Atomic Absorbtion

Introduction
 Analytical Chemistry deals with methods
for determining the chemical composition of
samples.
• Qualitative Analysis (identification)
provides information about the identity of
species or functional groups in the
sample (an analyte can be identified).
• Quantitative Analysis provides numerical
information of analyte (quantitate the
exact amount or concentration).

Analytical Methods
•Classical Methods: Wet chemical
methods such as precipitation, extraction,
distillation, boiling or melting points,
gravimetric and titrimetric measurements.
•Instrumental Methods: Analytical
measurements (conductivity, electrode
potential, light absorption or emission,
mass-to-charge ratio, fluorescence etc.)
are made using instrumentation.

Types of Instrumental Methods
1. Spectroscopic methods:
a. Atomic spectroscopy
b. Molecular spectroscopy
2. Chromatographic methods
(separations):
3. Electrochemistry:

Block diagram of an instrumental measurement

Applications of Instrumental
Methods
1. Bioanalytical: biological molecules and/or
biological matrices (e.g., proteins, amino
acids, blood, urine)
2. Environmental: pesticides, pollution, air,
water, soil
3. Material science: polymers,
characterization of new materials
4. Forensic science (application of science to
the law): body fluids, DNA, gun shot
residue, hair, fibers, elemental analysis,
drugs, alcohols, poisoning, fingerprints, etc.

Analytical Methodology
1. Plan: Qualitative or quantitative or both; what
kind of information have; which technique is
suitable etc.
2. Sampling: Accuracy depends on proper
sampling, characteristic of sample is very
important, required good representative
sample (from top, middle and bottom and mix
up and take average sample).
3. Sample preparation: depends on analytical
techniques.
4. Analytical measurement:
5. Data Analysis: Whether the data make sense

Selecting an Analytical Method
1. Accuracy and precision required
2. Available sample amount
3. Concentration range of the analyte
4. Interference in sample
5. Physical and chemical properties of the sample
matrix
6. Number of sample to be analyzed
7. Speed, ease, skill and cost of analysis

Types of Instrumental Methods:
• Emission of radiation
• Absorption of radiation
• Scattering of radiation
• Refraction of radiation
• Diffraction of the
radiation
• Rotation of the
radiation
• Electrical potential
• Electrical current
• Electrical resistance
• Mass
• Mass to charge ratio
• Rate of reaction
• Thermal charact.
• radioactivity

Atomic Spectroscopy Based on Ultraviolet
and Visible Radiation:
• Qualitative and Quantitative
determination of more than 70
elements.
• Sensitivities of Atomic methods lie
in the ppm, ppb and ppt range.
• Fast, High selective and moderate
instrument costs.

Atomic Absorption Spectroscopy:
A= ε b C
Bear-Lambert Law

Types of Atomic Absorption
Spectroscopy:
1- Flame Atomic Absorption
Spectreoscopy (FAAS)
2- Electrothermal ( Flame-less )
Atomic Absorption Spectroscopy
(EAAS)

Solution of
analyte
Spray
Nebulization
Desolvation
Solid/gas
aerosol
Atomic ions
Ionization
(reversible)
Dissociation
(reversible)
Atoms
Gaseous
molecules
Volatilization
Excited
molecules
hn atomic
Excited ions
Excited
atoms
hn molecular
hn ionic
Atomization
Processes

kT
E
e
g
g
N
N



0
*
0
*
Where g = degeneracy

• In this method, light from a source is
directed through the sample to a
detector.

