EI2302 Notes

UNIT I

COLORIMETER AND SPECTROPHOTOMETERS

PART A

1. List any four elements used in spectrophotometers.

1.Radiant source

2.wavelength selector

3.photodetector

4.sample

2. What is meant by flame emission spectrometry?

When an analytical sample is introduced into the flame, the flame atomizes the

Sample and subsequently they are de-excited with the emission of atomic lines

corresponding to different elements present in the sample

3. State Beers law.

The intensity of a beam of monochromatic light decreases exponentially

with increase in concentration of the absorbing substance arithmetically.

4. What is monochromators?

Monochromators are the optical systems which provide better isolation of spectral energy

than the optical filters and are therefore preferred where it is required to isolate narrow bands of

radiant energy.

5. What is analytical instrumentation?

It is the field of science which deals with the technique by which the sample

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(chemical species) is analyzed quantitatively. Instruments used for analysis of

chemical sample are analytical instrument

6. What is electromagnetic radiation?

It is a type of energy that is transmitted through space at a speed of approximately 3x108

m/s. Such radiation doesnt require a medium of propagation and can readily travel through

vacuum. It may be considered as discrete packets of energy called photons.

7. Define wave number.

It is defined as the number of waves per centimeter.

= 1/

8. Draw the spectrum of EM regions.

Given in the notes.

9. State Lamberts law.

When a beam of light is allowed to pass through a transparent medium the rate

of decrease of intensity with the thickness of medium is directly proportional to

the intensity of light.

10. What is Absorption Spectroscopy?

The measurement method based on absorption of radiation of a substance is known as

absorption spectroscopy.




PART B

1. Explain with necessary diagrams the instrumentation involved in uv visible

spectrophotometer?

Introduction

schematic diagram of a double-beam UV-Vis. spectrophotometer

Instruments for measuring the absorption of U.V. or visible radiation are made up of the

following components;

1. Sources (UV and visible)

2. Wavelength selector (monochromator)

3. Sample containers

4. Detector

5. Signal processor and readout

Instrumental components

Sources of UV radiation

It is important that the power of the radiation source does not change abruptly over it's

wavelength range.

The electrical excitation of deuterium or hydrogen at low pressure produces a continuous UV

spectrum. The mechanism for this involves formation of an excited molecular species, which

breaks up to give two atomic species and an ultraviolet photon. This can be shown as;

D2 + electrical energy D2* D' + D'' + hv

Both deuterium and hydrogen lamps emit radiation in the range 160 - 375 nm. Quartz

windows must be used in these lamps, and quartz cuvettes must be used, because glass

absorbs radiation of wavelengths less than 350 nm.




Sources of visible radiation

The tungsten filament lamp is commonly employed as a source of visible light. This type of

lamp is used in the wavelength range of 350 - 2500 nm. The energy emitted by a tungsten

filament lamp is proportional to the fourth power of the operating voltage. This means that

for the energy output to be stable, the voltage to the lamp must be very stable indeed.

Electronic voltage regulators or constant-voltage transformers are used to ensure this

stability.

Tungsten/halogen lamps contain a small amount of iodine in a quartz "envelope" which also

contains the tungsten filament. The iodine reacts with gaseous tungsten, formed by

sublimation, producing the volatile compound WI2. When molecules of WI2 hit the filament

they decompose, redepositing tungsten back on the filament. The lifetime of a

tungsten/halogen lamp is approximately double that of an ordinary tungsten filament lamp.

Tungsten/halogen lamps are very efficient, and their output extends well into the ultra-violet.

They are used in many modern spectrophotometers.

Wavelength selector (monochromator)

All monochromators contain the following component parts;

An entrance slit

A collimating lens

A dispersing device (usually a prism or a grating)

A focusing lens

An exit slit

Polychromatic radiation (radiation of more than one wavelength) enters the monochromator

through the entrance slit. The beam is collimated, and then strikes the dispersing element at

an angle. The beam is split into its component wavelengths by the grating or prism. By

moving the dispersing element or the exit slit, radiation of only a particular wavelength

leaves the monochromator through the exit slit.

Czerney-Turner grating monochromator




Cuvettes

The containers for the sample and reference solution must be transparent to the radiation

which will pass through them. Quartz or fused silica cuvettes are required for spectroscopy in

the UV region. These cells are also transparent in the visible region. Silicate glasses can be

used for the manufacture of cuvettes for use between 350 and 2000 nm.

Detectors

The photomultiplier tube is a commonly used detector in UV-Vis spectroscopy. It consists

of a photoemissive cathode (a cathode which emits electrons when struck by photons of

radiation), several dynodes (which emit several electrons for each electron striking them) and

an anode.

A photon of radiation entering the tube strikes the cathode, causing the emission of several

electrons. These electrons are accelerated towards the first dynode (which is 90V more

positive than the cathode). The electrons strike the first dynode, causing the emission of

several electrons for each incident electron. These electrons are then accelerated towards the

second dynode, to produce more electrons which are accelerated towards dynode three and so

on. Eventually, the electrons are collected at the anode. By this time, each original photon has

produced 106 - 10

7 electrons. The resulting current is amplified and measured.

Photomultipliers are very sensitive to UV and visible radiation. They have fast response

times. Intense light damages photomultipliers; they are limited to measuring low power

radiation.

Cross section of a photomultiplier tube




The linear photodiode array is an example of a multichannel photon detector. These

detectors are capable of measuring all elements of a beam of dispersed radiation

simultaneously.

A linear photodiode array comprises many small silicon photodiodes formed on a single

silicon chip. There can be between 64 to 4096 sensor elements on a chip, the most common

being 1024 photodiodes. For each diode, there is also a storage capacitor and a switch. The

individual diode-capacitor circuits can be sequentially scanned.

In use, the photodiode array is positioned at the focal plane of the monochromator (after the

dispersing element) such that the spectrum falls on the diode array. They are useful for

recording UV-Vis. absorption spectra of samples that are rapidly passing through a sample

flow cell, such as in an HPLC detector.

Charge-Coupled Devices (CCDs) are similar to diode array detectors, but instead of diodes,

they consist of an array of photocapacitors.




2. Explain the construction and working of UV-Visible Spectrophotometer

Introduction

If you pass white light through a coloured substance, some of the light gets

absorbed. A solution containing hydrated copper(II) ions, for example, looks pale

blue because the solution absorbs light from the red end of the spectrum. The

remaining wavelengths in the light combine in the eye and brain to give the

appearance of cyan (pale blue).

Some colourless substances also absorb light - but in the ultra-violet region. Since

we can't see UV light, we don't notice this absorption.

Different substances absorb different wavelengths of light, and this can be used to

help to identify the substance - the presence of particular metal ions, for example, or

of particular functional groups in organic compounds.

The amount of absorption is also dependent on the concentration of the substance if

it is in solution. Measurement of the amount of absorption can be used to find

concentrations of very dilute solutions.

An absorption spectrometer measures the way that the light absorbed by a

compound varies across the UV and visible spectrum.

A simple double beam spectrometer

We'll start with the full diagram, and then explain exactly what is going on at each

stage.




Working

The light source

You need a light source which gives the entire visible spectrum plus the near ultra-

violet so that you are covering the range from about 200 nm to about 800 nm. (This

extends slightly into the near infra-red as well.)

