1

ATOMIC ABSORPTION AND EMISSION SPECTROSCOPY

Principle

The absorption of energy by ground state atoms in the gaseous state forms the of

atomic absorption spectroscopy

When a solution containing metallic species is introduced into a flame, the vapor

of metallic species will be obtained. Some of the metal atoms may be raised to an energy

level sufficiently high to emit the characteristic radiation of the metal-a phenomenon that

is utilised in the familiar technique of emission flame photometry. But a large percentage

of the metal atoms will remain in the non-emitting ground state. These ground stage atoms

of a particular element are receptive of light radiation of their own specific resonance

wavelength (in general, the same wavelength as they would emit if excited). Thus, when a

light of this wavelength is allowed to pass through a flame having atoms of the metallic

species, part of that light will be absorbed and the absorption will be proportional to the

density of the atoms in the flame. Thus, in atomic absorption spectroscopy, one

determines the amount of light absorbed. Once this value of absorption is known, the

concentration of the metallic element can be known because the absorption is proportional

to the density of the atoms in the flame. Mathematically, the total amount of light

absorbed may be given by the expression as follows:

At ν the total amount of light absorbed = Nfmcπe2

……. (1)

where e is the charge on the electron of mass m, c the speed of light, N the total number of

atoms that can absorb at frequency ν in the light path and f the oscillator strength or ability

for each atom to absorb at frequency, ν. As π, e, m and c are constants, equation (1) can be

simplified to the following expression:

Total amount of high absorbed = constant x N x f ……. (2)

From expression (2), it follows that

(a) Firstly, there is no term involving the wavelength (or frequency) of absorption other

than the indication of the actual absorption wavelength.

(b) Secondly, there is no term involving the temperature.

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Fig. 1. Arrangement for Atomic Absorption Spectroscopy

From above, it follows that absorption by atom is independent of the wavelength of

absorption and the temperature of the atoms. These two features provide atomic

absorption spectroscopy distinct advantages over flame emission spectroscopy.

In should be pointed out, however, that although the temperature does not affect the

process of absorption by atoms, it does affect the efficiency with which atoms are

produced from a sample and therefore indirectly affects the signal quite significantly.

Furthermore, some atoms, particularly those of the alkali metals, easily ionize at high

temperatures. Ions do not absorb at atomic absorption wavelengths.

Atoms that become ionized are effectively removed from the absorbing population

resulting in a loss of signals.

Instrumentation:

A schematic diagram of the absorption spectrometer is shown in Fig.1. The

Principle of the instrumentation is similar to other spectroscopic absorption methods.

Light of a certain wavelength (produced by a special kind of lamp), which is able

to emit the spectrallines corresponding to the energy required for an electronic transition

from the ground state to an excitedstate, is allowed to pass through the flame. Meanwhile

the sample solution is aspirated into to the flame. Before it enters the flame, the solution

gets dispersed into a mist of very small droplets which evaporatesin the flame to give the

dry salt, and then the vapor of the salt. At least a part of this vapor will bedissociated into

atoms of the element to be measured. Thus, the flame possesses free unexcited

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atomswhich are capable of absorbing radiation, from an external source when the

radiation corresponds exactlyto the energy required for a transition element from the

ground electronic state to an upper excited level. Then the unabsorbed radiation from the

flame is allowed to pass through a monochromator which isolatesthe exciting spectral

lines of light source. From the monochromator the unabsorbed radiation is led into the

detector which is then registered by a photo-detector, the output of which is amplified and

measuredon a recorder. Absorption is measured by the difference in transmitted signal in

the presence and absenceof test element.

For all types of atomic absorption spectrometer, the following components are required:

1. Radiation Source. The radiation source for atomic absorption spectrophotometer

should emitstable, intense radiation of the element to be determined, usually a resonance

line of the element. Preferably, the resonance spectral lines should be narrow as compared

with the width of the absorption lines to bemeasured. These lines should not be interfered

from other spectral lines which are not resolved byspectrophotometer. There should be no

general background or other extraneous lines emitting within theband pass of the

monochromator. The problem of using such narrow spectral lines has been solved

byadopting a hollow cathode lamp as the radiation source.

