Atomic absorption spectroscopy From Bpharm IIIrd yr students

Each element emits and absorbs its own unique wavelength

Atomic absorption spectroscopy (AAS) 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. 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

When a solution containing metallic species in introduced into a flame, the vp of the metallic species is obtained. Desirable in AAS Energy Aq sample molecules atoms Excited atoms* Desirable in AES

In AAS the amount of light absorbed in determined When light of a particular wavelength is allowed to pass through the flame (having atoms of sample), Part of the light will be absorbed . The absorption will be proportional to the density of the atoms ion the flame. In AAS the amount of light absorbed in determined

Grotrian diagram AAS for sodium

Spectra

Features of AAS spectra The spectrum consists of a small number of discrete absorption lines corresponding to transitions between the ground state (the 3s atomic orbital) and the 3p and 4p atomic orbitals. Absorption from excited states, such as the 3p → 4s and the 3p → 3d transitions included in Figure are too weak to detect. Because an excited state's lifetime is short

Instrumentation

Atomic absorption spectroscopy is based on the same principle as the flame test used in qualitative analysis.

Light Source Hollow-cathode lamp (HCL): The cathode contains the element that is is to be analysed.

Hollow cathode sourse lamp for Na Hollow-cathode lamps are a type of discharge lamp that produce narrow emission from atomic species. They get their name from the cup-shaped cathode, which is made from the element of interest. The electric discharge ionizes rare gas atoms, which are accelerated into the cathode and sputter metal atoms into the gas phase. Collisions with gas atoms or electrons excite the metal atoms to higher energy levels, which decay to lower levels by emitting light. Windows -- Two window materials, quartz and borosilicate glass, are available on standard types. Special window materials have been developed, such as MgF2, and LiF. Gas Fills -- Two gas fills, argon and neon, are available on standard types. In general, neon will give the highest spectral output and lamps containing it are recommended

Electrodeless discharge lamp The internal electrodeless lamp or induction light is a gas discharge lamp in which the power required to generate light is transferred from outside the lamp envelope to the gas inside via an electric or magnetic field, in contrast with a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope. There are two advantages to elimination of the internal electrodes: Extended lamp life higher efficiency Used for volatile elements such as As, Cd, Pb

2. Chopper (source modulation correction ) In case of emission measurements, there will always be a positive error since emission from flame is an additive value to the actual sample emission. It is therefore obvious that we should get rid of this interference from emission in flames. Excluding the emission signal from flames can easily be done by an addition of a chopper to the instrumental design. The chopper is a motor driven device that has open and solid (mirrors in some cases) The function of the chopper is to chop the light leaving the source so that when the incident beam hits the chopper at the solid surface, the beam will be blocked and detector will only read the emitted signal from the flame. As the chopper rotates and the beam emerges to the detector, the detector signal will be the sum of the transmitted signal plus that emitted from the flame. The signal processor will be able to subtract the first signal from the second one, thus excluding the signal from emission in flames.

Single beam AAS Light from the source is modulated electronically or chopped mechanically by rotating chopper. This helps isolate and remove sample cell emissions from light emitted by the source. The specific wavelength isolated by monochromator is led to the detector and the electrical signal generated is proportional to the elemental concentration in the sample

Double beam AAS Light beam from source is split into two beams by the chopper. One beam passes directly through the flame and the other beam passes round the flame. Detector response represents the ratio of sample and reference beams. Fluctuations in light intensity are eliminated electronically to get greater reliability of results.

Sample introduction: Atomizers1) FLAME ATOMIZEER 2) NON-FLAME ATOMIZER

Burner system Burner system The burner assembly comprises of nebulizer to reduce the liquid sample to a fine aerosol, a spray chamber and a burner head which is used to generate a flame to produce atoms of the same elements that are present in the sample. TOTAL CONSUMPTION BURNER PREMIXED BURNER TOTAL CONSUMPTION BURNER

PREMIXED BURNER

Pre-mixed burner The sample is nebulized and mixed with the fuel and oxidant prior to introduction into the flame, with the use of a series of baffles. the sample is drawn from the sample container via the vacuum created by the rushing fuel and oxidant (aspiration). A drain line is required in this design in order to remove sample solution droplets that do not make it all the way to the flame. The fuel, oxidant and sample all meet at the base of the flame and is premixed prior to entering the flame. 

Graphite furnace atomizer With the graphite atomizer, a discreet volume of sample solution is vaporized and wastage is virtually eliminated.

Application Biological samples can include both human tissue samples and food samples. In human tissue samples, AAS can be used to determine the amount of various levels of metals and other electrolytes, within tissue samples. These tissue samples can be many things including but not limited to blood, bone marrow, urine, hair, and nails. Some examples of trace elements that samples are analyzed for are arsenic, mercury, and lead.

An example of an application of AAS to human tissue is the measurement of the electrolytes sodium and potassium in plasma. This measurement is important because the values can be indicative of various diseases when outside of the normal range In the food industry, AAS provides analysis of vegetables, animal products, and animal feeds. Samples are also analyzed to determine heavy metals which can be detrimental to consumers.

Environmental and marine analysis typically refers to water analysis of various types. Water analysis includes many things ranging from drinking water to waste water to sea water. water analysis is an analysis of leaching of lead and zinc from tin-lead solder into water. analysis of lake and river sediment for lead and cadmium.