You can't get this range of wavelengths from a single lamp, and so a combination of

two is used - a deuterium lamp for the UV part of the spectrum, and a tungsten /

halogen lamp for the visible part.

The combined output of these two bulbs is focussed on to a diffraction grating.

The diffraction grating and the slit

You are probably familiar with the way that a prism splits light into its component

colours. A diffraction grating does the same job, but more efficiently.




The blue arrows show the way the various wavelengths of the light are sent off in

different directions. The slit only allows light of a very narrow range of

wavelengths through into the rest of the spectrometer.

By gradually rotating the diffraction grating, you can allow light from the whole

spectrum (a tiny part of the range at a time) through into the rest of the instrument.

The rotating discs

This is the clever bit! Each disc is made up of a number of different segments.

Those in the machine we are describing have three different sections - other designs

may have a different number.

The light coming from the diffraction grating and slit will hit the rotating disc and

one of three things can happen.

1. If it hits the transparent section, it will go straight through and pass through

the cell containing the sample. It is then bounced by a mirror onto a second

rotating disc.

This disc is rotating such that when the light arrives from the first disc, it




meets the mirrored section of the second disc. That bounces it onto the

detector.

It is following the red path in the diagram:

2. If the original beam of light from the slit hits the mirrored section of the first

rotating disc, it is bounced down along the green path. After the mirror, it

passes through a reference cell (more about that later).

Finally the light gets to the second disc which is rotating in such a way that

it meets the transparent section. It goes straight through to the detector.

3. If the light meets the first disc at the black section, it is blocked - and for a

very short while no light passes through the spectrometer. This just allows

the computer to make allowance for any current generated by the detector in

the absence of any light.




The sample and reference cells

These are small rectangular glass or quartz containers. They are often designed so

that the light beam travels a distance of 1 cm through the contents.

The sample cell contains a solution of the substance you are testing - usually very

dilute. The solvent is chosen so that it doesn't absorb any significant amount of light

in the wavelength range we are interested in (200 - 800 nm).

The reference cell just contains the pure solvent.

The detector and computer

The detector converts the incoming light into a current. The higher the current, the

greater the intensity of the light.

For each wavelength of light passing through the spectrometer, the intensity of the

light passing through the reference cell is measured. This is usually referred to as

Io - that's I for Intensity.

The intensity of the light passing through the sample cell is also measured for that

wavelength - given the symbol, I.

If I is less than Io, then obviously the sample has absorbed some of the light. A

simple bit of maths is then done in the computer to convert this into something

called the absorbance of the sample - given the symbol, A.

An absorbance of 0 at some wavelength means that no light of that particular

wavelength has been absorbed. The intensities of the sample and reference beam are

both the same, so the ratio Io/I is 1. Log10 of 1 is zero.




An absorbance of 1 happens when 90% of the light at that wavelength has been

absorbed - which means that the intensity is 10% of what it would otherwise be.

In that case, Io/I is 100/I0 (=10) and log10 of 10 is 1.

The chart recorder

Chart recorders usually plot absorbance against wavelength. The output might look

like this:

This particular substance has what are known as absorbance peaks at 255 and 395

nm.

3. Explain the construction and working of FTIR?

Introduction

FTIR spectrometers (Fourier Transform Infrared Spectrometer) are widely used in organic

synthesis, polymer science, petrochemical engineering, pharmaceutical industry and food

analysis. In addition, since FTIR spectrometers can be hyphenated to chromatography, the

mechanism of chemical reactions and the detection of unstable substances can be investigated

with such instruments. FTIR spectrometers (Fourier Transform Infrared Spectrometer) are

widely used in organic synthesis, polymer science, petrochemical engineering,

pharmaceutical industry and food analysis. In addition, since FTIR spectrometers can be




hyphenated to chromatography, the mechanism of chemical reactions and the detection of

unstable substances can be investigated with such instruments.

The Components of FTIR Spectrometers

A common FTIR spectrometer consists of a source, interferometer, sample compartment,

detector, amplifier, A/D convertor, and a computer. The source generates radiation which

passes the sample through the interferometer and reaches the detector. Then the signal is

amplified and converted to digital signal by the amplifier and analog-to-digital converter,

respectively. Eventually, the signal is transferred to a computer in which Fourier transform is

carried out. Figure 2 is a block diagram of an FTIR spectrometer.

Figure 2. Block diagram of an FTIR spectrometer

Michelson Interferometer

The Michelson interferometer, which is the core of FTIR spectrometers, is used to split one

beam of light into two so that the paths of the two beams are different. Then the Michelson

interferometer recombines the two beams and conducts them into the detector where the

difference of the intensity of these two beams are measured as a function of the difference of

the paths. Figure 3 is a schematic of the Michelson Interferometer.




Figure 3. Schematic of the Michelson interferometer

A typical Michelson interferometer consists of two perpendicular mirrors and a beamsplitter.

One of the mirror is a stationary mirror and another one is a movable mirror. The

beamsplitter is designed to transmit half of the light and reflect half of the light.

Subsequently, the transmitted light and the reflected light strike the stationary mirror and the

movable mirror, respectively. When reflected back by the mirrors, two beams of light

recombine with each other at the beamsplitter.

If the distances travelled by two beams are the same which means the distances between two

mirrors and the beamsplitter are the same, the situation is defined as zero path difference

(ZPD). But imagine if the movable mirror moves away from the beamsplitter, the light beam

which strikes the movable mirror will travel a longer distance than the light beam which

strikes the stationary mirror. The distance which the movable mirror is away from the ZPD is

defined as the mirror displacement and is represented by . It is obvious that the extra

distance travelled by the light which strikes the movable mirror is 2. The extra distance is

defined as the optical path difference (OPD) and is represented by delta. Therefore,




=2

It is well established that when OPD is the multiples of the wavelength, constructive

interference occurs because crests overlap with crests, troughs with troughs. As a result, a

maximum intensity signal is observed by the detector. This situation can be described by the

following equation:

=n

(n = 0,1,2,3...)

In contrast, when OPD is the half wavelength or half wavelength add multiples of

wavelength, destructive interference occurs because crests overlap with troughs.

Consequently, a minimum intensity signal is observed by the detector. This situation can be

described by the following equation:

=(n+12)

(n = 0,1,2,3...)

These two situations are two extreme situations. If the OPD is neither n-fold wavelengths nor

(n+1/2)-fold wavelengths, the interference should be between constructive and destructive.

So the intensity of the signal should be between maximum and minimum. Since the mirror

moves back and forth, the intensity of the signal increases and decreases which gives rise to a

cosine wave. The plot is defined as an interferogram. When detecting the radiation of a broad

band source rather than a single-wavelength source, a peak at ZPD is found in the

interferogram. At the other distance scanned, the signal decays quickly since the mirror

moves back and forth. Figure 4(a)shows an interferogram of a broad band source.

Fourier Transform of Interferogram to Spectrum

The interferogram is a function of time and the values outputted by this function of time are

said to make up the time domain. The time domain is Fourier transformed to get a frequency

domain, which is deconvoluted to product a spectrum. Figure 4 shows the Fast Fourier

transform from an interferogram of polychromatic light to its spectrum.




