1. INTRODUCTION
Some abbreviations used: A =
mass number, Ar = relative atomic mass,
Z = atomic number (NOT z)
m/z = relative molecular mass or isotopic mass /
electric charge for ions formed in a mass spectrometer
Mass spectrometry gives accurate
information on the relative masses of isotopes and their relative abundance
(proportions).
Mass spectrometry is an important method of analysis in chemistry and
can be used to identify elements and compounds by their characteristic mass spectrum
pattern - the technique is used in planetary space probes e.g. mass
spectrometer instrumentation is incorporated in the Mars explorer vehicles,
mass spectrometers can monitor the concentration of air pollution molecules
and detect traces of illegal drugs in the urine of athletes.
What is Mass Spectroscopy and
a mass spectrum?
A mass spectrometer
is an instrument of analysing particles of different relative mass.
The
instrument used is called a mass spectrometer,
of which there are several types.
All
types of mass spectrometers involve vaporising atoms or
molecules in high vacuum and
subjecting the vapourised particles to electron bombardment to
generate a beam of positive ions,
a process called ionisation.
The mass spectrometer, by several different means,
separates and
counts the numbers of different positive ion particles produced.
The
resulting data from the detector is called a mass
spectrum (plural
mass spectra)
which gives you lots of data including:
the accurate
relative masses (based
on 12C = 12.0000)
of all the positive ions generated from individual atoms
(isotopes), whole molecules and fragments of molecules,
the relative
numbers of each particle
(listed above) generated by the electron bombardment of the
original atoms or molecules.
Uses of mass spectrometry include:
the determination of
very accurate relative
isotopic masses (these days to at least 9 significant figures with high resolution
mass spectrometers,
the relative abundances of
the isotopes for a specific element - from this you can calculate the
relative atomic mass of an element (which can also be measured from
chemical analysis),
identifying molecular formula using a high resolution mass
spectrometer
identification of organic molecules from fragmentation patterns
(each has a mass spectra fingerprint)
See separate page for more
on
details on uses
and applications of mass spectrometry
Advantages of using mass
spectrometry as an analytical technique
Like other modern instrumental analytical techniques used in
chemistry in the 20th-21st centuries, mass spectroscopy has several
advantages over traditional methods of chemical analysis e.g.
It is a very sensitive
technique, only requiring tiny amounts of material for analysis
and only tiny amounts might be available e.g. in forensic
analysis of a crime scheme.
It is a very accurate
technique, but the mass spectrometer does require careful
calibration e.g. relative to carbon-12 isotope given a value of
12.0000 atomic mass units and quality instruments rarely make a
mistake.
The analysis can be done
quickly AND continuously. A sampling plus a mass spectrometer
system could monitor pollution or a chemical production process
24/7!
A mass spectrometer can be
linked to other analytical instruments e.g. you can set up a
mass spectrometer to sample the separate molecules exiting from
a gas/liquid chromatograph column.
See separate page for more
on
details on uses
and applications of mass spectrometry
TOP OF PAGE
and sub-index
2. Method (1)
Magnetic field
DEFLECTION MASS SPECTROMETER
(NOTE:
Most mass spectrometers these days are of the
TOF type,
and students now, and in the future, should be expected know how a TOF
works, the results are ultimately the same, but I've described the older
type of mass spectrometer as an introduction to mass spectroscopy of the
atoms and molecules of elements and compounds)
My
use of the
word 'deflection'
in describing this type of mass spectrometer is NOT official, but I
couldn't think of any other way of distinguishing it from a 'time of
flight' mass spectrometer described in method (2) at the end of this
page!
The substance to be analysed is introduced/injected into a
high vacuum
(extremely low pressure) tube system (at K
left diagram) where the particles are ionised
by colliding with beam of high speed electrons (at Q
in left diagram).
Note:
If the sample is not already a gas,
then a liquid or solid substance must be vapourised, i.e.
the
material must be in the gaseous state
to be analysed in a mass spectrometer.