(a) Hollow Cathode Lamp. Hollow cathode lamp is shown is Fig. 2. The cathode

consists of a hollowcup. In the cup is the element which is to be determined, in this case

sodium. The anode is a tungstenwire. The two electrodes are housed in a tube containing

an inert gas. The lamp window is constructedof either quartz, silica, or glass. The exact

material depends upon the wavelength which is to betransmitted. When a potential is

applied between the two electrodes, a current in the milliampere range arises, the inert gas

is charged at the anode, and the charged gas is attracted at high velocity to thecathode.

The impact with the cathode vaporizes some of the sodium atoms. These are exited and

upon returning to the ground state give rise to the sodium emission spectrum.

The manufacturer’s recommended current for the lamp should not be exceeded or

the lifetime of the lamp may be considerably shortened. Excess current may also rise to a

self-absorption process wherein the sputtered ground-state atoms absorb some of the

emitted energy, resulting in a lessening of intensity of the emitted the (see fig. 3). On the

other hand, if the source lamp is run below the recommended current, a loss of intensity

and a corresponding loss of sensitivity will result.

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Fig.2. Schematic diagram of a hollow cathode lamp for atomic absorption spectroscopy.

Fig.3. Typical absorbance vs. lamp current for hollow cathode lamps. Note that the lamp which gives curve C would have an optimum current at 8.6

mA.

When a dc voltage of 300-500 V is put across the anode and the cathode, the

atoms of the filler gas undergo ionisation at the anode and are rapidly attracted by the

cathode. Then, the fast moving ions strike the surface of the cathode and physically

displace the surface metal atoms of the cathode into neon (or argon) gas. Further

collisions of the vaporised metal atoms from the cathode with energetic filled gas ions

result in excited metal atoms which emit the characteristic spectrum of the metal used to

construct the cathode. Thus, the emission spectrum produced by a hollow cathode lamp is

a sharp line spectrum of the cathode material and the filled gas. The neon or helium gas

filled in the hollow cathode lamp performs three functions

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(i) It is the main source of current-carryingcapacity in the hollow cathode.

(ii) It dislodges atoms from the surface of the cathode.

(iii) It is primarily responsible for excitation of the ground state metal atoms.

The pressure maintained in the hollow cathode lamp is 1 to 5 torn. If higher

pressures are maintained, the discharge tends to be unstable and if lower pressures are

maintained, the vaporisation of the hollow cathode metal increases and the operating

temperature also increases.

The spectral lines produced by the hollow cathode lamp are so narrow that they

are completely absorbed by the atoms. By this method, one can easily detect and measure

the atomic absorption.

Each hollow cathode lamp emits the spectrum of that metal which is used in the

cathode. For example, copper cathode emits the copperspectrum; zinc cathode emits the

zinc spectrum and so on. At the same time, the narrow spectral lines emitted by copper

cathode are only absorbed by the copper atoms present in the sample to be analysed by

atomic absorption spectroscopy. Similarly, zinc atoms will absorb spectral lines emitted

by zinc cathode. For this reason, a different hollow cathode lamp has to be used for each

element to be analysed by atomic absorption spectroscopy. This is not very convenient

In atomic absorption spectrophotometer, gaseous discharge lamps are also used.

These are also called arc lamps. Gaseous discharge lamps contain an inert gas at low

pressure and a metal or metal salt. These lamps are useful for the alkali metals, zinc,

cadmium and mercury.

(b) Electrodeless Discharge Lamp. It is difficult to make stable hollow cathodes from

certain elements, particularly those that are volatile, such as arsenic, germanium, or

selenium. An alternative light source has been developed in the electrodeless discharge

lamp (EDL). This is illustrated in Fig.5. It consists of an evacuated tube in which the

metal of interest is placed. The tube is filled with argon at low pressure and sealed off.

The sealed tube is then placed in a microwave discharge cavity. Under these conditions

the argon becomes a plasma and causes excitation of the metal sealed inside the tube. The

emission from the metal is that of its spectrum, including the resonance line. The intensity

of these lamps is very high, and they have been made quite stable in recent years.

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Fig.5. Electrodeless discharge lamps with vacuum jackets: (a) Dismountable and (b) Permanent.

2. Chopper. A rotating wheel is interposed between the hollow cathode lamp and the

flame. Thisintermittent wheel is known as chopper and is interposed to break the steady

light from the lamp into anintermittent or pulsating light. This gives a pulsating current in

the photocell. There is also a steady currentcaused by light which is emitted by a flame.

But only the pulsating (or alternating) current is amplified andrecorded and, thus, the

absorption of light will be measured without interference from the light emitted bythe

flame itself.