(a)

(b)

Figure 4. (a) Interferogram of a monochromatic light; (b) its spectrum

4. Derive Beer Lamberts law and explain about deviations from Beers Law?

Introduction

The Beer-Lambert law (also called the Beer-Lambert-Bouguer law or simply Beer's law) is

the linear relationship between absorbance and concentration of an absorber of

electromagnetic radiation. The general Beer-Lambert law is usually written as:

where A is the measured absorbance, a is a wavelength-dependent absorptivity coefficient,

b is the path length, and c is the analyte concentration. When working in concentration units

of molarity, the Beer-Lambert law is written as:

where is the wavelength-dependent molar absorptivity coefficient with units of M-1

cm-1

.

The subscript is often dropped with the understanding that a value for is for a specific

wavelength. If multiple species that absorb light at a given wavelength are present in a

sample, the total absorbance at that wavelength is the sum due to all absorbers:




where the subscripts refer to the molar absorptivity and concentration of the different

absorbing species that are present.

Theory

Experimental measurements are usually made in terms of transmittance (T), which is defined

as:

where P is the power of light after it passes through the sample and Po is the initial light

power. The relation between A and T is:

The figure shows the case of absorption of light through an optical filter and includes other

processes that decreases the transmittance such as surface reflectance and scattering.




Derivation of the Beer-Lambert law

The Beer-Lambert law can be derived from an approximation for the absorption coefficient

for a molecule by approximating the molecule by an opaque disk whose cross-sectional area,

, represents the effective area seen by a photon of frequency w. If the frequency of the light

is far from resonance, the area is approximately 0, and if w is close to resonance the area is a

maximum. Taking an infinitesimal slab, dz, of sample:

Io is the intensity entering the sample at z=0, Iz is the intensity entering the infinitesimal slab

at z, dI is the intensity absorbed in the slab, and I is the intensity of light leaving the sample.

Then, the total opaque area on the slab due to the absorbers is * N * A * dz. Then, the

fraction of photons absorbed will be * N * A * dz / A so,

Integrating this equation from z = 0 to z = b gives:




or

Since N (molecules/cm3) * (1 mole / 6.023x10

23 molecules) * 1000 cm

3 / liter = c

(moles/liter) and 2.303 * log(x) = ln(x), then

or

where = * (6.023x1020

/ 2.303) = * 2.61x1020

Limitations of the Beer-Lambert law

The linearity of the Beer-Lambert law is limited by chemical and instrumental factors. Causes

of nonlinearity include:

deviations in absorptivity coefficients at high concentrations (>0.01M) due to

electrostatic interactions between molecules in close proximity

scattering of light due to particulates in the sample

fluoresecence or phosphorescence of the sample

changes in refractive index at high analyte concentration

shifts in chemical equilibria as a function of concentration

non-monochromatic radiation, deviations can be minimized by using a relatively flat

part of the absorption spectrum such as the maximum of an absorption band

stray light




5. Explain with neat diagram the construction and working of atomic absorption

spectrophotometer?

A diagram of a Flame Atomic Absorption Spectrometer is shown in figure 11. The light

source is a cold cathode lamp that produces (almost exclusively) the light that would be

naturally emitted by the element to be measured at a high temperature. A large range of such

lamps are available that includes the vast majority of the elements of general analytical

interest. Consequently, the light will contain specifically those wavelengths that the element

in the flame will selectively absorb. The light passes through the flame, which is usually

rectangular in shape so as to provide an adequate path length of flame for the light to be

absorbed, and then into the optical system of the spectrometer.

The flame is fed with a combustible gas, customarily air/acetylene, nitrous oxide/acetylene or

air/propane or butane. The sample, dissolved in a suitable solvent, is nebulized and fed into

the gas stream at the base of the burner. The light, having passed through the flame, can be

focused directly onto a photo-cell or onto a Diffraction Grating by means of a spherical

mirror. The Diffraction Grating can be made movable, and so it can be set to monitor a

particular wavelength that is characteristic of the element being measured, or it can be

scanned to produce a complete absorption Spectrum of the sample. After leaving the grating,

light of a selected wavelength, or range of wavelengths, is focused onto the photocell. The

position of the Diffraction Grating determines the wavelength of the light that is to be

monitored.




Figure 1. The Basic System of a Flame Atomic Absorption Spectrometer

The flame absorption spectrometer is fairly sensitive, and can be readily used as a tandem

instrument combined with a chromatograph, providing that an appropriate interface is

employed. This instrument is normally fitted with 4 different cold cathode lamps and, thus,

can examine 4 different elements, automatically, from the same analysis; 8 lamp units are

also available. A diagram of a hollow cathode lamp is shown in figure 13.

The cylindrical hollow cathode of the lamp contains one or more of the elements of interest in

the analysis. The cylindrical cathode is screened from the anode connections by means of a

ceramic cylinder The anode is situated above the cathode and is made of tungsten or nickel

and the whole electrode system is enclosed in a glass envelope. The glass envelope is filled

with neon or argon to a pressure of 1kPa and on applying a potential of 100 to 200 volts

across the electrodes a glow discharge is formed.




Figure 2. The Hollow Cathode Element Lamp.

A simple explanation of the process is as follows, Accelerated electrons from the cathode

collide with the gas atoms and produce ions. The ions are accelerated to the cathode by the

electric field and when they strike the cathode surface the elements of interest are ejected

from the surface and are excited to provide radiation in the discharge environment. These

lamps can be combined in groups so that a given instrument can determine a number of

different elements by merely switching the lamps.

A single lamp can be made to generate characteristic radiation for up to two or three elements

without interference problems. Modern atomic adsorption spectrometers are deigned to

provide extremely simple operation, easy maintenance and, when required, fast and

inexpensive servicing. Many ancillary and alternative devices are available to suit specific

applications.

A number of different sampling devices are available including very sophisticated automatic

samplers that allow instruments to be operated 24 hours a day.




6. With a neat sketch explain flame emission spectroscopy

The analytical apparatus

The functions of an analytical flame spectrometer in general are:

(a) transformation of the solution to be analyzed into a vapour containing free atoms

or molecular compounds of the analyte in the flame;

(b) Selection and detection of the optical signal (arising from the analyte vapour)

which carries information on the kind and concentration of the analyte;

(c) Amplification and read-out of the electrical signal.

Transformation of sample into vapour

With a pneumatic nebulizer operated by a compressed gas, the solution is aspirated

from the sample container and nebulized into a mist or aerosol of fine droplets.

By desolvation, i.e., evaporation of the solvent from the droplets, this mist is

converted into a dry aerosol which is volatilized in the flame.

The atomization, i.e., the conversion ofvolatilized analyte into free atoms is

performed by the flame or other atomizer.

The total consumption time is the time required to consume the sample entirely. The

minimum consumption time is the time for which nebulization must be carried out to

perform an analysis with a given precision.

Nebulizers can be described as follows:

According to the source of energy used for nebulization as, for example,




pneumatic or ultrasonic nebulizers.

According to the way the liquid is taken up, e.g., suction, gravity-fed, controlled

flow, and reflux-nebulizers.

According to the relative position of the capillaries for the nebulizing gas and the

aspirated liquid, e.g., angular and concentric nebulizers.

