Nikhilbinoy.CAssistant Professor

ICE Department

Analytical Instrumentation:Mass Spectroscopy

IntroductionThe sample to be analyzed is bombarded with an electron

beam to produce ionic fragments of the original molecule. The ions are sorted out by accelerating them through electric

and magnetic fields, according to their mass/charge ratio. No two molecules will be fragmented and ionized in exactly

the same manner.Highly sensitive.Application:1) Determination of molecular weight.2) Functional group.3) Investigation of interconnection and reaction

mechanism.

Introduction Analysis by MS does not require:

Chemical modification of the analyte.Any unique or specific chemical properties.

In theory, MS is capable of measuring any gas-phase molecule that carries charge.

Analyzed moleculesrange in size to mega-Dalton DNA and intact virus.

Mass Spectroscopy:Example

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Principle of OperationThe molecules in the gas sample

are bombarded with electrons to produce ions. These ions are accelerated in high

vacuum into a magnetic field, which deflects them into circular path.

The deflection of light ions is greater than that for heavy ions.

The ion stream separate into beams of different molecular weights.

A suitably placed slit allows a beam of a selected mass-charge ratio to pass through to a collection electrode.

Another beam of higher ratio is selected by gradually reducing the accelerating voltage.

Source

Detector

Analogy Between Mass Spectroscopy and Optical Spectroscopy

Detector

RadiationSource

Slit Sample Prism SlitOptical Spectrometer

Detector

RadiationSource

Slit Sample Prism SlitMass Spectrometer

IonSource Magnet

Magnetic Deflection Mass Spectrometer

Steps

Steps/Stages:1) Starts by converting the substance into gaseous ion state by mean of an electron beam.2) The positive ions formed are deflected and focussed by means of suitable magnetic and

electric fields.3) For a given accelerating voltage, only positive ions of a specific mass pass through a slit and

reach the collecting plate. By varying the accelerating voltage, ions from other mass species may be collected.4) The ions produced are measured by using a sensitive electrometer tube.

The Need for VacuumIt is important that, the ions produced in the ionisation chamber have a free run through the machine without hitting air molecules

Ionisation The vaporized sample passes into the

ionization chamber. The electrically heated metal coil

gives off electrons which is attracted to the electron trap which is a positively trapped plate.

The particles in the sample (atoms or molecules) are bombarded with a stream of electrons.

Some collisions are energetic enough to knock one or more electrons out of the sample particles to make positive ions.

Most of the positive ions carry a charge of +1 because it is much more difficulty to remove further electrons.

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

Acceleration The positive ions are

repelled away from the very positive ionisation chamber and pass through three slits. The final slit is at 0volts. The middle slit carries

some intermediate voltage.

All ions are accelerated into a finally focussed beam.

Deflection 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 the heavier ions.

The charge on the ion. Ions with 2 or more positive charges are deflected more than one with only 1 positive charge. These two factors are combined into

the mass/charge ratio.

PrincipleWhen ions of mass ‘m’

and charge ‘e’ pass through an accelerating electrical field with accelerating voltage V, they would attain kinetic energy with velocity ‘v’.

the velocity

In accelerating field

PrincipleWhen the ions enter a magnetic

field of constant intensity, which is applied at right angles to their direction of motion, the velocity remains constant, but the ions travels in a circular path.

The magnetic sector follows an arc.Equating the centrifugal and

centripetal forces:

The radius of curvature of the ion trajectory through magnetic sector is given by

In magnetic field

Principle

The radius of orbit is a function of the mass per charge ratio of the particle.

If all variables, except ‘m’ and ‘V’ kept constant, then by varying the accelerating voltage ’V’, it is possible to cause an ion of any mass to follow the path which may coincide with the arc of the analyser tube in the magnetic field.

Combining the effect of accelerating field and magnetic field

PrincipleUnder specified conditions, the ions which

would follow the following equation will be collected, and ions of different mass per charge ratio strike the tube at some point and would get grounded.

For obtaining a mass spectrum, the accelerating voltage or the magnetic field strength can be varied.Usually, magnetic field is kept constant, and

the voltage is adjusted to bring to focus a specific mass per charge ratio.

Magnetic field must be uniform over a large area.

