IB Chemistry on Mass Spectrometer and Isotopes

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Prepared by Lawrence Kok

Tutorial on Mass Spectrometer and Isotopes

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Atomic Structure and Isotopes

Isotopes – Atoms of same element with • Different number of neutrons• Same number of proton• Same number of electronsDue to presence of isotopes, when calculating RAM, the weighted average/mean of all isotopes present will have to used.

X - No isotopes

RAM/Ar X• Mass of 1 atom X Mass of 1/12 of 12C• Mass of 1 atom X relative to 1/12 mass of 1 atom 12C

Relative Abundance 75% 25%

Mass number = proton + neutron

Proton number = proton Z = 29 protons

A= 29 protons + 35 neutrons = 64

Isotopes

Y - TWO isotopes

RAM/Ar Y• Average Mass of 1 atom Y Mass of 1/12 of 12C• Average mass of 1 atom Y relative to 1/12 mass of 1 atom 12C

RAM for CI • Weighted average mass of 2 isotopes present= (mass 53CI x % Abundance) + (mass 37CI x % Abundance) = (35 x 75/100) + (37 x 25/100)= 35.5

CI - TWO isotopes

Mg has 3 Isotopes

Relative Abundance

Pb has 4 Isotopes

Relative Abundance

Mg has 3 Isotopes

24 Mg – (100/127.2) x 100% - 78.6%25 Mg – (12.8/127.2) x 100% - 10.0%26 Mg – (14.4/127.2) x 100% - 11.3%

Relative Abundance % Abundance

Pb has 4 Isotopes

204Pb – (0.2/10) x 100% - 2%206Pb – (2.4/10) x 100% - 24%207Pb – (2.2/10) x 100% - 22%208Pb – (5.2/10) x 100% - 52%

Convert relative abundance to % abundance



Mg has 3 Isotopes

24 Mg – (100/127.2) x 100% - 78.6%25 Mg – (12.8/127.2) x 100% - 10.0%26 Mg – (14.4/127.2) x 100% - 11.3%

RAM for Mg := (Mass 24Mg x % Abundance) + (Mass 25Mg x % Abundance) + (Mass 26Mg x % Abundance)= (24 x 78.6/100) + (25 x 10.0/100) + (26 x 11.3/100) = 24.30


Pb has 4 Isotopes

204Pb – (0.2/10) x 100% - 2%206Pb – (2.4/10) x 100% - 24%207Pb – (2.2/10) x 100% - 22%208Pb – (5.2/10) x 100% - 52%RAM for Pb :

= (Mass 204Pb x % Abundance) + (Mass 206Pb x % Abundance) + (Mass 207Pb x % Abundance) + (Mass 208Pb x % Abundance)= (204 x 2/100) + (206 x 24/100) + (207 x 22/100) + (208 x 52/100) = 207.20




Mass spectrophotometer Separates particles according to their masses and record their relative abundance• Use to determine presence of isotopes and its abundance • Use to calculate relative atomic mass /relative molecular mass of substance• Use to determine structural of organic molecules• Use to distinguish between structural isomers

Mass Spectrometer

Parts of Mass Spectrometer

Sample injection

Vaporization Chamber • Sample heat to vapour state

Ionization Chamber • Molecule bombard

with electrons form positive ions

Accelerator Chamber• M+ ions accelerated by

Electric field

Deflector • M+ ions

deflected by magnetic field

Detector • Measure m/z ratio• Relative abundance of

ions

http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch13ms.html

Sample X bombarded by electron • Form positive M+ ion• Accelerated (Electric Field)• Deflected (Magnetic Field) and DetectedX + e- → X+ + 2e-

Ionization and Fragmentation Process

Ionization Process - CH3CH2CH2CH3

• Bombarded by electron forming cation• Molecular ion, M+ = 58• (CH3CH2CH2CH3)

+ = 58

Fragmentation Process CH3CH2CH2CH3 • Molecular ion, M+ undergo fragmentation • Cation and Radical formed• Cation will be detected• Radical - Not charged –Not detected

