Figure1. Elements detectable by atomic absorption are highlighted in pink in this periodic table.
Flame atomic absorption is a very common technique for detecting metals and metalloids in environmental samples. It is very reliable and simple to use. Figure 1 shows which elements are commonly detected through atomic absorption. The technique is based on the fact that ground state metals absorb light at specific wavelenths. Metal ions in a solution are converted to atomic state by means of a flame. Light of the appropriate wavelength is supplied and the amount of light absorbed can be measured against a standard curve.
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The history of spectroscopy starts with the use of the lens by Aristophanes about 423 B.C.; and the studies of mirrors by Euclid (300 B.C.) and Hero (100 B.C.). Seneca (40 A.D.) observed the light scattering properties of prisms, and in 100 A.D. Ptolemy studied incidence and refraction.
Alhazen in 1038 studied reflection and refraction of light, and in 1250 Roger Bacon determined the focal points of concave mirros.
Around 1600, the telescope was developed in Holland and by 1610, Galileo had made improvements on the telescope design. Sir Isaac Newton (1642-1727) performed many experiments on the separation of light to obtain a spectrum and the indices of refraction of different colors of light; he applied those principles to the telescope.
Fraunhofer, about 1814-15, observed diffraction phenomena and was able to measure wavelength instead of angles of refraction. Herschel (1823) and Talbot (1825) discovered atomic emission when certain atoms were placed in a flame. Wheatstone concluded in 1835 that metals could be distinguished from one another on basis on the wavelengths of this emission. In 1848, Foucault observed atomic emission from sodium and discovered that the element would absorb the same rays from an electric arc.
In the later 1800, scientists such as Kirchoff, Bunsen, Angström, Rowland, Michelson and Balmer studied the composition of the sun based on their emissions at different wavelengths. Kirchoff summarized the law which states that, "Matter absorbs light at the same wavelength at which it emits light". It is under this law that atomic absorption spectroscopy works.
Woodson was one of the first to apply this principle to the detection of mercury. In 1955, Walsh suggested the use of cathode lamps to provide an emission of appropriate wavelength; and the use of a flame to produce neutral atoms that would absorb the emission as they crossed its path. Instrumentation and applications for atomic absorption greatly expanded after the 1950s.
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The technique of flame atomic absorption spectroscopy (FAAS) requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide. The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 oC.
During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths, as shown in figure 3.
Figure 3. Operation principle of an atomic absorption spectrometer.
The characteristic wavelengths are element specific and accurate to 0.01-0.1nm. To provide element specific wavelengths, a light beam from a lamp whose cathode is made of the element being determined is passed through the flame. A device such as photonmultiplier can detect the amount of reduction of the light intensity due to absorption by the analyte, and this can be directly related to the amount of the element in the sample.
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Figure 4 shows an atomic absorption spectrometer. This instrument in particular is designed to operate either with a flame or with a graphite furnace (see atomic absorption methods other than flame). The graphite furnace is additionally equipped with an auto sampler.
Flame atomic absorption hardware is divided into six fundamental groups that have two major functions: generating atomic signals and signal processing. Signal processing is a growing additional feature to be integrated or externally fitted to the instrument. The instrument parts are shown in figure 5.
A cathode lamp (1), shown in figure 6, is a stable light source, which is necessary to emit the sharp characteristic spectrum of the element to be determined. A different cathode lamp is needed for each element, although there are some lamps that can be used to determine three or four different elements if the cathode contains all of them. Each time a lamp is changed, proper alignment is needed in order to get as much light as possible through the flame, where the analyte is being atomized, and into the monochromator.
The atom cell (2), shown in figure 7, is the part with two major functions: nebulization of sample solution into a fine aerosol solution, and dissociation of the analyte elements into free gaseous ground state form. Not all the analyte goes through the flame, part of it is disposed as shown in the figure.
As the sample passes through the flame, the beam of light passes through it into the monochromator (3). The monochromator isolates the specific spectrum line emitted by the light source through spectral dispersion, and focuses it upon a photomultiplier detector (4), whose function is to convert the light signal into an electrical signal.
The processing of electrical signal is fulfilled by a signal amplifier (5). The signal could be displayed for readout (6), or further fed into a data station (7) for printout by the requested format.
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A) Types of flame
Different flames can be achieved using different mixtures of gases, depending on the desired temperature and burning velocity. Some elements can only be converted to atoms at high temperatures. Even at high temperatures, if excess oxygen is present, some metals form oxides that do not redissociate into atoms. To inhibit their formation, conditions of the flame may be modified to achieve a reducing, nonoxidizing flame. Table 1 shows the characteristics of various flames.
B) Ultrasonic Nebulization
Proper nebulization is required to break up an aqueous sample into a fine mist of uniform droplet size that can be readily burned in the flame. Most instruments utilize the direct aspiration. During aspiration, the gas flow breaks down the liquid sample into droplets, and the nebulization performance depends on the physical characteristics of the liquid. Only about 10% of the sample gets into the flame. Another option for nebulization is the use of an ultrasonic wave beam, which generates high frequency waves in the liquid sample. This causes very small liquid particles to be ejected into a gas current forming a dense fog.
C) Slotted Tube Atom Trap
This device is a heated quartz tube that can be placed in a conventional flame. As the dissociated ground state atoms pass into the tube, they are delayed and stay longer in the optical path, increasing the sensitivity of the instrument.