The material being analysed must in the form of free moving gaseous
atoms or molecules which can be then bombarded with electrons to produce
equally free moving positive ions which can rapidly be accelerated in a
powerful electric field. It is the manipulation of the stream of gaseous
ions that forms the basis of mass spectrometry.
You cannot analyse any
liquid or solid material in this way unless it is vapourised.
The resulting (+) ions are
accelerated
down a tube (from + to - plates, P
in left diagram) and then through a powerful magnetic field.
The charged or ionised particles are
deflected
by this powerful magnetic field (R
in left diagram).
How much they are deflected depends on the particle mass
and the speed of the particle and the strength of a magnetic field i.e.
lighter particles of lower mass (and momentum) are deflected more than
heavier particles of bigger mass (see right diagram below) for a given
set of conditions.
By
varying the strength of the magnetic field,
it is possible to bring into focus onto an ion detector
(N
in diagram above right) at the end of the tube (effectively an electrical event
is detected), every possible mass in turn and a measure the strength of the ion current,
which is a measure of how much of that ion
has been formed from the sample under analysis.
Simplified diagrams
of a mass spectrometer tube system are shown above and below with further explanation as
to what is going on and an extra diagram to show the relative paths of
light to heavy ions for a given strength of magnetic field.
KEY TO DIAGRAM
and more detail of each component's function
K
= sample injection
point, it must be a gas, so a liquid/solid must be vaporised at the
injection point.
IONISATION
Q
= high voltage (high +/- p.d.) electron gun which fires a beam of high
speed/energy electrons from a heated 'metal element' into the vaporised sample under analysis and causes
ionization of the atoms (or molecules) forming
positive ions
(mainly monopositive in charge).
The collision of
high KE electrons with atoms or molecules causes another electron to be
knocked off the particle leaving a negative deficit i.e. a positively
charged particle is formed e.g.
M(g) +
e- ==> M+(g) + 2e-,
usually written as just
M(g) ==> M+(g) + e- (M might
represent e.g. a metal atom or a molecule)
The ions formed
should be written as [M]+, a notation that is handy if
you are dealing with ionised molecule fragments with an overall single
positive charge e.g. [CH3]+ is seen in the
mass spectrum of methane gas, CH4.
The low pressure (~vacuum) is needed
to prevent the ions from colliding with air particles which would stop
them reaching the ion detector system.
ACCELERATION
P
= are negative plates which accelerate the positive ions
down the tube (there are positive plates at the start of the tube). A
moving beam of charged particles creates a magnetic field around itself, and
this 'ion beam' magnetic field interacts with the magnetic field at
R.
DEFLECTION-SEPARATION
R = the magnetic field that causes
deflection of ions,
this is can be varied to change the extent of deflection for a given mass
and to focus a beam of ions of particular mass down onto the detector.
Hence, by programming the mass spectrometer to 'sweep' through all
likely particle masses, in terms of the right hand diagram, you can increase the
strength of the magnetic field to bring into focus onto the ion
detector monopositive ions of increasing mass.
DETECTION
N
= an ion detection
system which essentially generates a tiny electrical current when the
ions hit it. The minute
electric current which can be amplified. The strengths of the 'electronic'
signals from the various ion peaks are sent to a computer for analysis,
computation and display. They tell you the particle masses present and their
relative abundance (see the mass spectrum diagram for the element strontium
below). The data is then presented as an m/z
versus peak height.
m/z
means the relative mass of the ion over its charge, which for our
purposes the electric charge is +1 (lower case z) and the mass (lower
case m) is the relative
atomic/formula mass of the particle ionised. You
should write the structure of the ion in square brackets and put the
charge on the outside of them in the top right - this is an important
and universally accepted notation in mass spectrometry.
Examples of m/z values (mass/charge
ratio) m/z values apply to
ALL methods of mass spectrometry (see TOF later)
ion |
relative mass
(m) |
positive ion
charge (z) |
m/z ratio |
[14N]+ |
14 |
1 |
14/1 = 1 |
[56Fe]+ |
56 |
1 |
56/1 = 56 |
[56Fe]2+ |
56 |
2 |
56/2 = 28 |
[35Cl]+ |
37 |
1 |
35/1 = 35 |
[35Cl2]+ |
70 |
1 |
70/1 = 70 |
[35Cl2]2+ |
70 |
2 |
70/2 = 35 |
[CH3]+ |
15 |
1 |
15/1 = 15 |
Note that you can get multiple charged ions, but
most mass spectral analysis is based on mono-positive ions.
