This article throws light upon the top five instrumental techniques for monitoring industrial pollutants. The techniques are: 1. Nuclear Magnetic Resonance [NMR] Spectroscopy 2. X-Ray Fluorescence 3. Infrared Spectroscopy 4. Emission Spectroscopy 5. Flame Photometry Coupled with GC.
Instrumental Technique # 1. Nuclear Magnetic Resonance [NMR] Spectroscopy:
The discovery of nuclear magnetic resonance spectroscopy in 1946 by Purcell and Bloch is now recognized as a very important tool for the advancement of inorganic and organic chemistry- determination of various compounds present in industrial pollutants.
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NMR studies are helpful in predicting:
(a) The presence of functional groups,
(b) The relative position of these groups and
(c) The relative number of nuclei in these groups.
NMR spectroscopy has also been proved helpful in measuring the rates of fast reactions in air and in the determination of moisture present in the compound containing polluted contaminants.
Definition:
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(1) Nuclear magnetic resonance is defined as a condition when the frequency of the rotating magnetic field becomes equal to the frequency of the processing nucleus.
(2) If ratio frequency energy and a magnetic field are simultaneously applied to the nucleus, a condition as given by the equation v = ϒH0/2r is met.
The system at this condition is said to be in resonance [V = frequency of radiation associated with transition from one state to the other; ϒ∆∆ = proportionality constant and H0 = magnetic field].
Principle:
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The principle of nuclear magnetic resonance is based on the spins of atomic nuclei. The magnetic measurements depend upon the spin of unpaired electron whereas nuclear magnetic resonance measures magnetic effect caused by the spin of protons and neutrons. Both these nucleons have intrinsic angular momenta or spins and hence act as elementary magnet.
The existence of nuclear magnetism was revealed in the hyper fine structure of spectral lines. If the nucleus with a certain magnetic moment is placed in the magnetic field, we can observe the phenomenon of space quantization and for each allowed direction there will be a slightly different energy level.
Theory of NMR:
The hydrogen nucleus or protons can be regarded as a spinning positively charged unit and so it will generate a tiny magnetic field H0 along its spinning axis (as shown in figure 1). Now if this nucleus is placed in an external magnetic field H0, it will naturally line up either parallel A or antiparallel B to the direction of external field. The A will be more stable, being of lower energy.
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The energy difference ∆E between two states will be absorbed or emitted as the nucleus flips from one orientation to the other. Then ∆E = hv
where v = a radiation frequency and h = Planck’s constant
If correct frequency is applied to the sample containing hydrogen nuclie and sample is placed in the external field H0, then low energy nuclie A will absorb ∆E = hv, and flips to B.
Thus on flipping back down, they remit hv as a radiation signal which is picked up by the instrument. In other words, if both radio frequency and magnetic field are simultaneously applied to the nucleus, transition from lower to higher level will occur when equation (1) will be equal to (2).
δ = Gyromagnetic ratio, a constant characteristic of a particular nucleus.
where ∆E = energy difference between two spin states,
h = Planck’s consult,
v = frequency of resonance absorption,
H = strength of applied magnetic field at nucleus.
The system at this condition is said to be in resonance and hence the name nuclear magnetic resonance.
The observed value of H is therefore a function of molecular environment of proton affording the signal.
(1) Relaxation process:
Relaxation processes are defined as different types of radiation less transitions by which a nucleus in an upper spin state returns to a lower spin state.
Generally there are two types of relaxation processes:
(a) Spin-spin Relaxation:
It is affected by mutual exchange of spins by two processing nuclei in close proximity to each other.
(b) Spin Lattice Relaxation (lattice term refers to frame work of molecules containing the processing nuclei):
This process maintains an excess of nuclei in a lower state, which is the essential basic condition for the observation of nuclear resonance phenomenon.
In a NMR spectroscopy the sharp resonance lines are observed for states of extended excitation, and broad lines are observed for short-lived excited states. Both the processes, spin-spin relaxation and spin lattice relaxation contribute to the width of a spectral line.
(2) Condition of Resonance Signals:
The atoms like O16 and C12 which have even number of protons and neutrons have no magnetic moment and hence refuse to give resonance signals. While atoms such as P21, F19, which have odd number of protons and even numbers of neutrons, if any, generate nuclear magnetic moments and hence give resonance signals.
(3) Units of NMR:
The nuclear magnetic resonance values are expressed in any of three ways:
(a) δ:
The reference compound be quoted (8 denotes that chemical shift is independent of oscillator frequency).
(b) Cps:
The reference compound must be quoted and the oscillator frequency given.
(c) t-TMS (tetra methylsilane) or DSS (2, 2 dimethyl-2 silapentane-5 sulphonate) is assumed independent of both oscillator frequency and reference compound.
Nuclear Magnetic Resonance Spectrometers:
The basic elements of a typical n.m.r spectrometer consist of the main parts;
(1) A magnet with strong, stable homogeneous field. The field must be constant over the area of the sample.
(2) A radio frequency oscillator (transmitter) connected to coil which transmits energy to the sample in a direction perpendicular to the magnetic field.
