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Chetan SharmaQuality control department

Introduction Flame photometry (more accurately called flame atomic

emission spectrometry)

It is a branch of atomic spectroscopy in which the species

examined in the spectrometer are in the form of atoms

Based on measurement of intensity of the light emitted when a

metal is introduced into flame

• The wavelength of color tells what the element is .

• The color's intensity tells us how much of the element present.

History

The history of spectroscopy starts

with the use of the lens by

Aristophanes about 423 B.C.; and

the studies of mirrors by Euclid (300

B.C.) and Hero (100 B.C.).

Seneca (40 A.D.) observed the light

scattering properties of prisms.

In 100 A.D. Ptolemy studied

incidence and refraction.

Fraunhofer , about 1814-15, observed diffraction phenomena and

was able to measure wavelength instead of angles of refraction.

Herschel (1823) and Talbot (1825) discovered atomic emission

when certain atoms were placed in a flame.

Wheatstone concluded in 1835 that metals could be distinguished

from one another on basis on the wavelengths of this emission.

In 1848, Foucault observed atomic emission from sodium and

discovered that the element would absorb the same rays from

an electric arc.

In the later 1800, scientists such as Kirchoff, Bunsen, Angstrm, Rowland, Michelson and Balmer studied the composition of the sun based on their emissions at different wavelengths.

Kirchhoff summarized the law which states that, "Matter absorbs light at the same wavelength at which it emits light". It is under this law that atomic absorption spectroscopy works.

Woodson was one of the first to apply this principle to the detection of mercury.

In 1955, Walsh suggested the use of cathode lamps to provide an emission of appropriate wavelength; and the use of a flame to produce neutral atoms that would absorb the emission as they crossed its path

Principle

Liquid sample contaning metal salt solution introduced into a flame:

Solvent is vaporised , leaving particles of solid salt

Salt is vaporised into gaseous state

Gaseous molecule dissociate to give neutral atoms

Neutral atoms are excited by thermal energy of flame

The unstable excited atoms emit photons while returning to lower energy state

The measurement of emitted photons forms the basis of flame photometry.

Structure of flame

As seen in the figure, the flame may be divided into the following regions or zones.

i) Preheating zones

ii) Primary reaction zone or

inner zone

iii) Internal zone

iv) Secondary reaction zone

preheating zone- In this combustion mixture is heated to the ignition temperature by thermal conduction from the primary reaction zone.

primary reaction zone- This zone is about 0.1 mm thick at atmospheric pressure

There is no thermodynamic equilibrium in this zone and the concentration of ions and free radicals is very high.

This region is not used for flame photometry.

interconal zone – It can extend up to considerable height. The maximum temperature is achieved just above the tip of the inner zone.

This zone is used for flame photometry.

secondary reaction zone - In this zone, the products of the combustion processes are burnt to stable molecular species by the surrounding air.

Instrumentation1. Sample Delivery

2. Source

3. Monochromator

4. Detector

5. Read out device

Schematic diagram showing the layout of various components of a flame photometer

Sample deliveryThere are three components for introducing liquid sample.

Nebulizer – it breaks up the liquid into small droplets

Aerosol modifier – it removes large droplets from the stream and allow only smaller droplets than a certain size to pass

Flame or Atomizer – it converts the analyte into free atoms

Sample Introduction TechniquesLiquid:

Pneumatic Nebulization

Ultrasound Nebulization

Electro thermal Vaporization

Solid:

Direct Insertion

Electro thermal Vaporization

Laser Ablation

Types of sample introduced

Nebulization Nebulization: is conversion of a sample to a fine mist of

finely divided droplets using a jet of compressed gas.

The flow carries the sample into the atomization region.

Pneumatic Nebulizers: (most common)

Four types of pneumatic nebulizers:

Concentric tube

Cross flow

Fritted disk

Babington

• Concentric tube - the liquid sample is sucked through a capillary tube by a high pressure jet of gas flowing around the tip of the capillary.

-This is also referred to aspiration. The high velocity breaks the sample into a mist and carries it to the atomization region.

• Cross-flow -The jet stream flows at right angles to the capillary tip. The sample is sometimes pumped through the capillary.

• Fritted disk -The sample is pumped into a fritted disk through which the gas jet is flowing. Gives a finer aerosol than the others.

• Babington - Jet is pumped through a small orifice in a sphere on which a thin film of sample flows. This type is less prone to clogging and used for high salt content samples.

Ultrasonic Nebulizer-The sample is pumped onto the surface of a vibrating piezoelectric crystal.

-The resulting mist is denser and more homogeneous than pneumatic nebulizers.

Electro-thermal Vaporizers (Etv)-An electro thermal vaporizer contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into the atomizer.

Source

Source

Burner-which are used to spray the sample solution

into fine droplets.

Several burners and fuel – oxidant combinations have been used to produce analytical flame

Premixed burner

Mecker burner Total consumption burner

Lundergarh burner

Shielded burner

Nitrous oxide-acetylene flames

Premixed burner

widely used because uniformity in flame intensity

In this energy type of burner , aspirated sample , fuel and oxidant are thoroughly mixed before reaching the burner opening.

Total consumption burner

In this fuel and oxidant are hydrogen and oxygen gases

Sample solution is aspirated through a capillary by high pressure of fuel and Oxidant and burnt at the tip of burner

Entire sample is consumed.

Mecker burner - Generally used for study of alkali metals only.

Lundergarh burner-In this sample must be in liquid form

• Large droplets condense on the side or drain away

• Small droplets and vaporised sample are swept into the base of flame in form of cloud

• Devices such as ultrasonic vibrators are used to enhance the nebulization stage in this burner.

