Lecture 5
 Three
techniques (methods) included in
atomic spectroscopy:
1) atomic absorption spectroscopy
2) atomic emisssion spectroscopy
3) atomic fluorescene spectroscopy
 In
order to perform atomic spectroscopy,
atoms of the analyte must first be formed,
usually in the form of an atomic vapor.
Atomization – the process by which a sample is
converted to an atomic vapor.
Atomizer – a device used to converted a
sample to an atomic vapor.
 Three
1)
2)
3)
types of atomizer:
flame atomizer
plasma atomizer
electrothermal atomizer
 Solution
of the analyte is evaporated rapidly
at an elevated temperature to yield a finely
divided solid.
 Further
atoms.
heating will break down into gaseous
Process occuring during atomization
Nebulization – Conversion of the
liquid sample to a fine spray.
Desolvation - Solid atoms are mixed
with the gaseous fuel.
Volatilization – Solid atoms are
converted to a vapor in the
flame.

There are three types of particles
that exist in the flame:
1) Atoms
2) Ions
3) Molecules
1. Types of Flames
Fuel / Oxidant
H–CC-H

acetylene / air
acetylene / N2O
acetylene / O2
Temperature
2100 – 2400 C (most common)
2600 – 2800 C
3050 – 3150 C
Selection of flame type depends on the volatilization of interest.
2.
Flame Structure

Interzonal region is the hottest part of the flame
and best for atomic absorption.

Oxidation of the atoms occurs in the secondary
combustion zone where the atoms will form
molecular oxides and are dispersed into the
surroundings.
Temperature Profile
Temperature profile in degrees Celsius for a natural gas-air flame
3.
Burners
 Two
types of burners in flame spectroscopy
i. Turbulent flow (total consumption burner)
ii. Laminar flow (premix burner)
Turbulent Flow Burner
 Nebulizer
& burner are
combined into a single unit.
 Sample
is drawn up the
capillary & nebulized
 Sample
mL/min
flow rate: 1 to 3
Turbulent Flow Burner
 Advantage:

introduce relatively large & representative sample
into the flame.
 Disadvantages:



A relatively short path length through flame
Problems with clogging of the tip
Burners noisy from electronic and auditory stand
point
Laminar Flow Burner

Sample is nebulized by the flow
of oxidant past a capillary tip.

Resulting aerosol then mixed
with fuel & flows past a series of
baffles that remove all but the
finest droplets.

Majority of the sample collects
in the bottom of mixing
chamber, drained to a waste
container.

Aerosol, oxidant & fuel are
burned in a slotted burner that
provided a flame, 5 – 10 cm in
length.
Laminar Flow Burner

Advantages:
i.
Provide quiet flame
ii. Longer path length, enhance sensitivity &
reproducibility

Disadvantages:
i.
Lower rate of sample introduction
ii. Possibility of selective evaporation of mixed
solvents in the mixing chamber, create analytical
uncertainties.
iii. Mixing chamber contains a potentially explosive
mixture that can flash back if the flow rates are
too low.
Atomic Absorption Spectroscopy
(based on flame)

Currently the most widely used of all the atomic
methods.

Simplicity, effectiveness, relatively low cost.

Flame atomization best for reproducibility
(precision)(<1%)
Atomic Absorption Spectroscopy
(based on flame)

The spectra result form the atomized sample
absorbing photons of radiation of the appropriate
energy (wavelength).

Energy of radiation absorbed by a vaporized atom
is identical with that needed to bring about
excitation to a higher electronic state.

Transitions: excitation of an electron from ground
state to a higher energy level
E1
E0
Spectrophotometers

Contain same basic components as an instrument
designed for absorption analysis of solution.

Source
sample container (flame or hot
surface)
 selector
detector
readout system
AA Spectrophotometer Design
1. Single Beam Instrument
• The modulated power source can be replaced by a chopper
AA Spectrophotometer Design
2. Double-Beam Instrument
Double-Beam Instrument

Radiation from hollow cathode lamp is split into
2 beams
One passes through the flame
 The other around the flame


A half-silvered mirror returns both beams to a
single path then pass through the
monochromator then detector.

