AAS - Graphite Furnace

Graphite furnace atomic absorption spectrometry (GFAAS) is also known by various other acronyms, including electrothermal atomic absorption spectrometry (ETAAS).

This technique is based on the fact that free atoms will absorb light at frequencies or wavelengths characteristic of the element of interest (hence the name atomic absorption spectrometry).

Within certain limits, the amount of light absorbed can be linearly correlated to the concentration of analyte present. Free atoms of most elements can be produced from samples by the application of high temperatures. In GFAAS, samples are deposited in a small graphite tube, which can then be heated to vaporize and atomize the analyte.

The first GFAAS systems were built 30 years ago, there is still room for improvement. An ideal graphite furnace should fulfill the following requirements:
A constant temperature in time and space during the interval in which free atoms are produced;
quantitative atom formation regardless of the sample composition;
separate control of the volatilization and atomization processes;
high sensitivity and good detection limits;
A minimum of spectral interferences. Developments in graphite furnace design made at Umeå University have been incorporated in many modern commercial instruments.

AAS- Analysis interference

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 effectThis effect arises because atoms will have different components of velocity along the line of observation.
2. Lorentz effectThis 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 effectIn 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 effectThe 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.

AAS- Measurement Principle

A cathode lamp 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 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 the sample passes through the flame, the beam of light passes through it into the monochromator. The monochromator isolates the specific spectrum line emitted by the light source through spectral dispersion, and focuses it upon a photomultiplier detector, whose function is to convert the light signal into an electrical signal.
The processing of electrical signal is fulfilled by a signal amplifier. The signal could be displayed for readout, or further fed into a data station for printout by the requested format.

AAS - Types of Flame

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.
Characteristics of different flames Source Reynolds et al., 1970.
----------------------------Max. flame speed (cm/s) ---------------Max. temp. (oC)
Air-Coal gas -----------------55------------------------------------1840
Air-propane -----------------82------------------------------------1925
Air-hydrogen----------------320-----------------------------------2050
Air-50% oxygen-acetylene---160-----------------------------------2300
Oxygen-nitrogen-acetylene--640-----------------------------------2815
Oxygen-acetylene------------1130----------------------------------3060
Oxygen-cyanogen------------140-----------------------------------4640
Nitrous oxide-acetylene------180-----------------------------------2955
Nitric oxide-acetylene---------90-----------------------------------3095
Nitrogen dioxyde-hydrogen---150----------------------------------2660
Nitrous oxide-hydrogen-------390----------------------------------2650

AAS - Basic Instrumentation

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.

AAS - Basic Principle

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 deg C.
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

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.

AAS - History of Spectroscopy

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, Angstr�m, 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.

AAS - Background

All atoms and their components have energy. The energy level at which an atom exists is referred to as its state. Under normal conditions, atoms exist in their most stable states. We refer to that most-stable level as the ground state. Al though we cannot measure the precise energy state for an atom, we can usually measure changes to its energy relative to its ground state.

Certain processes can change the energy state for an atom. For example, adding thermal energy (heat) can cause an atom to increase to a higher energy state. This change in energy is written as DE. We refer to energy states which are higher than the ground state as excited states. In theory, there are infinite excited states, however there are decreasing numbers of atoms from a population that reach higher excited states.
The laws of quantum mechanics tell us that atoms do not increase their energy levels gradually. An atom goes directly from one state to another without going through intermediates. We refer to these "quantum leaps" as transitions. The transition from the ground state (written as Eo) to the first excited state (E1) requires some form of energy input. This energy is absorbed by the atom. That energy absorption is equal to ^E0-1. When this energy absorption takes place in the presence of ultraviolet light, some of that light will be absorbed. This uv absorption occurs at a specific wavelength.
Each element in the periodic table will have a specific ^ E that will absorb a specific wavelength of uv light. The relationship between the energy transition and the wavelength (l) can be described by ^ E=h/ l .

where h is Planck's constant. Atomic absorption uses this relationship to determine the presence of a specific element based on absorption in a specific wavelength.

For example, calcium absorbs light with a wavelength of 422.7 nm. Iron absorbs light at 248.3 nm.

AAS - Introduction

Atomic absorption spectroscopy (AAS) determines the presence of metals in liquid samples.

Metals include Fe, Cu, Al, Pb, Ca, Zn, Cd and many more. It also measures the concentrations of metals in the samples. Typical concentrations range in the low mg/L range.
In their elemental form, metals will absorb ultraviolet light when they are excited by heat. Each metal has a characteristic wavelength that will be absorbed.

The AAS instrument looks for a particular metal by focusing a beam of uv light at a specific wavelength through a flame and into a detector.

The sample of interest is aspirated into the flame. If that metal is present in the sample, it will absorb some of the light, thus reducing its intensity.

The instrument measures the change in intensity. A computer data system converts the change in intensity into an absorbance. As concentration goes up, absorbance goes up.