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

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Atomic Absorption Spectroscopy

Prof Mark A. BuntineSchool of Chemistry

Dr Vicky BarnettUniversity Senior College

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“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

Badger Room 232

[email protected]

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•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

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Atomic Absorption Spectroscopy:An Aussie Invention

•Developed by Alan Walsh (below) of the CSIRO in early 1950s.

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Electromagnetic Radiation

Sinusoidally oscillating electric (E) and magnetic (M) fields.

Electric & magnetic fields are orthogonal to each other.

Electronic spectroscopy concerns interaction of theelectric field (E) with matter.

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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 IRMicro-wave

Frequency (Hz)

Wavelength (m)10-11 10-8 10-6 10-3

Visible

400 500 600 700 800 nm

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The Visible Spectrum

< 400 nm, UV 400 nm < < 700 nm, VIS > 700 nm, IR

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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:

E h

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Energy & Frequency

• Note that energy and frequency are directly proportional.

• Consequence: higher frequency radiation is more energetic.

E.g. X-ray radiation ( = 1018 Hz): 4.0 x 106 kJ/mol IR radiation ( = 1013 Hz): 39.9 kJ/mol

(h = 6.626 x 10-34 J.s)

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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 107 kJ/mol Orange light ( = 600 nm): 199.4 kJ/mol

(h = 6.626 x 10-34 J.s c = 2.998 x 108 m/s)

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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

Groundstate

Excitedstate

photon

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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.

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• 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).

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Absorption and Emission

Ground State

Excited States

Absorption Emission MultipleTransitions

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Absorption and Emission

Ground State

Excited States

Absorption Emission

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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.

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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.

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Atomic Absorption/Emission/Fluorescence Spectroscopy

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• The analyte concentration is determined from the amount of absorption.

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• The analyte concentration is determined from the amount of absorption.

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• 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


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• It is possible to measure the concentration of an absorbing species in a sample by applying the Beer-Lambert Law:

Abs logIIo

Abscb

= extinction coefficient

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• But what if is unknown?• Concentration measurements can be

made from a working curve after calibrating the instrument with standards of known concentration.

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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

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Calibration Curve for Sodium

Concentration (ppm)

Absorbance

2 4 6 8

0.2

0.4

0.6

0.8

1.0

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Use of Calibration curve to determine sodium concentration {sample

absorbance = 0.65}

Concentration (ppm)

Absorbance

2 4 6 8

0.2

0.4

0.6

0.8

1.0

Concentration

Na+ = 7.3ppm

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• Instrumentation

• Light Sources

• Atomisation

• Detection Methods

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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.

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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.

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• 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.

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• 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.

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MM

MM**

M + e MM + e M**

M + ArM + Ar** M M**

M

M*

MM** M + M + hh

collision-inducedexcitation

spontaneousemission

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Hollow-Cathode Spectrum

Harris Fig. 21-3:Steel hollow-cathode

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Atomisation

• Atomic Absorption Spectroscopy (AAS) requires that the analyte atoms be in the gas phase.

• Vapourisation is usually performed by:– Flames– Furnaces– Plasmas

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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.

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Flame Atomisation

Harris Fig 21-4(a)

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Flame Atomisation

• Degree of atomisation is temperature dependent.

• Vary flame temperature by fuel/oxidant mixture.

Fuel Oxidant Temperature (K)Acetylene Air 2,400 - 2,700Acetylene Nitrous Oxide 2,900 - 3,100Acetylene Oxygen 3,300 - 3,400Hydrogen Air 2,300 - 2,400Hydrogen Oxygen 2,800 - 3,000Cyanogen Oxygen 4,800

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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.

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Furnaces

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Furnaces

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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.

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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.

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Detection• Photomultiplier Tube (PMT).

pp 472-473 (Ch. 20) Harris

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Photomultiplier Tubes

• Useful in low intensity applications.

• Few photons strike the photocathode.

• Electrons emitted and amplified by dynode chain.

• Many electrons strike the anode.

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

Documents

nm slide

absorption of light

solution slide

ir slide

ms slide

visible light

energy wavelength

incident light