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Chapter: 3 Chemistry
    Section: 3.7 Properties of chemical bonds
        SubSection: 3.7.8 Atomic spectroscopy

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3.7.8 Atomic spectroscopy

Atomic spectroscopy, i.e. emission, absorption and fluorescence, involves the input of energy (e.g. electro­magnetic, thermal, chemical or electrical) into an atomic population, which is then converted into light energy by various atomic and electronic processes before the final measurement. The light energy is manifest in the form of a spectrum consisting of radiation at a number of discrete wavelengths. The quantized energy levels involved may be expressed as:

ΔE = E1E2 = hv

where E1 and E2 are the energies in the initial and final states respectively, h is Planck’s constant (6.626 076 × 10−34 Js) and v is the frequency of the radiation.

An atom is said to be in the ground state when its electrons are at their lowest energy level. When energy is transferred to such atoms, as in emission spectroscopy, the energy transfer to individual atoms will vary and hence the resulting radiation will be at a number of different frequencies and will give rise to a complex emission spectrum.

The proportion of excited to ground state atoms in a population at a given temperature is expressed by the Boltzmann relationship:

   

 

Nm

  =  

gm

  exp

(EmEn)

Nn

gn

kT

where N is the number of atoms in a state n or m, g is the statistical weight for a particular state and k is the Boltzmann constant (1.380 658 × 10−23 JK−1).

Atomic absorption spectrometry is the measurement of the absorption of optical radiation by atoms in the gaseous state. Usually only absorptions involving the ground state, known as resonance lines, are observed. The absorption coefficient is determined by the product of the total number of atoms present per unit volume and the oscillator strength of the resonance line. The excitation energy is provided by a radiation quantum and no temperature factor is involved.

In quantitative spectroscopy, the absorbance A is often defined by:

A = log(Io/I)

where Io is the intensity of the incident beam and I is the intensity of the transmitted light. Thus we obtain a linear relationship where:

A = kv log e

= 0.4343 kvl

In this case l is the path length and so in practical terms kv, the absorption coefficient at frequency v, is proportional to the number of atoms per cubic centimetre in the flame and A is therefore proportional to analyte concentration. The atoms excited by absorption of resonance radiation may also re-emit the energy giving rise to atomic fluorescence. The re-emitted energy may be of the same wavelength as the absorbed energy, or as is more often the case at a longer wavelength indicating that an intermediate state is involved with the partial loss of energy in some other form.




References

S. J. Hill (1999) Inductively Coupled Plasma Spectroscopy and its Applications, Sheffield Academic Press.
L. Ebdon, E. H. Evans, A. Fisher and S. J. Hill (1998) An Introduction to Atomic Absorption Spectroscopy, Wiley.
W. J. Price (1979) Spectrochemical Analysis by Atomic Absorption, Heyden.
B. Welz (1985) Atomic Absorption Spectrometry, 2 edn, VCH.




Values of the ratios of atoms in the excited state (Nj) to the number of atoms in the ground state (N0) for typical elemental resonance lines

Line, nm

g j /g0

Nj/N0

    2000 K 3000 K 4000 K 5000 K

Cs 852.1

2

     4.44 × 10−4

     7.24 × 10−3

     2.98 × 10−2

     6.82 × 10−2

Na 589.1

2

     9.86 × 10−6

     5.88 × 10−4

     4.44 × 10−3

     1.51 × 10−2

Ca 422.7

3

     1.21 × 10−7

     3.69 × 10−5

     6.03 × 10−4

     3.33 × 10−3

Zn 213.9

3

     7.29 × 10−15

     5.58 × 10−10

     1.48 × 10−7

     4.32 × 10−6

Note: gj/g0 are the statistical weights of the excited and ground state.



Detection limits for flame atomic absorption spectrometry   

Element 

Wavelength 
(nm) 

Characteristic 
concentration 
(μg ml−1)a

Detection 
limit 
 (μg m1−1)a

Normal 
range 
 (μg ml−1)

Flame Type

 