Note: monochromator placed between sample
and detector

Eliminates most of the radiation originating from the flame.
A.
Radiation Sources for AAS
LINE SOURCE – narrow emission bands.
Examples of line source used in AAS:
Hollow cathode lamp
(HCL)
Electrodeless
discharge lamp
(EDL)
1. Hollow Cathode Lamp (HCL)

light source in AA instrument

light from this lamp exactly light required for the analysis,
even no monochromator is used.

atoms of metal tested are present within the lamp.
How does the Hollow Cathode Lamp works?

When lamp on, atoms are supplied with energy

Causes atoms elevate to excited states

Upon returning to ground state, exactly the same
wavelength that are useful in the analysis are emitted

Since the analyzed metal with exactly the same
energy levels that undergoes excitation

Hollow cathode lamp MUST contain the element to be
determined
Mechanism of excitation process in Hollow
Cathode Lamp

Lamp is a sealed glass envelope filled with argon (or
Neon) gas

When lamp is ON, Ne atoms are ionized with
electrons drawn to anode (+ charge electrode)

The Ne ions, Ne+ bombard the surface of the cathode
(- charge electrode)

Metal atoms, M, in the cathode are elevated to the
excited state and are rejected from the surface as a
result of this bombardment
Mechanism of excitation process in Hollow
Cathode Lamp

When atoms return to ground state, characteristic line
spectrum of that atom is emitted

This light is directed at the flame, where unexcited
atoms of the same element absorb the radiation and
are themselves raised to the excited state

Absorbance is measured, and related to the
concentration
Illustration of mechanism of excitation
process in HCL
1.
Electrodeless Discharge Lamp (EDL)

Constructed of a metal or salt of interest sealed in a quartz
tube filled with a noble gas (Ne or Ar) at low pressure (1 – 5
torr).

The noble gas is ionized and accelerated by a strong radiofrequency (RF) or microwave field and in turn excite the
metal or salt of interest.

Provide radiant intensities usually one to two orders of
magnitude greater than HCL.
Source of Modulation
 Function:
-
Employed in AAS to distinguish between the
component of radiation arising from the
source and the component of radiation
arising from the flame background
-
Device: light chopper
Source Modulation
Light chopper

A chopper or a modulated power supply is used to
modulate the source radiation that passes through
the atomizer (flame).

The chopper rotates resulting in an alternating
atomic absorption and atomic emission signal.

The signal from emission of radiation will be
continuous and can be substracted from the
modulated AA signal.
Light chopper
Eliminate the effects of radiation from the flame
 Light is “chopped” with a rotating half-mirror so
that detector sees alternating light intensities
 One moment, only light emitted by flame is read
since the light from the source is cut off
 Next moment, light from both the flame
emission & transmission of the source’s light is
measured since the source’s light is allowed to
pass
 The elements of the detector is such that the
emission signal is substracted from the total
signal & this difference is what we measured.

Questions
1.
In flame atomic absorption spectroscopy,
briefly describe the ‘atomization’ process
which the analyte undergoes.
2.
Why is source modulation employed in
atomic absorption spectroscopy? Name the
device used for this purpose.
3.
Name the common line source used in AAS.
INTERFERENCES
Spectral
interference
Chemical
interference
Spectral interference
 Arise
when particulate matter from
atomization scatters the incident radiation
from the source.
 Or
when absorption or emission of an
interfering species either overlaps or lies so
close to the analyte absorption that
resolution by the monochromator becomes
impossible.
Spectral interference (simple words)
 Is
one in which the spectral line of the
elements being determined overlaps with a
spectral line (or band) from another element
present in the sample.
 The
effect of the element will also be
measured & thus the results will be
incorrect.
Spectral interference
The most common method of solving this problem
i) Tune the monochromator to a different
spectral line for the element of interest so that
there is no overlap.
ii) secondary lines (can be found in the
literature)
Chemical interefrence
2
common types of chemical interferences
(that reduce the concentration of free
gaseous atoms)
(i) ionization
(ii) formation of molecular species
Chemical interference – Ionization
Sample (liquid/solid) must be vaporized &
atomized in a high temperature source such as
flame.
 This high temperature environment lead to
ionization of the analyte atoms.