(3) A sample container, usually a glass tube spun by an air-driven turbine to average the magnetic field over the sample dimensions.
(4) A radio frequency receiver connected to a coil encircling the sample. The two coils are perpendicular to each other and to the magnetic field.
(5) A read out system. The other supporting parts are— consisting of an amplifier, recorder and additional components for increasing sensitivity, accuracy or convenience.
(6) A sweep generator which supplies a variable d-c current to a secondary magnet so that the total applied magnetic field can be varied (swept) over a limited range.
Experimental Technique:
Always a dilute solution is analysed. The compound to be studied is generally mixed with a solvent like CCI4 or tetramethyl silane and the dilute solution is filled in a tube.
Now when a sample under investigation is placed in the magnetic field and subjected to rf field of oscillator then at particular combinations of the oscillator frequency and field strength, the rf. energy is absorbed by certain nuclei and an rf. signal is picked up by the detector.
Two ways have been employed in NMR experiments for getting the desired particular combinations:
(i) In one way, the magnetic field remains constant and radio frequency is varied.
(ii) In second, the radio frequency remains unchanged and magnetic field is varied till resonance conditions are obtained and there is detectable absorption by the nucleus.
Instrument:
The block diagram for a sample NMR spectrometer is shown in Fig. 3. In block diagram, the blocks labelled N and S represent the poles of the large HO magnet, which is generally an electromagnet operated through a stabilized power supply. A field of up to 1400 gauss and a pole of about 1.75—1.8 inch is necessary for high resolution spectra. The frequency and field strength are related to each other by Larmor condition.
[This equation represents the condition of resonance.]
where H0 = magnetic field,
v = is the frequency of radiation associated with transition from one state to another. It is generally known as Larmor frequency,
ϒ = proportionality constant or gyromagnetic ratio.
Experimental Parameter (Chemical shift):
The most important molecular parameter determined by NMR is the chemical shift. The chemical shift is defined as a measure of the resonance frequency of the nuclei in a given chemical environment
The magnitude of the chemical shift is proportional to the strength of applied field and is caused by the circulations of surrounding electrons about the protons.
The chemical shift parameter δ is defined
Where Hr and Hs are field strengths corresponding to resonance for a particular nucleus in the sample (Hs) and reference (Hr). But as spectra are usually calibrated in cycles per second (cps), the equation can be written as
where ∆v = Difference in absorption frequencies of the sample and the reference in cps; oscillator frequency is the characteristic of the instrument : For a 60 MHz instrument, the oscillator frequency is 60 x 106 cps.
The factor 106 has been included for convenience.
Units:
The units of 8, is expressed as parts per million (ppm). The tetra methyl silane (TMS) is generally taken as acceptable standard (because of low boiling point 27°C). If the compound has a symmetrical structure, each proton is identical to all others and is found in an identical electronic environment which gives a very high shielding. As a result, TMS gives a single sharp resonance line.
Chemical shift is also designated by r where r = 10 – δ.
The standard (CH3)4Si protons appear between 0 on 8 scale and 10 on r scale.
Measurement of Chemical Shift:
In fact the measurement of chemical shift gives information about the various types of magnetic environments. The chemical shift in simple molecules is fairly characteristics and may be used for analysis and characterization.
Factors which influence δ:
Actually the chemical shift parameter δ is a function of electron density around the nucleus as the electrons are directly involved in the diamagnetic shielding which acts to attenuate the applied magnetic field.
Hence following factors are responsible for influencing its value:
(a) Specific solvent,
(b) Bulk diamagnetic susceptibility effect,
(c) Temperature (only when change in temperature causes changes in some type of association equilibrium or changes in amplitude of torsional vibrations),
(d) Electron density,
(e) Inductive effect,
(f) Vander Waal de-shielding, and
(g) Hydrogen bonding.
Interpretation of NMR Spectrum several kinds of information:
(1) The number of signals (peaks) tells us how many kinds of protons (protons with different chemical environments) are present in a molecule.
(2) The position (chemical shift) of the signal informs about the bonding environment of each proton.
(3) The area under each signal tells us how many protons of each kind are in the molecule.
(4) All hydrogen’s with identical environments in a molecule have same chemical shift, e.g.,
(a) all the three protons of a methyl CH3;
(b) the protons of a methylene — CH2 ;
(c) one identical.
(5) Protons on heteroatoms (H—S, H —N, H—O etc.) show highly variable chemical shifts and sometimes broad peaks.
(6) Hydrogen on different carbons yield the same absorptional signal if they are structurally indistinguishable.
(7) Sometimes a proton exhibits an absorption signal which is split into several peaks because of coupling with its neighbouring protons. In such cases a coupling constant J is calculated.
(8) The number of peaks (N) into which a proton signal is split equals one more than the number of vicinal protons (n) (number of equivalent neighbours causing splitting):
N = n + 1
N = 2 (one vicinal H) = doublet (d)
= 3 (two vicinal H’s) = triplet (t)
= 4 (three vicinal H’s) = quartet (q).
