Slide 1

Mass Spectrometry

La Ode Kadidae, S.Si., M.Si., Ph.D.

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Mass Spectrometry

I. Introduction
A. General overview
1. Mass Spectrometry is the generation, separation and characterization of
gas phase ions according to their relative mass as a function of charge

2. Previously, the requirement was that the sample be able to be vaporized

(similar limitation to GC), but modern ionization techniques allow the
study of such non-volatile molecules as proteins and nucleotides

3. The technique is a powerful qualitative and quantitative tool, routine

analyses are performed down to the femtogram (10 -15 g) level and as low
as the zeptomole (10-21 mol) level for proteins

4. Of all the organic spectroscopic techniques, it is used by more divergent

fields – metallurgy, molecular biology, semiconductors, geology,
archaeology than any other
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Mass Spectrometry

II. The Mass Spectrometer

A. General Schematic
1. A mass spectrometer needs to perform three functions:
• Creation of ions – the sample molecules are subjected to a high
energy beam of electrons, converting some of them to ions

• Separation of ions – as they are accelerated in an electric field, the

ions are separated according to mass-to-charge ratio (m/z)

• Detection of ions – as each separated population of ions is

generated, the spectrometer needs to qualify and quantify them

2. The differences in mass spectrometer types are in the different means

to carry out these three functions

3. Common to all is the need for very high vacuum (~ 10 -6 torr), while still
allowing the introduction of the sample
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Mass Spectrometry

II. The Mass Spectrometer

B. Single Focusing Mass Spectrometer
1. A small quantity of sample is injected and vaporized under high vacuum

2. The sample is then bombarded with electrons having 25-80 eV of

energy

3. A valence electron is “punched” off of the molecule, and an ion is

formed
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Mass Spectrometry

II. The Mass Spectrometer

B. The Single Focusing Mass Spectrometer
4. Ions (+) are accelerated using a (-) anode towards the focusing magnet

5. At a given potential (1 – 10 kV) each ion will have a kinetic energy:

½ mv2 = eV

As the ions enter a magnetic field, their path is curved; the radius of the
curvature is given by: m = mass of ion
r = mv v = velocity
eH V = potential difference
e = charge on ion
If the two equations are combined to factor out velocity:

m/e = H2r2
2V H = strength of magnetic field
r = radius of ion path
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Mass Spectrometry

II. The Mass Spectrometer

B. Single Focusing Mass Spectrometer
6. At a given potential, only one mass would have the correct radius path
to pass through the magnet towards the detector

7. “Incorrect” mass particles would strike the magnet

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Mass Spectrometry

II. The Mass Spectrometer

B. Single Focusing Mass Spectrometer
8. By varying the applied potential difference that accelerates each ion,
different masses can be discerned by the focusing magnet

9. The detector is basically a counter, that produces a current proportional

to the number of ions that strike it

10. This data is sent to a computer interface for graphical analysis of the
mass spectrum
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Mass Spectrometry

II. The Mass Spectrometer

C. Double Focusing Mass Spectrometer
1. Resolution of mass is an important consideration for MS

2. Resolution is defined as R = M/D M, where M is the mass of the particle

observed and DM is the difference in mass between M and the next
higher particle that can be observed

3. Suppose you are observing the mass spectrum of a typical terpene (MW
136) and you would like to observe integer values of the fragments:
For a large fragment: R = 136 / (135 – 136) = 136

Even a low resolution instrument can produce R values of ~2000!

4. If higher resolution is required, the crude separation of ions by a single

focusing MS can be further separated by a double-focusing instrument
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Mass Spectrometry

II. The Mass Spectrometer

C. Double Focusing Mass Spectrometer
4. Here, the beam of sorted ions from the focusing magnet are focused
again by an electrostatic analyzer where the ions of identical mass are
separated on the basis of differences in energy

5. The “cost” of increased resolution is that more ions are “lost” in the
second focusing, so there is a decrease in sensitivity
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Mass Spectrometry

II. The Mass Spectrometer

D. Quadrupole Mass Spectrometer
1. Four magnets, hyperbolic in cross section are arranged as shown; one
pair has an applied direct current, the other an alternating current

2. Only a particular mass ion can “resonate” properly and reach the
detector

The advantage
here is the
compact size of
the instrument –
each rod is
about the size of
a ball-point pen
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Mass Spectrometry

II. The Mass Spectrometer

D. Quadrupole Mass Spectrometer
3. The compact size and speed of the quadrupole instruments leads them
to be efficient and powerful detectors for gas chromatography (GC)

4. Since the compounds are already vaporized, only the carrier gas needs
to be eliminated for the process to take place

5. The interface between the GC and MS is shown; a “roughing” pump is

used to evacuate the interface

Small He molecules are

easily deflected from their
flight path and are pulled
off by the vacuum; the
heavier ions, with greater
momentum tend to remain
at the center of the jet and
are sent to the MS
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Mass Spectrometry

III. The Mass Spectrum

A. Presentation of data
1. The mass spectrum is presented in terms of ion abundance vs. m/e ratio
(mass)

2. The most abundant ion formed in ionization gives rise to the tallest peak
on the mass spectrum – this is the base peak

base peak, m/e 43

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Mass Spectrometry

III. The Mass Spectrum

A. Presentation of data
3. All other peak intensities are relative to the base peak as a percentage

4. If a molecule loses only one electron in the ionization process, a

molecular ion is observed that gives its molecular weight – this is
designated as M+ on the spectrum