In the chamber-type nebulizer, the nebulizing gas-jet stream emerges from the sprayer

into a spray chamber.

Special devices are the nebulizer with heated spray chamber, the

twin nebulizer, and the drop generator.

Construction and working:

Flames are produced by means of a burner to which fuel and oxidant are supplied in

thenform of gases.

With the premix burner, fuel and oxidant are thoroughly mixed inside the burner

housing before they leave the burner ports and enter the primary combustion or inner

zone of the flame. This type of burner usually produces an approximately laminar

flame, and is commonly combined with a separate unit for nebulizing the sample.

In contrast, a direct-injection burner combines the function of nebulizer and burner.

Here oxidant and fuel emerge from separate ports and are mixed above the burner

orifice to produce a turbulent flame. Most commonly, the oxidant is also used for

aspirating and nebulizing the sample.

However, when the fuel is used for this purpose, the term reversed direct-injection

burner is applied. In each case, the mist droplets enter the flame directly, without

passing through a spray chamber.

The term total-consumption burner, which is often used, is not recommended.

Premix burners are distinguished as Bunsen-, Meker-, or slot-burners according to

whether they have one large hole, a number of small holes, or a slot as outlet for the

gas mixture, respectively. When several parallel slots are present, they are identified

as multislot burners (e.g., a three-slot burner).

The small diameter of the holes in the Meker burner or the narrowness of the slot in

the slot-burner prevents the unwanted flash-back of the flame into the burner housing.

At the edge of the flame where the hot gas comes into contact with the surrounding

air, secondary combustion occurs and the secondary combustion or outer zone is

formed.




The region of the flame confined by the inner and outer zones, where in many

instances the conditions for flame analysis are optimum, is called the interzonal

region, or, when the combustion zones have the form of a cone, the interconal zone.

Sometimes provision is made to screen the observed portion of the flame gases from

direct contact with the surrounding air. This may be done either mechanically, by

placing a tube on the top of the burner around the flame, which produces a zonal

separation (separated flame), or aerodynamically, by surrounding the flame with a

sheath of inert gas that emerges from openings at the rim of the burner top (shielded

flame).

Observations can thus be made without disturbances from the secondary-combustion

zone.




Unit II

CHROMATOGRAPHY

PART A

1. What is chromatogaraphy ?

chromatography is a physical method of seperation of the commponents of a mixture

by distribution between two phases, of which one is stationary bed of a large surface

area and other fluid phase (Mobile phase) that percolates through or along the

stationay bed.

2. What are the types of chromatography?

3. What is Retention Time?

The retention time is the total time that a compound spends in both the mobile phase

and stationary phase.

The time between sample injecction and an analyte peak reaching a detector at the end

of the column is termed as the retention time (tR).

4. What is Gas Chromatography ?

Gas Chrromatography is an analytical technique used for seperating compounds based

primarily on their volatalities. Gas chromatography provides both qualitative and

quantitative information for individual compounds present in a sample.

5. List out the types of colummns used in gas chromatography

Packed Column

Capillary Column

6. List out the various detectors used in gas chromatography

The katharometer or Thermal conductivity detector

Flame ionization Detectors

Chromatography

Gas chromatography

Gas-Liquid (GLC) Gas_Solid (GSC)

Liquid Chromatography

Liquid-Liquid (LLC)

Bonded Phase (BPC)

Liqid _Solid (LSC)Ion Exchange

(IEC)

Ion Pair Chromatography

Exclusioin (EC)




Flame Photometric Detectors

7. What are the advantages of Gas Chromatography?

High flow rates of mobile phase is possible

Several methods of detecting components in flowing gas stream are available

8. What is meant by efficiency in chromatography?

The efficiency is related to the number of compounds that can be seperated by the

column.It is expressed as the number of Theoritical plates or as the highest equivalent

to a theortical plate.

9. Why High pressure pumps are used in HPLC?

Liquid chromatography consists of columns with packing, through which the mobile

phase and the sample flows. The packing in the column reduces the flow rate of the

mobile phase. In order to ensure constant flow rate high pressure pumps are used to

pump the mobile phase into the column.

10. What are the advantages of Katharometer ?

It is a non destructive method of testing

High sensitivity can be achieved, which enables us to detect small quanitiy of

a compound coming out of a mixture

The measurement errors are minimized

PART B

1. With a neat block diagram explain the construction and working of gas

chromatography

Introduction

Gas chromatography - specifically gas-liquid chromatography - involves a sample being

vapourised and injected onto the head of the chromatographic column. The sample is

transported through the column by the flow of inert, gaseous mobile phase. The column itself

contains a liquid stationary phase which is adsorbed onto the surface of an inert solid.




Instrumental components

Carrier gas

The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium,

argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of

detector which is used. The carrier gas system also contains a molecular sieve to remove

water and other impurities.

Sample injection port

For optimum column efficiency, the sample should not be too large, and should be introduced

onto the column as a "plug" of vapour - slow injection of large samples causes band

broadening and loss of resolution. The most common injection method is where a

microsyringe is used to inject sample through a rubber septum into a flash vapouriser port at

the head of the column. The temperature of the sample port is usually about 50C higher than

the boiling point of the least volatile component of the sample. For packed columns, sample

size ranges from tenths of a microliter up to 20 microliters. Capillary columns, on the other

hand, need much less sample, typically around 10-3

mL. For capillary GC, split/splitless

injection is used. Have a look at this diagram of a split/splitless injector;




The injector can be used in one of two modes; split or splitless. The injector contains a heated

chamber containing a glass liner into which the sample is injected through the septum. The

carrier gas enters the chamber and can leave by three routes (when the injector is in split

mode). The sample vapourises to form a mixture of carrier gas, vapourised solvent and

vapourised solutes. A proportion of this mixture passes onto the column, but most exits

through the split outlet. The septum purge outlet prevents septum bleed components from

entering the column.

Columns

There are two general types of column, packed and capillary (also known as open tubular).

Packed columns contain a finely divided, inert, solid support material (commonly based on

diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m

in length and have an internal diameter of 2 - 4mm.

Capillary columns have an internal diameter of a few tenths of a millimeter. They can be one

of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT).

Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary

phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of

support material such as diatomaceous earth, onto which the stationary phase has been

adsorbed. SCOT columns are generally less efficient than WCOT columns. Both types of

capillary column are more efficient than packed columns.

In 1979, a new type of WCOT column was devised - the Fused Silica Open Tubular (FSOT)

column;




These have much thinner walls than the glass capillary columns, and are given strength by the

polyimide coating. These columns are flexible and can be wound into coils. They have the

advantages of physical strength, flexibility and low reactivity.

Column temperature

For precise work, column temperature must be controlled to within tenths of a degree. The

optimum column temperature is dependant upon the boiling point of the sample. As a rule of

thumb, a temperature slightly above the average boiling point of the sample results in an

elution time of 2 - 30 minutes. Minimal temperatures give good resolution, but increase

elution times. If a sample has a wide boiling range, then temperature programming can be

useful. The column temperature is increased (either continuously or in steps) as separation

proceeds.