Example:
Spectrum of isomers—dimethyl ether and ethanol.
The low resolution spectra of isomers dimethyl ether and ethanol are shown in figures 4 (a) and (b). The spectrum of dimethyl ether shows only one signal because all 6H atoms are equivalent. The 3H atoms in a given CH3 group are indistinguishable, as are the two CH3 groups.
The spectrum of ethanol has three signals-one each for CH3, CH2 and OH protons. The relative areas under the peak for the ethanol are 3: 2: 1 for CH3, CH2, and OH groups respectively. CH2 signal is far away than CH3 signal because of electron withdrawing effect of two adjacent atoms.
In a high resolution NMR spectrum of ethyl alcohol (CH3—CH2—OH), the methyl peak is associated with two peaks, white methylene and OH are associated with four and three peaks respectively.
The three equivalent methyl protons are split into a triplet (1 + 2) by two equivalent methyl protons. Similarly two equivalent methylene protons are split into a quartet (1 + 3) by three equivalent methyl protons.
Applications of N.M.R. Spectroscopy:
(1) Quantitative Analysis:
The area of peak is directly proportional to the number of nuclei responsible for that peak. Thus the concentration of species can be determined directly by making use of signal area per proton.
The signal area per proton can easily be calculated by use of a known concentration of an internal standard. Similarly, the concentration of new species formed during the reaction can also be calculated from the spectrum of parent compound.
(2) Qualitative Analysis:
The qualitative analysis of the compound can easily be made by knowing:
(i) Chemical shift 5 values of hydrogen containing groups,
(ii) The presence of particular functional group,
(iii) The relative position of these groups and
(iv) The relative number of nuclei in these groups.
Nuclear Magnetic Double Resonance (NMDR):
When two oscillating magnetic fields are simultaneously applied to the sample, the experiment is called double resonance, double irradiation, or spin decoupling. In the usual nuclear magnetic double resonance experiment, a strong rf. field H2 is used to eradicate the sample while a weak rf. field H1 induces the transitions to be observed. We can sweep the magnetic field holding Hi and H2 constant.
Electron Paramagnetic resonance (EPR):
The electron paramagnetic resonance (EPR) differs from NMR principally because in that the frequencies of electron resonance occur in microwave region for magnetic fields of the order of several thousand gauss. Therefore EPR spectrometer uses such components as Klystrons, wave guides and resonance cavities for the sample.
EPR method is applicable whenever the compound displays at least one unpaired electron, i.e., in free radicals, crystalline and amorphous solids subjected to irradiation or containing transition element ions and rare earths had some chelates. The other different examples are metals, odd molecules, graphite’s and impurities in semiconductors.
Instrumental Technique # 2. X-Ray Fluorescence:
The technique is used in the industrial pollution analysis of metals and non-metals with atomic number greater than 12. The X-ray region of electromagnetic spectrum consists of wavelengths in the region of about 0.1 to 100 A°. Generally the range between 0.7 to 2.0 A° is used as it is most useful region for analytical purposes.
Principle:
This depends on atomic number and wavelength of incident radiation. When an element is placed in a beam of X-rays then these rays arc absorbed. The absorbing atoms become excited and then emit X-rays with wavelengths characteristic of emitting atoms. This technique has been given the name X-ray fluorescence.
Apparatus and Working:
This diagram of the apparatus used for X-ray fluorescence is shown above. The fluorescence intensity is first measured which helps in the qualitative determination of the element. For this purpose X the wavelength of fluorescence is calculated by Bragg’s equation.
where d = spacing between crystal layers of analytical crystal and Φ = angle
The comparison of ϒ with known ϒ of elements given in chart help in identifying the element
Applications:
The X-ray fluorescence has been proved very helpful in the analysis of metals and non-metals with atomic number greater than 12.
(1) Qualitative Analysis:
The Qualitative analysis can be carried out by measuring the angle of diffraction of fluorescent X-rays. With the help of this angle, the wavelength of fluorescence can be easily calculated. As each element fluoresces at a characteristic wavelength hence fluorescing element can easily be identified in the air polluted samples.
(2) Quantitative Analysis:
By measuring the intensity of fluorescence at wavelength characteristic of element, Quantitative analysis from polluted samples of food, fertilizers, agricultural products, rocks, solids etc. can easily be carried out. The fluorescence spectra are very simple.
Sometimes back ground emission from one element may overlap line emission from another element. Hence to measure the intensity of peak Y, a correction must be made for the background of X.
This can be done by the following way:
Suppose intensity of X = 60 units
Suppose intensity of Y = 50 units
The true intensity of peak X = 10 units
(3) This technique is also used for the determination of trace elements in plants and foods; the detection of insecticides on fruit and leaves ; the determination of phosphorous in fertilizers; the detection of fodder elements, such as selenium, known to be harmful in large quantities.
Instrumental Technique # 3. Infrared Spectroscopy:
The infrared radiation was first discovered by William Herschel in 1800. Later on the technique was developed for industrial application near Second World War by William W. Coblentz. The spectra of the molecules which are observed in infrared region are called infrared spectra.
