Created with MindGenius Business 2005® Mass Spectrometry Mass Spectrometry.

Created with MindGenius Business 2005Created with MindGenius Business 2005®®

Mass Spectrometry Mass Spectrometry

Principles Mass Spectra

http://www.mrc-dunn.cam.ac.uk/facilities/mass_spectrometry.phphttp://www.mrc-dunn.cam.ac.uk/facilities/mass_spectrometry.php

Principles Principles

The study of ionised molecules in the gas phase

Principles date back to ~ 1897 Based on accelerating ions in a vacuum Joseph J. Thompson used an early MS to discover the electron .

Received the Nobel Prize in 1906 [Physics].

For a “friendly” practical introduction see: http://www.asms.org/whatisms/p1.html and following pages

USES Molecular weight determination Structural characterisation Gas phase reactivity study Qualitative and Quantitative analysis of mixtures

Principles Principles

Volatilisation Ionisation Separation Detection

Sample

Volatilisation

Ionisation

Separation

Detection

Volatilisation Volatilisation

Gaseous and volatile samples are readily drawn into the reservoir.

Less volatile solid samples require heating.

The volatile sample diffuses from the reservoir into the ionization chamber via a leak - a pin-hole restriction in a gold foil.

Ionisation (1) Ionisation (1)

Electron impact or EI (most common) Sample passes through an electron beam. energy of e- beam is increased until e- is ejected from the

target molecule (normally ~70eV) high energy causes substantial fragmentation

M + e [M]+. + 2e

Radical cation

[MOLECULAR ION]

Most MS are set up to detect positive ions.

Ionisation (2) Ionisation (2) Chemical ionisation (CI) Softer than EI (used for more sensitive compounds). Sample is introduced with an excess of a carrier gas (eg

NH3) which is ionised by the electron beam.

Lower energy means less fragmentation

Separation Separation

A stable controllable magnetic

field separates the ions

according to their momentum. Only ions of a single

mass/charge [m/z] ratio will

have the trajectory to be

detected. By varying the magnetic, field

ions with different m/z values

are focused on the detector

Detection Detection

The ion current will cause emission of secondary electrons

from a metal plate detector. The Faraday cup detector suppresses secondary ion

formation. The -ve plate also suppresses secondary ion formation

Mass Spectra Mass Spectra

Features Fragmentation

patterns

Features Features

After ionisation: Ejection of an electron from the parent molecule gives the

molecular ion (generally M+, but depends on ionisation

technique) M+ fragments giving rise to other peaks

Gives rise to a mass spectrum: Plotted as intensity (%) vs m/z (amu) The tallest peak is termed the base peak and is given an

intensity of 100%. Other peak intensities are given relative to this

Features Features

Fragmentation patterns (1)Fragmentation patterns (1)Occurs via two major routes:

[M]+. A+ (even electron cation) + B. (radical)

[M]+. C+. (cation radical) + D (molecule)

NB Only particles with positive charges are detected

Fragmentation patterns (2)Fragmentation patterns (2)

Characteristic fragmentation patterns arise as: Weak bonds tend to break Formation of stable fragments (ions, radicals and

molecules) is favoured Some molecules can form cyclic transition states

Fragmentation patterns Fragmentation patterns

Alkanes Alkenes and Aromatic compounds Heteroatoms Isotopes

Alkanes (1) Alkanes (1) Carbocation is an sp2 hybrid with an empty p orbital.

Stability of the cation depends on the number of alkyl groups, due to inductive effects

Alkanes (2) Alkanes (2)

+CH3 < CH3CH2CH2CH2+ < CH3CH2

+CHCH3 < (CH3)3C+

Increasing stability

CH3H3C

More stable

Less stable

Aliphatic carbon skeletons are readily cleaved at branches because it results in carbocations that are more stable

Alkanes (3) Alkanes (3)

Stable fragments formed from

branched alkyl chain results in

less strong Molecular Ion (MI)

NB Size of peak is relative

Alkenes and Aromatic compounds Alkenes and Aromatic compounds (1)(1)

Cleavage occurs β to double bonds because more stable carbocations are produced due to resonance stabilisation

Cyclic alkenes can undergo rearrangements

Double bonds easily migrate in MS and isomers may easily be difficult to identify

Aromatic hydrocarbons strong molecular ion β cleavage favoured strongly by resonance stabilisation fragmentation of ring energetically disfavoured

C C- e-

C C CC

Resonance

Aromatic hydrocarbons Characteristic m/z = 91 and 65 from tropylium ion and

breakdown product.