Analyte ionization can be supressed by
adding a source of electrons, which shifts the
equilibrium of the analyte from the ionic to
the atomic form.
 Cesium and potassium are common ionization
supressors that are added to analyte
solutions.

Analyte

Analyte+ +
e-
Chemical interference – Ionization
 Cesium
and potassium are common ionization
suppressors that are added to analyte
solutions
 These
atoms (Cs & K) are easily ionized &
produce a high concentration of free
electrons in the flame.
Chemical interference – Refractory Formation

Some elements can form refractory compounds
that are not atomized in flames.

E.g. in the presence of phosphates, which
interferes with calcium measurements due to
formation of refractory calcium phosphate.

Formation of refractory compounds can be
prevented or reduced by adding a releasing agent.
Chemical interference – Refractory Formation
 For
calcium measurement:

Adding lanthanum to the sample (and standard)
solutions binds the phosphates as LaPO4.

LaPO4 has a very high formation constant, Kf and
effectively ties up the phosphate interferent.
Quantitative Analysis


Calibration Curve
Standard Addition Method

Absorption behavior follows Beer’s Law &
concentration of unknown are determined the
same way.

All atomic species have an absorptivity, a

Pathlength, b is width of the flame

A = abc
Calibration Curve Method
A
general method for determining the
concentration of a substance in an unknown
sample by comparing the unknown to a set of
std sample of known concentration.
 Plot
is linear over a significant concentration
range (the dynamic range).
 Analysis
should never be based on the
measurement of a single standard with
assumption that Beer’s law is being followed.
Standard Addition Method
 Extensively
used in AAS
 Compensate for variation caused by physical
& chemical interferences in the analyte
solution
 The most common way involves adding one
or more increments of a standard solution to
sample aliquots of the same size.
 This process is often called spiking the
sample.
 Each solution is then diluted to a fixed
volume before measurement.
Example: Standard Addition Method
The chromium in an aqueous sample was determined
by pipetting 10.0 mL of the unknown into each of five
50.0 mL volumetric flasks. Various volumes of a
standard containing 12.2 ppm Cr were added to the
flasks, following which the solutions were diluted to
the volume.
1.
2.
Volume of
unknown (mL)
Volume of
standard (mL)
Absorbance
10.0
0.0
0.201
10.0
10.0
0.292
10.0
20.0
0.378
10.0
30.0
0.467
10.0
40.0
0.554
Plot the data.
Calculate the concentration of Cr in the sample.
Standard Addition
- if Beer’s law is obeyed,
A = bVstdCstd
Vt
= kVstdCstd
+
+
k is a constant equal to
bVxCx
Vt
kVxCx
b
Vt
 Standard
Addition
- Plot a graph: A vs Vstd (should yield a
straight line)
A = mVstd + b
slope;
intercept;
Concentration of unknown:
Cx = bCstd
mVx
m = kCstd
b = kVxCx
 Detection
Limits (ng/mL)
Element
AAS Flame
Element
AAS Flame
Al
30
Cu
2
As
100
Fe
5
Ca
1
Hg
500
Cd
1
Pb
10
Cr
3
Zn
2
Nanogram/mililiter = 10-3 g/mL = 10-3 ppm
Accuracy
 Relative
error associated with a flame
absorption analysis is the order of 1% to
2%
 With
special precautions, this figure can
be lowered to a few tenths of one
percent.

Both methods use atomization of a sample and
therefore determine the concentrations of elements.

For AAS, absorption of radiation of a defined
wavelength is passed through a sample and the
absorption of the radiation is determined. The
absorption is defined by the electronic transition for
a given element and is specific for a given element.
The concentration is proportional to the absorbed
radiation.