These spectra provide valuable information about the basic characteristics of the molecule, namely, the nature of the atoms, their spatial arrangement and their chemical linkage forces.
The infrared spectroscopy is now-a-days used for the qualitative and quantitative analysis of the organic compounds present either in air, liquid or solid state. The infrared spectrum causes when a molecule due to vibrations and rotations of atoms within the molecule produce a change in permanent electric dipole of the molecule.
The usual range of infrared spectrum is between 4000 cm-1 at high frequency end and 667 cm1 at lower frequency end. The position of an absorption band in spectrum is given by microns (µ) or reciprocal of wavelength, cm-1. Taking into consideration the electromagnetic theory, the wavelength ϒ is related to frequency v by the expression
The electromagnetic radiation constituting light can be divided into three regions:
(i) Visible Region:
4000°A — 8000°A (1Å = 10 -8 cm).
(ii) Infrared Region:
8000°A—35000°A.
(iii) Ultraviolet Region:
Below 4000°A.
Principle:
The atoms in a molecule are not still. They rotate and vibrate in different ways in certain quantized energy levels. It has been noticed that the molecular vibrations can occur by two different mechanisms.
Firstly, quanta of infrared radiation can excite atoms to vibrate directly— the absorption of infrared radiation gives rise to the infrared spectrum.
Secondly, quanta of visible light can achieve the same result indirectly—the Raman effect. Consider any two atoms which form a bond, these are the kinds of motions and infrared region in which they are excited.
(1) Stretching:
2.5 to 15 p (650 to 4000 cm1).
(2) Bending:
6.5 to 20 p (500 to 1550 cm1).
(3) Rotation: ≥ 12p (< 800 cm1).
In general the localized vibrations of methylene group can be named as stretching, bending, rocking, twisting or wagging.
These localized vibrations have been found very useful for the identification of functional groups.
It is found that the stretching modes are most important to structure determination and can be divided into four kinds:
In fact, the covalent bonds have characteristic and essentially invariant absorption wavelengths so the presence of a bond in the spectrum (specially below 1200 cm-1) indicates the presence of a bond in the molecule.
In the region below 1400 cm-1, there are many peaks to interpret because this is a lower-energy region. Many complex polyatomic vibrations, single bond stretching’s and bending frequencies occur together in this region.
This region (from 650—1550 cm-1) is very useful for characterizing compounds and is called fingerprint region. The particular bonds are unique to particular molecule and can be used for the identification of the molecule. The double bond region shows the absorptions due to carbonyl group in its many functional variations.
Hooke’s Law:
The stretching vibration of two bonded atoms cm be linked to vibrations of two balls at the end of a spring, for which Hooke’s Law can be applied.
Ball and spring are the representation of two atoms of a molecule vibrating in the direction of the bond
f = force constant and v = vibration frequency.
Thus from equation (1), vibration frequency v is directly related to force constant f of bond, which is a measure of bond energy and is inversely proportional to combined mass of two vibrating nuclei. Hence we can conclude that stronger bonds produce absorptions at lower wavelength (higher frequency) and this is confirmed in bond strengths.
triple > double > single
Electric Dipole:
According to theory, a molecule must have a changeable electric dipole for absorbing infra red radiation. If a molecule has slight positive and slight negative electrical charge on its component atoms it is said to have electrical dipole.
These slight changes are not equal to change of a whole electron or proton but represent a slight excess or depletion of electrons in some area. In principle, two adjacent fractional but opposite changes make a dipole. Now this dipole must change because of vibrational transition resulting from infra red absorption.
Conditions for the Infra Red Absorption:
The necessary conditions for the absorption of infra red radiation can be summarized below:
(1) The natural frequency of vibration of the molecule must be equal to the frequency of incident radiation.
(2) Changes in vibration must stimulate changes in the dipole moment of the molecule.
(3) The frequency of the vibration must satisfy the equation
E = hv
(4) The intensity of absorption must be proportional to square of the rate of change of dipole.
Instrument:
(1) Single Beam Infra Red Absorption Instrument:
In this instrument the radiation from a source passes through the sample and then through entrance slit to monochromatic. The selected wavelength is passed through the sample to the detector.
The main function of detector is to measure the intensity of radiation because with the help of this data we can measure how much radiation has been absorbed by the sample. Hence the absorption spectrum of sample can be studied by measuring the degree of absorption of different wavelengths.
We know that according to Beer-Lambert Law.
It/IO = transmittance
The IO, intensity of radiation is measured when there is no sample in the light beam. The sample is then put into light beam which absorbs radiation. This reading gives the value of It.
Hence by measuring It and Iq, the quantitative analysis of sample can be made.
In practice it is found that the intensity of radiation source varies slowly over long periods of time hence It/Io is difficult to measure accurately. Hence to win this difficulty a double beam instrument is used.
(i) Double Beam Infra Red Absorption Instrument:
In this instrument the source beam is separated into two half beams— sample beam and reference beam. The solvent which is used in the sample is placed in the reference beam and the sample is placed in the sample beam. The two half beams are recombined and pass along the optical to the detector. In this system when the sample half beam is absorbed, however, it decreases intensity.