3) Atomisers. In order to achieve absorption of atoms, it becomes necessary to reduce the

sampleto the atomic state. This is done by

a) Flame atomisers

b) Non-flame atomisers.

Flame Atomisers. The most common way is to use a flame which is used for converting

the liquidsample into the gaseous state and also for conversion of the molecular entities

into an atomic vapor. Thereare two types of burners in common use, the total

consumption burner and the premixed burner.

In the total consumption burner, the sample solution, the fuel and oxidising gases are

passed throughseparate passages to meet at the opening of the base of the flame. As the

sample containing metallicelement to be estimated by atomic spectroscopy is a liquid, the

flame breaks up the liquid sample intodroplets which are then evaporated or burnt, leaving

the residue which is reduced to atoms. Total consumption burners do use oxygen, with

hydrogen or acetylene, and give very hot flames. A typical totalconsumption burner is

illustrated in Fig. 6.

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Fig.6. A total consumption burner.

Fig.7. A premixed type.

A total consumption burner is noisy and hard to use. The efficiency of this burner

is not very good.

A typical premixed burner is illustrated in Fig. 7

In the premixed burner, a mixture of the sample (liquid) and premixed gases

(C2H2+O2) is allowed to enter the base M. From the base M, the gases enter the region A.

From the region A the unburnthydrocarbon gaseous mixture and liquid droplets are

allowed to enter the region B which is a region offree heating and about l mm in

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Fig.8. Beckman total consumption burner.

thickness. In the region B, the liquid is evaporated leaving a residue. Theheating in this

region is done by the heat obtained from the region C by conduction and convection and

by diffusion of radicals into it which initiate the combustion. After this the sample residue

is burnt into region C and D to produce atoms. The production of atoms is initiated in the

region C and is complete in the region D.

The premixed burner is very suitable for the atomic absorption studies of metals of

groups I A, I B, and II B, together with Ga, In, Ti, Pb, Te, Mn, Ni and Pd.

4. Nebulization of the Liquid Sample:

Before the liquid sample enters the burner, it is first of all converted in to small

droplets. This method of formation of small droplets from the liquid sample is called

nebulisation. A common method ofnebulisation is by use of a gas moving at high velocity,

called pneumatic nebulization; The Beckman total consumption burner is commonly used

in atomic absorption measurements (Fig. 8.)

The description of various parts of Beckman total consumption burner is as following:

i. A is a solution capillary.

ii. B is the aspirating gas inlet

iii. C is a fuel gas inlet.

iv. D is the centering screw.

v. E is the gas inlet.

vi. F is a jacket.

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Fig. 9. Photomultiplier tube with power supply.

In the burner, a back pressure of about 250 torr occurs at the tip of the burner due to

the high velocity of the aspirating gas as it emerges from the office. As the liquid is drawn

up the capillary, it is broken into droplets by the high velocity gas stream.

5. Monochromators

In atomic absorption measurements, the most common monochromarors areprisms

and grating. Commercially packaged atomic absorption instrumentation commonly

includes a monochromator of about ½ m focal length with a linear reciprocal dispersion in

the range 16-35 Ao/mm.

The function of a monochromator is to select a given absorbing line from spectral

lines of the hollow cathode. For many elements, high dispersion is not necessary. In such

cases a resolution of the order of 0.5 Ao is desirable.

When the cathode in the hollow cathode lamp is made up of transition metals, the

emission spectrum from the hollow cathode is so complicated that high dispersion is

essential. For such large dispersion and high resolving monochromators are advantageous

for resolving spectra.

6. Detectors

For atomic absorption spectroscopy, the photomultiplier tube is most suitable. It

has good spectrum if used with a stable power supply. It works satisfactorily and enables

to compare intense in a satisfactory manner.

In the photomultipliers tube, there is an evacuated envelope which contains a

photocathode, a seriesof electrodes called dynodes, and an anode. The photocathode is

fixed to the terminal of the power supply (Fig. 9). Photomultiplier tube with power supply

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Fig. 10. Block diagram of a lock-in amplifier.

As soon as a photon strikes the photocathode, an electron is dislodged and the

photon is acceleratedto dynode 1, resulting in the liberation of two or more electrons from

this dynode. Similarly, the electronsfrom this dynode 1 are accelerated to dynode 2,

resulting in the liberation of more electrons. Thus, thecurrent multiplied at each dynode

and the resultant electron current is received by the anode to producean EMF across RL

which goes to the external amplifier and read-out system.