Detectors

There are many detectors which can be used in gas chromatography. Different detectors will

give different types of selectivity. A non-selective detector responds to all compounds except

the carrier gas, a selective detector responds to a range of compounds with a common

physical or chemical property and a specific detector responds to a single chemical

compound. Detectors can also be grouped into concentration dependant detectors and mass

flow dependant detectors. The signal from a concentration dependant detector is related to the

concentration of solute in the detector, and does not usually destroy the sample Dilution of

with make-up gas will lower the detectors response. Mass flow dependant detectors usually

destroy the sample, and the signal is related to the rate at which solute molecules enter the

detector. The response of a mass flow dependant detector is unaffected by make-up gas.




The effluent from the column is mixed with hydrogen and air, and ignited. Organic

compounds burning in the flame produce ions and electrons which can conduct electricity

through the flame. A large electrical potential is applied at the burner tip, and a collector

electrode is located above the flame. The current resulting from the pyrolysis of any organic

compounds is measured. FIDs are mass sensitive rather than concentration sensitive; this

gives the advantage that changes in mobile phase flow rate do not affect the detector's

response. The FID is a useful general detector for the analysis of organic compounds; it has

high sensitivity, a large linear response range, and low noise. It is also robust and easy to use,

but unfortunately, it destroys the sample.

2. With a help of neat sketch, explain the construction and working of Katharometer

The thermal conductivity detector (TCD), also known as a Katharometer, is a bulk

property detector and a chemical specific detector commonly used in gas chromatography.

[1]This detector senses changes in the thermal conductivity of the column effluent and

compares it to a reference flow of carrier gas. Since most compounds have a thermal

conductivity much less than that of the common carrier gases of helium or hydrogen, when an

analyte elutes from the column the effluent thermal conductivity is reduced, and a detectable

signal is produced.




Operation

The TCD consists of an electrically heated filament in a temperature-controlled cell. Under

normal conditions there is a stable heat flow from the filament to the detector body. When an

analyte elutes and the thermal conductivity of the column effluent is reduced, the filament

heats up and changes resistance. This resistance change is often sensed by a Wheatstone

bridge circuit which produces a measurable voltage change. The column effluent flows over

one of the resistors while the reference flow is over a second resistor in the four-resistor

circuit.

TCD Schematic

A schematic of a classic thermal conductivity detector design utilizing a wheatstone bridge

circuit is shown. The reference flow across resistor 4 of the circuit compensates for drift due

to flow or temperature fluctuations. Changes in the thermal conductivity of the column

effluent flow across resistor 3 will result in a temperature change of the resistor and therefore

a resistance change which can be measured as a signal.

Since all compounds, organic and inorganic, have a thermal conductivity different from

helium, all compounds can be detected by this detector. The TCD is often called a universal

detector because it responds to all compounds. Also, since the thermal conductivity of

organic compounds are similar and very different from helium, a TCD will respond similarly

to similar concentrations of analyte. Therefore the TCD can be used without calibration and




the concentration of a sample component can be estimated by the ratio of the analyte peak

area to all components (peaks) in the sample.

The TCD is a good general purpose detector for initial investigations with an unknown

sample. Since the TCD is less sensitive than the flame ionization detector and has a larger

dead volume it will not provide as good resolution as the FID. However, in combination with

thick film columns and correspondingly larger sample volumes, the overall detection limit

can be similar to that of an FID. The TCD is not as sensitive as other detectors but it is non-

specific and non-destructive.

The TCD is also used in the analysis of permanent gases (argon, oxygen, nitrogen, carbon

dioxide) because it responds to all these pure substances unlike the FID which cannot detect

compounds which do not contain carbon-hydrogen bonds.

Process description

It functions by having two parallel tubes both containing gas and heating coils. The gases are

examined by comparing the rate of loss of heat from the heating coils into the gas. The coils

are arranged in a bridge circuit so that resistance changes due to unequal cooling can be

measured. One channel normally holds a reference gas and the mixture to be tested is passed

through the other channel.

Applications

In the oil industry katharometers have been used for a long time for hydrocarbon detection

but have a history of unstable calibrations in non-stationary oil-related applications. In

normal drilling practice, 5 hydrocarbon gases, plus a couple of non-hydrocarbon gases, are

expected in normal samples resulting in cross-talk between the methane absorption line and

the ethane. Hence the current use of flame ionization detectors.

Katharometers are used medically in lung function testing equipment and in gas

chromatography. The results are slower to obtain compared to a mass spectrometer, but the

device is inexpensive, and has good accuracy when the gases in question are known, and it is

only the proportion that must be determined.

Monitoring of hydrogen purity in hydrogen-cooled turbogenerators




3. Explain with a neat diagram the construction and working of flame ionization

detector

A flame ionization detector (FID) is a scientific instrument that measures the concentration

of organic species in a gas stream. It is frequently used as a detector in gas chromatography.

Standalone FIDs can also be used in applications such as landfill gas monitoring and fugitive

emissions monitoring in stationary or portable instruments.

Operating principle

The operation of the FID is based on the detection of ions formed during combustion of

organic compounds in a hydrogen flame. The generation of these ions is proportional to the

concentration of organic species in the sample gas stream. Hydrocarbons generally have

molar response factors that are equal to number of carbon atoms in their molecule, while

oxygenates and other species that contain heteroatoms tend to have a lower response factor.

Carbon monoxide and carbon dioxide are not detectable by FID.




Description of a generic detector

FID Schematic

The design of the flame ionization detector varies from manufacturer to manufacturer, but the

principles are the same. Most commonly, the FID is attached to a gas chromatography

system.

The eluent exits the GC column (A) and enters the FID detectors oven (B). The oven is

needed to make sure that as soon as the eluent exits the column, it does not come out of the

gaseous phase and deposit on the interface between the column and FID. This deposition

would result in loss of eluent and errors in detection. As the eluent travels up the FID, it is

first mixed with the hydrogen fuel (C) and then with the oxidant (D). The eluent/fuel/oxidant

mixture continues to travel up to the nozzle head where a positive bias voltage exists (E).

This positive bias helps to repel the reduced carbon ions created by the flame (F) pyrolyzing

the eluent. The ions are repelled up toward the collector plates (G) which are connected to a

very sensitive ammeter, which detects the ions hitting the plates, then feeds that signal (H) to




an amplifier, integrator, and display system. The products of the flame are finally vented out

of the detector through the exhaust port (J).

Operation

In order to detect these ions, two electrodes are used to provide a potential difference. The

positive electrode doubles as the nozzle head where the flame is produced. The other,

negative electrode is positioned above the flame. When first designed, the negative electrode

was either tear-drop shaped or angular piece of platinum. Today, the design has been

modified into a tubular electrode, commonly referred to as a collector plate. The ions thus are

attracted to the collector plate and upon hitting the plate, induce a current. This current is

measured with a high-impedance picoammeter and fed into an integrator. The manner in

which the final data is displayed is based on the computer and software. In general, a graph is

displayed that has time on the x-axis and total ion on the y-axis.

The current measured corresponds roughly to the proportion of reduced carbon atoms in the

flame. Specifically how the ions are produced is not necessarily understood, but the response

of the detector is determined by the number of carbon atoms (ions) hitting the detector per

unit time. This makes the detector sensitive to the mass rather than the concentration, which

is useful because the response of the detector is not greatly affected by changes in the carrier

gas flow rate.

