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UNIT V

ELECTRO MAGNETIC RESONANCE AND MICROSCOPIC TECHNIQUES

PART A

1. What is the principle of radio frequency mass spectrometer?The charged particles emerging from the ion source are all accelerated to the same

energy in an electrostatic field and then they pass through a system of RF electrodes. The

energy acquired by the ions in this process is a function of their specific mass/charge ratio.

2. What is NMR?It is the branch of spectroscopy in which the absorption of RF radiation by

nuclei of the sample subjected to magnetic field is studied.

3. What are the types of mass spectrometer?1.magnetic deflection mass spectrometers.

2. time of flight mass spectrometer.

3. radio frequency mass spectrometer

4. quadrapole mass analyzer

4. State the principle of mass analyzer.The sample to be analyzed is first bombarded with an electron beam to

produce ionic fragments of the original molecule .These ions are then sorted out by

accelerating them through electric field and magnetic field according to mass/charge ratio.

A record of numbers of different kinds of ions called mass spectrum.

5. State the limitations of quadrapole mass analyzer.The performance of a mass filter depends on the quality of the quadrapole rods, the

stability of the applied voltage and the field characteristic at the two ends of the rods.

Especially, it is essential that the Quadra pole rod assembly is produced with super high

precision

6. State the advantages and disadvantages of time of flight mass spectrometer.Advantages:

Speed, ability to record entire mass spectrum in one time. a conventional spectrometer

detects only one peak at a time. Its accuracy depends on electronic circuits than

mechanical alignment and on production of highly stable and uniform magnetic field.

Disadvantage:

Their poor resolution due to display on an oscilloscope.

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7. What is meant by nuclear shielding?The magnetic field at the nucleus is not equal to the applied magnetic field; electrons around

the nucleus shield it from the applied field. The difference between the applied magnetic

field and the field at the nucleus is termed the nuclear shielding.

8. What is the principle of Electron spin resonance?When the resonant frequency of the cavity matches with the frequency of the microwave

produced from the klystron, the incident microwave energy on the cavity will be absorbed

and the energy of the electron increases.

9. What are the rules to determine Nuclear spin If the number of neutrons and the number of protons are both even, then the nucleus

has NO spin.

If the number of neutrons plus the number of protons is odd, then the nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2)

If the number of neutrons and the number of protons are both odd, then the nucleus has aninteger spin (i.e. 1, 2, 3)

10. Draw the energy level of the nucleus with spin quantum number

PARTB

1. Explain in detail the NMR SpectrometerIntroduction:

NMR spectrometer is the effective device used to analyze the structural components of

compound especially organic compounds.

Principle:

The nucleus of the atom will be spinning and it posses its own magnetic field and frequency.

At a particular magnetic field strength, the compound absorbs resonant frequency.

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Schematic of NMR spectrometer:

Construction:

A solution of the sample in a uniform 5 mm glass tube is oriented between the poles of a

powerful magnet, and is spun to average any magnetic field variations, as well as tubeimperfections.

Radio frequency radiation of appropriate energy is broadcast into the sample from an antenna

coil

A receiver coil surrounds the sample tube, and emission of absorbed rf energy is monitored

by dedicated electronic devices and a computer.

An nmr spectrum is acquired by varying or sweeping the magnetic field over a small range

while observing the rf signal from the sample.

An equally effective technique is to vary the frequency of the rf radiation while holding the

external field constant.

Working:

The NMR spectrometer can be operated in two ways

Keeping magnetic field constant and varying electric field Keeping the electric field constant and varying the magnetic field1. The Magnetic field is kept constant and the electric field frequency is increased.

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2. The lattice vibration of the nucleus will be at a frequency fn3. If the frequency of the field is increased and if it is equal to fn resonance occurs.4. At resonance, the low energy nucleus will absorb the EM radiation and excites to

higher states.

5.

This can be graphically represented with ppm on x axis and energy of radiation on yaxis.

2. Explain with a neat sketch, the working of a scanning electron microscope

Introduction

SEM stands for scanning electron microscope. The SEM is a microscope that uses electrons

instead of light to form an image. Since their development in the early 1950's, scanning

electron microscopes have developed new areas of study in the medical and physical science

communities. The SEM has allowed researchers to examine a much bigger variety ofspecimens.