7. Amplifier

The electric current from the photomultiplier detector is fed to the amplifier which

amplifies the electriccurrent many times. Generally, "Lock-in" amplifiers are preferred

which provide a very narrow frequencyband pass and help to achieve an excellent signal-

to-noise ratio. A block diagram of a lock-in amplifieris shown in Fig. 10.

From Fig. 10, it clearly follows that the signal from the photomultiplier tube is led

to an AC amplifierto modulate the energy source. The modulated signal finally enters the

synchronous detector. At the sametime, the signal from a reference source (usually a

flashlight bulb) is modulated at exactly the samefrequency as the signal from the

photomultiplier tube undergoes by passing through an AC amplifier. Themodulated

reference signal also enters the synchronous detector through a phase shifter. The two

signalsare combined in a synchronous detector to produce "sum" and "difference"

frequencies, the "sum frequency"will be twice the chopping frequency and the "difference

frequency" will be zero. Thus the "differencefrequency" will be a DC signal. When the

signals from synchronous detector are allowed to pass througha low pass RC filter, it

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removes the "sum" frequencies and a DC "difference" signal is amplified by a

DCamplifier and then goes out to the read-out.

8. Read-out Device

In most of the atomicmenu, chart recorders are used as read-out devices. A

chartrecorded is a potentiometer using a servomotor to move the recording pen. The

displacement is directly proportional to the input voltage.

In some atomic absorption measurements, digital read-out devices are the used.

Differences between Atomic Absorption and Flame Emission Spectroscopy:

Emission is the process where a substance gives off or emits radiation when it is

heated or treated chemically. The level of emission of a substance depends on its

spectroscopic composition and temperature.

Absorption is the process where the electrons of a substance absorb or take up the

energy wavelengths incident on them. The atomic and molecular structure of the material

governs its level of absorption, along with the amount of electromagnetic radiation,

temperature, solid crystal structure, and intermolecular interactions.

Emission spectra get defined as the electromagnetic radiation that a source emits with

frequency. But on the other hand, Absorption Spectra gets defined as the

electromagnetic radiation that a substance emits and shows various dark colour lines

that result due to the absorption of wavelengths.

The lines that occur during the emission spectra show some spark whereas the lines

that occur during the absorption spectra shows some dip in the spectrum.

Emission does not depend on matching ones and carries out at any level; on the other

hand, absorption requires some degree of wavelength for the process to carry itself

out.

When an atom or molecule gets excited due to an external source, then the energy

gets emitted and cause the phenomenon of emission whereas when an atom or

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molecule comes back to the original position after the process, then the radiation gets

absorbed.

Emission spectrum may be visible at many levels of lines of frequencies as it does not

depend on any matching, whereas absorption spectrum only occurs at the frequencies

that match at the same time.

Different colors are present during the absorption spectrum as the frequencies will

have their own lines and colors depending on their nature, on the other hand,

emission spectrum does not have many color changes because it only focuses on a

path and few dark colors.

Applications of Emission and Absorption Spectroscopy:

To identify a substance: Every substance emits lights of different wavelengths. To

identify the given substance, light is focused on it or the substance is heated. This

causes the electrons to get excited and jump to a higher orbit. The energy emitted by

these electrons while returning to their ground states is compared to the

characteristic colors of the elements, and the chemical composition of the substance

is determined.

To study the composition of stars: The emission spectra of stars can be recorded

and then compared with standard emission spectra of known elements to determine

their chemical composition.

Absorption Spectrum

To identify a substance and determine its concentration: An unknown substance

can be identified by focusing light of a particular wavelength on it, and then

studying the absorption spectrum of the substance. Since substances absorb light

only from a particular wavelength or wavelength range, the wavelength of light

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focused on them is important. This spectrum can be compared with a set of

reference values for identification. These reference values are known absorbance

values of common elements and compounds. The concentration of the substance in

the sample can also be determined.

To study the composition of stars: The light emitted by stars and planets passes

through their atmosphere, where some of it is absorbed by the gases. When the

absorption spectra of these gases is recorded and compared to the reference spectra

values of gases, the composition of these planets or stars can be determined.

Differences between Emission and Absorption spectroscopy:

Flame Emission Spectroscopy Atomic Absorption Spectroscopy

1. The relation between absorbance and

concentration is almost linear.

1. The relation is not linear.

2. Emission intensity depends upon

number of excited atoms and therefore

depends upon temperature.

