# Atomic Absorption Spectroscopy

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Atomic Absorption Spectroscopy
Prof Mark A. Buntine School of Chemistry Dr Vicky Barnett University Senior College

Atomic Absorption Spectroscopy
“This material has been developed as a part of the Australian School Innovation in Science, Technology and Mathematics Project funded by the Australian Government Department of Education, Science and Training as a part of the Boosting Innovation in Science, Technology and Mathematics Teaching (BISTMT) Programme.”

Professor Mark A. Buntine

Atomic Absorption Spectroscopy
AAS is commonly used for metal analysis A solution of a metal compound is sprayed into a flame and vaporises The metal atoms absorb light of a specific frequency, and the amount of light absorbed is a direct measure of the number of atoms of the metal in the solution

Atomic Absorption Spectroscopy: An Aussie Invention
Developed by Alan Walsh (below) of the CSIRO in early 1950s.

Sinusoidally oscillating electric (E) and magnetic (M) fields. Electric & magnetic fields are orthogonal to each other. Electronic spectroscopy concerns interaction of the electric field (E) with matter.

The Electromagnetic Spectrum
Names of the regions are historical. There is no abrupt or fundamental change in going from one region to the next. Visible light represents only a very small fraction of the electromagnetic spectrum. 1020 1018 1016 1014 1012 108 -rays X-rays UV IR Micro- wave Frequency (Hz) Wavelength (m) 10-11 10-8 10-6 10-3 Visible 400 500 600 700 800 nm

The Visible Spectrum l < 400 nm, UV 400 nm < l < 700 nm, VIS
l > 700 nm, IR

The Electromagnetic Spectrum
Remember that we are dealing with light. It is convenient to think of light as particles (photons). Relationship between energy and frequency is:

Energy & Frequency Note that energy and frequency are directly proportional. Consequence: higher frequency radiation is more energetic. E.g. X-ray radiation ( = 1018 Hz): x 106 kJ/mol IR radiation ( = 1013 Hz): kJ/mol (h = x J.s)

Energy & Wavelength Given that frequency and wavelength are related: =c/ Energy and wavelength are inversely proportional Consequence: longer wavelength radiation is less energetic eg. -ray radiation ( = m): 1.2 x 107 kJ/mol Orange light ( = 600 nm): kJ/mol (h = x J.s c = x 108 m/s)

Absorption of Light When a molecule absorbs a photon, the energy of the molecule increases. Microwave radiation stimulates rotations Infrared radiation stimulates vibrations UV/VIS radiation stimulates electronic transitions X-rays break chemical bonds and ionize molecules Ground state Excited photon

Absorption of Light When light is absorbed by a sample, the radiant power P (energy per unit time per unit area) of the beam of light decreases. The energy absorbed may stimulate rotation, vibration or electronic transition depending on the wavelength of the incident light.

Atomic Absorption Spectroscopy
Uses absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms must be vapourised in a flame (or graphite furnace).

Absorption and Emission
Excited States Ground State Multiple Transitions Absorption Emission

Absorption and Emission
Excited States Absorption Emission Ground State

Atomic Absorption When atoms absorb light, the incoming energy excites an electron to a higher energy level. Electronic transitions are usually observed in the visible or ultraviolet regions of the electromagnetic spectrum.

Atomic Absorption Spectrum
An “absorption spectrum” is the absorption of light as a function of wavelength. The spectrum of an atom depends on its energy level structure. Absorption spectra are useful for identifying species.

Atomic Absorption/Emission/ Fluorescence Spectroscopy

Atomic Absorption Spectroscopy
The analyte concentration is determined from the amount of absorption.

Atomic Absorption Spectroscopy
The analyte concentration is determined from the amount of absorption.