Advantages

Flame ionization detectors are used very widely in gas chromatography because of a number

of advantages.

Cost: Flame ionization detectors are relatively inexpensive to acquire and operate.

Low maintenance requirements: Apart from cleaning or replacing the FID jet, these

detectors require no maintenance.

Rugged construction: FIDs are relatively resistant to misuse.

Linearity and detection ranges: FIDs can measure organic substance concentration at

very low and very high levels, having a linear response of 10^6.




Disadvantages

Flame ionization detectors cannot differentiate between different organic substances. They

also cannot detect inorganic substances. In some systems, CO and CO2 can be detected in the

FID using a methanizer, which is a bed of Ni catalyst that reduces CO and CO2 to methane,

which can be in turn detected by the FID.

Another important disadvantage is that the FID flame oxidizes all compounds that pass

through it; all hydrocarbons and oxygenates are oxidized to carbon dioxide and water and

other heteroatoms are oxidized according to thermodynamics. For this reason, FIDs tend to

be the last in a detector train and also cannot be used for preparatory work.

4. Explain the construction and working of Ion exchange chromatography with a neat

diagram

Ion exchange means exchange of ions from a medium. It has typical application in water

softening by exchange of alkaline metal ions like Ca2+, Mg

2+ by Na

+. Other common

applications are sugar processing, hydrometallurgical application, protein fractionation,

biological separation, etc.

Fundamentals:

Ion from a solution is removed when it is passed through a bed of exchangeable ions, called

resins.

In this reaction, R- is fixed negative charge on the resin. A

+ and B

+ are called counter-ions

and X- is called coion in resin phase

Ion Exchange Resin

Most popular base for ion exchange resin is polystyrene. Cross linking with divinylbenzene

(DVB) is done with resin to make it insoluble. About 2-10% DVB is used. Both (i)

macroporous and (ii) gel type resin beads are used.

Macroporous beads have pores inside the beads where ions can go in or get out.




Typical external porosity is about 0.40. Gel type resin have various degrees of swelling.

These may be polystyrene-sulfuric acid resin with various % of DVB, polyacrylic acid resin,

etc.

Acidic resins have negative fixed charges and can exchange cations. Basic resins have

positive fixed charges and can exchange anions. Exchangers can also weak or strong. Strong

resins are fully ionized and all the fixed groups are available to exchange cations. Strong base

resins can degrade at higher pH and temperature.

On the other hand, weak resins are only partially ionized at most pH values. They

have lower exchange capacity but they are easier to regenerate. Weak resins require less

regenerant than strong resins.

But weak resins swell or contract when ions are exchanged. They can rupture due to improper

stress distribution of during expansion/contraction cycle. In weak resin also the ions diffuse

slowly. So, mass transfer resistance is very high and time requirement is long.

Techniques of Ion Exchange Chromatography:

1. Preparation of Column

The ion exchange chromatography is carried out in a chromatographic column which usually

consists of a burette provided with a glass wool plug at the lower end. Generally a ratio of 10:

1 or 100:1 between height and diameter is maintained in most of the experiment. Too narrow

or too wide column give uneven flow of liquid and sometimes poor separation.

2. Preparation of Ion Exchange

Ion exchange materials are first allowed to swell in buffer or in HCl or NaOH solution for 2-3

hours or sometimes overnight. Almost all ion exchange resin swells when placed in buffer or

distilled water and this is due to hydration of their ions. In dry condition, the pore of resins is

restricted so in order to swell the pore of resin. Resins are suspended in buffer solution or in

distilled water.

3. Washing of Ion Exchangers




The ion exchange material is obtained in required ionic form by washing with appropriate

solution. For e.g. the H+

form of cation exchange resins is obtained by washing the material

with HCl then with water until the washings are neutral.

Anionic exchangers are generally supplied in the form of salt and amines. Similarly, Na+

form is prepared by washing the resins with NaCl or NaOH solution and then with water.

Figure 2: Ion exchange chromatography

4. Packing of Column

This is one of the most critical factors in achieving a successful separation. The column is

held in vertical position and the slurry of resins is poured into the column that has its outlet

closed. The column is gently tapped to ensure that no air bubbles are trapped and that packing

material settles evenly.

5. Sample Application

Sample can be loaded by using pipette or syringe. The amount of sample that can be applied

to a column is dependent upon the size of the column and the capacity of resins, If the

starting buffer is to be used throughout the development of column, the sample volume be 1

% to 5 % of bed volume.

6. Development an Elution of bound ions




Bound ions can be removed by changing the pH of buffer. E.g. separation of amino acid is

usually achieved by using a strong acidic cation exchanger. The sample is introduced onto the

column at pH of 1-2, thus ensuring complete binding of all of the amino acids.

Gradient elution used in increasing pH and ionic concentration results in the sequential

elution of amino acid. Then acidic amino acid such as aspartic acid and glutamic acid are

eluted first. The neutral amino acid such as glycine and valine are eluted. The basic amino

acid such as lysine and arginine retain their net positive charge at pH value of 9 to 11 and are

eluted at last.

7. Analysis of eluate

Equal fraction of each elute are collected at different test tube keeping the flow rate at 1 ml

per minute. The eluate collected in each fraction is mixed with ninhydrin color reagent. The

mixture is then heated to 105 C to develop the color and intensity of color is determined by

colorimeter method or spectrophotometer method at 540 to 570 nm.

5.Explain the construction and working of High Pressure Liquid Chromatography

(HPLC) with a neat sketch

Introduction

High-performance liquid chromatography (HPLC) is a form of liquid chromatography to

separate compounds that are dissolved in solution. HPLC instruments consist of a reservoir of

mobile phase, a pump, an injector, a separation column, and a detector. Compounds are

separated by injecting a plug of the sample mixture onto the column. The different

components in the mixture pass through the column at different rates due to differences in

their partitioning behavior between the mobile liquid phase and the stationary phase.

Instrumentation

Solvents must be degassed to eliminate formation of bubbles. The pumps provide a steady

high pressure with no pulsating, and can be programmed to vary the composition of the

solvent during the course of the separation.

The liquid sample is introduced into a sample loop of an injector with a syringe. When the

loop is filled, the injector can be inject the sample into the stream by placing the sample loop

in line with the mobile phase tubing.




The presence of analytes in the column effluent is recorded by detecting a change in

refractive index, UV-VIS absorption at a set wavelength, fluorescence after excitation with a

suitable wavelength, or electrochemical response.

Mass spectrometers can also be interfaced with liquid chromatography to provide structural

information and help identify the separated analytes.

Schematic of an HPLC instrument

Advantages

Speed (minutes)

High resolution

Sensitivity (ng to fg)

Reproducibility of +/- 1%

Accuracy

Automation




Disadvantages

Cost

Complexity

Low sensitivity for some compounds

Irreversibly adsorbed compounds not detected

Co-elution difficult to detect

6. Explain the construction and working of thin layer chromatography with a neat

diagram

Thin layer chromatography (TLC) is a chromatography technique used to separate non-

volatile mixtures.[1]

Thin layer chromatography is performed on a sheet of glass, plastic, or

aluminium foil, which is coated with a thin layer of adsorbent material, usually silica gel,

aluminium oxide, or cellulose. This layer of adsorbent is known as the stationary phase.