M+, m/e 114

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Mass Spectrometry

III. The Mass Spectrum

A. Presentation of data
5. In most cases, when a molecule loses a valence electron, bonds are
broken, or the ion formed quickly fragment to lower energy ions

6. The masses of charged ions are recorded as fragment ions by the

spectrometer – neutral fragments are not recorded !

fragment ions
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Mass Spectrometry

III. The Mass Spectrum

B. Determination of Molecular Mass
1. When a M+ peak is observed it gives the molecular mass – assuming
that every atom is in its most abundant isotopic form

2. Remember that carbon is a mixture of 98.9% 12C (mass 12), 1.1% 13C
(mass 13) and <0.1% 14C (mass 14)

3. We look at a periodic table and see the atomic weight of carbon as

12.011 – an average molecular weight

4. The mass spectrometer, by its very nature would see a peak at mass 12
for atomic carbon and a M + 1 peak at 13 that would be 1.1% as high

- We will discuss the effects of this later…

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Mass Spectrometry

III. The Mass Spectrum

B. Determination of Molecular Mass
5. Some molecules are highly fragile and M+ peaks are not observed – one
method used to confirm the presence of a proper M+ peak is to lower
the ionizing voltage – lower energy ions do not fragment as readily

6. Three facts must apply for a molecular ion peak:

1) The peak must correspond to the highest mass ion on the spectrum
excluding the isotopic peaks

2) The ion must have an odd number of electrons – usually a radical

cation

3) The ion must be able to form the other fragments on the spectrum
by loss of logical neutral fragments
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Mass Spectrometry

III. The Mass Spectrum

C. High Resolution Mass Spectrometry
1. If sufficient resolution (R > 5000) exists, mass numbers can be recorded
to precise values (6 to 8 significant figures)

2. From tables of combinations of formula masses with the natural isotopic

weights of each element, it is often possible to find an exact molecular
formula from HRMS

Example: HRMS gives you a molecular ion of 98.0372; from mass 98 data:
C3H6N4 98.0594
C4H4NO2 98.0242
C4H6N2O 98.0480
C4H8N3 98.0719
C5H6O2 98.0368  gives us the exact formula
C5H8NO 98.0606
C5H10N2 98.0845
C7H14 98.1096
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Mass Spectrometry

IV. The Mass Spectrum and Structural Analysis

A. Inferences from Isotopic Ratios
1. If a M+ can be observed at sufficient intensity, information leading to a
molecular formula can be attained

2. Consider ethane, C2H6 – on this mass spectrum a M+ ion would be

observed at 30:

(2 x 12C) + (6 x 1H) = 30

– However, 1.08% of carbon is 13C – there is a 1.08% chance that

either carbon in a bulk sample of ethane is 13C (2 x 1.08% or
2.16%)

– In the mass spectrum we would expect to see a peak at 31 (one of

the carbons being 13C) that was 2.16% of the intensity of the M+
signal - this is called the M+1 peak
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Mass Spectrometry

IV. The Mass Spectrum and Structural Analysis

A. Inferences from Isotopic Ratios
2. (cont.) Consider ethane, C2H6 – on this mass spectrum a M+ ion would
be observed at 30:
– There are also 6 hydrogens on ethane, 2H or deuterium is 0.016%
of naturally occurring hydrogen – the chance that one of the
hydrogens on ethane would be 2H is (6 x 0.016% = 0.096%)

– If we consider this along with the 13C to give a increased probability

of an M + 1 peak (31) we find (0.096% + 2.16% = 2.26%)

– There is a small probability that both carbon atoms in some of the

large number of ethane molecules in the sample are 13C – giving
rise to a M+2 peak: (1.08% x 1.08%)/100 = 0.01% - negligible
for such a small molecule

3. Many elements can contribute to M+1 and M+2 peaks with the
contribution of the heavier isotopes
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Mass Spectrometry

IV. The Mass Spectrum and Structural Analysis

A. Inferences from Isotopic Ratios
4. Natural abundances of common elements and their isotopes – (relative
abundance vs. a value of 100 for the most common isotope)

Element Isotope Relative Isotope Relative

M+1 abundance M+2 abundance
1H 2H 0.016
12C 13C 1.08
14N 15N 0.38
16O 17O 0.04 18O 0.20
19F

28Si 29Si 5.10 30Si 3.35

31P

32S 33S 0.78 34S 4.40

35Cl 37Cl 32.5
79Br 81Br 98.0
127I
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Mass Spectrometry

IV. The Mass Spectrum and Structural Analysis

A. Inferences from Isotopic Ratios
5. To calculate the expected M+1 peak for a known molecular formula:

%(M+1) = 100 (M+1) = 1.1 x # of carbon atoms

M + 0.016 x # of hydrogen atoms
+ 0.38 x # of nitrogen atoms…etc.

6. Due to the typical low intensity of the M+ peak, one does not typically
“back calculate” the intensity M+1 peak to attain a formula

7. However if it is observed, it can give a rough estimate of the number of

carbon atoms in the sample:

Example: M+ peak at 78 has a M+1 at 79 that is 7% as intense:

#C x 1.1 = 7%

#C = 7%/1.1 = ~6