ATOMIC ABSORPTION SPECTROSCOPY (AAS)

 

ATOMIC ABSORPTION SPECTROSCOPY (AAS) 

Atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES) is a spectroanalytical procedure for the quantitative determination of chemical elements using the absorption of optical radiation (light) by free atoms in the gaseous state. Atomic absorption spectroscopy is based on absorption of light by free metallic ions.

In analytical chemistry the technique is used for determining the concentration of a particular element (the analyte) in a sample to be analyzed. AAS can be used to determine over 70 different elements in solution, or directly in solid samples via electrothermal    vaporization, and is used in pharmacology, biophysics, archaeology and toxicology research.

Atomic emission spectroscopy was first used as an analytical technique, and the underlying principles were established in the second half of the 19th century by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff, both professors at the University of Heidelberg, Germany.

The modern form of AAS was largely developed during the 1950s by a team of Australian chemists. They were led by Sir Alan Walsh at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Division of Chemical Physics, in Melbourne, Australia.

Atomic absorption spectrometry has many uses in different areas of chemistry such as clinical analysis of metals in biological fluids and tissues such as whole blood, plasma, urine, saliva, brain tissue, liver, hair, muscle tissue, semen, in some pharmaceutical manufacturing processes, minute quantities of a catalyst that remain in the final drug product, and analyzing water for its metal content.

In order to analyze a sample for its atomic constituents, it has to be atomized. The atomizers most commonly used nowadays are flames and electrothermal (graphite tube) atomizers. The atoms should then be irradiated by optical radiation, and the radiation source could be an element-specific line radiation source or a continuum radiation source.

 

 

 

SCHEMATIC FLOW OF AAS MEASUREMENT

 

 

The atomizers most commonly used nowadays are (spectroscopic)

i. Flames

ii. electrothermal (graphite tube) atomizers. Other atomizers, such as glow-discharge atomization, hydride atomization, or cold-vapor atomization might be used for special purposes.

•      Flame atomizers

The oldest and most commonly used atomizers in AAS are flames, principally the air-acetylene flame with a temperature of about 2300 °C and the nitrous oxide system (N₂O)-acetylene flame with a temperature of about 2700 °C. The latter flame, in addition, offers a more reducing environment, being ideally suited for analytes with high affinity to oxygen.

•      Electrothermal atomizers/GFAA method development

Electrothermal AAS (ET AAS) using graphite tube atomizers was pioneered by Boris V. L’vov at the Saint Petersburg Polytechnical Institute, Russia, since the late 1950s, and investigated in parallel by Hans Massmann at the Institute of Spectrochemistry and Applied Spectroscopy (ISAS) in Dortmund, Germany

With this technique liquid/dissolved, solid and gaseous samples may be analyzed directly. A measured volume (typically 10–50 μL) or a weighed mass (typically around 1 mg) of a solid sample are introduced into the graphite tube and subject to a temperature program. This typically consists of stages, such as

i. Drying – the solvent is evaporated

ii. Pyrolysis – the majority of the matrix constituents are removed;

iii. Atomization – the analyte element is released to the gaseous phase; and

iv. Cleaning – eventual residues in the graphite tube are removed at high temperature.

•      The graphite tubes are heated via their ohmic resistance using a low-voltage high-current power supply;

iii. Cold-vapor atomization

•      The cold-vapor technique is an atomization method limited to only the determination of mercury, due to it being the only metallic element to have a large enough vapor pressure at ambient temperature. Because of this, it has an important use in determining organic mercury compounds in samples and their distribution in the environment.

 

OTHER INSTRUMENTS MEASURE HEAVY METALS.

Gas chromatography–mass spectrometry (GC-MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation, and identification of unknown samples, including that of material samples obtained from planet Mars during probe missions as early as the 1970s.

GC-MS can also be used in airport security to detect substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification. Like liquid chromatography–mass spectrometry, it allows analysis and detection even of tiny amounts of a substance.

 

High-performance liquid chromatography (HPLC) is an analytical technique to separate, identify, and quantify components in a mixture. It is the single biggest chromatography technique essential to most laboratories worldwide.

