Lecture Note on Mass Spectroscopy

M. Kaisarul Islam, Lecturer, Department of Pharmacy, Jagannath University, Dhaka-1100, Bangladesh

1

Mass spectroscopy

Mass spectroscopy is a quantitative and qualitative analytical technique by

which we can measure the molecular mass and formula of a compound and

the record is known as mass spectra.

Mass spectra is useful −

�� To establish the structure of a new compound

�� To give the exact molecular mass

�� To give the molecular formula

�� To indicate the presence of functional group in a compound

Principle/function:

The mass spectrometer is designed to perform four basic functions −

• To vaporize the compound by increasing volatility.

• To generate the ions from the neutral compound in resulting vapor

pressure

• To separate the ions according to their mass to charge ratio (m/z) in

a magnetic field.

• To collect the mass and record.

Theory:

1. Molecular ion production:

�� Mass spectrometer is a device for the production and weighing

of ions.

�� Molecules are subjected to bombardment by a stream of high-

energy electrons, converting some of the molecules to ions. The

molecular ions are usually radical cation and some may be

radical anion.

[M] [M] [M] [M]- e- + - e- -or,


2

2. Fragmentation:

�� When the molecule has been bombarded by high-energy

electrons to produce ions, the molecule absorbs sufficient

energy and undergo fragmentation.

B+ + neutral A+ C+ + neutral

Decompose to produce new ions D+ + neutral

3. Separation of ions:

�� The mixture of ions are separated according to the mass charge

ratio in the analyzer and then recorded.

�� The record is known as the mass spectrum. It is the record of

abundance of each ion against its m/z value.

4. Mass spectrum:

�� The mass spectrum is a plot of ion current intensity (ion

abundance) versus m/z value.

�� The most abundant peak will give the tallest peak of the mass

spectra. This peak is known as the base peak and its mass

arbitrarily assigned a value of 100%. The heaviest peak is the

molecular ion peak and its mass will give the mass of the

molecule.

C+

B+

Relative absorbance ↑ D

A+

m/z value →


3

Isotope peak:

Isotopes present in the molecule may generate additional peak. Due to the

occurrence of isotopes we also observe M+1, M+2, M+3 etc peaks. The

relative abundances of these isotopic peaks are proportional to the

abundance of the isotope in nature (e.g. the natural abundance of 13C is

1.1% of the 12C atoms. For an ion containing n number of carbon atoms, the

abundance of isotope peak is nX1.1% of the 12C containing peak.

M+1 peaks are made by − 13C, 2H, 15N, 33S

M+2 peaks are made by − 18O, 34S, 37Cl, 81Br)

Base peak

M+

peak

Relative abundance ↑

M +1

M + 2

m/z ratio →

M+1 and M+2 peak in benzene:

Benzene shows molecular ion peak at m/z value 78 due to C6H6. It will

also show M+1 peak at m/z 79 due to 13CC5H6+ or, C6H5D

+.

M+2 peak will also show at m/z 80 due to 13CC5H5D+ or, 13C2C4H6

+ or,

C6H4D2+.

The relative abundances of this isotopic can be used to determine

molecular formula.


4

Atomic weight and natural abundances of some isotope −

Isotope Atomic

weight

Natural

abundance (%) 1H 2H 12C 13C 14N 15N 16O 17O 18O

1.0078

2.014

12.000

13.003

14.003

15.0001

15.9949

16.999

17.999

99.985

0.015

98.9

1.1

99.64

0.36

99.8

0.04

0.2

Ionization method:

In ionization method compound are divided into 2 groups −

a. Ionization of volatile materials

b. Ionization of nonvolatile materials

a. Ionization of volatile materials −

Two methods are commonly used to produce ions from thermally

volatile compound ---

1. Electron impact ionization (IE)

2. Chemical ionization (CI)

1. Electron impact ionization:

•••• A direct probe tip is used near to a heated filament which

provides electron and is heated in the ionization chamber

causing vapor from the sample.