Schematic of SEM:

Construction and Working

The SEM is an instrument that produces a largely magnified image by using electrons instead

of light to form an image. A beam of electrons is produced at the top of the microscope by an

electron gun. The electron beam follows a vertical path through the microscope, which is held

within a vacuum. The beam travels through electromagnetic fields and lenses, which focus

the beam down toward the sample. Once the beam hits the sample, electrons and X-rays are

ejected from the sample.

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Detectors collect these X-rays, backscattered electrons, and secondary electrons and convert

them into a signal that is sent to a screen similar to a television screen. This produces the final

image.

Advantages

The scanning electron microscope has many advantages over traditional microscopes. The

SEM has a large depth of field, which allows more of a specimen to be in focus at one time.

The SEM also has much higher resolution, so closely spaced specimens can be magnified at

much higher levels. Because the SEM uses electromagnets rather than lenses, the researcher

has much more control in the degree of magnification. All of these advantages, as well as the

actual strikingly clear images, make the scanning electron microscope one of the most useful

instruments in research today.

3. Explain the construction and working of a Transmission electron microscope with a

neat sketch

Introduction:

A Transmission Electron Microscope (TEM) utilizes energetic electrons to provide

morphologic, compositional and crystallographic information on samples

Schematic of TEM:

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Construction:

TEMs consist of the following components:

An electron source Thermionic Gun Electron beam Electromagnetic lenses Vacuum chamber 2 Condensers Sample stage Phosphor or fluorescent screen Computer

A Transmission Electron Microscope functions under the same basic principles as an optical

microscope.

In a TEM, electrons replace photons, electromagnetic lenses replace glass lenses and images

are viewed on a screen rather than through an eyepiece.

Working:

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A Transmission Electron Microscope produces a high-resolution, black and white image from

the interaction that takes place between prepared samples and energetic electrons in the

vacuum chamber.

Air needs to be pumped out of the vacuum chamber, creating a space where electrons are able

to move.

The electrons then pass through multiple electromagnetic lenses. These solenoids are tubes

with coil wrapped around them.

The beam passes through the solenoids, down the column, makes contact with the screen

where the electrons are converted to light and form an image.

The image can be manipulated by adjusting the voltage of the gun to accelerate or decrease

the speed of electrons as well as changing the electromagnetic wavelength via the solenoids.

The coils focus images onto a screen or photographic plate.

During transmission, the speed of electrons directly correlates to electron wavelength; the

faster electrons move, the shorter wavelength and the greater the quality and detail of the

image.

The lighter areas of the image represent the places where a greater number of electrons were

able to pass through the sample and the darker areas reflect the dense areas of the object.

These differences provide information on the structure, texture, shape and size of the sample.

To obtain a TEM analysis, samples need to have certain properties. They need to be sliced

thin enough for electrons to pass through, a property known as electron transparency.

Samples need to be able to withstand the vacuum chamber and often require special

preparation before viewing.

Types of preparation include dehydration, sputter coating of non-conductive materials,

cryofixation, sectioning and staining.

TEM Applications

A Transmission Electron Microscope is ideal for a number of different fields such as life

sciences, nanotechnology, medical, biological and material research, forensic analysis,

gemology and metallurgy as well as industry and education.

TEMs provide topographical, morphological, compositional and crystalline information.

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The images allow researchers to view samples on a molecular level, making it possible to

analyze structure and texture.

This information is useful in the study of crystals and metals, but also has industrial

applications.

TEMs can be used in semiconductor analysis and production and the manufacturing of

computer and silicon chips.

Technology companies use TEMs to identify flaws, fractures and damages to micro-sized

objects; this data can help fix problems and/or help to make a more durable, efficient product.

Colleges and universities can utilize TEMs for research and studies.

Although electron microscopes require specialized training, students can assist professors andlearn TEM techniques.

Students will have the opportunity to observe a nano-sized world in incredible depth and

detail.