The are integer m/z values from a low resolution
mass spectrometer.
Other terms used in mass spectroscopy:
Monatomic
(mononuclear ions) are derived from single atoms eg [35Cl]+
or [88Sr]+
and a molecular ion (polynuclear ion) is derived from when the molecule
is more than one atom
i.e. a complete but ionised molecule (molecular ion) e.g.
the complete molecules minus one electron to give a singly charged positive
ion OR the positive residue left when one of more electrons are broken off
to leave a molecular fragment ion)
Molecular ions:
[Cl2]+
from chlorine molecules, [C6H5COOH]+ from
benzoic acid molecules
Fragment ions:
[CH3CH2]+, an
ethyl fragment from the fragmentation of a hydrocarbon in a mass
spectrometer.
For advanced level students only:
Spectroscopy indexes: IR,
mass, H-NMR & C-13 NMR spectra of organic compounds
Index of all mass spectroscopy notes
and examples of spectra explained
TOP OF PAGE
and sub-index
3. Examples of a MASS SPECTRUM
explained
The resulting record of the
ion peaks is called the mass spectrum
or mass spectra. The highest peak
is called the base peak
and is often given the relative and arbitrary value of
100, particularly in the mass
spectra of organic compounds.
MASS SPECTRA
For elements
you get a series of signals or ion peaks for each isotope present and
the ratio of peak heights gives you the relative proportion of each
isotope in the element so that you can calculate the relative atomic
mass of an element. This 'simple' spectra of mononuclear ions like
[Sr]+
is only true for non-molecular elements like metals (see
mass spectrum of strontium diagram below) or noble gases, but for molecular elements like
nitrogen or the halogens things are not so simple (see chlorine example below).
The
proportions or percentages of all the isotopes of an element is often called
the isotopic abundance.
For larger e.g. organic molecules, things can be very
complex indeed, as molecules fragment and many different ions can be formed
BUT you can get the relative molecular mass of a molecule
by identifying what is called the molecular ion peak, that is, when
one electron is knocked of the molecule but the molecule retains its full
molecular structure.
e.g.
benzoic
acid (Mr = 122) gives a molecular ion peak of m/z =
122, due to
[C6H5COOH]+
but you also get fragments such as
[C6H5]+
with an m/z of 77
as the molecule breaks up from further electron impacts on the molecular
ion and larger fragments formed, so you get quite a complex degradation
of the original molecule (diagram below showing many of the fragments
formed).
So thing get very complicated
with organic molecules!
(full
details of mass spectrum of benzoic acid)
The fragmentation pattern is characteristic for a particular molecule
(and can be used for identification), BUT
the fragmentation pattern is also dependent on experimental conditions
e.g. lower/higher laser or electron beam ionisation
energy results in lesser/more fragmentation.
'Soft ionisation' is where use a low ionisation energy to give a greater
chance of measuring, M,
the mass of the molecular ion peak, and a very accurate value of M, to
3-4 decimal places can itself be used to identify the molecule.
For
more on this see: section
8. Introduction to more details on the mass spectra of organic compounds and the
molecular ion peak and fragmentation - use in identification of organic molecules
and
9.
Isotopic masses and accurate molecular ion peaks
to identify molecules and molecular formulae
More highly charged ions can show up
in mass spectra
You
can get multiple ionisation e.g.35Cl2+(m/z = 35/2
= 17.5), 16O2+(m/z = 16/2 = 8), 32S2+(m/z
= 32/2 = 16) etc. These more highly charged ions would be deflected or
accelerated more in the mass spectrometer than the monopositive ions. In
the mass spectrometer the monopositive ions are selected to produce the
mass spectrum.