(Handle) (metal sheet) (Ceramic tip with sample)


5

• Electrons are accelerated from the hot filament to an anode, usually

through a potential difference of about 70ev.

• A 70ev electron has sufficient energy not only to ionize an organic

molecule but also to cause extensive fragmentation.

• Molecules are ionized due to bombardment with high-energy electrons

by removal of an electron. The product is cation radical.

M + e = M+ + 2e

2. Chemical ionization (CI):

• In this technique a reagent gas (methane, isobutane or ammonia) is

allowed to pass into the ion chamber at low pressure.

• The gas is ionized by using electron impact, which can then undergo

ion molecule reaction.

•+•

+

•+

+→+

+→+

3544

44

CHCHCHCH

2eCHeCH

If the sample molecules are volatilized into mixture of ions, +

5CH act

as a strong acid and protonates the sample.

45 CHMHCHM +→+ ++

Thus in positive ion CI-spectra, the observed m/z value is one unit greater

than the true molecular weight.

�� In CI-technique, negative ion CI-spectra may occur for molecules with

electron accepting properties like trifluoroacetates, quinones and nitro

compounds.


6

b. Ionization of nonvolatile materials −

�� molecules have low molecular weight but have numerous polar

functional groups, or

�� have high molecular weight; usually don’t pass into the gas phase

at high temperature and at low pressure.

1. Field desorption:

•••• Here, the probe tip is replaced by a thin wire on which sharp

needles have been grown.

•••• The wire is supported between two posts on the probe.

•••• A solution of small amount of a sample is deposited on the wire.

•••• In the mass spectrometry the wire is maintained at +8kv and can

be heated and this can cause the discharge of an electron from

the sample into the metal. Thus positive ions (M+) are created.

In this way molecules are thrown into the gas phase as a

positive molecular ion without thermal decomposition.

Cathode slit

+8KV +

wire (+8KV) +

+

Probe +

Needle Ionized molecule

Fig: Field desorption technique

2. Desorption ionization by particles or radiation:

I. Laser desorption (LD)

II. Fast atom bombardment (FAB)

III. Californium plasma desorption

IV. Secondary ion mass spectrometry (SIMS)


7

�� based upon giving a large pulse of energy to the sample

�� here intermolecular bonds are broken and the sample is desorped

from its environment into the gas phase within 10-12 sec.

�� so thermal decomposition doesn’t occur.

� Laser desorption →

� In this technique, the sample is bombarded with short duration,

intense pulses of laser light.

� Efficient and controllable energy transfer to the sample requires

resonant absorption of the molecule at the laser wavelength.

� Therefore, lasers emitting either in the UV or IR are employed.

� Laser pulses are applied for 1-100ns.

� One disadvantage is some thermo labile compounds may be

degraded with the laser beam resulting in a spectrum of

fragment ions. To overcome this problem, a matrix is used and

the technique is known as matrix assisted laser desorption

ionization technique (MALDI).

In MALDI, a low concentration of the sample is embedded either in a

liquid or a solid matrix (molar ratio 1:100 to 1:50000) which is

selected to absorb strongly the laser light. In this way, the energy is

transferred indirectly to the sample, and in a controlled manner

which avoids sample decomposition.

� Used in conjunction with a suitable method for ion analysis,

MALDI can give approximate molecular weight determinations

for biomolecules, even in the range 100000 – 200000 Daltons.

Some common MALDI matrices:

Matrix Application

2,5-dihydroxy benzoic acid (DHB) Peptides, protein, lipids,

oligosaccharides

3,5-dimethoxy-4-hydroxy cinnamic

acid (Sinapinic acid) Peptides, protein, glycoproteins

αααα-cyano-4-hydroxy cinnamic acid

(CHCA)

Peptides, protein, lipids,

oligonucleotides


8

� FAB →

� Here the energy is provided by a beam of neutral atom. The

sample is dissolved in a matrix of low volatility. A few µgm of

the sample are dissolved in a few µl of glycerol as matrix and a

beam of fast xenon atoms bombards the solution.