Ionization , M+ ,m/z = 58Ionization and Fragmentation of M+

• Forming - m/z = 58, 43 and 15

Ionization and Fragmentation of M+

• Forming- m/z = 58 and 29

• Ionization forming M+

CH3CH2CH2 : CH3 + e → CH3CH2CH2+.CH3 + 2e

• Fragmentation of M+ producing 43CH3CH2CH2

+·CH3 → CH3CH2CH2+ + ·CH3


+·CH3 → CH3CH2CH2· + +CH3




+ = 58


H H | |CH3CH2CH2 C:H + e → CH3CH2CH2

C+.H + 2e | | H H


CH3CH2:CH2CH3 + e → CH3CH2+·CH2CH3

+ 2e

• Fragmentation of M+ producing 29CH3CH2

+·CH2CH3 → CH3CH2+

+ .CH2CH3

Ionization , M+ ,m/z = 58

CH3CH2CH2CH3 + e → CH3CH2CH2CH3

+ + 2e


• Forming - m/z = 58, 43 and 15

m/z = 58

m/z = 43

m/z = 15



m/z = 58

m/z = 58

m/z = 29


CH3CH2CH2 : CH3 + e → CH3CH2CH2+.CH3 + 2e


+·CH3 → CH3CH2CH2+ + ·CH3


+·CH3 → CH3CH2CH2· + +CH3




+ = 58


H H | |CH3CH2CH2 C:H + e → CH3CH2CH2

C+.H + 2e | | H H


CH3CH2:CH2CH3 + e → CH3CH2+·CH2CH3

+ 2e

• Fragmentation of M+ producing 29CH3CH2

+·CH2CH3 → CH3CH2+

+ .CH2CH3

Ionization , M+ ,m/z = 58

CH3CH2CH2CH3 + e → CH3CH2CH2CH3

+ + 2e


• Forming - m/z = 58, 43 and 15

m/z = 58

m/z = 43

m/z = 15



m/z = 58

m/z = 58

m/z = 29

Ionization and Fragmentation

Ionization of CI2 into CI2+

CI:CI + e- → CI+.CI + 2e [35CI+.35CI] – 70CI:CI + e- → CI+.CI + 2e [35CI+.37CI] – 72CI:CI + e- → CI+.CI + 2e [37CI+.37CI] – 74

Fragmentation of CI2+ into CI+

CI+.CI → CI+ + ·CI [35CI+ + 35CI·] – 35

CI+.CI → CI+ + ·CI [37CI+ + 37CI·] – 37

m/z = 35

m/z = 37


CI2 molecule (Isotopes) undergo Ionization and Fragmentation


CI+.CI → CI+ + ·CI [35CI+ + 35CI·] – 35

CI+.CI → CI+ + ·CI [37CI+ + 37CI·] – 37

m/z = 35

m/z = 37


Mass spectrum for CI2 molecule


Ratio for 35CI35CI: 35CI37CI: 37CI37CI is 9:6:1Ratio for 35CI : 37CI is 3:1



Deflection based on mass/charge ratio or (m/z) ratio• Ion – LOW ↓ mass (light) + HIGH ↑ charge – Deflected ↑ more (LOW ↓

(m/z) ratio )• Ion – HIGH ↑ mass (heavy) + LOW ↓ charge – Deflect ↓ less (HIGH ↑

(m/z) ratio )

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LOW ↓(m/z) ratio – HIGH ↑ Deflection

HIGH ↑ (m/z) ratio - LOW ↓Deflection


CI+.CI → CI+ + ·CI [35CI+ + 35CI·] – 35

CI+.CI → CI+ + ·CI [37CI+ + 37CI·] – 37

37CI+

35CI+

35CI2+

37CI2+

m/z = 35

m/z = 37


Mass spectrum for CI2 molecule


Ratio for 35CI35CI: 35CI37CI: 37CI37CI is 9:6:1Ratio for 35CI : 37CI is 3:1



http://www.chemguide.co.uk/analysis/masspec/howitworks.html

http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch13ms.html#isotopes

Mass spectra for CH3(CH2)8CH3

Mass spectra for C6H5CH2OH

Molecular Ion peak, M+ = [C6H5CH2OH]+ = 108Fragmentation peaks :(M- 17)+ = (C6H5CH2)+ = 91

