Flame emission & atomic absorption spectroscopy

Flame emission and Atomic
absorption Spectroscopy
Presented by: Guided by:
Himal Barakoti Arup Chakroborty
M. Pharm, 1st Sem Assistant Professor
Department of Pharmacy Department of Pharmacy
Assam Down Town University Assam Down Town University

Contents:
 Introduction
 Theory and principle of Flame emission Spectroscopy
 Instrumentation
 Interferences
 Application
 Atomic Absorption Spectroscopy
 Theory and Principle
2

Contents:
 Instrumentation
 Interferences
 Application
 Comparison between Flame emission and Atomic absorption
Spectroscopy
 References
3

Introduction
 Atomic spectroscopy is thought to be the oldest
instrumental method for the determination of
elements.
 These techniques are introduced in the mid of 19th
Century during which Gustav Kirchhoff and Robert
Bunsen showed that the radiation emitted from the
flames depends on the characteristic element present in
the flame.
 The developments in the instrumentation area led to the
widespread application of atomic spectroscopy.
 The absorption and emission of radiant energy by
atoms provide powerful analytical tools for both
quantitative and qualitative analysis of substance.
4

Flame emission Spectroscopy
 Flame emission spectroscopy (FES) is a method of chemical analysis that
uses intensity of light emitted from flame, arc or spark at particular
wavelength to determine quantity of element in sample.
 The basis of flame photometric working is that, the species of alkali metals
(Group 1) and alkaline earth metals (Group II) metals are dissociated due to
the thermal energy provided by the flame source.
 Due to this thermal excitation, some of the atoms are excited to a higher energy
level where they are not stable.
 The absorbance of light due to the electrons excitation can be measured by using
the direct absorption techniques.
5

 The subsequent loss of energy will result in the movement of excited atoms
to the low energy ground state with emission of some radiations, which can
be visualized in the visible region of the spectrum.
 The intensity of radiation emitted by these excited atoms returning to the
ground state provides the basic for analytical determination in FES.
 The wavelength of emitted light is specific for specific elements.
 In flame emission spectroscopy,
o Wavelength of spectral lines give identity of elements.
o Intensity of emitted light is directly proportional to the number of atoms
present.
6

Instrumentation
 Flame photometers are the simplest type of atomic spectrometers.
 Solution is introduced into a fine spray.
 Solvents evaporates leaving dehydrated salt.
 Certain fraction of atoms absorbs energy and are raised to excited state.
 These excited atoms on returning to ground state emits photons of certain wavelength.
 Flame emission passes through monochromator which filters all emitted light expect the
wavelength of our interest.
 Photoelectric Detector measures the intensity of filtered light.
7

1. Atomizer/Nebulizer
 Process of conversion of sample to a fine mist of finely divided droplets using a
jet of compressed gas.
o Pneumatic nebulizers
o Electro thermal vaporizer
o Ultrasound nebulizer
2. Burner
 They must have ability to evaporate the liquid droplets from sample solution and
capacity to excite atoms formed and cause them to emit radiant energy.
o Mecker burner
o Total consumption burner
o Laminar flow burner
o Shileded burner
o Lundergraph burner
8

3. Monochromator (Grating or prism monochromator)
They disperse radiation coming from the flame and falling on it. Dispersed
radiation goes to detector from exit slit.
Filters chosen has wavelength range transparent to emission from element
of interest.
4. Detectors (photoemissive cells or photomultiplier tubes)
They measure intensity of radiation falling on it
5. Amplifier & readout devices
It suitably amplify and display detector’s output.
Amplifier sensativity can be changed to be able to analyze sample of
varying concentration.
9

Types of flame used
The most common instrument uses air as oxidant. The temperature of the
flames produced is relatively low so the technique is only suitable for the
elements that are easily excited such as alkali and alkali earth elements.
10
Flame Temp/˚C
Gas/Air 1700-1900
Gas/O2 2700-2800
H2/Air 2000-2100
H2/O2 2550-2700
C2H2/Air 2100-2400
C2H2/O2 3050-3150
C2H2/N2O 2600-2800

 A higher temperature will tend to increase the number of atoms in the excited
state and hence the signal.
 Some detection limits:
11
Elements Spectral lines/nm Flame Detection limits/ppm
Al 396 C2H2/N2O 0.1
Ba 553 C2H2/N2O 0.01
K 766 C2H2/O2 0.001
Li 671 C2H2/N2O 0.0001