2. Absorption depends upon number of

unexcited atoms so absorption intensity

does not depend upon temperature.

3. Signal comes completely from emitted

atoms.

3. The signal is obtained due to difference

between intensity of source in absence

of metallic element and decreased

intensity when metallic elements are

present in optical path.

4. Detection limit for Cd and Zn is very

poor.

4. Detection limit for Cd and Zn is 10

times better than flame emission.

5. Sulphur and Prosperous give elemental

interferences.

5. Sulphur and Prosperous can be

determined without interferences by

precipitation as Barium sulphate and

phosphate.

6. Emission varies with the position of

electrodes.

6. Absorption varies without position of

the burner.

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Determination of Lead in Petrol

In petrol the two anti-knocking additives are tetraethyl and tetramethyl lead. In

order to apply atomic absorption spectroscopy to the analysis of lead in petrol,

theanalytical chemist must know which of these (tetraethyl or tetramethyl lead) or what

mixture of the two hasbeen included in the sample to be analysed. For this the direct

method is applicable. If the analytical chemistpossesses no information about the nature of

the lead additive, direct determination is not reliable. In suchcases, indirect method is

preferred in which the destruction of tetraethyl and tetramethyl lead is donefollowed by

extraction into aqueous phase. We will now discuss the direct as well as indirect

methodsseparately.

a) Direct method. If the nature of the antiknock additive is known, one can determine

lead directly in petrol by atomic absorption. This involves the following steps:

i. First of all standard solutions containing tetraethyl and tetramethyl lead (known

concentrations) is prepared in cyclohexanone. The standards should cover the

range 0-5 µg/ml lead if the tetramethyl lead is being estimated and 0-50 µg/ml if

the tetraethyl lead is to be assessed. Then, the standards are aspirated to the atomic

absorption spectrophotometer as 2833 Ao using air-acetylene and the response

curves for solutions of tetraethyl lead, tetramethyl lead and mixtures of these two

compounds are drawn between the absorbance and µg/ml. The typical response

curves are shown in Fig. 11.

ii. The petrol sample is diluted with cylohexanone and is then aspirated into the

absorption spectrophotometer at 2833 Ao. The value of absorption is noted and is

then compared with the readings on the standard curves shown in Fig. 11. In this

way one may obtain the lead content of the petrol.

b) Indirect method. If the analyst possesses no informationabout the nature of the lead

additive, indirect method isused. This method involves the following steps:

i. Standard samples of lead containing 50, 25, 10, and 5 µg/mlare prepared in the

deionized water. Then, the standardsamples are aspirated to the atomic absorption

spectrophotometers at 2833 Ao to note down the absorbancecorresponding to each

standard sample.

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Fig. 11. Typical response curves for TEL/TML.

ii. The petrol sample is treated with bromine to convert the lead to lead bromide

which is extracted with dilute nitric acid. Then the extract is aspirated to the

atomic absorption spectrophotometer to record the absorbance. Finally, the lead

content in the petrol sample is calculated by employing the following expression:

petrolin present mpleoriginalsa of s.g.10

1CSSSS (wt/wt) µg/ml as Pb

20

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whereS0 = scale reading for blank, i.e., 1% nitric acid.

S1 = scale reading for sample extract

S2 = scale reading for standard

C = concentration of standard Pb µg/ml (wt/wt)

s.g. = specific gravity.

Excitation and Ionization Potentials The energy required to excite an atom to emit a line (required) is referred to as

“Excitation Potential” of the line and is usually expressed in electron volts.

For a given line, the height of the upper level above the ground state corresponds

to its excitation potentials.

It is evident from Fig. 12 that the excitation potential of Au 2428 is about 5eV,

whereas Au 4241 and Au 3650 each have excitation potentials of about 8 volts.

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Figure 12

Volt

It is also clear that Au 2428 is a ground state line, as its low level is that of the

ground state of an atom.

In the case of Hydrogen atom, Lyman series are considered as ground state. The

most easily excited line of an element is that one whose upper level is lower than that of

any other upper level, such a line is commonly referred to as the “Resonance Line” and its

excited potential as the resonance potential.

In the low level Hydrogen spectrum H 1215.7 is the resonance line; its excitation

potentials is a little over 10 volts. The resonance lines of the alkali metals are the most

easily excited lines of any elements; their resonance potentials range from 1.4 volts (Cs)

to 2.1 volts (Na).

