Atomic Absorption Spectroscopy

Introduction

Atomic absorption spectroscopy (AAS) determines the presence of metals in liquid samples. Metals include Fe, Cu, Al, Pb, Ca, Zn, Cd and many more. It also measures the concentrations of metals in the samples. Typical concentrations range in the low mg/L range.

In their elemental form, metals will absorb ultraviolet light when they are excited by heat. Each metal has a characteristic wavelength that will be absorbed. The AAS instrument looks for a particular metal by focusing a beam of uv light at a specific wavelength through a flame and into a detector. The sample of interest is aspirated into the flame. If that metal is present in the sample, it will absorb some of the light, thus reducing its intensity. The instrument measures the change in intensity. A computer data system converts the change in intensity into an absorbance.

As concentration goes up, absorbance goes up. The researcher can construct a calibration curve by running standards of various concentrations on the AAS and observing the absorbencies.

Background

It is important to understand the theory behind any instrument if you are to be successful using that instrument. However, AAS theory can be looked at either on a simplified basis or a complex basis. Try looking at the complex first--you may surprise yourself by understanding it!

Atomic Absorption Spectroscopy 

Background 

We describe the motion of macroscopic objects using Newton's laws of physics. We can easily measure velocity, acceleration, force, kinetic and potential energies as well as quantify gravitational effects. However, when we look at atomic particles (i.e., protons, electrons,

neutrons, etc.), we can no longer describe motion and energy using Newtonian physics. The characteristics of atomic particles are described using the theorems of quantum mechanics.

Quantum mechanics--or quantum chemistry--describes the geometry of atoms and molecules in terms of complex mathematical expressions. It also describes the relative states of atomic matter. The atomic absorption spectrometer uses the principals of quantum chemistry to detect the presence of certain metals (i.e., iron, aluminum, copper, etc.) and determines the concentration of those metals in samples. Energy Transitions All atoms and their components have energy. The energy level at which an atom exists is referred to as its state. Under normal conditions, atoms exist in their most stable states. We refer to that most-stable level as the ground state. Although we cannot measure the precise energy state for an atom, we can usually measure changes to its energy relative to its ground state.

Certain processes can change the energy state for an atom. For example, adding thermal energy (heat) can cause an atom to increase to a higher energy state. This change in energy is written as ∆E. We refer to energy states which are higher than the ground state as excited states. In theory, there are infinite excited states, however there are decreasing numbers of atoms from a population that reach higher excited states.

The laws of quantum mechanics tell us that atoms do not increase their energy levels gradually. An atom goes directly from one state to another without going through intermediates. We refer to these "quantum leaps" as transitions. The transition from the ground state (written as Eo) to the first excited state (E1) requires some form of energy input. This energy is absorbed by the atom. That energy absorption is equal to ∆E0 to 1.

When this energy absorption takes place in the presence of ultraviolet light, some of that light will be absorbed. This uv absorption occurs at a specific wavelength.

Each element in the periodic table will have a specific ∆ E that will absorb a specific wavelength of uv light. The relationship between the energy transition and the wavelength (λ) can be described by

Δ E=h/ λ and frequency is Δ E=h ν

where h is Planck's constant. Atomic absorption uses this relationship to determine the presence of a specific element based on absorption in a specific wavelength. For example, calcium absorbs light with a wavelength of 422.7 nm. Iron absorbs light at 248.3 nm.

Atomic Absorption Spectroscopy

Instrument Design

In atomic absorption, there are two methods of adding thermal energy to a sample. A graphite furnace AAS uses a graphite tube with a strong electric current to heat the sample. In flame AAS, we aspirate a sample into a flame using a nebulizer. The flame is lined up in a beam of light of the appropriate wavelength. The flame (thermal energy) causes the atom to undergo a transition from the ground state to the first excited state. When the atoms make their transition, they absorb some of the light from the beam. The more concentrated the solution, the more light energy is absorbed!

 

The light beam is generated by a lamp that is specific for a target metal. The lamp must be perfectly aligned so the beam crosses the hottest part of the flame and travels into the detector. The detector then measures the intensity of the beam of light. When some of the light is absorbed by a metal, the beam's intensity is reduced. The detector records that reduction as absorption. That absorption is shown on a read out by the data system. The figure above shows the schematic diagram of a flame AAS. As the diagram indicates, there are four primary parts to the system--the light source, the flame apparatus, the detector, and the data system.

Fortunately, we do not need to separate solutions containing different metals. No chromatography is required for this instrument. We merely change lamps and adjust the detector wavelength.

We can find the concentrations of metals in a sample running a series of calibration standards through the instrument. The instrument will record the absorption generated by a given concentration. By plotting the absorption versus the concentrations of the standards, a calibration curve can be plotted. We can then look at the absorption for a sample solution and use the calibration curves to determine the concentration in that sample.