Advantages

A Transmission Electron Microscope is an impressive instrument with a number of

advantages such as:

TEMs offer the most powerful magnification, potentially over one million times or more TEMs have a wide-range of applications and can be utilized in a variety of different

scientific, educational and industrial fields

TEMs provide information on element and compound structure Images are high-quality and detailed TEMs are able to yield information of surface features, shape, size and structure They are easy to operate with proper training

Disadvantages

Some cons of electron microscopes include:

TEMs are large and very expensive Laborious sample preparation Potential artifacts from sample preparation Operation and analysis requires special training

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Samples are limited to those that are electron transparent, able to tolerate the vacuumchamber and small enough to fit in the chamber

TEMs require special housing and maintenance Images are black and white

Electron microscopes are sensitive to vibration and electromagnetic fields and must be

housed in an area that isolates them from possible exposure.

A Transmission Electron Microscope requires constant upkeep including maintaining

voltage, currents to the electromagnetic coils and cooling water.

4. Explain with neat sketch, electron spin resonance spectrometer

Introduction:

Electron Spin resonance spectroscopy is based on the absorption of microwave radiation by

an unpaired electron when it is exposed to a strong magnetic field. Species that contain

unpaired electrons (namely free radicals, odd-electron molecules, transition metal complexes,

rare earth ions, etc.) can therefore be detected by ESR.

Principle:

When the resonant frequency of the cavity matches with the frequency of the microwave

produced from the klystron, the incident microwave energy on the cavity will be absorbed

and the energy of the electron increases.

Schematic of Electron spin resonanace:

Construction:

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Klystron Tube:

There are three electrodes: a heated cathode from which electrons are emitted, an anode to

collect the electrons, and a highly negative reflector electrode which sends those electrons

which pass through a hole in the anode back to the anode. The motion of the charged

electrons from the

hole in the anode to the reflector and back to the anode generates a oscillating electric field

and thus electromagnetic radiation. The transit time from the hole to the reflector and back

again corresponds to the period of oscillation (1/n). Thus the microwave frequency can be

tuned (over a small range) by adjusting the physical distance between the anode and the

reflector or by adjusting the reflector voltage. In practice, both methods are used: the metal

tube is distorted mechanically to adjust the distance (a coarse frequency adjustment) and the

reflector voltage is adjusted as a fine control.

Attenuator:

The power level of microwaves will be very high. To reduce the power level of themicrowave, the attenuator is used.

Cavity:

The sample is mounted in the microwave cavity

The cavity is a rectangular metal box, exactly one wavelength in length.

An X-band cavity has dimensions of about 1 2 3 cm.

The sample is mounted in the electric field nodal plane, but at a maximum in the magnetic

field.

Working:

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Microwaves are generated by the Klystron tube and the power level adjusted with the

attenuator. The Circulator behaves like a traffic circle: microwaves entering from the

Klystron are routed toward the Cavity where the sample is mounted. Microwaves reflected

back from the cavity (less when power is being absorbed) are routed to the diode detector,

and any power reflected from the diode is absorbed completely by the Load. The diode ismounted along theE-vector of the

plane-polarized microwaves and thus produces a current proportional to the microwave

power reflected from the cavity. Thus, in principle, the absorption of microwaves by the

sample could be detected by noting a decrease in current in the microammeter.

Since the cavity length is not adjustable but it must be exactly one wavelength, the

spectrometer must be tuned such that the klystron frequency is equal to the cavity resonant

frequency. The tune-up procedure usually includes observing the klystron power mode. That

is, the klystron reflector voltage is swept, and the diode current is plotted on an oscilloscope

or other device. When the klystron frequency is close to the cavity resonant frequency, much

less power is reflected from the cavity to the diode, resulting in a dip in the power mode. The

"cavity dip" is

centered on the power mode using the coarse mechanical frequency adjustment with

the reflector voltage used to fine tune the frequency.

5. Explain the conceptual aspects of Mass spectrometer with necessary diagramAtoms can be deflected by magnetic fields - provided the atom is first turned into an

ion. Electrically charged particles are affected by a magnetic field although electrically

neutral ones aren't.

The sequence is :

Stage 1: I onisation

The atom is ionised by knocking one or more electrons off to give a positive ion. This is

true even for things which you would normally expect to form negative ions (chlorine,

for example) or never form ions at all (argon, for example). Mass spectrometers always

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work with positive ions.

Stage 2: Acceleration

The ions are accelerated so that they all have the same kinetic energy.