Why we need accelerating voltage?The magnetic field is enough for the analysis, but

the accuracy is decreased.For an accurate analysis, the magnetic field is not

enough.The resolution will be limited by the fact that ions

leaving the ions source do not all have exactly the same energy, and therefore do not have exactly the same velocity. Inorder to achieve better resolution, add an electric

sector which will make the kinetic energy constant for all ions which have same mass per charge ratio.

Like magnetic field, electrical field is applied perpendicular to the direction of charge path.

The Time-of-Flight Mass Spectrometer

Ions of different m/e ratio are separated by the difference in time they taken to travel over an identical path from ion source to the collector.

The starting time at which the ions leave the ion source to be well defined.

The ions are either formed by:1. A pulsed ionization method.

Usually matrix assisted laser desorption ionization.2. Various kinds of rapid electric field switching.

Used as a ‘gate’ to release the ions from the ion source in a very short time.

In the pulsed mass spectrometer, ion packets of a few microseconds duration are emitted at intervals of few milliseconds from a voltage source.

The ions traverse through an evacuated tube called as drift tube.

The detector is sensitised for a brief instant. Accurate measurement is required since the ions arrives at

different time. The signal from the ion reaching the detector is amplified, and

applied to the vertical plates of oscilloscope. The horizontal plates of oscilloscope commence as the ion

packets start out.The device gives a mass spectrum in a very short time. Necessitates the use of a wide-band amplifier.

Advantages: -1) Speed.2) Ability to record the entire mass spectrum at one time.

PrincipleAn ion with mass ‘m’ and charge ‘e’ in an

accelerating field with voltage ‘V’ has a velocity:

If ‘L’ is the length of the drift tube in cm, and ‘t’ is the transmit time in microseconds, then

The time resolution will increase with increased drift tube length, and will decrease with increasing accelerating voltage.

Reflectron:

The ions leaving the ion source have neither exactly the same starting times nor exactly same kinetic energies.

Design known as ‘reflectron’ is used to compensate these.

The ions pass through the reflectron (optical mirror), and their flight is reversed.

Ions with kinetic energies to penetrate deeper into the reflectron than ions with smaller kinetic energies.

The ions that penetrate deeper will take longer to return to the detector.

Quadrupole Mass Spectrometer

Quadrupole mass spectrometers are:Simple in construction.Light weight.High speed electronic

scanning.Low cost.

Consists: An ion source. A quadrupole mass filter. Lens system to focus the ions

into the quadrupole filter.The quadrupole mass filter

shown consists four cylindrical rod shaped electrodes. Provides a potential field

distribution, periodic in time and symmetric with respect to the axis, which will transmit a selected mass group and cause ions of improper mass to be deflected away from the axis.

The mass filter uses a combination of dc potential plus a radio frequency (rf) potential.

In practice, opposite electrodes of the filter are connected together to one voltage, and other pairs of electrodes to the same potential with the opposite sign.

By proper selection of potentials and frequency, an ion of the desired mass can be made to pass through the system.

Pyrolysis Mass Spectrometry(Modification)Used, if the differentiation of very similar

substances is very difficult.In this arrangement, the sample to be

analyzed usually a solid or in-volatile liquid, is rapidly heated in a vacuum to a precise temperature.This causes thermal breakdown and

subsequent gas phase reactions.The gas or pyrolysate is then analyzed

directly in a mass spectrometer.

Pyrolysis Mass Spectrometry(Modification)

The sample is held on a metal substrate, usually a wire of foil, made of a magnetic alloy.

The sample is heated by induction from an external coil operating in the 1MHz range.

Causes rapid heating to a specific temperature, known as Curie point at which the magnetic permeability drops abruptly.

The gas produced during pyrolysis is then held in a small buffer volume before being formed into a molecular beam.

The liquid cooled nitrogen cooled shield surrounds the ion source and reduces source contamination.

The gas produced has entered to the quadrupole mass analyzer.

Components of Mass Spectrometer

The Inlet Sample System:Gaseous SamplesThe sample for a gas bulb is transferred to

the small glass manifold of known metering volume.The pressure is kept constant within the

range: 30 to 50Torr.Mercury manometer is used.

The gas sample is introduced into the mass spectrometer ion source through a leak of some kind.Generally, the leak is a pin-hole in metal foil.

The Inlet Sample System:Liquid SamplesLiquid sample may be introduced:

a) Either by hypodermic needle and injected through a silicon rubber dam, or

b) By break-off device which in touching a micropipette to a sintered glass disc under mercury.

