FLAME ATOMIC ABSORPTION SPECTROMETRY

HISTORY
Aristophanes (423 B.C )- use of the lens
Euclid (300 B.C.) and Hero (100 B.C.) – studied mirrors
Seneca (40 A.D.) - observed the light scattering properties of prismS
Ptolemy (100 A.D.) - studied incidence and refraction
Alhazen (1038)- studied reflection and refraction of light
Roger Bacon (1250 ) - determined the focal points of concave mirros.
Galileo (1610) - made improvements on the telescope design
Sir Isaac Newton (1642-1727) – worked on the separation of light to obtain a spectrum
Fraunhofer (1814-15) - observed diffraction phenomena; measured wavelength instead of angles of refraction.
Herschel (1823) and Talbot (1825) - discovered atomic emission when certain atoms were placed in a flame.
Wheatstone (1835) – concluded that metals could be distinguished from one another on basis on the wavelengths of this emission
Foucault (1848) - observed atomic emission from sodium; discovered that the element would absorb the same rays from an electric arc.
Kirchoff (1800) - 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 - one of the first to apply this principle to the detection of mercury.
Walsh (1955) - 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.
After 1950’s - instrumentation and applications for atomic absorption greatly expanded


BASIC PRINCIPLE
The technique of flame atomic absorption spectroscopy 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. 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.


THE ATOMIC ABSORPTION INSTRUMENTATION HARDWARE
Flame atomic absorption hardware is divided into six fundamental groups that have two major functions: generate atomic signals and process signal. Signal processing is a growing additional feature to be integrated or externally fitted to the instrument.
Parts:
cathode lamp - is a stable light source necessary to emit the sharp characteristic spectrum of the element to be determined.
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.
monochromator – isolates the specific spectrum line emitted by the light source through spectral dispersion
photomultiplier detector - converts the light signal into an electrical signal.
signal amplifier – processes the electrical signal generated.
readout – displays the data.
data station – for data storage


ATOMIC ABSORPTION METHODS OTHER THAN FLAME
1. Electrothermal atomization 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.
2. Carbon rod analyzer 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.
3. Tantalum boat analyzer 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.


TECHNIQUES OF MEASUREMENT
1. 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.
2. Calibration and standard curves
As with other analytical techniques, atomic absorption spectrometry requires careful calibration. Several steps include: interference check sample, calibration verification, calibration standards, bland control, linear dynamic range

Idealized calibration or standard curve is stated by Beer's law that the absorbance of an absorbing analyte is proportional to its concentration. Deviations from linearity may occur due to unabsorbed radiation, stray light or disproportionate decomposition of molecules at high concentration.

INTERFERENCES
These are factors that affect the ground state population of the analyte element since the concentration of the analyte element is considered to be proportional to the ground state atom population in the flame.
• 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.
• Formation of compounds that do not dissociate in the flame. The most common example is the formation of calcium and strontium phosphates.
• Ionization of the analyte reduces the signal. This is commonly happens to barium, calcium, strontium, sodium and potassium.
• Matrix interferences due to differences between surface tension and viscosity of test solutions and standards.
• 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 arises because atoms will have different components of velocity along the line of observation.
2. Lorentz 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 occurs 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 occurs when atoms of the same kind as that emitting radiation 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.

MINIMIZING THE EFFECTS OF ERRORS
• 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.

Releasing Agents - used to control chemical interferences due to stable compounds formed in the desolvation process during the sample preparation; acts by forming a stable oxysalt with the interference ion and the analyte is released for the atomic process. Lanthanum or strontium are the most frequently used, although calcium, magnesium, and rare earth elements have been used.

Ionization suppression - added in order to prevent ionization. Commonly used at high concentration of another more easily ionized element. The elements used normally are the alkaline elements (potassium or cesium).

Method of Standard Addition
The major element chemical matrix of the standards and samples are matched. Matching is accomplished best by the method of standard additions, where a small spike of the standard is added to a split of the sample solution.

LEUCH, Kathryn K
LLORENTE, Cindy C
NATIVIDAD, James Thomas G