If the low level of a line is the ground state, the excitation potential of the line (in

volts) is obtained by substitution in the ratio 12,380/λ (oA) =12,380/ λ (oA), ground state

lines with wavelengths of about 6000 oA have excitation potentials of 2 volts, whereas

lines in the far ‘Vacuum” ultraviolet (say 1000-2000 oA) have excitation potentials of 12-

6 volts. In the low level of the line is not the ground state, the value of the lower level

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above the ground state must be taken into consideration when calculating the excitation

potentials. Each element has its own excitation potentials.

If during excitation the atom is ionized, the resultant ion may also be excited to

emit a spectrum, but because atom and corresponding ion have different electronic

structure. The spectrum of the ion will differ from that of the atom.

In the d-c arc a considerable degree of ionization may take place and consequently

lines from both atom and ion may be present. The minimum energy required to remove an

electron from an atom/molecule is called ‘Ionization Potential”.

The spectrum of a singly ionized element resembles that emitted by the atom of

the element of the preceding atomic number (Z). thus the spectra of Na, Mg+, Al2+, Si3+

and P4+ are similar because their electronic structures are similar, they are said to be

isoelectronic.

The first ionization potential of an element which is present in the d-c arc

discharge is an extremely important properly for determining the condition of excitation

in the arc discharge. The ionization potential may be used as a rough measure of the ease

with which an element is excited: a high ionization potential indicates that the elements

the electron is fairly held in the atom and not therefore easily moved to high levels,

whereas low ionization potential indicates that the electron is loosely bound and that the

element is therefore easily excited.

The alkali metals leave the lowest ionization potentials and are most easily

excited, whereas the noble gases, Hydrogen and such gases as F, Cl, Br, O and N have

highest ionization potentials and are the most difficult to excite.

SPECTROGRAPH:

Emission spectra may be examined by means of an optical arrangement that will

identify components frequencies and their intensities. This optical arrangement is known

as several names. For example, it is termed as a spectroscope if it involves a visual device;

it is termed as a spectrograph if it records many wavelengths simultaneously either by

photographic means; it is termed as spectrometer if it scan a spectrum. Except

spectroscopy, both spectrograph and spectrometer determine intensities. For this reason

the terms spectrograph and spectrometer will be used nearly interchangeably.

The various spectrographs may be distinguished chiefly by whether they use a

prism or grating as a dispersing medium.

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Prism instruments:

The optical system of a simple prism spectrometer is shown in Fig.3.1. It consists

of an excitation source, an entrance slit, a collimator lens, adispersing prism and an eye

piece through which spectrum is observed. Sometimes eye piece is replaced by a camera

so that entire spectra can be photographed directly without any difficulty.

In this spectrograph, light from the excitation source is allowed to pass through a

slit and then rendered parallel by the collimating lens L1. This parallel beam after passing

through the prism is reflected. By the help of camera lens L2, different colours are focused

on a photographic plate.

The slit used in a spectrograph should be straight and have parallel and sharp

edges to avoid reflections from the edges. The slit should be adjustable and bilateral,i.e.,

both sides should open or close rather than just one side.

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The prism is generally made from quartz or fused silica. When an emission line is

passed through a quartz prism, it undergoes splitting by polarisation and emerges from the

prism as two lines. This drawback can be overcome by using Cornu type prism having

two half prisms. One half polarises light in a right hand fashion whereas the other half

polarises light in a left-hand fashion. Thus, the first half prism splits the light into two

beams whereas the second half recombines them to give a single line.

A spectrograph involving Cornu type prism in fig.3.2.

In place of Corny type prism, Littrow type can be used. This type of prism

employs one piece of quartz with its back and forth through the same prism. A schematic

optical diagram of a Littrow-type spectrograph is shown in fig. 3.3.

A typical large Littrow spectrograph with a quartz covers a wavelength range from

2000 to 8000Ao in about three settings.

Grating instrument:

In principle, a grating could be substituted for the prism in prism spectrograph

keeping the rest of arrangement the same. A grating spectrograph is performed because a

grating gives linear dispersion and therefore the resolution is constant and independent of

wavelength.

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The Rowland mounting is so arranged that the film and grating are at right angle to

the slit. For a given grating, the slit grating and film fall on a circle known as Rowland

circle. The diameter of the circle is equal to the radius of curvature of the surface of the

concave grating. The image of the slit can be brought to focus somewhere on the Rowland

circle if the grating and slit lie on the same circle. These images can be detected by

keeping a photographic detector or a series of photomultiplier tubes on the Rowland

circle. The actual positon can be found by moving the detector along the circle until the

maximium signal is detected. A Rowland mounting for a concave grating is shown in fig.

