E = hν = hc/ λ

The interaction of radiation with matter is produced throughout the electromagnetic spectrum which ranges from cosmic rays with wavelengths of 10-9 nm to radio waves with lengths over 1000 Km. Between both extremes, and from the shortest upwards, can be found the following: gamma rays, X-rays, ultraviolet rays (far, mid and near), the visible portion of the spectrum, infrared rays and radio microwaves. All radiations are of the same nature and travel at the speed of light, being differentiated by the frequency, wavelength and the effects they produce on matter (Skoog et al., 2008).

The Bouguer-Lambert-Beer Law is fundamental in molecular absorption spectrophotometry. According to this law, absorbance is directly proportional to the trajectory of the radiation through the solution and to the concentration of the sample producing the absorption although there are limitations to its application. The Law is only applied to monochromatic radiation although it has been demonstrated experimentally that the deviations with polychromatic light are unappreciable (Skoog & West, 1984).

Atomic Absorption Spectroscopy: Fundamentals and Applications in Medicine 5

In atomic absorption spectrometry, no ordinary monochromator can give such a narrow band of radiation as the width of the peak of the line of atomic absorption. In these conditions the Beer Law is not followed and the sensitivity of the method is reduced. Walsh demonstrated that a hollow-cathode, made of the same material as the analyte, emits narrower lines than the corresponding lines of atomic absorption of the atoms of the analyte in flame, this being the base of the instruments of atomic absorption. The main disadvantage is the need for a different lamp source for each element to be analysed, but no alternative to

The energy source most frequently used in atomic absorption spectroscopy is the hollow-

The field of atomic absorption spectroscopy (AAS) includes: flame (FAAS) and electrothermal (EAAS or ETAAS) atomic absorption spectroscopy (Skoog et al., 2008). The base is the same in both cases: the energy put into the free atoms of the analyte makes its electrons change from their fundamental state to the excited state, the resulting absorbed radiation being detected. However the fundamental characteristic of the FAAS is the stage of atomization which is performed in the flame and which converts the analyte into free atoms, whereas in the EAAS the stage of atomization goes through successive phases of drying, calcination and carbonization, and it is not required to dissolve the sample in the

Prior steps to the stage of atomization in flame are the treatment of the sample, dissolving it in a convenient matrix, and the stage of pneumatic nebulisation. In FAAS the stage of atomization is performed in flame. The temperature of the flame is determined by the fuel/oxidant coefficient. The optimum temperatures depend on the excitation and

The concentration of excited and non-excited atoms in the flame is determined by the fuel/oxidant coefficient and varies in the different regions of the flame (Willard et al., 1991).

The electrothermal atomic absorption spectrophotometer has three part: the atomizing head, the power source and the controls for feeding in the inert gas. The atomizing head replaces the nebulising-burning part of the FAAS. The power source supplies the work current at the correct voltage of the atomizing head. The computer control of the atomizing chamber ensures reproducibility in the heating conditions, establishing a suitable profile of temperatures in the heating scale from environmental temperature to that of atomization so that the successive stages of drying, calcination and carbonization the sample must go through are those required. The working temperature and the duration of each stage of the electrothermal process must be carefully selected taking into account the nature of the analyte and the composition of the matrix of the sample. The control unit which measures and controls the flow of an inert gas within the atomizing head is designed to avoid the

destruction of the graphite at high temperatures due to oxidation with the air.

convenient matrix as occurs with the FAAS (Skoog et al., 2008; Vercruysse, 1984).

**1.2.1 Flame atomic absorption spectroscopy** 

**1.2.2 Electrothermal atomic absorption spectroscopy** 

ionization potentials of the analyte.

this procedure improves the results obtained with individual lamps.

cathode lamp.

**1.2 Types** 

According to the Bouguer-Lambert-Beer Law:

A = abc

when b (trajectory of the radiation) is expressed in cm and c (concentration of the substance) in g.L-1, the units of a (absorptivity) are L.g-1.cm-1, or

A = εbc

when b is expressed in cm and c in mol.L-1, a (absorptivity) is called molar absorptivity and it is represented by the symbol ε and its units are L.mol-1.cm-1

The absorption of light (A) = log Po/P ( = Optical density or extinction)

where:

Po: Incident radiation P: Transmitted radiation Absorptivity, a, is A/bc (= Coefficient of extinction) Molar absorptivity, ε, is A/bc (= Coefficient of molar extinction)

The Bouger-Lambert-Beer Law is fulfilled with limitations in molecular absorption spectrophotometry (Skoog et al., 2008).

In 1927, Werner Heisenberg proposed the principle of uncertainty, which has important and widespread implications for instrumental analysis. It is deduced from the principle of superposition, which establishes that, when two or more waves cross the same region of space, a displacement is produced equal to the sum of the displacements caused by the individual waves. This is applied to electromagnetic waves in which the displacements are the consequence of an electric field, as well as to various other types of waves in which atoms or molecules are displaced. The equation ∆t x ∆E = h, expresses the uncertainty principle, signifying that, for finite periods, the measurement of the energy of a particle or system of particles (photons, electrons, neutrons, protons) will never be more precise than h/∆t, in which h is the Planck´s constant. For this reason, the energy of a particle may be known as a zero uncertainty only if it is observed for an infinite period (Skoog et al., 2001).

In 1953, the Australian Physicist Alan Walsh laid the foundations and demonstrated that atomic absorption spectrophotometry could be used as a procedure of analysis in the laboratory (Willard et al., 1991). The theoretical background on which most of the work in this field was based is due almost entirely to this author (Elwell & Gidley, 1966).