In AES, the element is excited. A rapid relaxation is
accompanied by emission of UV or visible radiation is
used to identify the element. The intensity of the
emitted photon is proportional to element
concentration.
 Basic
Schematic
 Scanning
instruments can detect multiple
elements.
 Many
lines detected so sometimes it is a
quantitatively difficult method.
 Source
can be flame, but more commonly
plasma because it is much hotter.
1. Plasma Sources
Plasma – Highly ionized, electrically neutral
gaseous mixture of cations and electrons that
approaches temperature  10,000 K.

There are three types of plasma sources:
A) Inductively coupled plasma (ICP)
B) Direct current plasma (DCP)
C) Microwave induced plasma (MIP)

ICP is the most common plasma source.
Inductively Coupled Plasma (ICP)

Constructed of three concentric
quartz tube.

RF current passes through the
water-cooled Cu coil, which induces
a magnetic field.

A spark generates argon ions which
are held in the magnetic field and
collide with other argon atoms to
produce more ions.

Argon in outer tube swirls to keep
plasma above the tube.

The heat is produced due to the
formation of argon ions.
Inductively Coupled Plasma (ICP)
1. Sample Introduction
a. Liquid Sample
- Nebulizer similar to FAAS
- Sample nebulized in a
stream of argon with a
flow rate of 0.3 – 1.5 L/min.
- Sample aerosol enters the
plasma at the base through the
central tube.
b. Solid Samples
- Sample atomized by
electrothermal atomization a
and carried into the plasma by
a flow of argon gas.
Inductively Coupled Plasma (ICP)
Analyte Atomization and Ionization
By the time the analyte reaches the observation zone
it has resided in the excitation zone for  2 ms.
 Advantages of ICP over flame:
a) Temperature is two to three times higher than in a
flame or furnace, which results in higher
atomization and excitation efficiencies.
b) There is little chemical interference.
c) Atomization in the inert (argon) atmosphere
minimizes oxidation of the analyte.
d) Short optical path length minimizes the probability
of self-absorption by argon atoms in the plasma.
e) Linear calibration curves can cover up to five
orders of magnitude.

Inductively Coupled Plasma (ICP)
Plasma Appearance
a. Excitation Region



The bright, white, donut shaped
region at the top of the torch.
Radiation from this region is a
continuum with the argon line
spectrum superimposed.
Temperature: 8000 – 10 000 K
b. Observation Region


The flame shaped region above
the torch with temperatures 
1000 – 8000 K.
The spectrum consists of emission
lines from the analyte along with
many lines from ions in the torch.
2. Flame Emission Spectroscopy
Measure the intensity of emitted radiation
 Consists
1.
2.
3.
4.
5.
of:
Nebulizer
Burner
Monochromator
Detector
Readout device / computer

Sample is sprayed by the nebulizer into burner

Carried into the flame

Atomized & excited

The emission from the excited atoms passes into
the monochromator where the selected
wavelength is passed through for measurement

Intensity of the emitted wavelength is measured
by the detection system & indicated on the
readout/computer
Relationship Between Atomic Absorption and Flame
Emission Spectroscopy

Flame Emission -> it measures the radiation emitted
by the excited atoms that is related to concentration

Atomic Absorption -> it measures the radiation
absorbed by the unexcited atoms that are
determined.

Atomic absorption depends only upon the number of
unexcited atoms, the absorption intensity is not
directly affected by the temperature of the flame.

The flame emission intensity in contrast, being
dependent upon the number of excited atoms, is
greatly influenced by temperature variations.
Flame Emission Spectroscopy
Flame Emission Spectroscopy is based upon those
particles that are electronically excited in the
medium.
The Function of Flame
1. To convert the constituents of liquid sample
into the vapor state.
2. To decompose the constituents into atoms or
simple molecules:
M+ + e- (from flame)  M + hv
3.
To electronically excite a fraction of the
resulting atomic or molecular species:
M  M*
Flame Atomic
Absorption
Flame Atomic
Emission
Process measured
Absorption (light
Emission (light
absorbed by unexcited emitted by excited
atoms in flame)
atoms in a flame)
Use of flame
Atomization
Atomization &
excitation
Instrumentation
Light source
No light source
Beer’s Law
Applicable
Not applicable
Data obtained
A vs c
I vs c