The low pressure in the reservoir draws in the liquid and vaporizes it instantly.

The Inlet Sample System:Solid SamplesSolid samples are vaporized to gaseous

ions by instantaneous discharges with a power upto 100KW by using a radio frequency (1MHz) spark.All constituents of the sample are converted

into gaseous form at an equal rate with out regard to their vapour pressure.Eliminates the possibility of preferential

vaporization.

Ion SourcesFollowing the gas leak is the ionization chamber.

Maintained at low pressure ( to mm Hg), and at a temperature of .The electron gun is located perpendicular to the incoming gas

stream.Electrons are emitted from a filament normally of carbonized

tungsten filament. For special purpose, tantalum or oxide coated filaments may be used.

Electrons are drawn off by a pair of positively charged slits, through which they pass into the body of the chamber. The potential presents in the slits controls the electron emission, and the

energy of the electron. The electric field applied between these slits accelerates the electron, which

on subsequent collision with molecules of the passing gas stream, produces ionization and fragmentation..

The electric field is kept between 50 to 70V.The electron beam is usually collimated by a magnetic field.

Ion Source:ICPPotentially effective ion source.Generates plasma.

Gas in which atoms are present in an ionized state.

Ion Source:Glow Discharge Ion Source

Low energy ion source. Simple and inexpensive. Two electrode device. Filled with a rare gas to about

0.1 to 10torr. Sample to be analysed serves as

the cathode. Anode material is not particularly

critical. A few hundred volts applied

across the electrodes cause breakdown of the gas and formation of the ions.

The sample is atomized into the discharge by a process known as sputtering.

The glow discharge not only atomizes the solid sample, but also provides the mean by which these atoms are ionized.

Sputtered atoms diffuse into the negative glow, which contain energetic electrons, ions, and metastable atoms.

The glow discharge is adaptable to many discharge types and ion source configuration.

DC/PulsedPowerSupply

A

V

Limitting Resistor

Anode

Faraday/dark spaceNegative glowCathode dark space

Cathode

Ion Source:Electron ImpactElectron beam is used to ionize gas-phase

atoms or molcules.Electron beam is usually generated from a

tungsten filament.Electron from the beam knocks an electron

off of analyte atoms or molecules to create ions.

Ion Source:Electro-Spray IonizationConsists of a very fine needle and a series

of skimmers.Sample solution is sprayed into the source

chamber to form droplets.Droplets carry charges when they exit the

capillary.As the solvent evaporates, the droplets

disappear leaving highly charged analyte molecules.

Particularly applicable for large biological molecules which are difficult to ionize or vaporize.

Ion Source:Fast Atom BombardmentA high energy beam of natural atoms,

typically or , strikes a solid or low vapour pressure liquid sample causing desorption and ionization.

Used for large biological molecules that are difficult to get into the gas phase.

Ion Source:Field IonizationMolecules can lose an electron when placed

in a very high electric field.High fields can be created in an ion source

by applying a high voltage between the electrodes.Cathode.Anode called as ‘field emitter’.

Consists of a wire covered with microscopic carbon dendries, which greatly amplify the effective field at the carbon points.

Ion Source:Laser IonizationA laser pulse ablates material from the

surface of a sample, and creates a micro plasma that ionizes some of the sample constituents.

Accomplishes both vaporization and ionization of the sample.

Ion Source:Thermal IonizationThe sample is deposited on a metal ribbon,

such as Pt or Re.An electric current heats the metal to high

temperature.The ribbon is often coated with graphite to

provide a reducing effect.Thermal ionization is used for elemental or

refractory materials.

Electrostatic Accelerating SystemPositive ions are accelerated in a strong

electrostatic field between the first and second accelerating slits.Voltages of the order of 400-4000V.

Accelerates the ions to 150000miles/sec.Acquire a kinetic energy of a few thousand

electron volts.Relatively high kinetic energy is imparted to

the ions to produce an almost mono-energetic beam, when it finally emerges out of the final accelerating slit.

Electrostatic voltages are highly stabilized to an accuracy of better than 0.01%.

Ion Detectorsa) Faraday Cup.b) Channelbron.c) Electron multiplier tube.d) Micro-channel plate.

Vacuum SystemMass spectrometer requires a good vacuum

system.To prevent undue scattering by collision of

ions with residual gas molecules.Generally, separate mercury or oil diffusion

pumps are employed in the source and analysing regions of the spectrometer.

End!