Optical ATOMIC
SPECTROSCOPY:
1- Atomic Absorption Spectroscopy (AAS)
2- Atomic Emission Spectroscopy (AES)
3- Atomic Fluorescence Spectroscopy (AFS)

• Light source
Burner assembly
Optics
Detector
Signal processing
Instrumentation

1- Hollow Cathode Lamps (HCL)
Multielement lamps are available.
2- Electrodeless Discharge
Lamps (EDL)
Instrumentation
Light source

• The source of light is a lamp whose
cathode is composed of the element being
measured.
• Each element requires a different lamp.
Instrumentation
Light source-Hollow cathode lamp

Glass
shield
Anode
Quartz or pyrex
window
Ne or Ar at
1-5 torr
Hollow cathode
Instrumentation

Sputtering
Instrumentation

Instrumentation

• Microwave excited discharge tubes
• Intensities 10-100 x greater than from HCL
• Small amount of element or halide of an element
in a small sealed tube containing a few torr of
inert gas
• Placed in microwave cavity (2450 MHz)
• Argon is ionized, the ions are accelerated and
excite the metal atoms
• Less stable than HCL, but more intense.
• Not available for all elements
Instrumentation

RF Coil
Lamp
Ceramic
holder
Quartz
window
Instrumentation
Light source-Elctrodless Discharge lamp

Instrumentation
Light source-Elctrodless Discharge lamp

Burner assembly
Instrumentation

Fuel Oxidant
The Premixed Burner
Burner assembly
Instrumentation

1- Introduction of solution samples:
a) Pneumatic Nebulizers
b) Electrothermal (furnace) evaporator
c) Hydride generation (As, Sb, Sn, Se, Bi,
Pb )
d) Cold vapor atomization ( Hg )
Burner assembly
Instrumentation

Burner assembly-Pneumatic Nebulizers
Instrumentation

Flame pneumatic nebulizer
http://www.chemistry.nmsu.edu/Instrumentation
Typical uptake rate: ~5 mL/min
Typical delivery efficiency: ~5%
Adjustment of position of inner capillary

Properties of Flames
Fuel Oxidant Temperature o
C Max. burning
Velocity (cm/s)
Natural gas Air 1700-1900 39-43
Natural gss Oxygen 2700-2800 370-390
Hydrogen Air 2000-2100 300-440
Hydrogen Oxygen 2550-2700 900-1400
Acetylene Air 2100-2400 158-266
Acetylene Oxygen 3050-3150 1100-2480
Acetylene Nitrous oxide 2600-2800 285

Regions in a Flame
Secondary
combustion zone
Primary
combustion zone
Interzonal
region
C2, CH and other
radicals
Rich in free atoms
Stable molecular
oxides

1400
1700
1600
1750
1800
1858
1830
1863
1.5 1.0 0.5 0 0.5 .0 1.5
0
1
2
3
4
5
Distance
above
orifice
cm
Burner tip
Temperature Profile in oC for a
Natural Gas /Air Flame

Graphite Furnace
oC
150
Dry 600
Ash 3000
Atomize

Electrothermal evaporator :
There is no nebulziation, etc. The sample is
introduced as a drop (usually 10-50 uL)
• The furnace goes through several steps:
a- Drying (usually just above 110 deg. C.)
b- Ashing (up to 1000 deg. C)
c- Atomization (Up to 2000-3000 C)
d- Cleanout (quick ramp up to 3500 C or so). Waste is
blown out with a blast of Ar.

Graphite Furnace
L’vov platform
Graphite tube
Cooled electrical
connections
Cooled electrical
connections
Internal gas inlet
External gas inlet
Entrance
window
Exit window

From spectrophototmeter To detector
Hydride Generation Atomizer
Stirrer
Sample and
NaBH4
Inert gas
Quartz absorption
tube

Sample atomization techniques:
1- Flame atomization
2- Electrothermal atomization
3- Glow discharge atomization
4- Hydride atomization
5- Cold – vapor atomization

Flame absorbance profile for
three elements

Optical elements of
monochromators
1- An entrance slit
2- A collimating lens or mirror
3- A prism or grating
4- A focusing element
5- An exit slit
Optics

Abs = 0.762
Lamp Zn
Current = 3.0
+ =
Lamp supply
Burner Ebert monochromator
Detector
Phase sensitive amplifier
Source Modulation

Lamp supply
Burner Ebert monochromator
Detector
Typical Single Beam Flame
Atomic Absorption
Spectrophotometer

