mass spectrum

Presented By :- Ms. ARTI R RAJPUT M.Pharm, (SUCOP,Pune)

Mass Spectrometry

1

Contents

Introduction of Mass spectrum. Types of Ions Molecular ion, Metastable ions,

Fragment ions. Fragmentation procedure Fragmentation patterns Fragment characteristics Relative abundances of isotopes.

2

Introduction of MS

The impact of a stream of high energy electrons causes the molecule to lose an electron forming a radical cation.

A species with a positive charge and one unpaired electron

+ e-C H

H

HH H

H

H

HC + 2 e-

Molecular ion (M+)

m/z = 16

3

Introduction of MS

Only cations are detected. - Radicals are “invisible” in MS

The amount of deflection observed depends on the mass to charge ratio (m/z). -Most cations formed have a charge of +1 so the amount of deflection observed is usually dependent on the mass of the ion.

.

4

Molecular ion The ion obtained by the loss of an electron from the molecule also called parent ion

Base peak The most intense peak in the MS, assigned 100% intensity

M+ Symbol often given to the molecular ion.Mol. With an unpaired e-

Radical cation

+ve charged species with an odd number of electrons

Fragment ions

Lighter cations formed by the decomposition of the molecular ion. also called daughter ion

Mass Spectrum

The resulting mass spectrum is a graph of the mass of each cation vs. its relative abundance.

Relative abundance of an ion means the % of total ion

current.

Mass spectrum is an analytical techniques which can provide information concerning the molecular structure of organic comp.

Base peak is the highest peak or the most intense peak in the spectrum.

6

Types of Ion

Types of ion produced in MS 1.Molecular ions (parent ion)

2.Metastable ions

3.Fragment ions (Dissociation process)

4.Rearrangement ions

5.Multiple charged ions

6.Isotopes ions

7.Negative ions

8.Base peak7

Molecular ion

Molecular ion (parent ion):

-The radical cation corresponding to the mass of the original molecule

The molecular ion is usually the highest mass in the spectrum

HH

H

HC H C

H

H

C

H

H

H

8

Molecular ion

When a sample sub.is bombarded with electrons of energies of 9 to 15eV, the molecular ion is produced by loss of a single electron.

This will give rise to a very simple mass spectrum with essentially all of the ion appearing in one peak called parent peak.

M + e = M+ + 2e-

Most important ion.

9

Molecular ion

In organic compound there is generally a small peak appearing one mass unit higher than the parent peak (M+1) due to small but observable ,natural abundance of C13 and H2 in these compound.

The relative height of parent peak decreases in the following order,

aromatic>conjugated olefins>sulphides> unbranched>hydrocarbon>ketones>amine>ester> ethers >carboxylic acid>branched hydrocarbons.

10

Molecular ion

If a molecule yields the parent peak due to molecular ion ,the exact molecular weight can be calculated.

Molecular ion are formed in the ground state and in the electronically excited state.

11

Mass Spectrum

• Mass spectrum of ethanol (MW = 46)M+

Mass spectrum of ethanol (MW = 46)

M+

12

Introduction of MS

.

The mass spectrum of ethanol

base peakM+

13

Fragment ion

The molecular ion produced in MS is generally left with considerable excess energy.

This energy is rapidly lost by the molecular ion resulting in one or more cleavages in it with or without some rearrangement.

One of the fragment retains the charge where as the remaining fragment may be stable molecule or radicals.

14

Fragment ion

If the electron beam energy is further increased to apparent potential of a molecule ,then the excited molecule ions undergoes decomposition to give rise to variety of fragment ions which leaves smaller masses than the molecular ion.

Formed by both heterolytic and homolytic cleavage of bond.

They are formed by simple cleavage and rearrangement process.

15

Fragment ion

Bond dissociation energy stability of neutral fragment are steric factors are some of the major factor which determine formation of fragment ions.

E.g. : Ethyl chloride.

CH3-CH2-Cl + e- = CH3-CH2-Cl + + 2e-

CH3-CH2-Cl + = CH3-CH2+ + Cl. Or

CH2-CH2+ + HCl. (Fragment ion)

16

M + e- M+* + 2e-

+M+*1

M2

M4+

Fragmentation Process

OR

M3*+

Metastable ion

The ions in a mass spectrometer that have sufficient energy to fragment sometime after leaving the ion source but before arriving at the detector.

M+ A+ + N

(m1/z) (m2/z) (m1-m2)

M+ with large amount of internal energy will fragment in the ionization source, producing “normal” A+ ions. These A+ ions will be seen as narrow peaks at m/z values correct for the mass and charge on the ion A+.