(M- 31)+ = (C6H5)+ = 77

(M- 77)+ = (CH2OH)+ = 31

Molecular Ion peak, M+ = [CH3(CH2)8CH3]+ = 142

Fragmentation peaks :(M-15)+ =(CH3CH2CH2CH2CH2CH2CH2CH2CH2)

+ = 127(M-29)+ =(CH3CH2CH2CH2CH2CH2CH2CH2)

+ = 113(M-43)+ =(CH3CH2CH2CH2CH2CH2CH2)

+ = 99(M-57)+ =(CH3CH2CH2CH2CH2CH2)

+ = 85(M-71)+ =(CH3CH2CH2CH2CH2)

+ = 71(M-85)+ =(CH3CH2CH2CH2)

+ = 57(M-99)+ = (CH3CH2CH2)

+ = 43(M-113)+ = (CH3CH2)

+ = 29(M-127)+ = (CH3)

+ = 15

Loss of Methylene gp, CH2 = 14

Loss of CH2OH

Loss of OH

Loss of C6H5


Mass spectra for CH3(CH2)8CH3

127

Mass spectra for C6H5CH2OH

Molecular Ion peak, M+ = [C6H5CH2OH]+ = 108Fragmentation peaks :(M- 17)+ = (C6H5CH2)+ = 91

(M- 31)+ = (C6H5)+ = 77

(M- 77)+ = (CH2OH)+ = 31

Molecular Ion peak, M+ = [CH3(CH2)8CH3]+ = 142

Fragmentation peaks :(M-15)+ =(CH3CH2CH2CH2CH2CH2CH2CH2CH2)

+ = 127(M-29)+ =(CH3CH2CH2CH2CH2CH2CH2CH2)

+ = 113(M-43)+ =(CH3CH2CH2CH2CH2CH2CH2)

+ = 99(M-57)+ =(CH3CH2CH2CH2CH2CH2)

+ = 85(M-71)+ =(CH3CH2CH2CH2CH2)

+ = 71(M-85)+ =(CH3CH2CH2CH2)

+ = 57(M-99)+ = (CH3CH2CH2)

+ = 43(M-113)+ = (CH3CH2)

+ = 29(M-127)+ = (CH3)

+ = 15

Loss of Methylene gp, CH2 = 14

Loss of CH2OH

Loss of OH

Loss of C6H5


Mass spectra for CH3CH(CI)CH3

Molecular Ion peak, M+ = [CH3CH(35CI)CH3]+

= 78 (35CI)Molecular Ion peak, M+ = [CH3CH(37CI)CH3]

+ = 80 (37CI)Fragmentation peaks :(M-15)+ =(CH3CH35CI)+ = 63(M-15)+ =(CH3CH37CI)+ = 65

(M-35/37)+ =(CH3CHCH3)+ = 43

Loss of methyl gp CH3

Loss of CI gp

Mass spectra for CH3CH2CH3Br

Molecular Ion peak, M+ = [CH3CH2CH2Br]+ = 122 (79Br)Molecular Ion peak, M+ = [CH3CH2CH2Br]+ = 124 (81Br)Fragmentation peaks :(M-79/81)+ =(CH3CH2CH2)

+ = 43

(M- 29)+ =(CH279Br)+ = 93

(M- 29)+ =(CH281Br)+ = 95

Loss of Br gp

Loss of ethyl gp CH3CH2


Isotopic peak – (M+ + 2) peak present due to 35CI and 37CI

Mass spectra for CH3CH(CI)CH3

Molecular Ion peak, M+ = [CH3CH(35CI)CH3]+

= 78 (35CI)Molecular Ion peak, M+ = [CH3CH(37CI)CH3]

+ = 80 (37CI)Fragmentation peaks :(M-15)+ =(CH3CH35CI)+ = 63(M-15)+ =(CH3CH37CI)+ = 65