Ag 

328.1

        0.03 

        0.002

  0.02–10

     Air–C2H2

Al

309.3

        0.8

        0.03

0.3–200

     N2O–C2H2

As

193.7

        0.5

        0.3

3–150

     N2O–C2H2

Au

242.8

        0.1

        0.01

0.1–30

     Air–C2H2

B

249.7

        8.0

        0.5

5–2000 

     N2O–C2H2

Ba

553.5

        0.2

        0.02

0.2–50

     N2O–C2H2

Be

234.9

        0.015

        0.001

0.01–4

     N2O–C2H2

Bi

223.1

        0.2

        0.05

0.5–50

     Air–C2H2

Ca

422.7

        0.01

        0.001

0.01–3

     N2O–C2H2

Cd

228.8

        0.01

        0.0015

0.02–3

     Air–C2H2

Co

240.7

        0.05

        0.005

0.05–15

     Air–C2H2

Cr

357.9

        0.05

        0.006

0.06–15

     Air–C2H2

Cs

852.1

        0.02

        0.004

0.04–5

     Air–C2H2

Cu

324.7

        0.03

        0.003

0.03–10

     Air–C2H2

Dy

421.2

        0.6

         0.03

0.3–150

     N2O–C2H2

Er

400.8

        0.5

        0.03

0.5–150

     N2O–C2H2

Eu

459.4

        0.3

        0.02

0.2–100

     N2O–C2H2

Fe

248.3

        0.05

        0.006

0.06–15

     Air–C2H2

Ga

287.4

        0.08

        0.08

1–200

     Air–C2H2

Gd

368.4

      20

        2.0

20–6000

     N2O–C2H2

Ge

265.1

        1.0

        0.2

2–300

     N2O–C2H2

Hf

307.3

      10

        2.0

20–300

     N2O–C2H2

Hg

253.7

        1.5

        0.15

2–400

     Air–C2H2

Ho

410.4

        0.7

        0.04

0.4–200

     N2O–C2H2

In

303.9

        0.15

        0.04

0.4–40

     Air–C2H2

Ir

208.8

        0.8

        0.5

5–200

     Air–C2H2

K

766.5

        0.007

        0.003

0.03–2

     Air–C2H2

La

550.1

      40

         2.0

20–10 000

     N2O–C2H2

Li

670.8

        0.02

        0.002

0.02–5

     Air–C2H2

Lu

336.0

        7.0

        0.3

3–2000 

     N2O–C2H2

Mg

285.2

        0.003

        0.0003

0.003–1

     Air–C2H2

Mn

279.5

        0.02

        0.002

0.02–5

      Air–C2H2

Mo   

313.3

        0.3

        0.02

0.2–100

      N2O–C2H2

Na    

589.0

        0.003

        0.0002

0.002–1

      Air–C2H2

Nb    

334.9

      20

        2.0

20–6000

      N2O–C2H2

Nd    

492.5

        6.0

        1.0

10–1500

      Air–C2H2

Ni     

232.0

        0.07

        0.01

0.1–20

      Air–C2H2

Os    

290    

        1.0

        0.1

1–300

      N2O–C2H2

P      

213.6

    120

     40

400–30 000

      N2O–C2H2

Pb    

217.0

        0.1

        0.01

0.1–30

      Air–C2H2

Pd    

247.6

        0.05

        0.01

0.1–15

      Air–C2H2

Pr     

495.1

      20

        8.0

100–5000

      N2O–C2H2

Pt     

265.9

        1.0

        0.1

1–300

      Air–C2H2

Rb    

780.0

        0.05

        0.009

0.1–15

      Air–C2H2

Re    

346.0

        8.0

        0.8

10–2000

      N2O–C2H2

Rh    

343.5

        0.1

        0.005

0.05–30

      Air–C2H2

Ru    

349.9

        0.4

        0.08

1–150

      Air–C2H2

Sb    

215.6

        0.3

        0.04

0.4–100

      Air–C2H2

Sc    

391.2

        0.3

        0.05

0.5–80

      N2O–C2H2

Se    

196.0

        1.0

        0.5

5–250

      N2O–C2H2

Si     

251.6

        1.5

        0.25

3–400

      N2O–C2H2

Sm   

429.7

        6.0

        1.0

10–1500

      N2O–C2H2

Sn    

235.5

        0.7

        0.1

1–200

      N2O–C2H2

Sr     

460.7

        0.04

        0.002

0.02–190

      N2O–C2H2

Ta     

271.5

      10

        2.0

20–3000

      N2O–C2H2

Tb     

432.7

        7.0

        0.7

7–2000

      N2O–C2H2

Te     

214.3

        0.2

        0.03

0.3–60

      Air–C2H2

Ti      

364.3

        1.0

        0.08

1–300

      N2O–C2H2

Tl      

276.8

        0.2

        0.02

0.2–50

      Air–C2H2

Tm    

371.8

        0.3

        0.02

0.2–100

      N2O–C2H2

U      

358.5

    100

      40

400–30 000

      N2O–C2H2

V      

318.5

        0.7

        0.07

1–200

      N2O–C2H2

W     

255.1

        5.0

        1.0

10–1500

      N2O–C2H2

Y      

410.2

        2.0

        0.2

2–500

      N2O–C2H2

Yb    

398.8

        0.06

        0.004

0.04–15

      N2O–C2H2

Zn     

213.9

        0.008

        0.008

0.01–2

      N2O–C2H2

Zr      

360.1

        9.0

        1.0

10–2000

      Air–C2H2

 

