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 Atomic Absorption Spectroscopy: Instrumental

Atomic identification and quantification





Atomic spectroscopy is the determination of elemental composition by its electromagnetic or mass spectrum. Atomic spectroscopy is closely related to other forms of spectroscopy. It can be divided by atomization source or by the type of spectroscopy used. The basic principle is that light is passed through a collection of atoms. If the wavelength of the light has energy corresponding to the energy difference between two energy levels in the atoms, a portion of the light will be absorbed. The relationship between the concentration of atoms, the distance the light travels through the collection of atoms, and the portion of the light absorbed is given by the Beer-Lambert law.


 

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Origins of Atomic Spectroscopy

Spectroscopy of atoms or ions do not involve vibrations or rotation transitions. Transition involves promoting an electron from a ground state to a higher empty atomic state orbital, this state is referred to as the excited state. 

Shown to the right is the three sodium absorption and emission process and the emission lines. Atomic p-orbitals are in fact split into two energy levels for the multiple spins of the electron. The energy level is so small however that a single line observed. A high resolution would show the line as a doublet.


 

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Resolution (Width) Lines Spectra

Atomic spectral lines have finite widths with factors to line broadening due to:
• Natural Broadening - The lifetime of the excited states lead to uncertainty leading to broadening due to shorter excited state lifetimes. Lifetimes of 10-8 s lead to width of 10-5 nm.

• Collisional Broadening -Also referred to as Pressure Broadening is the result of collision of the excited state leads to shorter lifetimes and broadening of the spectral lines.

• Doppler Broadening - When molecules are moving towards a detector or away from a detector the frequency will be offset by the net speed the radiation hits the detector. This is also known as the Doppler effect and the true frequency will ether be red shifted (if the chemical is moving away from the detector) or blue shifted (if the chemical is moving towards the detector)


 

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Instrumentation
 
Atomic spectroscopy begins with atomizing the sample.

• Sample introduction - Atomizer devices are either continuous or discrete. Continuous are in the form of plasmas and flame. Discrete are in the form of electrothermal. Nebulizers are the method to introduce samples into the atomizer. Direct nebulizers creates fine droplets by aerosol.

• Shown is the continuous sample method. Samples are frequently introduced into plasmas or flame by means of nebulizer which takes the sample and convert it to a spray or mist.


 

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Plasma Source

• Plasma is the phase of matter with its electrons stripped. In argon plasma, argon ions and electrons act as the conducting species. Three power sources are dc-electric, radio and microwave frequency generators. The most advantageous is the radio or inductively coupled plasma (ICP) because of sensitivity and minimal interference. DC plasma source (DCP) are also advantageous and is also simple and less expensive.

•Inductive Coupled Plasma consist of three concentric quartz tubes in which streams of argon flow. Ionization of the argon is initiated by a spark from a Tesla coil.

 

The geometries of CP source, in radial geometry or axial geometry.


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Flame Atomizer

• Flame atomizers contains a pneumatic nebulizer, which converts the sample solution into a mists or aerosol. Shown is a diagram of a three electrode dc plasma jet. Two separate dc plasmas have a single common cathode. The overall plasma burns n the form of an inverted Y. Samples are introduced as aerosol from the are between the two graphite anodes. Observation of emission in the region beneath the strongly emitting plasma core avoids much of the plasma background emission.

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When a nebulized sample is carried into a flame, desolvation of the droplets occurs in the primary combustion zone, located in the tip of the burner. The fine solid particles are carried to a region in the center of the the flame called the inner core.



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Electrothermal Atomizer

Electrothermal atomizer deposit a few microliters of sample in the furnace with a syringe or an autosampler. This is followed by drying ashing and atomization steps that is carried out by instrument programming. .

There are other type of atomizers devices. Examples are the gas discharge which results in glow discharge. Early atomizers include dc and ac arcs which have been replaced almost entirely by ICP.

 

 

Shown is the cross-sectional view of a graphite furnace atomizer. The L’vov platform and its position in the graphite furnace.y ICP.



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Atomic Emission Spectroscopy

Atomic emission spectroscopy is widely used in elemental analysis.
Shown is the block diagram of a typical ICP atomic emission spectrometer.



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Atomic Absorption Spectroscopy

Flame atomic absorption spectroscopy (AAS) is the most used of atomic methods.
Block diagram of a single-beam atomic absorption spectrometer. Radiation from a line source is focused on the atomic vapor in aflame or an electrothermal atomizer. The attenuated source radiation then enters a monochromator, which isolates the line of interest. Next the radiant power from the source, attenuated by absorption, is measured by the photomultiplier tube (PMT). The signal is then processed and directed to a computer system for output.