After two half beams are recombined, they produce an oscillating signal. The detector system here actually measures the degree of oscillation which then becomes a direct measure of ratio the transmittance of the degree of absorption. Hence by knowing this ratio, we can calculate the concentration of the sample. We will describe in detail here the main components of the apparatus which form the basis of the instrument.
(1) Radiation Source:
Generally two sources of ir radiation are used:
(i) Nernst Glowers.
(ii) Globars.
(i) Nernst Glower:
It is actually a bar made up of cerium oxide, zirconium oxide and thorium oxide. Such a bar is heated electrically to the temperature between 1000° to 1800°C.
(ii) Globar : It is a bar of sintered silicon carbide which is also heated electrically to temperature between 1000°-1800° C.
(2) Monochromators:
(i) Prism Monochromators :
Prism monochromators are most effective in separating ir radiation according to wavelength. The prisms are generally used for various forms of radiation including visible, IR and ultra violet. Hence if we want to get ir radiation then the material used in prism should be of metal salts, such as calcium fluoride, potassium bromide, sodium chloride or thallous bromide.
This material should be transparent to ir radiation, it has been found that sodium chloride is transparent between 2.5µ and 1.5 µ. KBr can be used over the range of 2.1 µ —2.6 µ and calcium fluoride in the range 2.4—7.7 µ .The surface of the prism should be smooth to prevent random scattering of radiation by prism surface.
(ii) Grating Monochromators:
In recent times grating monochromators are very popular in ir spectroscopy. The main advantage of these gratings is that they are made up of aluminum which is stable in atmosphere and not attacked by moisture. The another advantage is that these gratings can also be used over considerable wavelength range. This is in contrast to salt prism as they have only reduced wavelength range in which they can be used.
(3) Detectors:
The common detectors which are generally used in spectroscopy are:
(i) Bolometers,
(ii) Thermistors.
(i) Bolometers:
It is a very sensitive resistance thermometer which is used to detect and measure feeble thermal radiation. Actually speaking, bolometers are best suited for ir study.
This is because of the fact that when ir radiations fall on this conductor, it becomes warmer due to which its temperature changes. Its electrical resistance also changes. The degree of change in resistance is a measure of amount of radiation that has fallen on the detector.
(ii) Thermistors:
They are made of a fused mixture of metal oxides. It is found that as the temperature increases, its electrical resistance also increases. This relationship helps to IR detectors in same way as bolometers.
(4) Sample Cells:
The compounds can be examined in vapour phase, as pure liquids, in solution and in solid state.
Working:
(i) In Solution:
Here the given compound is dissolved to give a 1—5% solution in CCl4 or chloroform free from alcohol. This solution is introduced into a special cell, 0.1 to 1 mm. thick, made of sodium chloride. A second cell of equal thickness, but containing pure solvent, is placed in path of other beam of spectrometer in order that solvent absorption should be balanced.
(ii) In Vapour Phase:
In this case the vapour is introduced into a special cell about 10 cm. long which can then be placed directly in the path of one of infra red beams. Generally the end walls of cell are made up of sodium chloride, which is transparent to infra red.
(iii) In Solid State:
(a) When C—H vibrations are to be examined then 1 mg of solid is finely ground in a mortar with 1 drop of liquid hydrocarbon. The mull which is obtained in this way is then pressed between flat plates of sodium chloride,
(b) Now to prevent bands due to mailing agent, the solid is ground with 10 to 100 times its bulk of pure potassium bromide and the mixture which is obtained in this way is passed into a disc using a special mould through a hydraulic press.
Applications:
Interpretation of Infrared λ Spectra:
Each functional group has a characteristic absorption band in the so called group frequency region (4000 cm-1 — 1400 cm-1) while direct comparison of finger print regions (1200 cm-1 to 500 cm-1) between unknown and standard reference spectra give the true identification.
The position of various groups can be identified from the tables but there are some variations. For example C = O peak in carboxylic acids varies from 1700 cm-1 to 1715 cm-1 depending on other functional groups.
The position of bond is same for aliphatic aldehydes, 1740 cm1 to 1720 cm-1 and ketones, 1725 to 1705 cm-1. But it appears at different locations for conjugated unsaturated systems, carboxylic acids, anhydrides, esters and acetyl halides. Hence the location of various C = O stretching vibrations should be studied with great precaution.
(1) Qualitative Analysis:
The qualitative analysis can easily be done by comparing the spectrum of unknown sample with the standard one.
(2) Quantitative Analysis:
The quantitative determination is based on the determination of concentration of one of functional groups of compound being estimated. Suppose there is a mixture of hexane and hexanol and then concentration of hexanol can be determined by measuring absorption of OH bond.
The following formula is used for calculating the concentration:
A = — log I\IO = abc
where A = absorbance,
I = intensity of radiation after leaving the sample,
lo = intensity of radiation before entering the sample,
a = absorptivity of cell (It is property of molecular species being transmitted)
b = initial path length of sample cell and
c = concentration of solution.
