Quadrupole and Time of Flight Mass analysers.

Topic: Quadrupole and Time Of Flight mass
analyzers
Presented by:
Ramesh S. Gagan S.
Dept. of Pharmacology. Dept. of Pharmaceutical
Chemistry
Government college of Pharmacy, Bengaluru.
Mass spectroscopy

CONTENTS
Introduction
Principle
Theory
Construction and working
variants
Advantages
Disadvantages
Applications

INTRODUCTION
Mass spectrometry is an analytical chemistry technique that helps
identifying the amount and type of chemicals present in a sample
by measuring the mass-to-charge ratio and abundance of gas
phase ions.

Typical representation of mass spectrometer

MASS ANALYSERS
Mass analysers are called heart of mass
spectrometer .
These are the instruments which separates
the mixtures of ions that are generated
during the ionization step by m/z in order to
obtain a spectrum.

Types of mass analysers
Magnet sector mass spectrometer.
Quadrupole mass spectrometer.
Ion trap mass spectrometer.
Time of flight mass spectrometer.
Fourier transform mass spectrometer.

Table : Summary of mass analysers.
MASSS MASS RANGE RESOLUTION SENSITIVITY
MAGNETIC
SECTOR
1-15,000 m/z 0.0001 Low
QUADRUPOLE 1-5000 m/z Unit High
ION TRAP 1-5000 m/z Unit High
TIME OF FLIGHT Unlimited 0.0001 High
FOURIER
TRANSFORM
Up to 70 k Da 0.0001 High

QUADRUPOLE MASS SPECTROMETER
Quadrupole mass analysers concept was first reported
by Paul and Steinwedel in the 1950s.
Quadrupole consists of four rods which are arranged
parallelly.
Rods opposite to one another are electrically
connected.
Ions are separated based on m/z at a particular DC and
Radiofrequency voltage.

PRINCIPLE:
 . IONS FROM THE IONS SOURCE
QUADRUPOLE ANALYSER WITH A
CONSTANT DC AND RF VOLTAGE
AT GIVEN COMBINATION OF DC AND RF
VOLTAGE
IONS WITH CERTAIN m/z
VALUES REACH THE
DETECTOR
IONS WITH DIFFERENT m/z
VALUES TRAVEL UNSTABLE
PATH

It consists of four electrically conducting parallel rods.
One diagonally opposite pair of rods is held at +Udc
volts and the other pair at –Udc volts.
A radiofrequency oscillator supplies +Vcosωt to the first
pair and –Vcosωt to the second pair.
Ions enter to the quadrupole through a circular aperture.

WORKING:
 When ions enter into the quadrupole ions get deflected by
the positive and negative electrodes and ion with a certain
m/z value will reach the detector.
 This can be explained by taking individual pair of rods.

Rods connected with +DC and +RF Voltage :

Rods connected wit –DC and –RF voltage

In combination of both the pair of rods

FACTORS AFFECTING FUNCTION OF QUADRUPOLE:
 Focussing slits
 Rod length
 Rod diameter
 Increased rf frequency

ADVANTAGES:
 High sensitivity.
 They can operate close to ground potential.
 Ideal for interfacing to LC system and for atmospheric
pressure ionization (API) techniques such as electrospray.
 Well suited for the negative ions ,since the analyser does not
discriminate between the polarity of the ions.
 Economic, robust and scanning is very fast.

DISADVANTAGES:
Quadrupole is generally inferior to magnetic sector
analyser with respect to resolution and mass range.
Peak height variable as a function of mass.
Not well suited for pulsed ionization methods.

APPLICATIONS:
In liquid chromatography-Mass spectrometry.
In Gas chromatography-Mass spectrometry.
To measure the relative amounts of different gases in
a mixture.
Proteomics.

TIME OF FLIGHT MASS SPECTROMETR
Introduction:
Stephens first described the concept of time of flight in 1946.
As the name implies, time of flight mass spectrometer
separates ions and measure their m/z based on the time they
take to pass ( “fly” ) from the ion source to the detector.

PRINCIPLE:
Ions with different masses and same kinetic energy
travel when accelerated and the ions with smaller
masses (less m/z value)reaches early to the detector
than the bigger masses .

THEORY:
The kinetic energy of ions of mass (m), charge (z) and accelerating potential (V) is given by
KE = zV = ½ mv2 (1)
Where (v) is velocity of the ions which is given by
v = l/t
where, l = length of the tube
t = time taken by the ions to pass the length of the tube
Therefore, equation 1 becomes
zV = ½ ml2/t2
m/z = 2Vt2/l2
2V/l2 = k (constant)
therefore, m/z = kt2
m/z α t2

LINEAR TIME OF FLIGHT MASS ANALYSER
Construction:

Working:
.
IONS FROM THE ION
SOURCE
TOF
ACCELERATION OF
IONS
ACQUIRES
DIFFERENT
VELOCITIES
SPATIAL
SEPARATION OF
IONS
IONS WITH HIGHER
VELOCITY REACH
THE DETECTOR
FIRST

VARIANTS OF LINEAR TOF ANALYSER
1.Ion mirror / ion reflectron / reflectron.
2.Time-lag focussing

NEED FOR VARIANT TYPE OF TOF ANALYSER
Ions with very similar m/z ratio may have relatively poor
mass resolutions .
This is due to the spatial distribution in the ion source and
their proximity to the applied electric field, not all the ions
receive the same kinetic energy.
This leads to components in mixtures being unresolved
from one another and large errors in molecular weight
measurements.

ION MIRROR / ION REFLECTRON / REFLECTRON
Construction and working:

.
It consists of stack of donut shaped lens connected by a
series of resistors across which voltage is applied.
These lenses repell the ions entered.
These ions are reflected down at second flight tube to a
second detector.
Mass resolution is achieved as ions of different kinetic
energies penetrate the mirror to differing degrees.

.
For example consider two ions of the same m/z ratio that
have slightly different kinetic energies.

TIME-LAG FOCUSSING / PULSED ION EXTRACTION /
DELAYED EXTRACTION
Construction and working:

.
In time-lag focusing TOF applying accelerating potential is
delayed.
Ions from the ion source will have some kinetic energy and
moves with different velocities
Application of potential gives more energy to the ions which
are away from the detector than closer to it.
The amplitude is adjusted in such a way that all the ions
reach the detector at the same time.

ADVANTAGES OF TOF MASS ANALYSER
Mass range is unlimited.
Excellent sensitivity due to lack of resolving slits.

DISADVANTAGES OF TOF MASS ANALYSER
Limited use in case of pulsed ionization techniques.
Mass resolution is usually less than 20000.
Fast electronics are necessary for adequate resolution.

APPLICATION OF TOF MASS ANALYSER
Due to its fast scanning capability it is increasingly being
used in LC-MS instrumentation.
Generally it is used in case of fast analysis or in high
chromatographic resolution techniques.

REFERENCE:
1. Spectrometric identification of organic compounds by Robert M Silverstein
and Francis X Webster,pg. no. 10-12.
2. Instrumental methods of analysis ,7th edition by Willard Merrit Dean Settle,
pg. no. 480-483.
3. Instrumental analysis by Skoog, Holler, Crouch, India edition pg. no. 627.
4. Mass spectroscopy, A foundation course by K.Downard, pg. no. 40-43.
5. Organic spectroscopy, William Kemp, Third edition, pg. no.286-288
6. An introduction to Quadrupole Time of flight mass spectroscopy ,Journal of
mass spectroscopy, Willey analytical science.

Quadrupole and Time of Flight Mass analysers.

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