Bombarding atom beam

Probe sample ion Mass analyzer

Sample in matrix

� This fast xenon atoms are prepared by accelerating xenon ions

and then neutralizing these ions by charge exchanger at a low

pressure.

++ +→+ XeXeXeXe

Another matrixes used in FAB are ~

� Thioglycerol : Diglycerol (1:1)

� Tetracol

� Teracol

� Glycerol

� Californium plasma desorption →

� Here the sample to be analyzed is deposited on a thin metal foil

(nickel).

� Spontaneous fission of the radioactive Californium nucleus

(252Cf) occurs, and each fission event gives rise to two fragments

travelling in opposite directions. A typical pair of fission

fragments are +Ba142

18 and +Te106

22 of high kinetic energy.

� When such a high energy fission fragment passes through the

sample foil, produce a high temperature of 10000K.


9

� Consequently the molecules in this plasma zone are desorbed

from the foil with the production of both positive and negative

ions. These ions are then accelerated out of the source into the

analyzer system.

252

Cf fission + −

or

Sample

Ni foil (10–3

mm)

� Californium plasma desorption technique produces better

molecular peak for molecules having molecular weight between

10000 – 20000 Dalton.

Different technique for analyzing ions in a mass spectrometer:

a. Magnetic sector

b. Time of flight

c. Quadrupole

d. Ion cyclotron resonance

e. Ion trap

Magnetic sector analyzer/mass analyzer:

�� The ions may be separated according to their mass to charge ratio

(m/z) using a magnetic field.

�� Here the ions of larger mass are deflected less than the ions of

smaller mass according to the equation –

2v

rH

z

m 22

=

Where,

r = radius of circular of path in which ion is traveling

H = magnetic field strength

V= potential difference of ion.


10

The equation clarifies that by varying the magnetic field strength or

accelerating potential, the ions of all m/z value can be successively

allowed to pass through the detector slit & mass spectrum recorded.

Time of flight mass analyzer:

�� TOF mass analyzer separates or resolves the ion beam by measuring

the flight time of the ions. The technique requires that all the ions

produced in the ion source should leave at the same time.

�� The ions are accelerated by a potential difference and then allowed

to pass into the filed free region.

Since all the singly charged ions will acquire the same kinetic

energy, the largest mass will have the lowest velocity and the longest

time of flight over a given distance. Mathematically,

2V

1

e

mLt ×=

It is quicker than any other mass analyzer and applicable for all

masses.

Quadrupole:

�� Here four parallel rods arranged symmetrically around an ion flight

path and a direct current and a radio frequency are applied to the

rods resulting an oscillating electrostatic field.

�� The ions when pass through the region, will acquire an oscillation in

the electrostatic field. The ions of incorrect m/z ratio undergo an

unstable oscillation and strikes one of the rods.

�� Ions of correct m/z ratio undergo stable oscillation of constant

amplitude and pass through analyzer to reach the recorder.

Quadrupole analyzer is a relatively compact instrument and

inexpensive.

where t = time of flight L = distance in which a ion travel m = mass of the ion e = charge of the ion V = velocity in which the ion travel


11

Low-resolution mass spectrometer (LRMS):

LRMS employs a single stage analyzer. It resolves only integral masses

and it can’t differentiate the molecules e.g. CO (28), CH2=CH2 (28), N2

(28). As all have the same integral mass 28. since it can’t give exact

masses molecular formula can’t be determined.

High-resolution mass spectrometer (HRMS):

HRMS employs multiple stage analyzer such as magnetic and

quadropole sectors linked in series. The accuracy of these types of

instruments enables the distinction between different isotopes such as 13C vs. 12C. The high-resolution data are obtained at an accuracy of

0.0001 amu (atomic mass unit) and consequently this permits a

distinction between species of the same mass unit such as - CO (28),

CH2=CH2 (28), N2 (28). Therefore data from HRMS are essential for

unambiguous determination of molecular data.

