Transcript Document

Chapter 8 & 9 Atomic
Absorption Spectroscopy
Atomic Absorption Process
• A neutral atom in the gaseous state can absorb
radiation and transfer an electron to an excited
state.
• Simple electronic transitions possible with no
vibrational and rotational energy levels possible.
Bandwidth much narrower!
• Occur at discreet l
• Na(g) 3s  3p and 3p  5s as well as other
transitions are possible at the correct photon
energy a transition.
Chapter 8&9 - 2
Atomic Absorption Transitions
Chapter 8&9 - 3
THE FLAME AND EXCITED
STATES
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3 steps required before measurements are possible in an A.A. experiment: 1. vaporization 2.
reduction to the elemental state and then 3. exposure to radiation.
The first two steps are accomplished by a flame.
Effect of flame temperature: Since flame is at high temperature might have an effect on fraction
of atoms in excited state.
Boltzmann's equation describes effect of flame temperature: where
Eex
Nu gu
=
 e kt
No go
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N = # of atoms in a given state;
g = statistical factor for a given level and measures the number of possible electrons in each level;
g = 2J + 1 where J = Russel-Saunders coupling constant and is given by J = L + S or L  S where
L = orbital angular momentum quantum # (=0,1,2,3 for s, p, d, f respectively) and S = spin = ±½.
E.g. For the Na transition
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–
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3s½ 3p3/2
3s½  3p½
gu = 2(L+S) + 1 = 2(1 + ½) + 1 = 4 and
go = 2(0 + ½) + 1 = 2.
go = 2 and gu = 2(1  ½) + 1 = 2.
Overall population of both of these states: since they are only separated by 5Å, let's use average
of their wavelengths and add population for the two excited states:
g = 4 + 2 = 6 and go = 2 (as before); lave 5892Å.
E
hc 6.626x1027 erg  sec 2.998x1010

l
5892X10 8 cm
Chapter 8&9 - 4
cm / sec
= 3.37  10 12 erg
The Flame and Excited States
• Assume Air-acetylene flame (2400°C):
Temperatures for different flames used in
AA are listed in text. T = (2400 + 273)K =
2673K;
• Substituting into the Boltzmann equation:

 = 3.23x104
3.37x1012erg


Nu 6

No

2
e
 1.38x1016erg 1  2673K 
K


• Very small fraction of the atoms in the
flame are excited to this excited state.
Chapter 8&9 - 5
Relative population of higher
energy transitions
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3p  5s transition is also possible and has l = 6161Å (E = 3.22x1012 erg.
The fraction of 3p electrons excited to the 5 s orbital is calculated as before:


3.22x1012erg


Nu 2

  e  1.38x 16erg 12673K  = 5.34x10-5
10
K


No 6
Fraction involved in this transition even smaller.
Finally, we can estimate the fraction of electrons in the 5s state relative to
the 3s state:
N5s N3p N5s