Atomic Absorption Spectroscopy
Emission lamp produces light frequencies unique to the element under investigation When focussed through the flame these frequencies are readily absorbed by the test element The ‘excited’ atoms are unstable- energy is emitted in all directions – hence the intensity of the focussed beam that hits the detector plate is diminished The degree of absorbance indicates the amount of element present

Atomic Absorption Spectroscopy
It is possible to measure the concentration of an absorbing species in a sample by applying the Beer-Lambert Law: e = extinction coefficient

Atomic Absorption Spectroscopy
But what if e is unknown? Concentration measurements can be made from a working curve after calibrating the instrument with standards of known concentration.

AAS - Calibration Curve
The instrument is calibrated before use by testing the absorbance with solutions of known concentration. Consider that you wanted to test the sodium content of bottled water. The following data was collected using solutions of sodium chloride of known concentration Concentration (ppm) 2 4 6 8 Absorbance 0.18 0.38 0.52 0.76

Calibration Curve for Sodium
1.0 0.8 0.6 0.4 0.2 2 4 6 8 Concentration (ppm)

Use of Calibration curve to determine sodium concentration {sample absorbance = 0.65}
1.0 0.8 0.6 0.4 Concentration Na+ = 7.3ppm 0.2 2 4 6 8 Concentration (ppm)

Atomic Absorption Spectroscopy
Instrumentation • Light Sources • Atomisation • Detection Methods

Light Sources Hollow-Cathode Lamps (most common).
Lasers (more specialised). Hollow-cathode lamps can be used to detect one or several atomic species simultaneously. Lasers, while more sensitive, have the disadvantage that they can detect only one element at a time.

Hollow-Cathode Lamps Hollow-cathode lamps are a type of discharge lamp that produce narrow emission from atomic species. They get their name from the cup-shaped cathode, which is made from the element(s) of interest.

Hollow-Cathode Lamps The electric discharge ionises rare gas (Ne or Ar usually) atoms, which in turn, are accelerated into the cathode and sputter metal atoms into the gas phase.

Hollow-Cathode Lamps

Hollow-Cathode Lamps The gas-phase metal atoms collide with other atoms (or electrons) and are excited to higher energy levels. The excited atoms decay by emitting light. The emitted wavelengths are characteristic for each atom.

Hollow-Cathode Lamps M + e M* M + Ar* M* M* M + hn M*
collision-induced excitation M* spontaneous emission M* M + hn M

Hollow-Cathode Spectrum
Harris Fig. 21-3: Steel hollow-cathode

Atomisation Atomic Absorption Spectroscopy (AAS) requires that the analyte atoms be in the gas phase. Vapourisation is usually performed by: Flames Furnaces Plasmas

Flame Atomisation Flame AAS can only analyse solutions.
A slot-type burner is used to increase the absorption path length (recall Beer-Lambert Law). Solutions are aspirated with the gas flow into a nebulising/mixing chamber to form small droplets prior to entering the flame.

Flame Atomisation Harris Fig 21-4(a)

Flame Atomisation Degree of atomisation is temperature dependent.
Vary flame temperature by fuel/oxidant mixture.

Furnaces Improved sensitivity over flame sources.
(Hence) less sample is required. Generally, the same temp range as flames. More difficult to use, but with operator skill at the atomisation step, more precise measurements can be obtained.

Furnaces

Furnaces

Inductively Coupled Plasmas
Enables much higher temperatures to be achieved. Uses Argon gas to generate the plasma. Temps ~ 6,000-10,000 K. Used for emission expts rather than absorption expts due to the higher sensitivity and elevated temperatures. Atoms are generated in excited states and spontaneously emit light.

Inductively Coupled Plasmas
Steps Involved: RF induction coil wrapped around a gas jacket. Spark ionises the Ar gas. RF field traps & accelerates the free electrons, which collide with other atoms and initiate a chain reaction of ionisation.

Detection Photomultiplier Tube (PMT). pp (Ch. 20) Harris

Photomultiplier Tubes
Useful in low intensity applications. Few photons strike the photocathode. Electrons emitted and amplified by dynode chain. Many electrons strike the anode.