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Atomic Absorption Spectroscopy:

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

Atomic Absorption Spectroscopy:

2 “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 .” Atomic Absorption Spectroscopy

Slide 3:

Professor Mark A. Buntine Badger Room 232 mark.buntine@adelaide.edu.au

Atomic Absorption Spectroscopy:

4 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:

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

Electromagnetic Radiation:

6 Electromagnetic Radiation 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:

7 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. 10 20 10 18 10 16 10 14 10 12 10 8  -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:

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

The Electromagnetic Spectrum:

9 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:

10 Energy & Frequency Note that energy and frequency are directly proportional. Consequence: higher frequency radiation is more energetic. E.g. X-ray radiation (  = 10 18 Hz): 4.0 x 10 6 kJ/mol IR radiation (  = 10 13 Hz): 39.9 kJ/mol (h = 6.626 x 10 -34 J.s)

Energy & Wavelength:

11 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 (  = 10 -11 m): 1.2 x 10 7 kJ/mol Orange light (  = 600 nm): 199.4 kJ/mol (h = 6.626 x 10 -34 J.s c = 2.998 x 10 8 m/s)

Absorption of Light:

12 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 state photon

Absorption of Light:

13 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:

14 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:

15 Absorption and Emission Ground State Excited States Absorption Emission Multiple Transitions

Absorption and Emission:

16 Absorption and Emission Ground State Excited States Absorption Emission

Atomic Absorption:

17 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:

18 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:

19 Atomic Absorption/Emission/ Fluorescence Spectroscopy

Atomic Absorption Spectroscopy:

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

Atomic Absorption Spectroscopy:

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

Atomic Absorption Spectroscopy:

22 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

Atomic Absorption Spectroscopy:

23 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:

24 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:

25 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:

26 Calibration Curve for Sodium Concentration (ppm) A b s o r b a n c e 2 4 6 8 0.2 0.4 0.6 0.8 1.0

Use of Calibration curve to determine sodium concentration {sample absorbance = 0.65}:

27 Use of Calibration curve to determine sodium concentration {sample absorbance = 0.65} Concentration (ppm) A b s o r b a n c e 2 4 6 8 0.2 0.4 0.6 0.8 1.0  Concentration Na + = 7.3ppm

Atomic Absorption Spectroscopy:

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

Light Sources:

29 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:

30 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:

31 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:

32 Hollow-Cathode Lamps

Hollow-Cathode Lamps:

33 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:

34 Hollow-Cathode Lamps M M * M + e M * M + Ar * M * M M * M * M + h n collision-induced excitation spontaneous emission

Hollow-Cathode Spectrum:

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

Atomisation:

36 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:

37 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:

38 Flame Atomisation Harris Fig 21-4(a)

Flame Atomisation:

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

Furnaces:

40 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:

41 Furnaces

Furnaces:

42 Furnaces

Inductively Coupled Plasmas:

43 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:

44 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:

45 Detection Photomultiplier Tube (PMT). pp 472-473 (Ch. 20) Harris

Photomultiplier Tubes:

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

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