Shielded Burner• In this flame was shielded from the ambient

atmosphere by a stream of inert gas.

• Shielding is done to get better analytical sensitivity.

• Following results are obtained with shielded burner

Element Sensitivity(ppm)

Ba 0.05

Ca 2

Cr 0.002

Pb 0.05

Mg 0.3

Nitrous oxide-Acetylene flame• These flames were superior to other flames for

effectively producing free atoms

• Eg.-metals with very reflective oxides such as aluminium and titanium.

The drawback of it is:

• the high temperature reduces its usefulness for the determination of alkali metals as they are easily ionized

• Intense background emission, which makes the measurement of metal emission very difficult

MonochromatorConverts a polychromatic light into monochromatic light.

It is of two types:

1. Prism : Quartz material is used for making prism, as quartz is transparent over entire region

2. Grating : it employs a grating which is essentially a series of parallel straight lines cut into a plane surface

Prism monochromator

Grating monochromator

Detectors

Photomultiplier tubes

Photo emissive cell

Photo voltaic cell

Flame Photometric detector

Photovoltaic cell

• It has a thin metallic layer coated with silver or gold act as electrode , also has metal base plate which act as another electrode

• Two layers are separated by semiconductor layer of selenium, when light radiation falls on selenium layer.

• This creates potential diff. between the two electrode and cause flow of current.

Read Out Device It is capable of displaying the absorption spectrum as well

absorbance at specific wavelength

Nowadays the instruments have microprocessor

controlled electronics that provides outputs compatible

with the printers and computers thereby minimising the

possibility of operator error in transferring data.

Applications Qualitative application:

• used for the determination of alkali and the alkaline earth metals

• elements can be detected visually by the colour in the flame, e.g. sodium produces yellow flame

• In this flame photometer with a filter or monochromator of separate radiation with the wavelength characteristic of the different metals are used . If the radiation of the characteristic wavelength is detected, it will indicate the presence of the metal in the sample.

The table below gives details of the measurable atomic flame emissions of the alkali and alkaline earth metals in terms of the emission wavelength and the colour produced.

Elements Emission Wavelength(nm)

Flame Colour

Sodium(Na) 589 Yellow

Potassium(k) 766 Violet

Barium(Ba) 544 Lime green

Calcium(Ca) 422 Orange

Lithium(li) 670 Red

Quantitative application

It is done by determining the concentration of various alkali and alkaline earth metals

It is done by two methods:

I. Standard addition method

II. Internal standard method

Elements, their characteristic emission wavelengths and detectionlimits

Element wavelength

Detection limit

Element wavelength

Detection limit

Al 396 0.5 Pb 406 14

Ba 455 3 Li 461 0.067

Ca 423 0.07 Mg 285 1

Cu 325 0.6 Ni 355 1.6

Fe 372 2.5 Hg 254 2.5

Other applications Useful in determination of Na , K , Al, Ca , B:

• In biological fluids and tissues

• In soil analysis

Used for natural and industrial waters, glass , cement , petroleum products.

INTERFERENCES IN QUANTITATIVE DETERMINATIONS The interferences encountered can be classified as

follows.

• Spectral interferences

• Ionised interferences

• Chemical interferences

Spectral interferences The first type of interference arises when two

elements exhibit spectra, which partially overlap, and both emit radiation at some particular wavelength.

eg. - the Fe line at 324.73 nm overlaps

with the Cu line at 324.75 nm.

• It can overcome either by taking measurements at an alternative wavelength which has no overlap, if available, or by removing the interfering element by extraction.

The second type of spectral interference deals with spectral lines of two or more elements which are close but their spectra do not overlap.

• It can be reduced by increasing the resolution of the spectral isolation system.

A third type of spectral interference occurs due to the presence of continuous background which arises due to high concentration of salts in the sample, especially of alkali and alkaline earth metals

• This type of interference can be corrected by using suitable scanning technique.

Ionisation interferences high temperature flame may cause ionisation of some

of the metal atoms, e.g. sodium

Na Na+ + e_

The Na+ ion possesses an emission spectrum of its own with frequencies, which are different from those of atomic spectrum of the Na atom.

• The addition of potassium salt suppresses the ionisation of sodium, as the potassium atom itself undergoes ionisation due to low ionisation energy.

Chemical Interferences:

The chemical interferences arise out of the reaction between different interferents and the analyte . These are of different types:

Cation-anion interference• The presence of certain anions, such as oxalate,

phosphate, sulphate , in a solution may affect the intensity of radiation emitted by an element, resulting in serious analytical error.

• For example, calcium in the presence of phosphate ion forms a stable substance, as Ca3(PO4)2 which does not decompose easily, resulting in the production of lesser atoms.

Cation-cation interferences• Due to mutual interferences of cations

• These interferences are neither spectral nor ionic in nature

• Eg. aluminum interferes with calcium and magnesium.

Interference due to oxide formation:

It arises due to the formation of stable metal oxide if oxygen is present in the flame

Limitations The temperature is not high enough to excite

transition metals, therefore the method is selective towards detection of alkali and alkaline earth metals.

The relatively low energy available from the flame leads to relatively low intensity of the radiation from the metal atoms.

The low temperature renders to interference and the stability of the flame and aspiration conditions.

Interference by other elements is not easy to be eliminated.

New technology in flame photometry

BWB XP Flame Photometer

It is the first and only 5 channel flame photometer Simultaneous detection and display of all 5 elements like potassium (K), Sodium (Na), Lithium (Li), Calcium (Ca) and Barium (Ba). A high quality and high performance flame photometer, which improve both accuracy and stability while significantly reducing analysis time.