When a sample is introduced into a flame, only a small fraction of the atoms are in an excited or high energy state [as per the Boltzmann equation] and most atoms of the element remain in the unexcited or ground state and are capable of absorbing energy. If light energy is supplied as a continuous spectrum, ground-state atoms will selectively absorb wavelengths of light that correspond with the energy intervals needed to excite electrons from low energy to higher energy levels. The wavelengths of light that are absorbed as electrons are excited from ground state to higher energy levels are analogous to the wavelengths of light emitted as high energy electrons return to the ground state. The key advantage of atomic absorption spectrometry is the most of the atoms in a flame remain in that unexcited ground state and are capable of light absorption, allowing this technique to be ~100-fold more sensitive than flame emission spectrometry.
Related: How Flame Photometers are used in Chemical Analysis
The absorbance of light, A, is defined as the logarithm of the ratio of initial light beam intensity I0 to the intensity of the beam after light absorption, I.
The absorbance of light is directly proportional to the concentration of absorbing atoms c and the path length of the light beam in the atoms d. The value k is a constant referred to as the absorptivity constant.
Components of a Flame Atomic Absorption Spectrometer
The components of a flame atomic absorption spectrometer are as shown in the figure below:
As shown in the figure above, flame atomic absorption spectrometer consist of a nebulizer, premix burner, hollow cathode lamp, modulation system or beam chopper, flame, monochromator, detector and readout system. Monochromator light is obtained by isolating a single spectral line emitted from a hollow cathode lamp using an interference filter. The detector is a photomultiplier tube and the results can be displayed by a visual meter, digital display or captured by a computer system. The flame serves as a sample compartment and light from the hollow cathode lamp is passed through the flame to measure absorption. Electronic modulation or a mechanical beam chopper is used to interrupt the light from the hollow cathode lamp into pulses so the extraneous light emitted from the flame alone can be measured and subtracted prior to measuring atomic absorption.
The electrothermal atomic absorption instruments usually use a graphite furnace to rapidly heat a sample in a uniform manner to approximately 2600 °C. Samples are normally introduced into the furnace on a carbon rod. As the temperature is raised, the samples are dried, charred, rendered to ash and atomized. Hollow cathode lamps are used as a source of monochromatic light to determine atomic absorption. In contrast to flame-based techniques that rapidly dilute and disperse analyte atoms, electrothermal methods retain atoms at higher concentration with longer residence time in the light beam. These advantages allow electrothermal atomic absorption instruments to measure many trace elements and heavy metals not capable of measurement with flame atomic absorption measurements.
Also read: How Spectrofluorimeters are used in Biomedical Measurements
Medical Applications of Flame Atomic Absorption Spectrometers
Both flame atomic emission spectrometry and flame atomic absorption spectrometry are sophisticated techniques for elemental analyses that are robust and flexible; that have been applied to analysis of a number of different types of biological or clinical samples.
While the use of atomic absorption spectrometers in clinical laboratories for the determination of calcium, magnesium, and other metal cations has largely been replaced by potentiometric techniques, it has remained an important platform for the determination of serum zinc, copper and lead.
Though inductively coupled plasma mass spectrometry provides greater sensitivity and flexibility of analysis, flame and electrothermal atomic absorption spectroscopic techniques remain cost-effective, precise analytic methods, widely employed in clinical reference laboratories to support the evaluation of serum or blood concentrations of zinc, copper, aluminium, chromium, cadmium, mercury and lead.
Related: Principle of Operation of a Medical Mass Spectrometer
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