3.4.

In Wadsworth mounting, a concave mirror is used along the concave grating. This

is done as illuminate the grating by parallel light. By this arrangement, the light –

gathering power of the set-up becomes high. One more advantagbe is that the light arising

from horizontal line and vertical line can be focused at the same distance from the grating.

A Wadsword mounting for a concave grating is shown in fig. 3.5.

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Jarrell-Ash 1.5 m- spectrograph used Wadsworth mounting. Its grating has 600

lines times per mm. It covers the range from 2200 to 7800 Ao in first order with a

dispersion of 10-9 Ao per mm.

A more compact instrument based on the Rowland circle is obtained by use of

eagle arrangement as shown in fig.3.6. In this instrument, all components lie on one side

of the Rowland circle giving a set up that resembles the Littrow prism arrangement. As

rays from the grating return over a path nearly the same as thet taken in incidence,

astigmatism is slight. By using a total reflecting prism the slit may be located within the

Rowland circle.

The Bausch and Lomb 1.5 m spectrograph employ an eagle mounting. This

instrument covers the range from 2250 to 6250 Ao. It has a dispersion of 16Ao per mm in

the first order.

In Ebert mounting, a plane grating rather thana concave one is used. This makes

the ruling of the grating easier. In the arrangement, a concave mirror is used to render the

radiation striking the grating parallel and to focus the dispersed beam on the plate at the

camera (fig. 3.7). Most of the large spectrograph uses Ebert mounting. Gratings used in

these spectrographs have 600 to 1200 lines per mm. These spectrographs have range of

1800-30000Ao. Spectrographs having Ebert mounting have the following advantages:

1) These spectrographs have a considerably greater wavelength range.

2) High – order visible and UV spectra can be efficiently observed.

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Applications of Emission Spectroscopy;

The history of science abounds with examples of men, who observe natural

phenomena but are unable to explain or apply them until a “genius” discovers a

generalization, principle or law. After more than 2000 years of astronomical observation

and speculation of remained for Copernicus and Kepler to announce the true laws of

planetary motion.

Qualitative analysis:

The identification of elements in any sample by spectroscopic method is called

“Spectro Chemical Analysis”. Instruments used for the analysis is spectroscope.

Any chemical element can be detected by a characteristic spectral line, and its

concentration determined from the intensities of the spectral line. When sample is exited

in an electric discharge, a complex spectrum is obtained.

Whether the sample is Geological (or) Biological (or) mixture of any salts (or)

Soil, it contains large number of elements. To identify most of the elements easily, R.U.

Spectra has been used.

Raies Ultima (or) Raies Ultimes powder contains 50 elements 9prepared by

Johnson Mathey. co., London) the standard spectrum of R.U. powder has been recorded

first (Note the elements present in it). The R.U. lines are particularly useful in detecting

small concentrations of impurities. If ones standard R.U. powder spectra and the elements

present in it are known, the unknown sample elements can be easily known by comparing

it with R.U. powder spectra.

Using Hartmann Diapharm the spectra’s were recorded.

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Exact coincidence of the lower spectral lines (sample) with upper spectral lines

(R.U. powdered spectra) given the elements in the sample. The above method describes

only the elements identification.

How wavelengths of the elements determined?

(1) Record Iron spectra (known spectra) using 1st Hartmann slit.

(2) Again record the spectra of the given sample using 2nd Hartmann slit.

The spectrum will look like as follows:

The spectral appearance will be the same both for prism (or) Grating elements.

Grating gives linear dispersion and prism gives Prismatic (or) angular dispersion.

Applying linear dispersion = dxdλ

The wavelength of the unknown line is determined. Linear dispersion for a

particular grating is constant and we have to measure the distance between known Iron

line to the unknown line of the sample (using comparator, Least count is 0.00001 cm).

This is called dx.

Linear dispersion is constant for a particular grating (i.e., 5.4 Ao/mm).

Then, 5.4=dxdλ

dλ = 5.4 × dx

Unknown lines wavelength can be calculated as follows: λ1+dλ. This estimated

wavelength of the unknown line is compared with the standard wavelengths (from

standard tables). In the case of Prismatic dispersion, Hartmann suggested an interpolation

formula for evolution of the unknown wavelengths, the formula needs three known

wavelengths and corresponding comparator readings.