Photon transducers:
1- Vaccum phototubes
2- Photomultipliers tubes

Mechanical modulator or
Choppers

Interferences in Atomic
Absorption Spectroscopy
• Spectral Interferences
• Chemical Interferences

Spectral Interferences
• Line
• Broadband
• Scattering
nm

Interferences in Atomic
Absorption spectroscopy:
1- Spectral interferences
a- overlapping of two lines(< 0.01 nm- 308.211
V ,308.215 Al )
b- presence of combustion products (broad
band absorption- scatter the radiation by
particulate products)
C- absorption or scattering (CaOH in Ba
absorption, Ti, Zr and W refractory oxides or
incomplete combustion of organic solvents)
by the matrix components

2- Chemical interferences
a- formation of compounds of low volatile
( Ca-PO4
3- or SO4
-2 )
b- Dissociation equilibria
c- Ionization equilibria

Background Correction
Signal = ( Background + Signal ) – ( Backgound )
• Two-line method
• Continuum source method
• Self-reversal method
• Zeeman method

Two-line method
Monochromator
bandwidth
• Monochromator at
analytical wavelength:
signal + background
are measured
• Monochromator is
scanned to nearby line
not absorbed by the
analyte:
backgound is measured
• Signal = The difference
of two measurements

Continuum Source Method
Monochromator
bandwidth
• Monochromator at analytical
wavelength: signal +
background are measured
• Slit is opened, source is
replaced by deuterium lamp
(continuum source):
– tiny amount of light is absorbed
by sample and the rest is
scattered by backgound.
• Signal = the difference

Analyte Hollow
cathode lamp
Deuterium lamp
Chopper
Electrothermal
atomizer
To monochromator
Schematic of a continuum source background
correction system

mA
Power Supply
Self Absorption
As the current
increases
Line width also
increase
As the current
increses further
Number of unexcited
atoms increase and
absorb the center of
the line.

Self-Reversal Method
• Monochromator at
analytical wavelength:
signal + background
are measured.
• Current is pulsed high
– Background is
measured.
• Signal = the difference
Monochromator
bandwidth

Chemical Interferences
• Formation of Compounds of Low Volatility
– Anion
– Cation
To eliminate the effect:
• Higher temperature
• Releasing agents
• Protective agents

Chemical Interferences
• Dissociation Equilibria
– MO M + O
– M(OH)2 M + 2 OH
• Ionization Equilibria
– M M+ + e-

Interferences
• Spectral
• Vaporization
• Chemical

Spectral
• Mg 285.21 nm
• Na 285.28 nm
• Not usually much of a problem – can
change to another wavelength
• Problem worse in emission because more
lines – High T – lots of excitation
• Choice of line dictates concentration range
able to be analyzed

Vaporization Interferences
• When one component of a sample
influences the rate of vaporization of the
species of interest
• Physical – changes matrix it vaporizes
from
• Chemical – changes the species to be
vaporized

Chemical Vaporization
Interferences
• Metal oxides form
• Metal ions form thermally stable
complexes with anions
• The effects usually occur during formation
of the solid particle
• CaPO4 formation – a well known example.
• CaPO4 is harder to vaporize than Ca2+

CaPO4 - Interference Prevention
• Put light path higher in flame to allow a longer
residence time
• Add releasing agent – La2+ or Sr2+ (added in
excess) will preferentially combine with PO4
3-
and leave Ca2+ free to be analyzed
• Protective agent – add EDTA. Ca-EDTA
complex is easily destroyed in flame
• Glucose – burns easily and helps droplets
shatter apart
• Hotter flame – then need ionization suppressor

Figures of Merit
 Precision
 Bias
 Sensitivity
 Detection limit
 Concentration range (Dynamic range)
 Selectivity

 Precision: How close the same measurements are
to one another. The degree of mutual agreement
among data that have been obtained in the same
way. Precision provides a measure of the random
or indeterminate error of an analysis.
 Accuracy: How close the measurement
approaches the real value.
 Bias: Bias provides a measure of the systematic,
or determinate error of an analytical method.
bias =  - xt, where,  is the population mean and
xt is the true value

