الطيف الذري

Atomic spectroscopy is the determination of elemental

composition by its electromagnetic or mass spectrum. The

study of the electromagnetic spectrum of elements is called

Optical Atomic Spectroscopy.

Electrons exist in energy levels within an atom. These levels

have well defined energies, and electrons moving between

them must absorb or emit energy equal to the difference

between levels. In optical spectroscopy, the energy absorbed to

move an electron to a more energetic level and/or the energy

emitted as the electron moves to a less energetic energy level

is in the form of a photon.

The wavelength of the emitted radiant energy is directly related

to the electronic transition which has occurred. Since every

element has a unique electronic structure, the wavelength of

light emitted is a unique property of each individual element. As

the orbital configuration of a large atom may be complex, there

are many electronic transitions which can occur, each transition

resulting in the emission of a characteristic wavelength of light,

as illustrated in Figure 1.

The science of atomic spectroscopy has yielded three

techniques for analytical use:

The processes of excitation and decay impunges on all three

techniques of atomic spectroscopy. Either the energy absorbed

in the excitation process, or the energy emitted in the decay

process is measured and used for analytical purposes.

If light of just the right wavelength impinges on a free, ground state

atom, the atom may absorb the light as it enters an excited state in

a process known as atomic absorption. This process is illustrated

in Figure 2.

Atomic absorption measures the amount of light at the resonant

wavelength which is absorbed as it passes through a cloud of

atoms. As the number of atoms in the light path increases, the

amount of light absorbed increases in a predictable way. By

measuring the amount of light absorbed, a quantitative

determination of the amount of analyte element present can be

made. The use of special light sources and careful selection of

wavelength allow the specific quantitative determination of

individual elements in the presence of others.

The atom cloud required for atomic absorption measurements is

produced by supplying enough thermal energy to the sample to

dissociate the chemical compounds into free atoms. Aspirating a

solution of the sample into a flame aligned in the light beam

serves this purpose. Under the proper flame conditions, most of

the atoms will remain in the ground state form and are capable of

absorbing light at the analytical wavelength from a source lamp.

The ease and speed at which precise and accurate determinations

can be made with this technique have made atomic absorption

one of the most popular methods for the determination of metals.

Figure 1 - Energy Transitions

Figure 2 - The atomic absorption process

Figure 3 - ICCD Quantum Efficiency relevant to Atomic

spectroscopy

C h e m i s t r y

In atomic emission, a sample is subjected to a high energy,

thermal environment in order to produce excited-state atoms,

capable of emitting light. The energy source can be an

electrical arc, a flame, or more recently, a plasma.

The emission spectrum of an element exposed to such an

energy source, consists of a collection of the allowable

emission wavelengths, commonly called emission lines,

because of the discrete nature of the emitted wavelengths. This

emission spectrum can be used as a unique characteristic for

qualitative identification of the element. Atomic emission using

electrical arcs has been widely used for qualitative analysis.

Emission techniques can also be used to determine how much

of an element is present in a sample. For a "quantitative"

analysis, the intensity of light emitted at the wavelength of the

element to be determined is measured. The emission intensity

at this wavelength will be greater as the number of atoms of

the analyte element increases. The technique of flame

photometry is an application of atomic emission for quantitative

analysis.

The third field of atomic spectroscopy is atomic fluorescence.

This technique incorporates aspects of both atomic absorption

and atomic emission. Like atomic absorption, ground state

atoms created in a flame are excited by focusing a beam of

light into the atomic vapor. Instead of looking at the amount of

light absorbed in the process, however, the emission resulting

from the decay of the atoms excited by the source light is

measured. The intensity of this "fluorescence" increases with

increasing atom concentration, providing the basis for

quantitative determination.

The source lamp for atomic fluorescence is mounted at an

angle to the rest of the optical system, so that the light detector

sees only the fluorescence in the flame and not the light from

the lamp itself. It is advantageous to maximize lamp intensity,

since sensitivity is directly related to the number of excited

atoms which inturn is a function of the intensity of the exciting

radiation.

While atomic absorption is the most widely applied of the three

techniques and usually offers several advantages over the other

two, particular benefits may be gained with either emission or

fluorescence in special analytical situations.

Considering the needs of atomic spectroscopy, Andor has

designed its line of products to fully cater to the needs of this

industry. Traditionally this application has used a combination of

a monochromator and a PMT, however the Andor solution

provides consists of a high throughput (F/4 aperture ratio)

imaging spectrograph (Shamrock SR-303i) coupled to an ICCD

camera (iStar).

The iStar comes with a variety of photocathode tubes which

can be selected depending on the range of wavelength desired

to be detected. These cameras have a single photon sensitivity

as found in PMTs and can be gated down to 2ns gate widths.

The advantage of using this combination of an imaging

spectrograph and ICCD detector, over the monochromator and

PMT, is that a number of wavelengths can be detected in one

shot, this drastically reduces the experiment time while

simultaneously provides for high spatial and spectral resolution.

Single photon sensitivity is offered by the gain multiplication

ability of the intensifier tube.

Figure 4 - How the three techniques are implemented.

SR-303i spectrograph fitted with iStar ICCD camera