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CHEM7113 - Atomic Spectroscopy !
!!

Analytical Techniques
"!

Absorption (AAS)
"! Flame AAS "! Vapour

generation AAS furnace (electrothermal) AAS

"! Graphite

"!

Emission (AES)
"! ICP-AES "! AFS

(atomic fluorescence)

Some diagrams and Tables are reproduced from Skoog ‘Principles of Instrumental Analysis’, 5th/6th ed. 1 !

Atomic Spectroscopy !
!!

Electronic transitions : UV - visible radiation excitation of outer-shell electrons of atoms in the gas phase

energy levels of atomic Na

Line thicknesses represent probability of transition!
2 !

Energy Levels of Na vs Mg+ !
energy levels of free Na and Mg+

Na!

Mg+!

Units here are angstroms (0.1 nm)!

3 !

Ca

Ar

Na

Hg

http://astro.u-strasbg.fr/~koppen/discharge/

4 !

Atomic Spectroscopy Options !
Atomic spectra can be measured in 3 ways
"! "! "!

Emission - from thermally excited atoms (and ions) (AES) Absorption - from ground-state atoms (AAS) Fluorescence - photo-excitation of ground-state atoms, followed by emission (AFS)

Fundamentals of AAS, Analytik Jena
5 !

Atomic Spectroscopy !

Atomic transitions suitable for analytical measurements have energies corresponding to UV or visible radiation (vacuum UV can also be used)

!E = h! = hc/" !
Lines used in AAS are usually given in wavelength units "/nm!

6 !

Line Spectra of Atoms !
!!

Atomic linewidths are very narrow ( < 0.01 nm )
Spectrum of a steel hollow cathode lamp

7 !

Band and Continuum Spectra !
Molecular species give band spectra; in the gas phase these can show vibrational structure Continuum emission is observed from hot materials (blackbody radiation); spectral profile depends on the temperature Both must be considered in the context of AAS (and later in ICP-AES), since they can affect the observed signal 8 !

Emission spectrum of brine!

Atomisation methods – overview "
!! !!

Flame:

2000 - 3400 K

Vapour generation: Hydride and cold vapour 298 - 800 K Electrothermal (or Graphite Furnace) Vaporisation 1500 - 3500 K ET-AAS or GF-AAS Plasmas: Inductively coupled plasma (ICP) AES ~ 6000 K

!!

!!

9 !

Atomisation methods – more examples "
Skoog, Ch 8

10 !

Absorption and Emission Lines !

Absorbance Emission Intensity

A = log (I0 / I) = a x b x c = k x c I=kxc (k is a constant)

In each case, the signal will also depend on the population of the initial state AAS : ground state population of atoms AES : excited state population of atoms So, atomisation efficiency and temperature are key parameters! 11 !

Boltzmann distribution !
Ni N0 = = pi exp(- ! Ei / kT) p0

i
! Ei

0!
Ni pi !Ei population of state i degeneracy of state energy gap between state i and the ground state Boltzmann constant

k = 1.38 x 10-23 JK-1

12 !

Populations of Excited States !
Calculate the population of the 3p level of Na relative to the ground state at 300 K, 2000 K and 2500 K! average " = 589.3 nm Ni N0 = 3 !E = 3.37 x 10-19 J T = 300 K

exp - 3.37 x 10-19 1.38 x 10-23 . 300

Ni / N0

=

1.33 x 10-35

ie very few (or no) atoms excited at 300 K
13 !

Populations of 3p level in Na !
300K 2000 K 2500 K
!!

1.33 x 10-35 1.49 x 10-5 1.72 x 10-4

What does this tell us about the temperatures (and atomisation methods) needed for absorption, emission and fluorescence spectroscopies ?

14 !

Flame AAS : Instrumentation !

Single Beam

Skoog Ch 8!

http://www.chem.agilent.com/en-US/Products/Instruments/ atomicspectroscopy/aas/systems/aa280fs/PublishingImages/AAgilent-280FS.jpg!

15 !

Flame AAS : Instrumentation !
! Double Beam!
! ! ! ! Has advantages in AAS # stability / precision! Allows continual check for baseline drift! Does not compensate for flame effects !

!

Skoog Ch 8!16!

Flame AAS : Lamps !
Hollow Cathode lamps (HCL)!
!! !!

One for each element; some dual or multi-element lamps available (but there are disadvantages)! “Super” lamps :...
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