a Both the characteristic concentration and detection limit are quoted for the most sensitive line.
Data supplied by Varian Ltd, for the Spectra AA series of spectrometers.
Note: The characteristic concentration is the concentration corresponding to an absorbance of 0.004 4.




Typical detection limits (in μg/L) attainable with various techniques of atomic emission spectrometry

Element

Flame AESa

Graphite
Furnace
AES
a

ICP-AESb

Ag   

20

 0

.45

3

 

Al   

10

 1

  

  1

.5

As   

50 000        

 

  

          12

 

Au   

500  

      160 

 

          20

 

B   

30 000        

      200 

 

1

.5

Ba   

  1

          4 

 

0

.07

Be   

40 000        

      460 

 

0

.2

Bi   

40 000        

        30 

 

12

 

Ca   

      0.1 

     

 

       0

.03

Cd   

2 000      

        50 

 

     1

.5

Co   

50

        10 

 

5

 

Cr   

  5

          1 

 

           4

Cs   

  8

        18 

 

3200

 

Cu   

10

          2 

 

2

 

Fe   

50

          7 

 

1

.5

In   

  5

          0

.65

18

 

Ir   

1 00 000           

      860 

 

3

.5

K   

   3

   0

.0015

10

 

Li   

         0.03 

0

.07

0

.6

Mg   

    5

          1

 

0

.1

Mn   

    5

          1

.5

0

.3

Mo   

100

        16

 

4

 

Na   

       0.1

    0

.0025

1

 

Ni   

  30

        15

 

5

.5

P   

 

     

 

18

Pb   

200

        27

 

 14

 

Pd   

  50

        60

 

7

 

Rb   

       0.3

          0

.1

 3

 

Si   

5 000     

        90

 

5

 

Sn   

300 

        15

 

15

 

Sr   

      0.1

          1

 

7

 

Ta   

18 000        

      

 

9

 

Ti   

200  

       17

 

0

.6

Tl   

20

         1

 

27

 

U   

10 000        

   2500

 

18

 

V   

10

         9

 

2

 

W   

500  

    

 

17

 

Zn   

50 000        

   1500

 

0

.9

Zr   

3 000      

     

 

1

.5

 

 

 

 

 

 

  
a
Reproduced with permission from B. Welz (1985) Atomic Absorption Spectrometry, VCH.
b Reproduced with permission from Varian Ltd. Data for Liberty series of  spectrometers.




Some detection limits in simultaneous multi-element fluorescence

Element

nm

Detection
limit ppm

               Flame

     

   

    

Aluminium   

396.1

 

0.3

  N2O–C2H2

Silver   

328.0

 

0.07

  Air–C2H2

Calcium   

422.7

 

0.003

  Air–C2H2

Cadmium   

228.8

 

0.03

  Air–C2H2

Cobalt   

240.7

 

0.06

  Air–C2H2

Chromium   

357.6

 

0.02

  Air–C2H2

Copper   

324.8

 

0.04

  Air–C2H2

Iron   

248.3

 

0.03

  Air–C2H2

Magnesium   

285.2

 

0.005

  Air–C2H2

Manganese   

279.5

 

0.003

  Air–C2H2

Molybdenum   

312.6

 

0.2

  Air–C2H2

  

 

 

1.0

  N2O–C2H2

  

  

 

0.1

  Argon-separated N2O–C2H2

Nickel   

232.0

 

0.08

  Air–C2H2

Lead   

405.8

 

0.07

  Argon-separated N2O–C2H2

Antimony   

217.6

 

0.1

  Air–C2H2

   

(in MIBK)  

 

Selenium   

204.0

 

1.5

  Air–C2H2

Zinc   

213.9

 

0.07

  Air–C2H2

    

  

 

 

Ref: W J Price (1979) Spectrochemical analysis by Atomic Absorption, Wiley.




S.J.Hill

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