 Sensitivity: Sensitivity of an instrument is a
measure of its ability to discriminate between
small differences in analyte concentration. The
change in signal per unit change in analyte
concentration. The slope of the calibration curve at
the concentration of interest is known as
calibration sensitivity.
S = mc + Sbl
S = measured signal; c= analyte concentration;
Sbl = blank signal; m = sensitivity (Slope of line)
Analytical sensitivity ()
 = m/ss
m = slope of the calibration curve
s = standard deviation of the measurement

 Detection Limit (Limit of detection, LOD): The
minimum concentration of analyte that can be
detected with a specific method at a known
confidence level.
LOD is determined by S/N, where, S/N = Signal-to-
noise ratio = (magnitude of the signal)/(magnitude of
the noise)
• Noise: Unwanted baseline fluctuations in the
absence of analyte signal (standard deviation of the
background)
• The detection limit is given by,
Cm = (Sm – Sbl)/m, where, Cm = minimum
concentration i.e., LOD, Sm = minimum
distinguishable analytical signal (i.e., S/N = 2 or S/N

 Dynamic Range: The lowest concentration at which
quantitative measurements can be made (limit of
quantitation, or LOQ) to the concentration at which
the calibration curve departs from linearity (limit of
linearity, or LOL).
The lower limit of quantitative measurements is
generally taken to be equal to ten times the standard
deviation of repetitive measurements on a blank or
10 Sbl.
Dynamic range is the range over which detector still
responds to changing concentration (at high
concentrations – usually saturates – quits
responding)
An analytical method should have a dynamic range
of at least two orders of magnitude, usually 2-6

Selectivity: Selectivity of an analytical
method refers to the degree to which the
method is free from interference by other
species contained in the sample matrix. No
analytical method is totally free from
interference from other species, and steps
need to be taken to minimize the effects of
these interferences. Selectivity coefficient
gives the relative response of the method to
interfering species as compared with analyte.
Selectivity coefficient can range from zero
(no interference) to values greater than unity.
A coefficient is negative when the
interference caused a reduction in the

Calibration of Instrumental
Methods
 All types of analytical methods require
calibration for quantitation. Calibration is a
process that relates the measured analytical
signal to the concentration of analyte. We
can’t just run a sample and know the
relationship between signal and concentration
without calibrating the response
The three most common calibration methods
are:
• Calibration curve

Calibration Curves
• Several standards (with different concentration) containing
exactly known concentrations of the analyte are measured
and the responses recorded.
• A plot is constructed to give a graph of instrument signal
versus analyte concentration.
• Sample (containing unknown analyte concentration) is run,
if response is within the LDR of the calibration curve then
concentration can be quantitated.
• Calibration curve relies on accuracy of standard
concentrations.
• It depends on how closely the matrix of the standards
resemble that of the sample to analyzed.
• If matrix interferences are low, calibration curve methods
are OK.
• If matrices for sample and standards are not same
calibration curve methods are not good.

Standard Addition Methods
 Better method to use when matrix effects can be
substantial
 Standards are added directly to aliquots of the
sample, therefore matrix components are the same.
 Procedure:
• Obtain several aliquots of sample (all with the same
volume).
• Spike the sample aliquots ==> add different volume of
standards with the same concentration to the aliquots
of sample
• Dilute each solution (sample + standard) to a fixed
volume

 Instrumental measurements are made on each solutions to
get instrument response (S). If the instrument response is
proportional to concentration, we may write,
S = (kVsCs)/Vt + (kVxCx)/Vt
Where, Vx =Volume of sample = 25 mL (suppose)
Vs = Volume of standard = variable (5, 10, 15, 20 mL)
Vt = Total volume of the flask = 50 mL
Cs = Concentration of standard
Cx = concentration of analyte in aliquot
k = proportionality constant
 A plot of S as a function of Vs is a straight line of the form,
S = mVs+b
Where, slope, m = (kCs)/Vt and intercept, b = (kVxCx)/Vt
Now, b/m = (kVxCx)/Vt x Vt/(kCs)
Cx = bCs /mVx