� Double focusing mass spectrometer are used to obtain high

resolution in which the beam ions are pass through an electric

field region before entering the magnetic field.

� In a single focusing mass spectrometer, there is a lack of

uniformity of ion energies that is all ions do not have precisely

same velocity. The result is peak broadening and low to

moderate resolution.

Electrospray ionization (ESI):

‘Electrospray’ is applied to the small flow of liquid (1-10µl/min) from a

capillary needle when a potential difference of 3-6kV is typically

applied between the end of the capillary and a cylindrical electrode

located 0.3-2 cm away.

� Under these circumstances, the liquid leaving the capillary does

not leave as a drop, but rather as a spray.


12

� The spray consists of highly charged liquid droplets, and these

droplets may be positively or negatively charged depending on

the sign of the voltage applied to the capillary.

� If the liquid spraying from the capillary contains sample

molecules, then a molecular ion of these sample molecules can

be obtained by evaporation of the solvent.

ESI is an excellent technique for the production of molecular ions from

large polar molecules, and it will be seen subsequently that, since it

frequently produces multiply charged ions, it is a very powerful tool in

the analysis of biopolymers. This is especially true since the method

can be conveniently used to analyze directly the effluent from an HPLC

column.

Gas chromatography-mass spectrometry (GC/MS) �� The separation and detection of components from a mixture of

organic compound is readily achievable by gas chromatography.

Furthermore, limited characterization of unknown components is

often possible from retention times appropriate to the particular

column used.

�� Mass spectrometry, because of its high sensitivity and fast scan

speeds, is the technique most suited to provide definite structural

information from the small quantities of material eluted from a gas

chromatograph.

�� The association of the two techniques provided a powerful means of

structure identification for the components of natural and synthetic

organic mixtures even though the components may be present in

nanogram quantities and eluted over periods of only a few seconds.

The interface between the GC and the MS is a jet separator.

Such a combination is useful as an aid in determining the structures

and chiralities of amino acids. The amino acids are first derivatized as


13

follows: by treatment with trifluoroacetic anhydride in the first step

and by isopropanol / HCl in the second (to render them volatile):

NH2CH(R)COOH → CF3CONHCH(R)COOH → CF3CONHCH(R)COOCH(CH3)2

Since trifluoroacetyl is a good electron capture group, the mass

spectra are determined in the negative ion mode. The mixture of

derivatized amino acids (frequently from 6N HCl hydrolysis) is simply

injected on to a chiral GC column, where the retention times are not

only dependent on structure but also on absolute configuration of the

amino acids. Thus separation, molecular weight, and chirality can all

be determined in one experiment.

Liquid chromatography-mass spectrometer (LC/MS)

• HPLC is a powerful method for the separation of complex

mixtures, especially when many of the components may have

similar polarities. In reverse-phase HPLC, the column substrate is

such that starting with an aqueous solution of a mixture of polar

components; the most polar components are eluated first. The

later-eluated hydrophobic components are often encouraged to

leave the column by gradually increasing the concentration of

acetonitrile (CH3CN) in the otherwise aqueous developing

solvent.

• If the mass spectrum of each component can be recorded as it

eluates from the HPLC column, quick characterization of the

components is greatly facilitated.

Tandem mass spectrometry (MS/MS):

• Tandem mass spectrometer uses two mass spectrometers is

tandem.

• It has a great potential value in the structure elucidation of

organic compounds.


14

• In this technique, a compound to be analyzed is subjected to

ionization and fragmentation. The mixtures of ions are then

separated in the first mass analyzer.

• From the mixture of ions, a specified ion is selected for the

second mass analyzer. The magnetic field is set to pass only the

selected ion through a slit into a collision chamber. This chamber

contains a high energy reagent gas like helium (He) or argon (Ar)

with which the ions collide. As a result, mixtures of ions are

produced. The process is known as collision activated

decomposition (CAD). The ions are then analyzed in the second

mass analyzer.

Example –

Penazitidine A is a heterocyclic compound with a long chain. It has a

methyl group on the side chain but the position was not established by

various technique (NMR & even by 2D NMR). But MS-MS can determine

the position of methyl group on the structure.