Stage 3: Def lection

The ions are then deflected by a magnetic field according to their masses. The lighter

they are, the more they are deflected.

The amount of deflection also depends on the number of positive charges on the ion - in

other words, on how many electrons were knocked off in the first stage. The more the

ion is charged, the more it gets deflected.

Stage 4: Detection

The beam of ions passing through the machine is detected electrically.

A full diagram of a mass spectrometer

Ionisation

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The vaporised sample passes into the ionisation chamber. The electrically heated metal

coil gives off electrons which are attracted to the electron trap which is a positively

charged plate.

The particles in the sample (atoms or molecules) are therefore bombarded with a stream

of electrons, and some of the collisions are energetic enough to knock one or more

electrons out of the sample particles to make positive ions.

Most of the positive ions formed will carry a charge of +1 because it is much more

difficult to remove further electrons from an already positive ion.

These positive ions are persuaded out into the rest of the machine by the ion repeller

which is another metal plate carrying a slight positive charge.

Acceleration

The positive ions are repelled away from the very positive ionisation chamber and pass

through three slits, the final one of which is at 0 volts. The middle slit carries some

intermediate voltage. All the ions are accelerated into a finely focused beam.

Deflection

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Different ions are deflected by the magnetic field by different amounts. The amount of

deflection depends on:

the mass of the ion. Lighter ions are deflected more than heavier ones.

the charge on the ion. Ions with 2 (or more) positive charges are deflected more than

ones with only 1 positive charge.

These two factors are combined into the mass/charge ratio. Mass/charge ratio is given

the symbol m/z (or sometimes m/e).

For example, if an ion had a mass of 28 and a charge of 1+, its mass/charge ratio would

be 28. An ion with a mass of 56 and a charge of 2+ would also have a mass/charge ratio

of 28.

In the last diagram, ion stream A is most deflected - it will contain ions with the

smallest mass/charge ratio. Ion stream C is the least deflected - it contains ions with the

greatest mass/charge ratio.

It makes it simpler to talk about this if we assume that the charge on all the ions is 1+.

Most of the ions passing through the mass spectrometer will have a charge of 1+, so

that the mass/charge ratio will be the same as the mass of the ion.

Assuming 1+ ions, stream A has the lightest ions, stream B the next lightest and stream

C the heaviest. Lighter ions are going to be more deflected than heavy ones.

Detection

Only ion stream B makes it right through the machine to the ion detector. The other

ions collide with the walls where they will pick up electrons and be neutralised.

Eventually, they get removed from the mass spectrometer by the vacuum pump.

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When an ion hits the metal box, its charge is neutralised by an electron jumping from

the metal on to the ion (right hand diagram). That leaves a space amongst the electrons

in the metal, and the electrons in the wire shuffle along to fill it.

A flow of electrons in the wire is detected as an electric current which can be amplified

and recorded. The more ions arriving, the greater the current.

6. Explain quadrapole Mass spectrometerIntroduction:

A Quadrupole is a mass analyzer that uses an electric field to separate ions.

Principle:

The ions, when passed in between the electric field, it will get deflected. The amount of

deflection is based on its mass.

Construction and Working:

The Quadrupole consists of 4 parallel rods/ poles, where adjacent rods have opposite

voltage polarity applied to them.

The voltage applied to each rod is the summation of a constant DC voltage (U) and a

varying radio frequency (Vrfcos(wt)),

where w = angular frequency of the radio frequency field.

The electric force on the ions causes the ions to oscillate/orbit in the area between the 4

rods, where the radius of the orbit is held constant.

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The ion moves in a very complex motion that is directly proportional to the mass of the

ion, voltage on the quadrupole, and the radio frequency.

The ions will remain orbiting in the area between the poles with no translation along the

length of the poles unless the ions have a constant velocity that is created as the ions

enter the quadrupole.

Before entering the analyzer, the ions travel through a potential of a certain voltage,

usually created by ring electrode, in order to give the ions a constant velocity so they

can transverse along the center of the quadrupole.

While in the quadrupole, the trajectories of the ions change slightly based on their

masses.

Ions of specific mass have a certain frequency by which they oscillate. The greater the

mass, the greater the frequency.

A certain limit is associated with each quadrupole and it selects ions which are within

the desirable frequency range.