After the sample has been applied on the plate, a solvent or solvent mixture (known as the

mobile phase) is drawn up the plate via capillary action. Because different analytes ascend

the TLC plate at different rates, separation is achieved.[2]

Thin layer chromatography can be used to monitor the progress of a reaction, identify

compounds present in a given mixture, and determine the purity of a substance. Specific

examples of these applications include: analyzing ceramides and fatty acids, detection of

pesticides or insecticides in food and water, analyzing the dye composition of fibers in

forensics, assaying the radiochemical purity of radiopharmaceuticals, or identification of

medicinal plants and their constituents [3]

A number of enhancements can be made to the original method to automate the different

steps, to increase the resolution achieved with TLC and to allow more accurate quantitative

analysis. This method is referred to as HPTLC, or "high performance TLC".




Plate preparation

TLC plates are usually commercially available, with standard particle size ranges to improve

reproducibility. They are prepared by mixing the adsorbent, such as silica gel, with a small

amount of inert binder like calcium sulfate (gypsum) and water. This mixture is spread as a

thick slurry on an unreactive carrier sheet, usually glass, thick aluminum foil, or plastic. The

resultant plate is dried and activated by heating in an oven for thirty minutes at 110 C. The

thickness of the absorbent layer is typically around 0.1 0.25 mm for analytical purposes and

around 0.5 2.0 mm for preparative TLC.[4]

Technique

Chromatogram of 10 essential oils coloured with vanillin reagent.

The process is similar to paper chromatography with the advantage of faster runs, better

separations, and the choice between different stationary phases. Because of its simplicity and

speed TLC is often used for monitoring chemical reactions and for the qualitative analysis of

reaction products.

To run a thin layer chromatography, the following procedure is carried out:[5]

A small spot of solution containing the sample is applied to a plate, about 1.5

centimeters from the bottom edge. The solvent is allowed to completely evaporate off,

otherwise a very poor or no separation will be achieved. If a non-volatile solvent was

used to apply the sample, the plate needs to be dried in a vacuum chamber.

A small amount of an appropriate solvent (eluent) is poured into a glass beaker or any

other suitable transparent container (separation chamber) to a depth of less than 1

centimeter. A strip of filter paper (aka "wick") is put into the chamber, so that its

bottom touches the solvent, and the paper lies on the chamber wall and reaches almost

to the top of the container. The container is closed with a cover glass or any other lid

and is left for a few minutes to let the solvent vapors ascend the filter paper and

saturate the air in the chamber. (Failure to saturate the chamber will result in poor

separation and non-reproducible results).

The TLC plate is then placed in the chamber so that the spot(s) of the sample do not

touch the surface of the eluent in the chamber, and the lid is closed. The solvent

moves up the plate by capillary action, meets the sample mixture and carries it up the




plate (elutes the sample). The plate should be removed from the chamber before the

solvent front reaches the top of the stationary phase (continuation of the elution will

give a misleading result) and dried.

Separation Process and Principle

Different compounds in the sample mixture travel at different rates due to the differences in

their attraction to the stationary phase, and because of differences in solubility in the

solvent.[6]

By changing the solvent, or perhaps using a mixture, the separation of components

(measured by the Rf value) can be adjusted. Also, the separation achieved with a TLC plate

can be used to estimate the separation of a flash chromatography column.[7]

Development of a TLC plate, a purple spot separates into a red and blue spot.

Separation of compounds is based on the competition of the solute and the mobile phase for

binding places on the stationary phase. For instance, if normal phase silica gel is used as the

stationary phase it can be considered polar. Given two compounds which differ in polarity,

the more polar compound has a stronger interaction with the silica and is therefore more

capable to dispel the mobile phase from the binding places. Consequently, the less polar

compound moves higher up the plate (resulting in a higher Rf value).[6]

If the mobile phase is

changed to a more polar solvent or mixture of solvents, it is more capable of dispelling

solutes from the silica binding places and all compounds on the TLC plate will move higher

up the plate. It is commonly said that "strong" solvents (eluents) push the analyzed

compounds up the plate, while "weak" eluents barely move them. The order of

strength/weakness depends on the coating (stationary phase) of the TLC plate. For silica gel

coated TLC plates, the eluent strength increases in the following order: Perfluoroalkane

(weakest), Hexane, Pentane, Carbon tetrachloride, Benzene/Toluene, Dichloromethane,

Diethyl ether, Ethylacetate, Acetonitrile, Acetone, 2-Propanol/n-Butanol, Water, Methanol,

Triethylamine, Acetic acid, Formic acid (strongest). For C18 coated plates the order is

reverse. Practically this means that if you use a mixture of ethyl acetate and hexane as the




mobile phase, adding more ethyl acetate results in higher Rf values for all compounds on the

TLC plate. Changing the polarity of the mobile phase will normally not result in reversed

order of running of the compounds on the TLC plate. An eluotropic series can be used as a

guide in selecting a mobile phase. If a reversed order of running of the compounds is desired,

an apolar stationary phase should be used, such as C18-functionalized silica.

Analysis

As the chemicals being separated may be colorless, several methods exist to visualize the

spots:

fluorescent analytes like quinine may be detected under blacklight (366 nm)

Often a small amount of a fluorescent compound, usually manganese-activated zinc

silicate, is added to the adsorbent that allows the visualization of spots under UV-C

light (254 nm). The adsorbent layer will thus fluoresce light green by itself, but spots

of analyte quench this fluorescence.

Iodine vapors are a general unspecific color reagent

Specific color reagents exist into which the TLC plate is dipped or which are sprayed

onto the plate.[8]

[9]

[10]

o Potassium permanganate - oxidation

o Bromine

In the case of lipids, the chromatogram may be transferred to a PVDF membrane and

then subjected to further analysis, for example mass spectrometry, a technique known

as Far-Eastern blotting.

Once visible, the Rf value, or retardation factor, of each spot can be determined by dividing

the distance the product traveled by the distance the solvent front traveled using the initial

spotting site as reference. These values depend on the solvent used and the type of TLC plate

and are not physical constants.

Applications

In organic chemistry, reactions are qualitatively monitored with TLC. Spots sampled with a

capillary tube are placed on the plate: a spot of starting material, a spot from the reaction

mixture, and a cross-spot with both. A small (3 by 7 cm) TLC plate takes a couple of minutes




to run. The analysis is qualitative, and it will show if the starting material has disappeared,

i.e. the reaction is complete, if any product has appeared, and how many products are

generated (although this might be underestimated due to co-elution). Unfortunately, TLCs

from low-temperature reactions may give misleading results, because the sample is warmed

to room temperature in the capillary, which can alter the reactionthe warmed sample

analyzed by TLC is not the same as what is in the low-temperature flask. One such reaction is

the DIBALH reduction of ester to aldehyde.




UNIT III

INDUSTRIAL GAS ANALYZERS AND POLLUTION MONITORING INSTRUMENTS

PART A

1. Define paramagnetic property and state the types of paramagnetic oxygen

analyzer

Those gases which seek the strongest part of the field are called paramagnetic gases.

Eg:nitric oxide, chlorine dioxide.