The ion at m/z 296 was selected. It was allowed to pass into the

collision chamber where it is subjected to CAD. The mixtures of ions

produced are analyzed in the 2nd mass analyzer. The intense peak at

m/z 182, m/z 210 indicates the position of methyl group at C12 of the

side chain.

Fragmentation patterns:

For most classes of compounds, the mode of fragmentation is

somewhat characteristic. The most common mode of fragmentation

involves the cleavage of one bond. In this process the odd-electron

molecular ion yields an odd-electron neutral fragment (a radical) and

an even electron fragment ion (carbonium type)


15

Fragmentation via the cleavage of one bond:

[ ]33

CH R CHR •+•+

+→−

[ ]ion]fragment electron [evene ion]molecular electron [old

radical]elecetron [oddX R XR •+•+

+→−

where, X = halogen, OR, SR or NR2, R = H, alkyl or aryl

Fragmentation via the cleavage of two bonds:

[ ] [ ] OH CHRRCH CHRRCH2

// +=→− •+

•+

[ ] [ ]fragment) neutralelectron (even n)fragmentioelectron (odd ion)molecular electron (odd

CHCHO CHRCH CHCOCHRCH3232

−−+=→−−−− •+

•+

In addition to this process, fragmentation process involving

rearrangements, migrations of groups and secondary fragmentations of

fragment ions are also possible.

Fragmentation patterns of different classes of compound:

Name of

compound

M+ peak Fragmentation

Alkane −

i) Branched

chain hydro-

carbon.

e.g.

Isobutane

CH3

CH3-CH-CH3

M+ peak is observed but less

intense than straight chain

compound

[ ]∗+

−−33

CHCHCH

C-C bonds leading to the formation of 20 & 3

0

carbonium ions which are more stable than 10.

As a result the M+ ion will become less intense

and undergo further fragmentation.

• Cleavage of a C-C bond yields an

isopropyl carbonium ion.

[ ]•+

− CHCH3

CH3

20 carbonium ion, m/z =43


16

Alkanes −

ii)Straight

chain

compound

e.g. − Butane

Molecular ion peak observed at

m/z = 58

[ ]•+

−−−3223

CHCHCHCH

43

29

15 58

11 15 25 30 40 45 50 55 60

(m/z) →

Fig: Mass spectrum of Butane

C-C bonds breaks resulting in a homologous

series of fragmentation products. Primary

carbonium ion is formed.

1. Cleavage of C-1 to C-2 bond result in loss of

a Me-radical and formation of propyl

carbonium ion.

[ ] [ ]•++−− 3223 CHCHCHCH (m/z = 43)

2. Cleavage of c-2 to c-3 bond result in loss of a

Et-radical and formation of ethyl carbonium

ion

[ ] [ ]•+−+− 2323 CHCHCHCH (m/z = 29)

3. Cleavage of c-3 to c-4 bond results in loss of

a propyl radical and formation of methyl

carbonium ion

[ ] [ ]•+−−+ 2233 CHCHCHCH (m/z = 15)

Alkenes −

E.g. − 1-butene

& 2-butene

CH2=CH-CH2-

CH3

1-butene

Distinction M+ peak

• 1-butene and 2-butene

give molecular peak at

m/z =56.

• Both produce allyl

carbonium ion at m/z =

41.

Alkene isomers show nearly identical mass spectra.

So double bond can’t be located. Also cis & trans

can’t be differentiated.

[ ]2222

CHCHCHRCHCHCHR =−+→=−− +••+

[ ][ 222222

222

CHCHCHCHCHCHCHCH

RRCHCHCHCH

−=↔=−

+→−−−= ••+

[ ] +••+

−=+→−−=223322

CHCHCHCCHCHCHCH H

(m/z = 41)

Alkynes −

E.g.− Propyne

• Molecular ion peak is

rather intense.

