Atomic absorption Spectroscopy

Atomic Absorption Spectroscopy

Elena Sevostianova

The study of absorption spectra by means of passing electromagnetic radiation through an atomic medium that is selectively absorbing; this produces pure electronic transitions free from vibrational and rotational transitions

(Academic Press Dictionary of Science and Technology)

1. Introduction

2. Instrumentation

3. Techniques of Measurement and EPA Methods Using FAAS

4. Operating instructions for Perkin-Elmer Spectrophotometer Model 460

5. Atomic Absorption Resources

1. Introduction.

Figure1. Elements detectable by atomic absorption are highlighted in pink in this periodic table

Atomic absorption methods measure the amount of energy (in the form of photons of light, and thus a change in the wavelength) absorbed by the sample. Specifically, a detector measures the wavelengths of light transmitted by the sample (the "after" wavelengths), and compares them to the wavelengths, which originally passed through the sample (the "before" wavelengths). A signal processor then integrates the changes in wavelength, which appear in the readout as peaks of energy absorption at discrete wavelengths (see schematic of an atomic-absorption experiment).

Any atom has its own distinct pattern of wavelengths at which it will absorb energy, due to the unique configuration of electrons in its outer shell. This allows for the qualitative analysis of a pure sample.

In order to tell how much of a known element is present in a sample, one must first establish a basis for comparison using known quantities. It can be done producing a calibration curve. For this process, a known wavelength is selected, and the detector will measure only the energy emitted at that wavelength. However, as the concentration of the target atom in the sample increases, absorption will also increase proportionally. Thus, one runs a series of known concentrations of some compound, and records the corresponding degree of absorbance, which is an inverse percentage of light transmitted. A straight line can then be drawn between all of the known points. From this line, one can then extrapolate the concentration of the substance under investigation from its absorbance. The use of special light sources and specific wavelength selection allows the quantitative determination of individual components of a multielement mixture.

The phenomenon of atomic absorption (AA) was first observed in 1802 with the discovery of the Fraunhofer lines in the sun's spectrum. It was not until 1953 that Australian physicist Sir Alan Walsh demonstrated that atomic absorption could be used as a quantitative analtical tool. Atomic absorption analysis involves measuring the absorption of light by vaporized ground state atoms and relating the absorption to concentration. The incident light beam is attenuated by atomic vapor absorption according to Beer's law.

The process of atomic absorption spectroscopy (AAS) involves two steps:

1. Atomization of the sample

2. The absorption of radiation from a light source by the free atoms

The sample, either a liquid or a solid, is atomized in either a flame or a graphite furnace. Upon the absorption of ultraviolet or visible light, the free atoms undergo electronic transitions from the ground state to excited electronic states.

To obtain the best results in AA, the instrumental and chemical parameters of the system must be geared toward the production of neutral ground state atoms of the element of interest. A common method is to introduce a liquid sample into a flame. Upon introduction, the sample solution is dispersed into a fine spray, the spray is then desolvated into salt particles in the flame and the particles are subsequently vaporized into neutral atoms, ionic species and molecular species. All of these conversion processes occur in geometrically definable regions in the flame. It is therefore important to set the instrument parameters such that the light from the source (typically a hollow-cathode lamp) is directed through the region of the flame that contains the maximum number of neutral atoms. The light produced by the hollow-cathode lamp is emitted from excited atoms of the same element which is to be determined. Therefore the radiant energy corresponds directly to the wavelength which is absorbable by the atomized sample. This method provides both sensitivity and selectivity since other elements in the sample will not generally absorb the chosen wavelength and thus, will not interfere with the measurement. To reduce background interference, the wavelength of interest is isolated by a monochromator placed between the sample and the detector.

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2. Instrumentation

Figure 2. Perkin-Elmer Spectrophotometer Model 460

In atomic absorption (see schematic of an atomic-absorption experiment), there are two methods of adding thermal energy to a sample. A graphite furnace AAS uses a graphite tube with a strong electric current to heat the sample. In flame AAS (see photo above), we aspirate a sample into a flame using a nebulizer. The flame is lined up in a beam of light of the appropriate wavelength. The flame (thermal energy) causes the atom to undergo a transition from the ground state to the first excited state. When the atoms make their transition, they absorb some of the light from the beam. The more concentrated the solution, the more light energy is absorbed!

The light beam is generated by lamp that is specific for a target metal. The lamp must be perfectly aligned so the beam crosses the hottest part of the flame. The light passed through the flame is received by the monochromator, which is set to accept and transmit radiation at the specified wavelength and travels into the detector. The detector measures the intensity of the beam of light. When some of the light is absorbed by metal, the beam's intensity is reduced. The detector records that reduction as absorption. That absorption is shown on output device by the data system.

We can find the concentrations of metals in a sample running a series of calibration standards through the instrument. The instrument will record the absorption generated by a given concentration. By plotting the absorption versus the concentrations of the standards, a calibration curve can be plotted. We can then look at the absorption for a sample solution and use the calibration curves to determine the concentration in that

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3. Techniques of Measurement and EPA Methods Using FAAS

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 1 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".

Table 1 EPA sample processing method for metallic element analysis
Analysis Target	Method Number	Environmental Matrice
total recoverable metals	3005	ground water/surface water
dissolved metals	3005	ground water/surface water
suspended metals	3005	ground water/surface water
total metals	3010	aqueous samples, wastes that contain suspended solids and mobility-procedure extracts
total metals	3015	aqueous samples, wastes that contain suspended solids and mobility-procedure extracts
total metals	3020	aqueous samples, wastes that contain suspended solids and mobility-procedure extracts
total metals	3050	sediments, sludges and soil samples
total metals	3051	sludges, sediment, soil and oil

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 3 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.

Figure 3. Idealized/deviation response curve

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 2 lists the available method for different metal analysis provided in EPA manual SW-846.

Table 2. EPA methods for determination of metals by direct aspiration
Analyte	Method number	Analyte	Method number	Analyte	Method number
aluminum	7020	antimony	7040	barium	7080A
beryllium	7090	cadmium	7130	calcium	7140
chromium	7190	cobalt	7200	copper	7210
iron	7380	lead	7420	lithium	7430
magnesium	7450	manganese	7460	molybdenum	7480
nickel	7520	osmium	7550	potassium	7610
silver	7760A	sodium	7770	strontium	7780
thallium	7840	tin	7870	vanadium	7910
zinc	7951

D) Interferences

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 interference. Factors that may affect the ability of the instrument to read this parameter can also be classified as 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 line width 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 vibration 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 center 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 vapor, which is absorbing radiation is considerably cooler than that which is emitting radiation.

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4. Operating instructions for Perkin-Elmer Spectrophotometer Model 460

Lamp installment

Gas ignition

Burner alignment

Calibration

5. Atomic Absorption Resources.

EPA document SW-846 "Test methods for evaluating solid wastes".

Textbooks:

Haswell, S.J., 1991. Atomic Absorption Spectrometry; Theory, Design and Applications. Elsevier, Amsterdam.

Reynolds, R.J. et al., 1970. Atomic Absorption Spectroscopy. Barnes & Noble Inc., New York.

Schrenk, W.G., 1975. Analytical Atomic Spectroscopy. Plenum Press, New York.