If a and b are constant then
A α C
hence C can be measured by knowing A.
The different values of A are plotted against respective concentrations to get a calibration curve from which the concentration of unknown solution is evaluated.
Some Examples of Infra Red Spectra:
We are giving here some examples of infra red spectra showing relative intensities of absorption peaks due to number of functional groups. The wide variety of finger prints also shows the usefulness of this region for the identification of certain groups.
Results:
A 3600 cm-1 — Free O—H
B 3460 cm-1 — Intermolecular and weakly bonded O—H
C 2940 cm ‘ — Saturated C—H
Instrumental Technique # 4. Emission Spectroscopy:
This is a technique mainly used in the identification and determination of elements present in industries. Changes in trace metal concentration can also be studied in polluted air, water and soil.
Principle:
An electron is said to be in the ground state when it is in the lowest energy level but when it gains thermal or radiant energy it jumps to any higher energy level and is called in the excited state. In the reverse case, when electron drops from higher level to lower level the radiant energy or light is emitted which depends upon the frequency or wavelength of the radiation.
These wavelengths of different radiations give spectrum which is measured by the spectroscope. Now this spectrum of emitted radiations is called Emission Spectrum.
Type of Emission Spectra:
(a) Continuous Spectrum:
It is observed that if solid is heated there is a continuous gradation throughout the spectrum from violet end to red and because of this continuity it is called continuous spectrum. For example, solids like iron or carbon emit continuous spectrum when they are heated until they glow.
(b) Discontinuous Spectrum:
When incandescent gases and vapours are heated in a flame or by means of an electric arc, they emit energy in the form of light When this light is examined by spectrometer, it is found that the spectrum is non continuous. The (i) line spectrum and (ii) band spectrum are the discontinuous spectra.
They can be explained as given below:
(i) Line Spectrum:
When a discontinuous spectrum consists of a series of lines then the spectrum is known as line spectrum. In other words a line spectrum consists of sharp bright lines separated from each other by dark spaces. The line spectrum is also called atomic spectrum because it originates in the atom of the element.
(ii) Band Spectrum:
Band spectra are characteristic of molecules and consist of broad luminous bands mostly well defined at one end which is called head of band. This spectrum is generally emitted by gases like nitrogen and oxygen at low temperature and high pressures and by various compounds.
Equipment for Emission Spectroscopy:
The following equipment is necessary for the spectral analysis of various materials:
Light sources, burners, electric discharge generators, arcs, spectral instruments.
(1) Excitation Source or Light Sources:
Generally discharges of glow or arc type and high frequency discharges are used as light sources in emission spectroscopy.
Any excitation source must accomplish the following conditions:
(i) The sample must be vaporized,
(ii) It must dissociate into atoms.
(a) Method of Production of Emission Spectra:
The emission spectra can be produced by various methods which are given below:
(i) Electric Discharge Method:
In case of solid samples, an electric discharge is passed between the portions of the sample or between sample and a counter-electrode generally graphite for getting the emission spectrum (1) Sample electrode (2) Graphite electrode. An electric discharge is passed between two electrodes. Gases also give emission spectra when passed through electric discharge.
(ii) A—C arc Method:
In this method a high voltage say 2000—45000 volts is obtained through a step up transformer and spark is energized from A—C power lines through Feussner circuit the diagram of which is exhibited above. The connections are made as shown in the circuit. The synchronous gap is only made to ensure reproducibility. This method is far superior than the above method as it does not destroy the sample.
Flames:
This method is used for those molecules which do not require very high temperature for excitation and dissociation. Here different flames of different temperatures are used in emission spectrography. The temperature of flame mostly depends upon the composition of fuel mixture. At particular temperature, the dissociation of the molecules and excitation of atoms take place.
Spectral Instruments:
This simplest spectroscope consists of entrance slit, lens, a dispersing element (prism or diffraction grating) 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. The diagrams of two spectral instruments are shown here.
Sample Handling and Working:
Generally solid or liquid sample is used in emission spectroscopy.
If a sample is solid having good conducting properties and can withstand high temperature then this can be used for making electrodes required for electric discharge.
If a solid is a bad conductor then it is first powdered and then mixed with powdered graphite and finally kept in a cup in the lower graphite electrode. After it is loaded, the electrode is placed in the circuit.
When the electric discharge occurs the sample is vaporized into the plasma of the discharge and spectrographic emissions take place.
The main function of adding powdered graphite to the powdered solid sample is to increase reproducibility, i.e., minimising aimless wandering of the arc.
For liquid samples, there are two sample holders.
The spectra obtained by sample are photographed. The emission lines are then compared with the lines of the known sample and thus qualitative analysis is made. If sample is gas, it is first changed to liquid and then analysed.
Measurement of Spectral Lines:
The measurement of spectral lines is made with the use of following method:
Comparison with Standards:
In this method first of all the photograph of known standard sample is taken and then it is compared with the photograph of unknown samples under similar conditions.
Applications:
(1) Determination of Concentration by Line Intensities (Quantitative Analysis):
There are various methods for the determination of concentration by line intensities:
(a) Homologous Pair Method.