• Give molecular ion peak

at m/z = 40

]HCCHCHCCC[H

RR]CHCC[H

22

2

==↔−≡−

+→−−≡−

+

••+

[ ]

39)(m/z (Propyne)

CHCCHHCCHCCHCHC 223

=

==↔=→−≡

++•+


17

Aromatic

hydrocarbon

i) containing

alkyl group

• Show distinct and intense

molecular ion peak.

Fragmentation occurs at benzylic position not at

phenolic position.

E.g.

Tolune

CH3

• Not so intense

• Shows very intense

molecular ion peak at m/z

= 92

CH3

+

Fragmentation occurs at benzylic position. Loss of

hydrogen gives a strong peak at m/z = 91

CH2

+

C7H7

+

Tropylium ion (m/z = 91)

+

Benzyl carbonium ion (m/z = 91)

Ethyl benzene

CH2-CH

3

Molecular ion peak gives at

m/z = 106

CH2-CH

3

+



CH2

CH3

+ +

+

Propyl

benzene

CH2-CH

2-CH

3

Molecular ion peak at m/z =

120

CH2-CH

2-CH

3

+



CH2

CH2-CH

3

+ +

+

Alcohol

1º & 2º

3º

Intensity of molecular ion

peak is usually low.

butanol)(1

OH]CHCHCH[CH 2223

−

−−−− •+

butanol)(2

CH

OH]CHCH[CH

3

23

−

−−− •+

• Loss of an alkyl group. C-C bond broken.

+

•

=+−− OHCH HCCHCH 2223 m/z=31

+

•

=+− OHCH-CH HCCH 323 m/z=45

+•

=+ OHC-)(CH HC 233 m/z=45


18

CH3-C-OH

CH3

CH3

*

+

• Dehydration: the importance of dehydration

increases as the chain length increases. The

dehydration is a 1,2 elimination of water.

RCH

H

CHR'

OH

(CH2)n

(CH2)n

CHR'RCH

+ H2O

General reaction -

CH2

H

CH2

OH

(CH2)2

CH2

CH2

CH2

+ H2O

Example - (1-butanol)

• Simultaneous loss of water & alkene usually

which contains more than four carbon.

CH2

CH2

CH2

O

HH

CH

CH2

CH2=CH

2

RCHR + + H2O

+

+

Ex − 1-butanol −

CH2

CH2

CH2

HCH2

O

H

CH2

CH2

CH

2=CH

2+ + H2O

+

+

m/z = 28

Phenols −

E.g.−Benzyl

alcohol

Exhibit intense molecular ion

peak.

108

(M)

79 107

77

Fig: Mass spectrum of

benzyl alcohol

• Loss of CO

OH

- CO

HH

m/z = 66

CH2OH

HHOH

C6H5

- COH

+

+

++

H2

m/z=107 m/z=79 m/z = 77

+


19

Ethers − Weak molecular ion peak. • Loss of alkyl group/cleavage of C−C bond to

the α-carbon

•+

•+

+=→− RR'OCH]OR'HC[R 22

α

• Cleavage of C−O bond

•+

•+

+−→− OR'RHC]OR'HC[R 22

α

Aromatic

ethers −

E.g. − Anisole

Molecular ion peak is

observed

• Loss of alkyl group

•+

•+

+→ R OHCOR]H[C 5656

• Loss of alkoxy group

•+•+

+→ OR HCOR]H[C 5656

• Example − Anisole

•

+•+

+→ 356356 CH OHC]CH-O-H[C

•+•

+

+→ 356356 OCH HC]CH-O-H[C

Aliphatic

aldehyde −

Molecular ion peak is

observed/ weak

• α-Cleavage: cleavage occurs in one of the two

bonds to the carbonyl group.

*R C H

O

R C O H+

+

*R C H

O

C O++H R

*R C H

O

C O+

+H R

• β-Cleavage: cleavage occurs in β- carbon.

•+ =+→−− CHOCHRCHOCHR 22

• Mclafferty rearrangement: when alkyl group

attached to the carbonyl carbon is large, a type

of rearrangement called Mclafferty

rearrangement occurs.