Interferences
 Number of factors beside the analyte affect the intensity of emitted radiation. The
analytical signals often include contributions from constituents other than
analyte termed as interferences and are found to interfere the outcome of
procedure.
 3 types:
1. Spectral Interferences: interference that affect spectral intensity or resolution.
a. 1st type of interference arises when two elements exhibit spectra, which
partially overlap, and both emit radiation at particular wavelength. The
detector cannot distinguish between the source of radiation and records the
total sign, thus resulting in incorrect answer. This type of interferences are
more common in high flame temperature as numerous spectral lines are
produced in high temperature.
E.g., Fe line at 324.73 nm overlaps with Cu line at 324.75 nm. This Can be
overcome by taking alternative wavelength with no overlap.
12

b. 2nd type of spectral interference deals with spectral lines of two or more
elements which are close but their spectra do not overlap. The filter may allow
spectral lines separated by 5.0-10.0 nm to pass through, thus resulting in error in
analysis. This can be reduced by increasing the resolution of spectral
isolation system.
c. 3rd type of spectral interference occurs due to presence of continuous
background which arises due to high concentration of salts in the sample. Some
organic solvents also produce continuous background. This can be corrected by
using suitable scanning technique.
2. Ionization Interference: In certain cases, high temperature flame may cause
ionization of the metal atoms. E.g., In case of sodium
Na Na+ + e-
The Na+ ion possesses an emission spectrum of its own frequencies, which
are different from those of Na atom. This reduces the radiant power of atomic
emission. This type of interference can be eliminated by addition of large
quantity of potassium salt to standard and sample solution. Potassium itself
undergoes ionization due to low ionization energy and suppresses the ionization
of sodium.
13

3. Chemical Interferences: This arises out of reaction between different
interferents and the analyte.
a. Cation-cation interference: The presence of certain anions, such as oxalate,
phosphate, sulphate and aluminate in a solution may affect the intensity of
radiation emitted by an element, resulting in serious analytical error. For e.g.,
calcium in presence of phosphate ion forms a stable substance as Ca3(PO4)2
which does not decompose easily, resulting in production of lesser atom. Thus
calcium signal is depressed. This can be removed either by extraction of anion
or by using calibration curve prepared from standard solutions containing
same concentrations of the anion as found in the sample.
b. Cation-cation interference: In many cases, mutual interferences of cations results
in reducing element signal intensity. These interference are neither spectral nor
ionic in nature and the actual mechanism has not been well understood. E.g.,
Al interferes with Ca and Mg. Na interferes with Mg.
c. Oxide formation: arises due to formation of stable metal oxide if oxygen is
present in flame, resulting in reduced signal intensity. E.g., Alkaline earth metal.
This can be eliminated by either using high flame temperature to dissociate
the oxides or by using oxygen-deficient environment to produce excited
atom.
14

Limitations
 As natural gas and air flame is employed for excitation, the temperature is not
high enough to excite transition metals, therefore the method is selective
towards detection of alkali and alkaline earth metals.
 Low temperature makes this method susceptible to certain disadvantages, most
of them related to flame stability and aspiration conditions. Fuel and oxidation
flow rates and purity, aspiration rates, solution viscosity affects these.
 FES is a means of determining the total metal concentration of a sample; it tells
us nothing about the molecular form of the metal in original sample.
 Only liquid samples can be used.
15

Applications
 Flame Photometers are widely used in quality control where a simple and quick
determination of alkali or alkali earth metal is required. They have the
advantage of being significantly lower priced than most other atomic
spectrometers.
 Biological/medical applications- notable applications are the determinations of
Na, K, Ca and Mg in body fluids and other biological samples.
 Food industry – Determination of calcium and iron in beer
16

Atomic Absorption Spectroscopy
 Guystav Kirchoff and Robert Bunsen first used atomic absorption
spectroscopy—along with atomic emission—in 1859 and 1860 as a means for
identify atoms in flames and hot gases.
 Modern atomic absorption spectroscopy has its beginnings in 1955 as a result of
the independent work of Alan. C. Walsh and C. T. J. Alkemade.
17

 Atomic absorption spectroscopy (AAS) is an absorption spectroscopic method
where radiation from a source is absorbed by non-excited atoms in vapour
state.
 AAS deals with the absorption of specific wavelength of radiation by neutral
atoms in the ground state.
 Reliable for detecting over 70 elements with metals and metalloids.
18

Theory and Principle
 The technique uses basically the principle that free atoms (gas) generated in an
atomizer can absorb radiations at specific frequency.
 AAS quantifies the absorption of ground state atoms in the gaseous form.
 Atoms absorb UV/visible light and make transition to higher electronic energy levels. The
analyte concentration is determined from the amount of absorption.
 Concentration measurement are usually determined from a working curve after
calibrating instrument with standard known concentration.
Liquid sample Formation of droplets Fine residue Formation of neutral ion
Neutral atom absorbs specific wavelength of radiation from hollow cathode lamp
Measurement of intensity of radiation absorbed by the neutral atom using the detector
19