(b)Line Appearance Method.
(c) Visual comparison of densities of analytical lines in spectra of samples and standards.
(а) Homologous Pair Method:
This technique depends on the fact that the ratio of intensities of line of two elements is based on the ratio of their concentrations in sample. Suppose there is an element E and the line of this element is λE, there is also an element F having lines of different intensity λF , λ”F , λ ‘’’F which are located next to each other in spectrum. Consider that at a concentration C1 of element E the intensity of line λE may be equal to that weakest line of element F or in other words, at concentration CI, IE = I’F.
(2) Quantitative Analysis:
The emission lines of the sample are identified by comparing with the standard sample. But this is a difficult procedure as it involves many lines hence this can be simplified by detecting few Raies Ultima (RU) lines.
The RU lines are also useful in detecting small concentrations of impurities.
Hertman used the following formula for measuring the wavelength of RU lines
where λO, c and do are constants which can be evaluated experimentally. The process can be carried out by substituting the wavelengths λX, and distance dx of three lines from an arbitrary point and solving the three equations.
(3) Emission spectroscopy has been proved very helpful in the analysis of copper alloys, zinc alloys, lead alloys, magnesium alloys and tin alloys.
(4) The technique is used in the analysis of metals like Na, K, Zn, Mg, Ni, Fe, Ca, present in tissues of animals and men.
(5) Changes in trace metal concentration can also be studied in polluted air, waters and effluents.
(6) Plants and Soils:
The technique has been proved as a boon to detect 40 Elements in plants and soils.
(7) The technique is successful for metals, alloys, soils, geological samples, biological samples and environmental samples.
Disadvantages:
(1) The sample gets destroyed in the process of analysis.
(2) The method is limited to the analysis of elements.
(3) The method fails in case of concentrated solutions. It is ideal for only trace element analysis.
Advantages:
(1) It is an excellent method for elemental trace analysis (a low a concentration level as 0.0001%).
(2) A sample (1-10 mg) is needed for analysis.
(3) The time required for analysis is short. Generally we can get results within 1 minute.
(4) It is an ideal method for environmental samples, biological samples and geological samples.
If I and Io give the light flux penetrating the marked and unmarked parts of the photographic plate, the blackening of the line is given by the formula,
D = log I0/I
Comparisons of the spectral lines from samples with those known standards give the quantitative results.
Instrumental Technique # 5. Flame Photometry Coupled with GC:
The first discoverer who used flame spectra for quantitative analysis in 1930 was Lundegradh.
It is a simple and rapid method for the determination of elements that can easily be excited. Flame photometry is also known as flame emission spectroscopy because of the use of flame to provide the energy of excitation to atoms introduced into the flame. It is very useful in industrial pollution, water pollution and agriculture.
Flame photometry coupled with GC is now-a-days used to give results within minutes.
Principle:
When a solution of the sample is sprayed into the flame, the water or other solvent evaporates, leaving the particles of the solute. Now at higher temperature of the flame, either decomposition products vaporise or dissociate into constituent atoms or radicals.
These vapours of metal atoms or of the molecules are then excited by the thermal energy of the flame. The emission spectra obtained may be atomic spectra due to lines originating from excited atoms or band spectra due to molecules.
It is always necessary to have high temperature of the flame between 1000°—3000°C so that the elements may be excited completely. The emission of characteristic radiation by each element and the correlation of the emission intensity with the concentration of that element form the fundamental basis of the subject matter of flame photometry.
Flame photometry is a technique whereby the concentration of a metal in solution may be determined by spraying the solution into a flame and comparing the intensity of the energy emitted which is characteristic of the metal with that of the energy of the metal are similarly treated.
In short the following events occur:
(i) The solvent is vaporised, leaving particles of solute only.
(ii) These solute particles are converted into gaseous state.
(iii) The gaseous molecules dissociate to give neutral atoms or radicals.
(iv) These neutral atoms are excited by the thermal energy of the flame.
(v) The excited atoms emit photons and return to lower energy state (unexcited state).
(vi) The emitted photons are measured.
Types of Instruments used for flame photometer and flame spectrophotometer:
(1) Flame Photometer:
In flame photometer, the intensity of the filtered radiation of the flame is measured with a photoelectric detector. The best detector which is used is the barrier-layer cell which has high temperature coefficient.
The diagram is shown below:
Generally the flame photometer has six parts:
(1) Pressure regulator.
(2) Atomizer.
(3) Burner.
(4) Optical system.
(5) Photosensitive detector, and
(6) Instrument for recording the output of the detector.
(1) Pressure Regulator:
When the instrument is in operation suitable gases arc provided to indicate the pressures so that proper adjustments can be made at every time.
(2) Atomizer:
There are two types of atomizer:
(i) The one which introduces the spray into a condensing chamber for removing large droplets.
(ii) And second which introduces the spray directly into flame.
Actually this is a device which introduces liquid into the flame.
(3) Burner:
The burner must produce a steady flame. For low temperature flame Meeker type burner is generally used.