M+ having only a small excess of internal energy, reach detector before decomposition can occur. Narrow peaks for “normal” M+ appear

18

Metastable ion

M+ which posses excesses of internal energy that are in between the those in above two cases, may fragment after leaving the ion source and before reaching the detector. The product ions, A+, are seen in the mass spectrum as broad peaks, centered at m/z values that are nor correct for the mass and charge on the ion A+.

These broad peaks are called “metastable ion peaks” These ““metastable ion peaks” do not represent metastable M+ ions, but represent products of decomposition of metastable ions. The cause of A+ ions from metastable ion decomposition being detected differently form “normal” A+ ions is due to their different momentum.

19

FRAGMENTATION MODES

The RA of fragment ion formed depends upon’1)The stability of the ion 2)Also the stability of radical lost.The radical site is reactive and can form a new bond.The formation of new bond is a powerful driving force for ion decompositions.The energy released during bond formation is available for the cleavage of some bonds in the ion.Some imp. Fragmentation modes are described below1)Simple cleavage : Involves i) Homolytic or ii) Heterolytic cleavage of a single covalent bond.

20

Fragmentation modes

1) Homolytic cleavage : odd electron ions have unpaired electron which is

capable of new bond formation. Bond is formed , energy is released , help offset the

energy required for the cleavage of some other bond in the ion.

Homolytic cleavage reactions are very common.

2) Heterolytic cleavage : It may be noted the cleavage of C-X (X=

0,N,S,Cl) bond is more difficult than that of C-C bond. In such cleavage , the positive charge is carried by the

carbon atom and not by the heteroatom. R-CH2-Cl.+ = Cl. + RC+H2

21

Fragmentation modes

2) Retro –Diels –Alder reaction

The reaction is an example of multicentre fragmentation which is characteristic of cyclic olefins.

It involves the cleavage of two bonds of a cyclic system , result the formation of 2 stable unsaturated fragment in which 2 new bonds are formed.

This process is not accompanied by any hydrogen

transfer rearrangement.

The charge can be carried by any one of the fragment.

22

3)Mc Lafferty Rearrangement:• This involves migration of hydrogen atom from one part of the ion to

another.• To undergo a Mc Lafferty Rearrangement a molecule must possess

a) An appropriately located heteroatom e.g. O, Nb) A pi electron system ( usually a double bond) &c) An abstractable hydrogen atom gamma to the C = X system

Gamma hydrogen atom is transferred through a six membered transition state to an electron deficient centre followed by cleavage at beta bond.

The reaction results in the elimination of a neutral molecule.

23

Rules

A number of general rules for predicting prominent peak in electron impact spectra are recorded and can be summarized below

1) most compound give molecule ion peak but some do not . Existence of molecular ion peak in the spectrum is dependent on the stability of molecule

2)In case of alkenes , the relative intensity of the molecule ion peak is greatest for the straight chain compound but,

a) The intensity decreases with increases degree of branching.

b) The intensity decreases with increasing molecular weight in a homologous series.

26

Rules

3) cleavage is favored at alkyl substituted carbons ,the more substituted ,the more likely is the cleavage .Hence

the tertiary carbocation is more suitable than secondary,

which is more turn stable then primary. The cation stability order is CH3 < R-CH2 <R2 CH+ < R3C+.Generally the largest substituent at a branch is eliminated most readily as a radical, presumably because a long chain radical can achieve some stability by delocalization of the lone electrons.

4)In alkyl substituted ring compounds, cleavage is favoured at the bound at the bond beta to the ring giving the resonance stabilized benzyl ion.

5)Saturated rings containing side chain, lose the side chains at the alpha bond. the ve+ charge tend to stay with ring fragment.

27

Rules

6)The cleavage of a C-X bond is more difficult than that of a C-C bond (X=O, N, S, F, CI, etc). If occurred ,the positive charge is carried by the carbon atoms, and not to the heteroatom.the halogens having great electron affinity do not have tendency to carry the positive charge.

7)Double bonds favour allylic cleavage and give the resonance stabilized allylic carbonium ion.

8)Compounds containing a carbonyl group tend to break at this group with positive charge remaining with the carbonyl portions.

28

Rules

9)During fragmentation, small, suitable neutral molecules e.g. water, carbon monoxide, alcohol, ammonia, hydrogen cyanide, carbon dioxide, ethylene etc, are eliminated from appropriate ions.

29

Fragmentation Pattern for org. comp.

Organic molecules will fragments in very specific ways

depending upon what functional groups are present in the

molecule.

These fragments (if positively charged are detected in

mass spectroscopy)

The presence or absence of various mass peaks in the

spectrum can be used to deduce the structure of the

compound in question.