(M-35/37)+ =(CH3CHCH3)+ = 43

Loss of methyl gp CH3

Loss of CI gp

Mass spectra for CH3CH2CH3Br

Molecular Ion peak, M+ = [CH3CH2CH2Br]+ = 122 (79Br)Molecular Ion peak, M+ = [CH3CH2CH2Br]+ = 124 (81Br)Fragmentation peaks :(M-79/81)+ =(CH3CH2CH2)

+ = 43

(M- 29)+ =(CH279Br)+ = 93

(M- 29)+ =(CH281Br)+ = 95Isotopic peak – (M+ + 2) peak present due to 79Br

and 81Br

Loss of Br gp

Loss of ethyl gp CH3CH2

93 95

Mass Spectrum for Isomers, 2 methylbutane and 2, 2 dimethylpropane

CH3 |CH3CHCH2CH3

CH3 |CH3C-CH3

| CH3

Isomers of C5H12

Vs

2 methylbutane 2, 2 dimethylpropane


CH3 |CH3CHCH2CH3

CH3 |CH3C-CH3

| CH3

Molecular Ion, M+ = [CH3CH(CH3)CH2CH3]+ =

72Fragmentation peaks :(M - 15)+ = (CH3CH(CH3)CH2)

+ = 57

(M - 29)+ = (CH3CH(CH3))+ = 43

(M - 43)+ = (CH3CH2)+ = 29

(M - 57)+ = (CH3)+ = 15

Isomers of C5H12

Molecular Ion, M+ = [CH3CH(CH3)CH2CH3]+

= 72Fragmentation peaks :(M - 15)+ = (C(CH3)3)

+ = 57

(M - 30)+ = (C(CH3)2)+ = 42

(M - 45)+ = (CH3C)+ = 27

(M - 57)+ = (CH3)+ = 15

Vs

Loss of CH3

Loss of CH3CH2

Loss of CH3CH(CH3)

Loss of CH3CH(CH3)CH2

Loss of CH3

Loss of T WO CH3

Loss of THREE CH3


Loss of C(CH3)3

Vs


CH3 |CH3CHCH2CH3

CH3 |CH3C-CH3

| CH3

Peak 29 absent• No CH3CH2 presentPeak 57 is higher• Loss of methyl radical produces a tertiary carbocation• Tertiary carbocation – More stable

Molecular Ion, M+ = [CH3CH(CH3)CH2CH3]+ =

72Fragmentation peaks :(M - 15)+ = (CH3CH(CH3)CH2)

+ = 57

(M - 29)+ = (CH3CH(CH3))+ = 43

(M - 43)+ = (CH3CH2)+ = 29

(M - 57)+ = (CH3)+ = 15

Isomers of C5H12

Molecular Ion, M+ = [CH3CH(CH3)CH2CH3]+

= 72Fragmentation peaks :(M - 15)+ = (C(CH3)3)

+ = 57

(M - 30)+ = (C(CH3)2)+ = 42

(M - 45)+ = (CH3C)+ = 27

(M - 57)+ = (CH3)+ = 15

Vs

Loss of CH3

Loss of CH3CH2

Loss of CH3CH(CH3)

Loss of CH3CH(CH3)CH2

Loss of CH3

Loss of T WO CH3

Loss of THREE CH3

CH3 |CH3C+·CH3 | CH3

m/z= 57

CH3 |CH3 C

+ + ·CH3

| CH3


Loss of C(CH3)3

Vs

Peak 29 absent• CH3CH2 present

Mass Spectrum for Isomers, Propan-1-ol and Propan-2-ol

Isomers of C3H8OH

CH3CH2CH2OH OH |CH3CHCH3

Propan-1-olPropan-2-ol


Molecular Ion, M+ = [CH3CH2CH2OH]+ = 60Fragmentation peaks :(M - 15)+ = (CH2CH2OH)+ = 45

(M - 29)+ = (CH2OH)+ = 31

(M - 31)+ = (CH3CH2)+ = 29

(M - 45)+ = (CH3)+ = 15

Isomers of C3H8OH

Molecular Ion, M+ = [CH3CH(OH)CH3]