N3s N3s N3p
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= 5.34x105×3.23x104 = 1.72x108 QED
Only very small proportion of the absorbing species is in the excited state
from excitation by flame; higher energy transitions much less likely than the
lower energy transitions.
Chapter 8&9 - 6
MEASURING ATOMIC
ABSORPTION
P
• Recall Beer's Law (A = log Po = ebC ) is obeyed when line width
small compared to absorption band.
• Atoms or molecules absorb radiation at discrete wavelengths.
• Broadband radiation contains photons of several wavelengths, some
of which may be useful but many of which will not. This will make Po
(= Pusable + Puseless) larger and the absorbance smaller than would be
expected with only the usable portion of the light available for
absorption.
• Besides the Pusable can be composed of wavelengths with different
absorptivities i.e. the sample does not absorb all radiation to the
same degree.
• Non-linear behavior observed when l range of excitation source is
greater than l range of absorber; bandwidth of excitation source
must be narrower than bandwidth of absorber.
Chapter 8&9 - 7
Linewidth of Atomic Transitions
• Natural linewidth of an absorption spectrum is
very small (104Å) but is broadened by
– Doppler broadening: Random thermal motions of
atoms relative to the detector
– Pressure broadening: In the atomic absorption
experiment the pressure is large enough that atoms
can undergo some interatomic collisions which cause
small changes in the ground state levels.
• Normal line width of excitation lines much
greater than this line width
• Monochromator cannot be used to select l
range in AA (bandwidth  few tenths of a nm).
Chapter 8&9 - 8
SOURCES
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Solution to line width problem: Use atomic source of same material.
e.g. For Na analysis Na vapor is used.
Atoms are excited by electrical discharge; the excited atoms emit a characteristic
l. The bandwidth of the source << sample linewidth since it is generated under
conditions where there is less broadening.
Hollow Cathode Tube : Hollow cathode made of the material needed is vaporized
and emits radiation of the characteristic wavelength.
The ion current to the cathode controls photon intensity; Increasing the voltage
between the anode and cathode will control the current and thus total photon flux.
Optimum current for each lamp (1-20ma).
Chapter 8&9 - 9
FORMATION OF ATOMIC VAPOR
Four methods used to vaporize sample from solution:
• Ovens: Sample placed in an oven; after evaporating
solvent, sample vaporized into irradiation area by rapidly
increasing temperature.
• Electric arc or spark: Sample subjected to high current or
high potential A.C. spark.
• Ion bombardment: Sample placed on cathode and
bombarded with + ions (Ar+). Sputtering process
dislodges them from cathode and directs them to
irradiation region.
• Flame atomization: Sample sprayed into flame where it
undergoes atomization and irradiation.
Chapter 8&9 - 10
FLAME ATOMIZERS
• Total consumption burner:
Separate channels bring sample,
fuel, and oxidant to combustion
area. All of the sample, that is
carried into the burner, is burned;
• Sensitivity is greater than in a
burner where the sample is not
completely burned.
• extra turbulence in the flame from
variations in droplet size increase
noise.
Chapter 8&9 - 11
Undergraduate Instrumental Analysis,
Robinson, p. 267.
Premix (laminar flow) burner
• Sample, fuel, and oxidant mixed prior to entering flame.
• Turbulence drastically reduced by removing larger
droplets.
• Mixing baffles insure only fine mist makes it through to
burner.
Instrumental Methods of Chemical Analysis, Ewing, p. 110.
Chapter 8&9 - 12
ELECTROTHERMAL
ATOMIZATION
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all of the sample used is atomized in
furnace (electrothermal) atomizer.
detection limit is 100-1000x lower than
with aspiration techniques.
only a few mL of solution is used.
Basic Principle:
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sample container resistively heated to
vaporize the metal atoms.
sample dried (evaporate solvent) at
110°C;
ash sample called "burn off" (200300°C);
atomization.(2000-3000°C)
comparison with flame atomization:
interaction with sample matrix and
electrode
poorer reproducibility
detection limits of 1010-1012g (or
1ppb) are possible.
Instrumental Methods of Analysis, Willard,Merritt, Dean and
Settle, p. 147
Chapter 8&9 - 13
FUELS/OXIDANTS
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Low T flames : easily reduced elements (Cu,
Pb, Zn, Cd)
High T flames: difficult to reduce elements
(e.g. alkaline earths).
Fuels: natural gas, propane, butane, H2, and
acetylene;
Oxidants- Air and O2 (low temperature
flames). N2O (high temperature flames).
Flame characteristics:
Sample enters flame, is vaporized, reduced
and eventually oxidized.
Exact region of flame in which each of these
occurs depends upon:
flow rate,
drop size, and
oxidizability of sample.
Optimum position for flame for many metals.
Chapter 8&9 - 14
Flame Profiles in AA
Chapter 8&9 - 15
MEASUREMENT OF AA
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Ideally, the amount of light reaching the detector is given by Beers Law: P =
Po×10ebC .
several interferences can change this to:
P = Po×10ebC + Pemission  Pbackground  Pscattering.
Pemission is due to analyte emission in the flame
eliminated from the absorbance by modulation of the light source: measures
only AC levels; emission DC level.
Pbackground, Pscattering: due to absorption by the flame or are induced by
sample matrix and are independent of the analyte.
Broad band in nature.
Flame interferences nulled by comparing a blank with sample
– Sample matrix is a problem. Caused by, for example, high salt content (e.g.
NaCl or KI). These have broad band absorption spectrum in flame since they
are not reduced by it. Most common approach uses secondary continuum
source (e.g. D2 lamp):
– Each lamp (D2 and HCT) modulated but are 180° out of phase with each other.
– Detection system measures difference between two absorbance signals: AHCT =
Asample + Abrdband while Acontinuum source = Abrd band. will be absorbance of sample.
Chapter 8&9 - 16
D2 Source Elimination of
Background
Chapter 8&9 - 17
MONOCHROMATOR
• Needed to choose one
of several possible
emission lines (lemitted)
associated with HCT.
• Since they are usually
reasonably well
separated from the line
of interest, it is
straightforward to use a
monochromator to
eliminate this
interference.
Chapter 8&9 - 18
ANALYTICAL TECHNIQUES
• Beer's law, A = k×C, not always true making a calibration
curve necessary.
• Standard addition method is used to minimize the effects
from the matrix
• Anion- height of the absorbance peak is influenced by
type and concentration of anion. It can reduce the
number of atoms made. An unknown matrix is thus hard
to correct for
• Cation: The presence of a second cation sometimes
causes stable compounds to form with the cation being
analyzed. e.g. Al + Mg produces low results for Mg due
to the formation of an Al/Mg oxide.
Chapter 8&9 - 19
Sample Problem
Determination of Nickel
Content by AA
120
y = 5.6x + 20
Absorbance Units
• The nickel content in river water
was determined by AA analysis
after 5.00 L was trapped by ion
exchange. Rinsing the column
with 25.0 mL of a salt solution
released all of the nickel and
the wash volume was adjusted
to 75.00 mL; 10.00 mL aliquots
of this solution were analyzed
by AA after adding a volume of
0.0700 g Ni/mL to each. A
plot of the results are shown
below. Determine the
concentration of the Ni in the
river water.
80
40
0
0
5
10
Volum e of Nickel Added(m L)
Chapter 8&9 - 20
15
Chapter 8&9 - 21
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