30

Fragmentation rules in MSFragmentation rules in MS

1. Intensity of MM..++ is Larger for linear chainLarger for linear chain than for branched compound

2. Intensity of MM..++ decreasedecrease with IncreasingIncreasing MW.MW. (fatty acid is an exception)

3. Cleavage is favored at branchingfavored at branching reflecting the Increased stability of the ionIncreased stability of the ion

Stability order: CH3+ < R-CH2

+ < RR

CH+ < C+

R

R

R

RRR”R”

CHR’R’

Loss of Largest Subst. is most favoredLoss of Largest Subst. is most favored31

Fragmentation Patterns

The impact of the stream of high energy electrons often breaks the molecule into fragments, commonly a cation and a radical.

- Bonds break to give the most stable cation.

Alkanes - Fragmentation often splits off simple alkyl groups:

Loss of methyl M+ - 15•Loss of ethyl M+ - 29•Loss of propyl M+ - 43•Loss of butyl M+ - 57

-Branched alkanes tend to fragment forming the most stable carbocation's.

32


Mass spectrum of 2-methylpentane

33


Alkenes: -Fragmentation typically forms resonance stabilized allylic carbocation.

34


Aromatics: -Fragment at the benzylic carbon, forming a resonance stabilized benzylic carbocation . (which rearranges to the tropylium ion)

CH

H

CH Br

HC

H

H

or

M+

35


Aromatics may also have a peak at m/z = 77 for the benzene ring.

NO2

77

77

M+ = 123

36


Alcohols :

-Fragment easily resulting in very small or missing parent ion peak

-May lose hydroxyl radical or water -M+ - 17 or M+ - 18

- Commonly lose an alkyl group attached to the carbinol carbon forming an oxonium ion.

-1o alcohol usually has prominent peak at m/z = 31 corresponding to H2C=OH+

37


MS for 1-propanol

CH3CH2CH2OH

H2C OH

M+-18

M+

38


Amines: -Odd M+ (assuming an odd number of nitrogen are present)

-cleavage dominates forming an iminium ion

CH3CH2 CH2 N

H

CH2 CH2CH2CH3 CH3CH2CH2N CH2

H

m/ z =72

iminium ion

39


86

CH3CH2 CH2 N

H

CH2 CH2CH2CH3

72

40


Ethers - -cleavage forming oxonium ion

- Loss of alkyl group forming oxonium ion

- Loss of alkyl group forming a carbocation

41


MS of diethyl ether (CH3CH2OCH2CH3)

H O CH2

H O CHCH3

CH3CH2O CH2

42


Aldehydes (RCHO) - Fragmentation may form acylium ion

- Common fragments

M+ - 1 for

M+ - 29 for

RC O

RC O

R (i.e. RCHO - CHO)

43


MS for hydrocinnamaldehyde

C C C H

H

H

H

H

O

133

105

91

91

105

M+ = 134

44


Ketones :

-Fragmentation leads to formation of acylium ion:

-Loss of R forming

-Loss of R’ forming

RCR'

O

R'C O

RC O

45


MS for 2-pentanone

CH3C O

CH3CH2CH2C O

M+

CH3CCH2CH2CH3

O

46


Esters (RCO2R’) -Common fragmentation patterns include:

Loss of OR’ -peak at M+ - OR’

Loss of R’ -peak at M+ - R’

47


C

O

O CH3

105

77

77

105

M+ = 136

48

•Summary of Fragmentation pattern:

49

Alkanesgood M+

14-amu fragments

Alkenes

distinct M+

m/e = 27 CH2=CH+

m/e = 41 CH2=CHCH2+

M-15, M-29, M-43, etc... loss of alkyl

Cycloalkanes

strong M+

M-28 loss of CH2=CH2


Aromatics

strong M+

m/e = 105 C8H9+

m/e = 91 C7H7+

m/e = 77 C6H5+

m/e = 65 (weak) C5H5+

Halides

M+ and M+2 Cl and Br

m/e = 49 or 51 CH2=Cl+

m/e = 93 or 95 CH2=Br+

M-36, M-38 loss of HCl

M-79, M-81 loss of Br·

M-127 loss of I·

Alcohols

M+ weak or absent


m/e = 31 CH2=OH+

m/e = 45, 59, 73, ... RCH=OH+

m/e = 59, 73, 87, ... R2C=OH+

M-18 loss of H2O

M-46 loss of H2O and CH2=CH2

Phenols

strong M+

strong M-1 loss of H·

M-28 loss of CO

Ethers

M+ stronger than alcohols


M-31, M-45, M-59, etc... loss of OR

m/e = 45, 59, 73, ... CH2=OR+

Amines

M+ weak or absent Nitrogen rule

m/e = 30 CH2=NH2+ (base peak)