 The extent to which radiation of a particular frequency is absorbed by an atomic
vapour is related to the length of the path transverse and to the concentration of
absorbing atoms in the vapour.
 This is analogues to the Beer-Lamberts law relating to samples in solution.
Thus, for a collimated monochromatic beam of radiation of incident Io passing
through an atomic vapour of thickness I
Iv = Io eл-kvI
Where,
Iv is the intensity of the transmitted radiation
Kv is absorption co efficient
20

1. Sharp line radiation source (hollow cathode lamp)
 Hollow cathode lamps are most commonly used radiation containing tungsten
anode and hollow cylindrical cathode made of element to be determined.
 These are sealed with gas tube filled with an inert gas.
 When a current flows between the anode and cathode in this lamps, metal atoms
are sputtered from the cathode cup, and collisions occur with the filler gas. A
number of metal atoms become excited and give off their characteristics radiation.
22

2. A solution Nebulizer and burner
 Nebulizer suck up liquid samples at controlled rate and create a fine aerosol spray
for introduction into flame.
 Aerosol, fuel and oxidant are thoroughly mixed for introduction into flame.
 Atomization is carried out by separation of particles into individual molecules and
breaking molecules into atoms. This is done by exposing analyte to high
temperature in flame or graphite furnace.
 In graphite furnace, samples are deposited in a small graphite coated tube which
can then be heated to vaporize and atomize the analyte.
 Graphite tubes are heated using a high current power supply.
23

3. Monochromator
 They are used to select specific wavelength of light which s absorbed by the
sample, and to exclude other wavelengths.
 Selections of specific light allows the determination of selected element in the
presence of others.
4. Detectors
 The light selected by monochromator is directed onto a detector that is typically a
photomultiplier tube which convert the light signal into electrical signal
proportional to the light intensity.
 The signal is amplified, displayed for readout of fed to data station for print out.
24

Interferences
 Spectral Interferences: Spectral interferences are caused by presence of
another atomic absorption line or a molecular absorbance band close to the
spectral line of element of interest.
 Chemical interference: If a sample contains a species which forms a thermally
stable compound with the analyte that is not completely decomposed by the
energy available in the flame then chemical interference exists. Refractory
elements such as Ti, W, Zr, Mo and Al may combine with oxygen to form thermally
stable oxides. Analysis of such elements can be carried out at higher flame
temperatures using nitrous oxide – acetylene flame instead of air-acetylene to
provide higher dissociation energy.
25

 Matrix interference: When a sample is more viscous or has different surface
tension than the standard it can result in differences in sample uptake rate due
to changes in nebulization efficiency. Such interferences are minimized by
matching as closely as possible the matrix composition of standard and
sample.
 Ionization interference: Excess energy of the flame can lead to excitation of
ground state atoms to ionic state by loss of electrons thereby resulting in
depletion of ground state atoms.
26

limitations
 Expensive
 Low precision
 Low sample throughput
 Requires high level of operator skill
 Sample must be in solution or at least volatile
 Individual source lamps required for each element
27

Applications
 Determination of metal at trace level in solution.
 Determination of purity:
o Presence of heavy metal in body fluids
o Pollution of water by metals
o Food stuffs
o Soft drinks and beer
o Analysis of Geochemical exploration for mineral
o Soils, crude oils, petroleum products, plastics.
28

Comparison between FES and AAS
Flame Emission spectroscopy Atomic Absorption spectroscopy
1. Amount of light emitted by excited atom is
measured.
1. Amount of light absorbed by ground state
atom is measured.
2. Absorption intensity and signal response
greatly influenced by temperature variation.
2. Absorption intensity and signal response
does not depend upon temperature.
3. Beer’s law is not obeyed. 3. Beers law is obeyed.
4. Intensity of emitted radiation is directly
proportional to the number of atoms in excited
state.
4. Intensity of absorbed radiation is directly
proportional to the number of atoms in ground
state.
5. Relation between emission intensity vs.
concentration is not much linear.
5. Absorption intensity vs concentration of
analyte is much linear.
6.Atomization and excitation flame used. 6. Atomization flame used.
7. Intensity vs concentration data is obtained. 7. Absorbance vs concentration data is
obtained
8. Limited to alkali and alkali earth metals. 8. Useful for more than 70 metals.
29

References
 G. Gauglitz and T. Vo-Dinh “Handbook of spectroscopy” pg 421-493
 J. Michael Hollas “Modern Spectroscopy” 4th Ed, pg 64-65
 https://socratic.org/questions/what-is-the-principle-of-flame-emission-spectroscopy
 http://vlab.amrita.edu/?sub=2&brch=294&sim=1351&cnt=1
 https://www.britannica.com/science/flame-emission-spectroscopy
 https://chem.libretexts.org/Textbook_Maps/Analytical_Chemistry_Textbook_Maps/
Map%3A_Analytical_Chemistry_2.0_(Harvey)/10_Spectroscopic_Methods/10.4%3
A_Atomic_Absorption_Spectroscopy
 https://web.nmsu.edu/~esevosti/report.htm
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