You
should note that e.g. the m/z for 32S2+(m/z = 32/2
= 16) is identical to the m/z for 16O+ (m/z = 16/1
= 16). In a low resolution mass spectrometer they would not be
distinguishable, but in a very modern high resolution mass spectrometer
they would be.
TOP OF PAGE
and sub-index
4. CHLORINE EXAMPLE
The mass spectrum of chlorine is a good example of a
molecular element
whose mass spectra can be a bit tricky when first encountered.
Chlorine
consists of two principal stable isotopes,
chlorine-37 (~25% is 37Cl) and
chlorine-35 (~75% is 35Cl).
Ar(Cl)
is ~35.5 using the above percentages
from Ar(Cl)
= [(75 x 35) + (25 x 37)] / 100
BUT, chlorine consists of Cl2
diatomic molecules, which may or may not split on ionisation, so how can
we explain the presence of five peaks and not just two for the two
isotopes?
The result of the ionisation process and subsequent
fragmentation of chlorine molecules is a series of
5
different mass peaks
from the various isotopic monatomic or molecular ion possibilities.
So, in order of
decreasing mass (m/z for a monopositive charge)
-
[37Cl37Cl]+
or
[37Cl2]+ m/z = 74
(1-3 are molecular ions)
-
[37Cl35Cl]+ m/z =
72 (note that you must show
the two isotopes separately in this molecular ion)
-
[35Cl35Cl]+
or
[35Cl2]+
m/z = 70
(molecular ion)
-
[37Cl]+ m/z = 37
(mononuclear ion, monatomic fragment)
-
[35Cl]+
m/z =35
(mononuclear ion, monatomic fragment)
Reminder: (i) m/z
means the relative mass of the ion over its charge (m/z
explained),
(ii)
monatomic/mononuclear ions are derived from one atom,
(iii)
a molecular ion is derived from more than one atom.
So,
the presence of five peaks is explained and the
ratio of the peak heights can be explained by considering a simple
probability table of all the permutations possible for the
monatomic or molecular ions - remember in a mass spectrometer you are
dealing with millions of 'randomised' particles.
m/z |
35Cl |
35Cl |
35Cl |
37Cl |
35Cl |
70 |
70 |
70 |
72 |
35Cl |
70 |
70 |
70 |
72 |
35Cl |
70 |
70 |
70 |
72 |
37Cl |
72 |
72 |
72 |
74 |
The
ratio of heights for peaks 4 and 5 of the monatomic ions is
1 : 3,
the ratio of the isotopic abundance in the original naturally occurring
sample of chlorine atoms in compounds..
For
the diatomic molecular ions, (left table of possibilities) we assume (for simplicity) that exactly 3/4 (75%) of the
chlorine isotopes are 35Cl and 1/4 (25%) of the isotopes are
37Cl.
This
gives an expected ratio of the molecular ions 70 : 72 : 74 of
9 : 6 : 1, and this is
what you observe for peaks 1 to 3.
The
ratio of the heights of the first set of peaks (1-3) to the heights of
the 2nd set (4-5) depends on the energy and intensity of the ionising
beam of electrons. The greater this is, the greater the fragmentation of
the molecules i.e. peaks 1-2 would increase and peaks 3-5 would decrease
relative to each other, BUT, the height ratios would stay the same in
each set i.e. the monatomic/mononuclear ions and the diatomic molecular
ions.
For
identifying molecules from a fingerprint pattern you should operate the
mass spectrometer under the same conditions i.e. standards and unknowns
compared under the same operating conditions to give reproducible mass
spectra.
Other examples
and explanation of the
calculation of the relative atomic mass of an element using % of
isotopes is given in
Part
1 of GCSE-AS (basic) calculations.
The simplest and best example on this
page of calculating
relative atomic mass from a mass spectrum is fully explained for the
metallic element strontium.
TOP OF PAGE
and sub-index
5.
STRONTIUM EXAMPLE
Using mass spectra data to calculate relative atomic mass from the mass
spectrum of strontium
A 'simple' element
mass spectrum to interpret AND a subsequent relative atomic mass calculation
based on the mass spectroscopy of the
element strontium