51

Aldehydes

weak M+

m/e = 29 HCO+

M-29 loss of HCO

M-43 loss of CH2=CHO

m/e = 44, 58, 72, 86, ... McLafferty rearrangement

strong M+ aromatic aldehyde

M-1 aromatic aldehyde loss of H·

Ketones

M+ intense


m/e = 43 CH3CO+

m/e = 55 +CH2CH=C=O

m/e = 42, 83 in cyclohexanone

m/e = 105, 120 in aryl ketones

Carboxylic Acids

M+ weak but observable

M-17 loss of OH

M-45 loss of CO2H

m/e = 45 CO2H+

m/e = 60 ·CH2C(OH)2+

M+ large aromatic acids

M-18 ortho effect

52

Esters

M+ weak but observable methyl esters

M-31 methyl esters loss of OCH3

m/e = 59 methyl esters CO2CH3+

m/e = 74methyl esters CH2C(OH)OCH3+

M+ weaker higher esters

M-45, M-59, M-73, etc... loss of OR

m/e = 73, 87, 101 CO2R+

m/e = 88, 102, 116 ·CH2C(OH)OR+

m/e = 61, 75, 89 RC(OH)2+ (long alkyl ester)

m/e = 108loss of CH2=C=O (benzyl,

acetate)

m/e = 105 C6H5CO+ (benzoate)

M-32, M-46, M-60 loss of ROH (ortho effect)

53

RA of Isotopes

RELATIVE ABUNDANCES OF ISOTOPES

Isotope peak : The isotope peak are obtained when a molecule contains heavier isotope of certain atoms than the common isotopes.

Commonly seen isotope peak are (M+1)+ peaks or (M+2)+ peaks

Intensity of an isotope peak is much lesser than that of the (M)+ peak except when Cl or Br is present in the molecule.

54

ISOTOPES

Most elements occur naturally as a mixture of isotopes.

-The presence of significant amounts of heavier isotopes leads to small peaks that have masses that are higher than the parent ion peak.

M+1 = a peak that is one mass unit higher than M+

M+2 = a peak that is two mass units higher than M+

55

RA of Isotopes


intensity of an isotope peak depends on the relative abundance of that isotope in nature. The relative abundance of an isotope is calculate on the basis of 100molecules. From RA, the intensity of (M+1)+, (M+2)+ peaks can be determined. For a compound containing one carbon atom , out of every 100 molecules, 98.892 molecule contain C12

isotope and 1.108 molecule contain C13 isotope

56

RA of Isotopes


Hence , the intensity of (M+1)+ peak is about 1.1% of the intensity of the (M) +peak and the ratio of the intensities of M+ and (M+1)+ peak is 98.892:1.108.

For compound containing silicon , the intensities of (M) + peak corresponding to Si28 isotope , (M+1) + peak corresponding to Si29 isotope and (M+2) + peak corresponding to Si30 isotope are in proportion of their relative abundance in the nature , i.e. 92.18:4.71:3.12.

57

RA of Isotopes


For compound containing sulphur , the ratio of intensities of (M) +: (M+2) + peaks , corresponding to S32 and S34 isotopes is 95.018:4.215. The height of the peak is the measure of intensity of that peak. Fluorine and iodine have only one naturally occurring isotope corresponding to atomic mass of 19 and 127, resp. Hence they produced only one peak corresponding to (M) + ion.

58

RA of Isotopes


S. Isotope Relative abundance peak

1 1H:2H 99.985:0.015 (M+1)

2 12C:13C 98.892:1.108 (M+1)

3 14N:15N 99.635:0.365 (M+1)

4 16O:17O:18O 99.759:0.037:0.204 (M+1) ,(M+2)

5 28Si:29Si:30Si 92.18:4.71:3:12 (M+1), (M+2)

6 32S:33S:34S 95.018:0.75:4.215 (M+1), (M+2)

7 35Cl:36Cl 75.529:24.471 (M+2)

8 79Br:81Br 50.52:49.48 (M+2)

59

References:1. Silverstein R.M., & Webster F.X, Spectrometric Identification of

Organic Compounds, Sixth edition 2006, Page no. 2 – 28.

2. Sharma Y. R. Elementary Organic Spectroscpoy Principles and Chemical Application, Fourth edition 2007, S. Chand & Company, Page no. 280 – 339.

3. Chatwal G.R., Aanand S.K., Instrumental Methods ofAnalysis, Himalaya Publishing House, 5th Edition, Page no. 2.272-2.302

4. http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html access date - 19 sept 2013

5. http://en.wikipedia.org/wiki/Mass_spectrometry access date – 19 sept 2013

6. Dr. supriya s. mahajan,Instuumental methods of analysis , page no. 125 -154

60

mass spectrum

Education

molecular ions parent

types of ions molecular

types of ion types of

molecular ion base peak

fragment ionthe molecular

ehmolecular ion

important ion

daughter ion