•      How Does HPLC Work?

In column chromatography a solvent drips through a column filled with an adsorbent under gravity. HPLC is a highly improved form of column chromatography. A pump forces a solvent through a column under high pressures of up to 400 atmospheres. The column packing material or adsorbent or stationary phase is typically a granular material made of solid particles such as silica or polymers.

The pressure makes the technique much faster compared to column chromatography. This allows using much smaller particles for the column packing material. The smaller particles have a much greater surface area for interactions between the stationary phase and the molecules flowing past it. This results in a much better separation of the components of the mixture.

 

Installation requirements

The condition for installation can be categorized into the following areas for laboratory Environment.

       i.            Suitability

     ii.             Environmental conditions

  iii.             Noise levels

  iv.            Suitability

Equipment class

 An instrument is designed for indoor use only and is classified suitable under Equipment Class category Example Agilent is I.

Installation category

The installation category implies the regulation for impulse withstand voltage. It is also called the ‘Over voltage category’. ‘II’ applies to electrical equipment with a nominal supply voltage up to 300 V. But in Tanzania normal voltage supply is 240V

Pollution level

Pollution level describes the degree to which a solid, liquid, or gas that deteriorates dielectric strength is adhering. ‘2’ applies to a normal indoor atmosphere, where only nonconductive pollution occurs.

Environmental conditions

Operating your instrument within the recommended cleanliness and temperature and humidity ranges ensures optimum instrument performance and lifetime.

Cleanliness

The area selected for operation of an AAS system must be free from drafts, corrosive atmospheres and vibration. Sample preparation areas and materials storage facilities should be located in a separate room. The area should have a dust-free, low-humidity atmosphere. Air conditioning is strongly recommended for control of the environment. The instrument should not be located near a window, door or any other area where drafts may cause unstable thermal conditions.

Noise levels

The sound pressure level (SPL) of a flame AA in a ‘normal’ laboratory environment (≈60 dBA) ambient noise measured at normal operator position is ≈65 dBA. At a distance of 1 meter from the instrument, the SPL is reduced to ≈62 dBA. The likely maximum SPL at a customer location will be greatly influenced by the extraction system.

 

For laboratory facilities

       i.            Exhaust system

     ii.            Electrical power supplies

  iii.             Water cooling system

  iv.            Waste fluids

     v.            i. Exhaust system

  vi.            Exhaust system Heat, vapors and fumes generated by flame, furnace and vapor generation methods can be toxic or corrosive and may be hazardous to personnel and must be extracted from the instrument by an exhaust system.

vii.            ii. Electrical power supplies

viii.            The installation of electrical power supplies must comply with the rules and/or regulations imposed by the local authorities responsible for the use of electrical energy in the workplace. Many AASs are supplied with a 2-meter (6 ft, 6 in) mains power cord terminated . All power supplies should be: Single phase AC, 3-wire system (active, neutral and ground, or two actives and ground) Terminated at an appropriate connection receptacle that is within reach of the system power cable assembly. In areas where 208/220/240 volt supplies are not normally available in a single phase configuration, supplies may be taken from two phases and ground of a three phase system.

 

Other power requirements

I.                   A separate connection receptacle should be provided for each unit in the system. Do not use double adapters or extension cords. For Furnace instruments, a separate mains circuit individually protected by fuses or circuit breakers must be used for the GTA accessory. It is preferable for the GTA and the instrument to share the same phase.

II.                iii. Water cooling system

III.             The Graphite Tube Atomizer must have a supply of cooling water to remove heat from the furnace workhead.

IV.            iv. Waste fluids

V.               The AA flame instrument atomizes only a small percentage of the sample taken up. The excess liquids from the spray chamber must be drained into a waste vessel. Suitable tubing is supplied with the spectrometer for use with inorganic solvents. If you use organic solvents, you will need different tubing, suitable for the solvent(s) of choice. The AA instrument also needs a drain or a sump for disposal of waste liquid during rinse cycles when flame or furnace autosamplers are used.