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Flame atomic absorption is very convenient and widespread, and has an acceptable level of accuracy for most analytes. However, there are other devices which allow for better sensitivity and more control over the chemical environment of the analyte:
A) Electrothermal atomisation
This type of atomisation requires a graphite furnace, where after thermal pre-treatment the sample is rapidly atomized. To maintain a dense fraction of free ground state elements in the optical path, an inert gas atmosphere is used. Since the dilution and expansion effects of flame cells are avoided, and the atoms have a longer residence time in the optical path, a higher peak concentration of atoms is obtained.
B) Carbon rod analyser
This device can be used to convert a powdered sample into atomic vapour. A current is applied to a very thin, heated carbon rod that contains the solid sample in order to vaporise it.
C) Tantalum boat analyser
This is another technique that produces an atomic vapour from a solid sample. A Tantalum boat is electrically heated in a manner similar to the carbon rod system, within an inert atmosphere.
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Atomic absorption spectrometry is a fairly universal analytical method for determination of metallic elements when present in both trace and major concentrations. The EPA employs this technique for determining the metal concentration in samples from a variety of matrices.
A) Sample preparation
Depending on the information required, total recoverable metals, dissolved metals, suspended metals, and total metals could be obtained from a certain environmental matrix. Table 2 lists the EPA method number for sample processing in terms of the environmental matrices and information required. For more detail information, readers could refer to EPA document SW-846 "Test methods for evaluating solid wastes".
Appropriate acid digestion is employed in these methods. Hydrochloric acid digestion is not suitable for samples which will be analyzed by graphite furnace atomic absorption spectroscopy because it can cause interferences during furnace atomization.
B) Calibration and standard curves
As with other analytical techniques, atomic absorption spectrometry requires careful calibration. EPA QA/QC demands calibration through several steps, including interference check sample, calibration verification, calibration standards, bland control, and linear dynamic range.
The idealized calibration or standard curve is stated by Beer's law that the absorbance of an absorbing analyte is proportional to its concentration.
Unfortunately, deviations from linearity usually occur, especially as the concentration of metallic analytes increases due to various reasons, such as unabsorbed radiation, stray light, or disproportionate decomposition of molecules at high concentrations. Figure 8 shows an idealized and deviation of response curve. The curvature could be minimized, although it is impossible to be avoided completely. It is desirable to work in the linearity response range. The rule of thumb is that a minimum of five standards and a blank should be prepared in order to have sufficient information to fit the standard curve appropriately. Manufacturers should be consulted if a manual curvature correction function is available for a specific instrument.
If the sample concentration is too high to permit accurate analysis in linearity response range, there are three alternatives that may help bring the absorbance into the optimum working range:
1) sample dilution
2) using an alternative wavelength having a lower absorptivity
3) reducing the path length by rotating the burner hand.
C) EPA method for metal analysis
Flame atomic absorption methods are referred to as direct aspiration determinations. They are normally completed as single element analyses and are relatively free of interelement spectral interferences. For some elements, the temperature or type of flame used is critical. If flame and analytical conditions are not properly used, chemical and ionization interferences can occur.
Graphite furnace atomic absorption spectrometry replaces the flame with an electrically heated graphite furnace. The major advantage of this technique is that the detection limit can be extremely low. It is applicable for relatively clean samples, however, interferences could be a real problem. It is important for the analyst to establish a set of analytical protocol which is appropriate for the sample to be analyzed and for the information required. Table 3 lists the available method for different metal analysis provided in EPA manual SW-846.
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Since the concentration of the analyte element is considered to be proportional to the ground state atom population in the flame, any factor that affects the ground state population of the analyte element can be classified as an interference. Factors that may affect the ability of the instrument to read this parameter can also be classified as an interference. The following are the most common interferences:
A) Spectral interferences are due to radiation overlapping that of the light source. The interference radiation may be an emission line of another element or compound, or general background radiation from the flame, solvent, or analytical sample. This usually occurs when using organic solvents, but can also happen when determining sodium with magnesium present, iron with copper or iron with nickel.
B) Formation of compounds that do not dissociate in the flame. The most common example is the formation of calcium and strontium phosphates.
C) Ionization of the analyte reduces the signal. This is commonly happens to barium, calcium, strontium, sodium and potassium.
D) Matrix interferences due to differences between surface tension and viscosity of test solutions and standards.
E) Broadening of a spectral line, which can occur due to a number of factors. The most common linewidth broadening effects are:
1. Doppler effect
This effect arises because atoms will have different components of velocity along the line of observation.
2. Lorentz effect
This effect occurs as a result of the concentration of foreign atoms present in the environment of the emitting or absorbing atoms. The magnitude of the broadening varies with the pressure of the foreign gases and their physical properties.
3. Quenching effect
In a low-pressure spectral source, quenching collision can occur in flames as the result of the presence of foreign gas molecules with vibrational levels very close to the excited state of the resonance line.
4. Self absorption or self-reversal effect
The atoms of the same kind as that emitting radiation will absorb maximum radiation at the centre of the line than at the wings, resulting in the change of shape of the line as well as its intensity. This effect becomes serious if the vapour which is absorbing radiation is considerably cooler than that which is emitting radiation.
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Sampling & Monitoring Primer Table of Contents
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Copyright © 1997 Daniel Gallagher
Last Modified: 09-10-1997