2. What is meant by magnetic susceptibility?

Magnetic susceptibility is defined as the ratio of intensity of magnetization induced in

the substance by the field and its field strength.

3. List the methods of NO2analyzer.

1.colorimetry

2.chemi luminescence

3.use of CO laser

4. Laser Opto acoustic spectroscopy.

4. What is chemiluminesence?

The phenomenon of emission of radiation from a chemically excited species is known

as chemiluminescence. It results due to the formation of new chemical bonds .The atoms

in the excited states posses higher energy levels than the ground state & while returning

to the ground state they emit radiations.

5. Define thermal conductivity.

Thermal conductivity is defined as the quantity of heat transferred in unit time in a gas

between two surfaces 1 cm apart, when the temperature difference between the surfaces is

1oC

6. State the principle of thermal conductivity analyzers.

The preheated metal conducts electricity at a particular rate. If the gas is passed over the

heated metal, the gas absorbs the heat from the metal. This action reduces the temperature

of the metal, in turn the conductivity. This change in conductivity gives the measure of

the gas passed over the metal.




7. State the limitations of magnetic wind type instrument.

Hydrocarbons & other combustible gases react with the heated filament & causes

changes in temperature resulting in large errors. Therefore sample shouldnt contain these

substances in it.

8. List the types of H2S analyzer.

1.Electrochemical cell

2.photometric analysis

3.gold film semiconductor sensor

4. solid state sensor

5.lead acetate tape staining method

9. Give the types of filaments used for thermal conductivity analyzers

Pt,W,Co,Ni,Fe

10. State the limitations of magnetic force type instrument.

1.Entry of moisture will change instrument response. Suitable filter must be used at

the sample inlet.

2.Any change in temperature of the gases cause a corresponding change in its

magnetic susceptibility .So suitable circuit has to be employed to maintain sample

temperature as constant.

PART B

1. Explain with neat sketch the construction and working of Infra red gas analyzers

Introduction:

The Infra red radiation can be effectively used to analyze the presence of diatomic components

such as O2, N2 present in the flu gas. The vibration energy of these diatomic components matches

with the IR radiation wave length.

Principle:

The diatomic molecules present in the sample absorb infra red radiation when exposed to it.

Hence there will be a reduction in the intensity of the IR radiation at the detector. The amount of

decrease in intensity is the measure of the amount of diatomic components present in the sample.




Schematic of IR gas analyzer:

Construction:

1. The IR gas analyzers consists of IR light source, a motor controlled chopper, a reference

cell, a sample cell and detector

2. The Motor controlled chopper is used to produce electric pulses rather than a continuous

beam of light

3. The reference cell consists of gases which do not absorb IR radiation

4. The sample cell has two openings. One for gas inlet and another is for gas outlet

5. The sample gas is made to pass through the sample continuously through these openings

6. An IR radiation detector is installed to capture the radiation passing out from the sample

cell and the reference cell

Working:

1. The IR radiation is passed through the chopper and pulses of radiation are produced

2. This radiation is passed through the sample cell and the reference cell

3. The reference cell do not absorb IR radiation and hence the intensity of IR radiation will

not change

4. The diatomic molecules present in the sample absorb the IR radiation and hence the

intensity of the radiation will be reduced

5. The detector produces an appropriate electrical signal for the amount of radiation it

received. This electrical signal will be the difference in intensity levels coming out from

reference cell and the sample cell. The electrical signal will be the measure of diatomic

components present in the sample




2.Explain in detail thermal conductivity sensors.

Introduction:

Thermal conductivity analyzer makes use of thermal conductivity principle to measure amount

of gas to be measured in the flu gas.

Principle:

When a cooled gas is made to pass through the heated metal connected to a power supply, the

gas will absorb the heat. This will change the current flow through the metal. It will give the

measure of the gas.

Schematic of Thermal Conductivity Analyzers

Construction:

1. The thermal conductivity sensors consist of a wheat stone bridge

2. Two arms of the bridge contain heated platinum wires in opposite direction over which

the sample and the reference gases are passed

3. An external power supply is used to excite the bridge

4. The output from the bridge is connected to amplifier and to signal conditioning circuit

Working:

1. The bridge has to be balanced before measuring the gas of our interest

2. Initially the reference gas is allowed to pass over the arms and the current flow through

the bridge is noted

3. If the galvanometer shows zero reading then the bridge is balanced. If the bridge is not

balanced external voltage is applied and the bridge is brought back to balanced condition

4. After attaining balance condition in the bridge, the sample gas is passed over one of the

arms of the bridge containing heated platinum wire




5. The sample gas absorbs the heat from platinum wire which in turn causes change in

current flow through the wire and hence the bridge balance is lost

6. This results in output Voltage and it is the measure of the sample gas concentration

3. Explain in detail how lead acetate tape staining is used to determine H2S in flu gas

Introduction:

Hydrogen sulphide is highly toxic gas which comes out as an effluent from the industries. Lead

acetate tape staining method is used to find out the concentration of H2S in the flu gas.

Principle:

When H2S reacts with lead acetate, it produces a brown patch of lead sulphide. This patch when

exposed to light absorbs radiation. The absorption of radiation is the measure of H2S present in

the flu gas.

Schematic of Lead acetate tape staining:




Construction:

1. The equipment consists of a humidifier containing 5% acetic acid in water to humidify

the H2S

2. A lead acetate tape is made to pass through a reaction window, where it is exposed to H2S

and light. This tape is driven by a motor

3. A measuring photocell and the sample photocell have been installed to measure the

radiation levels

4. A display device is deployed to give the readouts of the concentration of H2S

Working:

1. The H2S gas is humidified by acetic acid solution.

2. The humidified H2S is allowed pass through the lead acetate tape via a reaction window.

3. The H2S reacts with lead acetate tape and produces lead sulphide which is brown in color.

4. The brown patch will give the measure of H2S. To find out the concentration of H2S, a

light beam is passed over the brown patch and it is collected by a photocell.

5. The difference in intensities of radiation levels of reference beam and sample beam gives

the measure of H2S present in the solution.

4. What is Chemiluminescence? Explain how chemiluminescene can be used to determine

NO2 present in flu gas?

Chemiluminesence:

The phenomenon of emission of radiation from a chemically excited species is known as

chemiluminescence. It results due to the formation of new chemical bonds .The atoms in the

excited states posses higher energy levels than the ground state & while returning to the ground

state they emit radiations.




Schematic of NO2 Analyzer:

Construction:

1. NO2 is inert to O3 , Hence the NO2 converted into NO using appropriate catalyst and heat.

2. A O3 genereator is available to generate sufficient quantity of O3.

3. A reaction chamber is present where the NO and O3 reacts and produces light,

4. A Band pass filter is used to filter unwanted wavelengths

5. A photomultiplier tube is used to convert light into electric signals

Working:

1. The NO2 is converted into NO and it is allowed to react with O3 in a reaction chamber

2. When NO reacts with O3 it produces NO2 and light. This light is of unique wave length.

3. This light may contain multiple wavelengths and to remove unwanted wavelengths a

band pass filter is employed.

4. The filtered light is allowed to fall on photomultiplier tube, which converts the light into

appropriate electrical signals.

5. This electrical signal will give the measure of NO2 present in the flu gas.


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