Alkynes – strong MI, similar to alkenes, cleave at β site

CH2X CH2

Tropylium ion m/z = 91

H2C CH2

m/z = 65

Heteroatoms (1)Heteroatoms (1)

Cleavage may occur α, β or γ to heteroatoms depending on the functional groups involved. BEWARE: α to the carbonyl is β to the heteroatom!

a) α cleavage promoted by electronegativity eg ethers

ROC- e-

ROC C O R

Heteroatoms (2)Heteroatoms (2)b) β cleavage – promoted by resonance stabilisation

eg Carbonyl group

NB the most stable carbocation will be found in greatest abundance

R X C C C

X = O, N, S, halogen

R X C C C- e-

C O C R O C R

Acylium ionR = alkyl,- OH or -OR

Heteroatoms (3)Heteroatoms (3)c) McLafferty rearrangements give γ cleavage (β to carbonyl) occurs with carbonyl containing compounds depends on six membered transition state(i) Ketones, and carboxylic acids

1º carboxylic acids give a characteristic peak at m/z = 60

R = alkyl or OH

Heteroatoms (4)Heteroatoms (4)ii) Carboxylic esters

Can undergo two types of McLafferty rearrangement

Heteroatoms Heteroatoms

Oxygen heteroatoms Nitrogen heteroatoms Halogen heteratoms

Oxygen heteroatoms Oxygen heteroatoms Group Formul

aMI α β γ Other

Alcohols R-OH Weak or absent

Yes* Main *H2O Elimination

Aldehydes

RCHO weak No acylium ion

McLafferty

Ketones RCOR’ Strong No acylium ion

McLafferty

Carboxylic Esters

RCOOR’ Weak Not really

acylium ion

McLafferty

Carboxylic Acids

RCOOH Weak Not really

Mostly McLafferty

Ethers ROR’ Fairly weak

Yes Yes Rearrang-ements

Nitrogen heteroatoms Nitrogen heteroatoms

Weak or absent MI iminium ion (like acylium) gives most intense peak may also undergo cyclic rearrangements.

Nitrogen rule: Compounds with an odd number of nitrogens

have an odd molecular weight.

eg NH3 MW = 17;

CH3CH2NH2 MW = 45;

NH2CH2CH2NH2 MW = 60

Halogen heteroatoms Halogen heteroatoms

MI is strongest for Iodides less so for Bromides etc. More branched halides have weaker MIs. Cl and Br Isotopes give visible patterns

Fragmentation may include (in order of likelihood):

a) Loss of halogen

b) Loss of HX

c) β cleavage (as for oxygen) – loss of CH2X

d) Rearrangements

Isotopes (1)Isotopes (1)

It is important to use the correct method to calculate molecular

weight for MS. Average atomic masses take into account the

different abundances of isotopes. Need to use exact mass for

CARBON

1.1% of naturally occurring carbon is 13C: gives 1% abundance

m/z = 17 signal (M+H)+ in spectrum of Methane.

HALOGENS

Significant amounts of different isotopes, visible patterns within

spectrum.

Isotopes (2)Isotopes (2)

Chlorine: Average Atomic weight = 35.453

Isotope 35Cl 37Cl

Atomic weight

Abundance 75.8 24.2

Ratio ~ 3 1

Isotopes (3)Isotopes (3)Bromine: Average atomic weight = 79.904 Isotope 79Br 81Br

Atomic weight

Abundance 50.6 49.4

Ratio ~ 1 1

m/z m/z

Isotopic pattern for

one bromine atom

M+4 eg

CH3Br Mass:

50% CH379Br = 94

50% CH381Br = 96

CH2Br2 Mass:

25 % CH279Br79Br = 172

25 % CH279Br81Br = 174

25 % CH281Br79Br = 174

25 % CH281Br81Br = 176

Isotopic pattern for

one bromine atom

Characteristic signals in Characteristic signals in MSMS

Functional group Characteristic m/z

Alkanes 29, 43, 57, 71, 85

Alkenes Ions resulting from β cleavage

Amines 30, 44, 58, 72

Benzene 65, 77, 91

Halides Isotope peaks, loss of X and HX

Carbonyl, Ester, Acid Acylium ions and Mclafferty rearrangement products common

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