24

12

12

λλSSa

13

13

λλSSb

baSSλλ 23

10

1020 SaS

ba

SSSSC 2310

Iron known wavelengths Comparator reading

λ1 S1

λ2 S2

λ3 S3

The formulas are as follows:

SSCλλ

00

λ0, C and S0 are constants.

Take comparator readings for all unknown lines (i.e., S values) and substitute with

λ0, S0 and C evaluate the wavelengths for all spectral lines. Using R.U. powder, the

elements can be easily identified and employing Iron spectra, the wavelength of the

element can be evaluated. It is rapid method compare to all other spectroscopic method.

Quantitative analysis:

There are two methods:

(1) Comparison method

(2) Internal standard method

Principle: Concentration of the element is directly proportional to its Intensity.

The ideal properties of an internal standard are:

(1) It has the same concentration in both samples and standards.

(2) It has as similar as possible physical properties to those of the elements being

determined.

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(3) It should have an emission line that has about the same excitation energy as one

for the elements being determined so that the two lines are similarly affected by

temperature fluctuation in the source.

(4) Internal standard and the elements to be determined should have similar ionization

energies so that both have the same distribution ratio of atoms to ions in the

source.

(5) The lines of the standard and the element of interest should be in the same spectral

region in order to provide adequate compensation for emulsion varieties. This is,

however, not applicable to photo electric spectrophotometers.

(6) The intensity of the two selected lines should not be too widely diversity in order

to compensate for photographic process to be most effective.

After developing the film/plate, estimate the intensity of line with the standard

known concentrated spectral line intensity. Comparing the intensity of the desired

constituent with the known concentration of the constituent, its percentage can be

estimated. This is not a simple task (constant excitation and exposure conditions should be

maintained perfectly).

Internal Standard Method:

Sample contains much number of elements. Among all elements one must choose

one element is as an internal standard. The internal standard is a metal, an oxide or salt, its

concentration in the excitation source should be small. The lines of the elements to be

determined and of internal standard must have similar excitation potentials. The chosen

internal standard element amount is known (CE) and is added to the sample.

We know that

Intensity αion Concentrat

Then

ε

XEX I

I C=C

where C is concentration, E is internal

standard, x and ε refers to be determine elements

and internal standard element.

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We will attempt to measure manganese in Carbon steel. The internal standard

method is Iron. Add different amounts of Manganese to plain Carbon steel and record the

spectra.

Mn Fe 0.0025 0.005 0.01 0.02 0.03

i.e., the intensities of Manganese and Iron are determined on Micro-Photometer.

Intensity ratios of Mn/Fe are calculated. Draw a graph between Log intensity ratios of

Mn/Fe to the % of the Mn.

From the graph log (Mn/Fe) is known. We know the value of CE from beginning

(internal standard element concentration).

Then

Fe

MnE

ε

XEX I

I CII C=C

From the above, one can calculate % Manganese in Carbon steel.

Advantages of Emission Spectroscopy:

(1) It is an excellent method of trace analysis, typically detecting elements at the parts

per million levels. i.e., low concentration level at the order of 0.0001%.

(2) It can be used to analyze all metals and metalloids and occasionally other elements

as well.

(3) Usually no chemical separations or concentration steps are required. Samples are

generally analyzed as required.

(4) A small sample is sufficient for analysis i.e., few milligrams or occasionally less

than a milligram.

(5) If suitable comparison standards have been prepared, the time required for analysis

is short particularly if a direct reading instrument is available.

(6) The method can be used for routine quality control work. One can estimate

unknown element in a sample within short time.

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(7) This is an ideal method to analyze Geological (Rock), Biological, metals and

alloys, plants, forensic, soils and environmental samples.

(8) Astrophysical bodies like asteroids can also be analyzed.

(9) The photographic plate integrates the light intensity. Thus trace metals

determination is possible.

(10) Solid samples can be vaporized and excited by this process. Gases and solutions

can also be handled.

Disadvantages of Emission Spectroscopy:

(1) The equipment is relatively expensive.

(2) The technique taken much time, if only one analysis desired.

(3) The technique is semi quantitative.

(4) Standards must often be synthesized.

(5) The sample is destroyed in the process of analysis.

(6) The method is limited to the analysis of elements.

(7) The precision and accuracy are limited to roughly 5%.

(8) The method fails in case of concentrated solutions. It is only ideal for trace

element analysis.