2670(00)89174-6, ISSN 0003-2670, http://www.sciencedirect.com/science/article/pii/S0003267000891746