The various components of the instrument are described as follows:
The flame used in the flame photometer must possess the following functions:
(i) The flame must decompose the compounds in the solid residue formed in step (ii), resulting in the formation of atoms.
(ii) The flame should possess the ability to evaporate the liquid droplets from the sample solution.
(iii) The flame must have the capability to excite the atoms formed in step (ii) and cause them to emit radiant energy.
The temperature of the flame depends on several factors which are summarised as follows:
1. Amount of solvent which is entering into the flame,
2. Type of fuel and oxidant and fuel-to oxidant ratio,
3. Type of solvent for preparing the sample solution,
4. Type of burner employed in flame photometer, and
5. The particular region in flame which is to be focused onto the entrance slit of the spectral isolation unit.
Instrument for Recording the Output of the Detector:
Generally the spotlight type of galvanometer is used as a suitable measuring device for barrier layer cells.
Flame Spectrophotometer:
In flame spectrophotometer, light from burner passes into a monochromatic incorporating a spherical mirror and 60° prism. This in conjunction with a photomultiplier tube and a sensitive photocell is suitable for measurement from 250 to 1020 mp. The diagram is given below.
Experimental Technique:
First a solution containing the ion to be determined is sprayed into the flame, the galvanometer spot is adjusted to read full scale deflection and then distilled water is sprayed and galvanometer is adjusted to read zero by means of zero control.
Now solutions of known concentration are sprayed and galvanometer readings are noted at each concentration. A graph is plotted with galvanometer readings versus concentrations and with the help of this graph the concentration of unknown is evaluated.
Applications:
(1) The Qualitative Analysis:
Flame photometry coupled with GC is to detect elements of groups I and II of the periodic table. These elements are sodium, potassium, lithium, magnesium, calcium, strontium, and barium. If the radiation of the characteristic wavelength is detected this will indicate the presence of a metal in the sample.
Non-radiating elements, such as carbon, hydrogen and halides cannot be detected by flame photometer. They can only be detected under special circumstances.
For example, if chlorine is to be detected in a liquid sample, the best method is to precipitate it as silver chloride which is then aspirated into flame of a flame photometer to carry out the determination of silver. From the result, the chloride content can be easily calculated.
(2) Quantitative Analysis:
It is same as given in the experimental technique.
Factors which influence the intensity of emitted radiation in a flame photometer:
There are some factors which affect the intensity of emitted radiation.
They are as follows:
(i) Presence of Other Metals:
If other metals are present in the solution then they change the intensity of emitted radiation. Hence to remove this defect, special filters are used while performing the analysis work.
(ii) Viscosity:
Any substance which enhances the viscosity of the solution decreases the intensity of light emission.
(iii) Presence of Acids:
The presence of an acid in the sample solution results in the decrease of light intensity. This happens due to disturbance of initial dissociation equilibrium.
Interferences in Flame Photometry:
The main interferences which occur in the flame photometry are as follows:
(i) Ionisation interferences:
It has been noticed that in a few cases metal atoms ionise at high temperature of the flame:
Na ↔ Na+ + e–
The sodium has emission spectra of its own hence ionisation decreases the radiant power of atomic emission. This interference can be removed by adding an excess quantity of a potassium salt to all of the solutions—unknown as well as known (standard) solutions. When potassium is added, it decreases the ionisation of sodium but it itself ionises.
(ii) Oxide Formation Interference:
As there is formation of stable oxides with free metal atoms in presence of oxygen hence this interference occurs in this technique. This oxide formation lowers down the emission intensity. It has been observed that mainly alkaline earth elements form the oxides and hence cause the interference.
(iii) Cation-Cation Interferences:
An example of this type of interference is that aluminium interferes with magnesium and calcium. Potassium and sodium also have cation-cation interferences on one another. This cation-cation interference decreases the signal intensity of the element present in the sample solution.
(iv) Spectral Interference:
This interference occurs in three ways:
(a) If spectral lines of two or more elements are close but their spectra do not overlap then there occurs spectral interference.
The interference can be decreased by increasing the resolution of the spectral isolation system.
(b) This interference also takes place if two elements or compounds exhibit different spectra. Sometimes these spectra partly overlap and both are emitting at a definite wavelength.
This error can be minimised by using calibration curves which are prepared from a solution having similar quantity of interfering element.
(c) A third type of interference arises between a spectral line and a continuous background. This occurs due to high concentration of salts in the sample. This interference mainly takes place in the salts of alkali and alkaline earth metals.
(v) Cation-anion Interference:
Sometimes anions such as sulphate, phosphate, oxalate change the intensity of radiation emitted by an element. For example, in presence of phosphate ion, calcium forms a stable substance hence the calcium signal is depressed as it will not decompose easily, resulting in the production of lesser atoms.
Limitations:
(i) The technique fails in cases of halides and inert gases.
(ii) Only liquid sample can be used.
(iii) It can be used for direct determination of all metal atoms as there is a limitation on the number of elements to be analysed by this technique.
(iv) It does not give information about molecular form of metal ion present in the sample.