+ = 60Fragmentation peaks :(M - 15)+ = (CH3CH(OH))+ = 45

(M - 17)+ = (CH3CHCH3)+ = 43

(M - 33)+ = (CH3C)+ = 27

Vs

Loss of CH3

Loss of CH3CH2

Loss of CH2OH

Loss of CH2CH2OH

Loss of CH3


Loss of OH

Loss of OH, CH3, H


15

Vs


Peak 45 is higher• Loss of methyl radical at both sides produces (CH3CH(OH))+

• No m/z= 29 peak detected – No CH2CH3 found !

Molecular Ion, M+ = [CH3CH2CH2OH]+ = 60Fragmentation peaks :(M - 15)+ = (CH2CH2OH)+ = 45

(M - 29)+ = (CH2OH)+ = 31

(M - 31)+ = (CH3CH2)+ = 29

(M - 45)+ = (CH3)+ = 15

Isomers of C3H8OH

Molecular Ion, M+ = [CH3CH(OH)CH3]

+ = 60Fragmentation peaks :(M - 15)+ = (CH3CH(OH))+ = 45

(M - 17)+ = (CH3CHCH3)+ = 43

(M - 33)+ = (CH3C)+ = 27

Vs

Loss of CH3

Loss of CH3CH2

Loss of CH2OH

Loss of CH2CH2OH

Loss of CH3

OH OH | |CH3 C+·CH3 → CH3C

+ + ·CH3

| | H Hm/z= 45


Loss of OH

Loss of OH, CH3, H

Peak 29 and 31 are found• Inductive effect of OH causes splitting of CH3CH2-|-CH2OH• m/z =29 peak detected – CH2CH3 present

CH3CH2 +· CH2OH → CH3CH2

+ + ·CH2OH

m/z= 29

CH3CH2 +· CH2OH → CH3CH2 ·

+ +CH2OH

m/z= 31


15

Vs

Normal Mass Spectrometer Vs High Resolution Mass spectrometer

Normal Mass Spectrometer • Determination molecular formula/weight by adding all relative atomic mass • RMM for molecule = Sum of all RAM • RMM for O2 = 16 + 16 = 32• RMM for N2H4 = (14 x 2) + (1 x 4) =32• RMM for CH3OH = (12 + 3 + 16 + 1) = 32• Molecular ion peak, for O2, N2H4, CH3OH is the

SAME = 32

RAM, O = 16RAM, N = 14RAM, H = 1RAM, C = 12

High Resolution Mass Spectrometer -Measure to RMM to 4/5 decimal places• Determination molecular formula/weight by adding all relative atomic mass • RMM for molecule = Sum of all RAM • RMM for O2 = 15.9949 + 15.9949 = 31.9898• RMM for N2H4 = (14.0031 x 2) + (1.0078 x 4) = 32.0375• RMM for CH3OH = (12.0000 )+ (3 x 1.0078) + 15.9949 = 32.0262• Molecular ion peak, for O2, N2H4, CH3OH is the NOT the same

RAM, O = 15.9949RAM, N = 14.0031RAM, H = 1.0078RAM, C = 12.0000Vs



SAME = 32



RAM, O = 15.9949RAM, N = 14.0031RAM, H = 1.0078RAM, C = 12.0000Vs

Vs



SAME = 32



RAM, O = 15.9949RAM, N = 14.0031RAM, H = 1.0078RAM, C = 12.0000

(M+1 )– due to isotope 13C as Carbon has 3 isotopes, 12, 13 and 14Naturally occuring carbon made up of 98.9% 12C and 1.1% 13CRelative abundance for M+1 (13C) peak is very small

59

Vs

CH3CH2CH2CH3 • Molecular ion peak/parent ion – Highest m/z value = RMM of compound• Molecular ion, M+ = 58• (CH3CH2CH2CH3)

+ = 58

Mass spectrum for CH3CH2CH2CH3

Vs

Acknowledgements

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Prepared by Lawrence Kok

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Education

IB Chemistry on Mass Spectrometer and Isotopes