CH2

CH

CH

CH2

C

O

H

R1

R2

H

+

R1-CH

R2CH

+

CH-OH

+

m/z=44


20

e.g. mass spectrum of Butyraldehyde: molecular ion peak at m/z=72.

α-Cleavage:

CH3-CH

2-CH

2-C-H

HCH3CH

2CH

2=OO

+ ++

+

H-C O++ CH3CH2CH2

m/z=71

m/z=29

β - Cleavage:

CH3-CH

2-CH

2-C-H

CH

3CH

2

O+

++ CH2=CHO m/z=29

Mclafferty rearrangement:

CH2

CH2

CH2

CH

O

H +

CH2 CH2

+CH2 CH-OH m/z=44

+

+

Aromatic

aldehyde −

e.g.

Benzaldehyde

C

O

H

• Intense molecular ion

peak.

• Benzaldehyde show

molecular peak at m/z =

106

106(M+)

105

77

C6H5+ C6H5C≡O

+

Fig: mass spectrum of

Benzaldehyde

• Loss of hydrogen show molecular peak at

m/z = 105

C

O

H

+

O+

+ H

C

m/z=105

• Loss of CHO group may lead to C6H5+

which show molecular peak at m/z = 77

Aromatic

ketones −

Intensed molecular ion peak

• loss of alkyl radical giving ArCO+ (α -

Cleavage)

*C R

O+

C O+

+

m/z=105

+ R

m/z=77

• loss of COR giving Ar+ (Mclafferty

rearrangement)

C

O

+

+

m/z=105

m/z=120

CH2

CH2

CH

H R

C

O

+

CH2

H

C O

+ CHR CH2


21

Acyclic

Ketones −

• Intense molecular ion

peak.

1. α-Cleavage

R---C---R '

*O

+

R -C O + R '

+

R---C---R '

*O

+

R '-C O + R+

2. Mclafferty rearrangement:

R1CH

CH

CH2

CR3

O

H

+

R2CH

R1CH

R2CH

+

OH

C CH2

R3

+

e.g.−

2-butanone

CH3-C-CH

2CH

3

O

CH3-C CH

3CH

2O

+

++

m/z=43larger gr. will be predominant

CH3--C-CH

2CH

3

O

CH3CH

2-C O CH

3

+

++

m/z=57

the peak at m/z =43 is more intense than the peak at

m/z =57

Aliphatic

amines −

May be very weak or even

absent.

β - Cleavage:

R-CH2-NH

2

* *+

R + CH2 NH2

+

m/z=30

Aromatic

amines −

Intense molecular ion peak Loss of hydrogen atom.

*

*

*

*

** *

NH2+ NH

+H H

+ H+

+ HH HCN

m/z=92 m/z=66 m/z=65

Aromatic

Carboxylic

acids −

Intensed molecular ion peak Loss of OH to form C6H5CO+ ion (m/z = 105)

followed by loss of CO to form the C6H5+ ion

* *

*

C OH

O +

C O+

+

+

OH

m/z=105

+

m/z=77


22

Aromatic

esters −

E.g. − Methyl

benzoate.

Weak molecular ion peak

105

77

136

(M+)

Fig: mass spectrum of

methyl benzoate

α - Cleavage gives to the formation of C6H5CO+

*

*

C

O +C O

++

OCH3

m/z=105

+

m/z=77

OCH3

Esters − Weak but noticeable ion

peak

• α-Cleavage: loss of the alkoxy group to form

corresponding acylium ion RCO+

*

*

C-OR

'

R

O +

R+

+ C-OR'O

*

* R

O +

R

+

+ C-OR'

O

COR'

*

*

R-C OR'

O+

R C O + OR'

+

• β-Cleavage: Mclafferty rearrangement

**R1CH

CH

CH2

O

H +

R2CH

R1CH

R2CH

+

OH

C CH2

+

OR'

COR'

Documents

Lecture Note on Mass Spectroscopy