Standard Addition Method
Another approach to determine Cx
• Extrapolate line on plot to x-intercept
• Recall: At Vs = 0  instrument response (relating
to concentration of x in sample)
• At x-intercept, you know the volume of analyte
added to (i.e., inherent in) the sample.
• Another way: This value S = 0 (no instrument
response)  no analyte present in sample
In any case, Since S = 0,
Therefore, S = (kVsCs)/Vt + (kVxCx)/Vt = 0
Solve for Cx,
Cx = - (Vs)oCs / Vx

• In the interest of saving time or sample, it is possible to
perform standard addition analysis by using only two
increments of sample. A single addition of Vs mL of
standard would be added to one of the two samples
and we can write, S1 = (kVxCx)/Vt and S2 = (kVxCx)/Vt +
(kVsCs)/Vt
S
S
k V C V C
V
X
V
kV C
V C
V C
V C
V C
S S
S
C
S VC
V S S
x x s s
t
t
x x
s s
x x
s s
x x
x
s
x
2
1
2 1
1
1
2 1
1


 




( )
( )

Internal standard Method
An Internal Standard is a substance that is
added in a constant amount to all samples,
blanks and calibration standards in an
analysis.
Calibration involves plotting the ratio of the
analyte signal to the internal standard signal
as a function of analyte concentration of the
standards.
This ratio for the samples is then used to
obtain their analyte concentrations from a
calibration curve.
Internal standard can compensate for

Atomic Absorption Analytical
methodes :
1- Calibration curves
2- Standard addition method

A calibration curve and the equation for
the line can be used to determine an
unknown concentration based on its
absorbance.

Standard additions often used with GFAAS

ICP
Inductively Coupled Plasma
–much higher temperatures than AA
–thus higher number of atoms in
excited state
–thus stronger signal

Ar
sample solution
load coil
plasma
atomic emission
Inductively Coupled Plasma “torch”

Characteristic of the ICP:
• High temperature
• Long residence time
• High electron number densities (few
ionization interferences)
• Free atoms formed in nearly chemically inert
environment
• Molecular species absent or present in very
low levels
• No electrodes
• No explosive gases

Advantages of plasma:
1- more complete atomization
2- fewer chemical interferences
3- low ionization interference effects
4- atomization occurs in a chemically
inert environment
5- temperature cross section of plasma is
relatively uniform
6- wider linear range

Disadvantages of ICP:
• Expensive
• Spectral overlap
• Is not simple to operate
– Considerable training is required to become
an efficient and knowledgeable user of ICP

Plasma Application:
1- Useful for both quantitative
and qualitative determination in
liquids ( organic or aqueous
solvent)
2- All metal elements can be
determined.

Advantages of Plasma, Arc and
Spark emission spectrometry
(vs. to FAAS and EAAS):
1- Lower interelement interference
2- Obtaining good emission spectra for most
elements under a single set of excitation
conditions
3- Determination of low concentrations of
elements that form refractory compounds.
4- Determination of nonmetals such as F,Cl,I
and S by plasma based AES.
5- Methods based upon plasma sources have
concentration ranges of several decades

Advantages of FAAS and EAAS
vs. to Plasma, Arc and Spark
emission spectrometry:
1- Simple
2- Less expensive equipment
requirements
3- Lower operating costs
4- Procedures that require less operator
skills

Summary & Comparison of
Common Atomic Spectrocopies
FAAS ETA-AAS ICP-AES
Qualitative
Abilities:
Fair Fair Good
Quantitative
Abilities:
Very Good Excellent Excellent
Expense: $15,000+ $30,000+
$50 – 100
K+
Simultaneous
Multielement
Analysis: Difficult Very Difficult Easy
Detection Limits: ppb - ppm < ppb < ppb - ppt

Atomic Absorbtion

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