14-1 CH-4 Part 1 Mass Spectroscopy Prepared By Dr. Khalid Ahmad Shadid Islamic University in Madinah Department of Chemistry

14-1

CH-4 Part 1

Mass Spectroscopy

Prepared By

Dr. Khalid Ahmad Shadid

Islamic University in MadinahDepartment of Chemistry

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Mass Spectrometry (MS)

An analytical technique for measuring the mass-to-charge ratio (m/z) of ions in the gas phase. Mass spectrometry is our most valuable analytical tool

for determining accurate molecular masses. Also can give information about structure.

electron ionization mass spectrometer (EI-MS)

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Mass Spectrometry (MS)

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A Mass Spectrometer

A mass spectrometer is designed to do three things: Convert neutral atoms or molecules into a beam of

positive (or rarely negative) ions. Separate the ions on the basis of their mass-to-charge

(m/z) ratio. Measure the relative abundance of each ion.

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A Mass Spectrometer Electron Ionization MS

In the ionization chamber, the sample is bombarded with a beam of high-energy electrons.

Collisions between these electrons and the sample result in loss of electrons from sample molecules and formation of positive ions. Methane for example has 4 + 4 = 8 valence electrons. One is knocked out leaving a radical cation, which has only seven valence electrons

++

+

C HH

H

H

CH

H

He 2eH

Molecular ion(a radical cation)

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Molecular Ion

Molecular ion (radical cation) (M): A radical cation formed by removal of a single electron from a parent molecule in a mass spectrometer.

For our purposes, it does not matter which electron is lost; radical cation character is delocalized throughout the molecule. Therefore, we write the molecular formula of the parent molecule in brackets with: a plus sign to show that it is a cation. a dot to show that it has an odd number of electrons.

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Molecular Ion

At times, however, we find it useful to depict the radical cation at a certain position in order to better understand its reactions.

CH3CH2OCH(CH3)2CH3CH2OCH(CH3)2 .

The radical cation is unstable and undergoes fragmentation to give smaller ions

MHIGH ENERGY

70 eVM+ . + e-

FRAGMENTATION

UNKNOWNCOMPOUND

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Electron Impact Ionization

e- + H C

H

H

C

H

H

H

H C

H

H

C

H

H

H

H C

H

H

C

H

H

+ H

H C

H

H

C

H

H

H

+

Other fragments can be formed when C—C or C—H bonds are broken during ionization. Only the positive fragments can be detected in MS.

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Mass Spectrum

Mass spectrum: A plot of the relative abundance of ions versus their mass-to-charge ratio.

Base peak: The most abundant peak. Assigned an arbitrary intensity of 100.

The relative abundance of all other ions is reported as a % of abundance of the base peak.

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Molecular (or Radical cation)Ion (Parent) – ion having same mass as the analyte

Daughter Ion – ion having lower mass Base Peak – biggest peak

Size of peaks depends on relative natural abundance of isotopes

The base peak in electron-impact spectra arise from fragments rather than from molecular ion.

The molecular ion peak provides the MWt of the unknown.

Conclusion about types of MS Peaks:

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MS of dopamine

Figure 13.2 Partial mass spectrum of dopamine showing all peaks equal or greater than 0.5% of the base peak.

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MS of Dopamine

The number of peaks in the mass spectrum of dopamine is given here as a function of detector sensitivity.

HO

HO

NH2Number of PeaksRecorded

Peak IntensityRelative to Base Peak

> 5%> 1%> 0.5%> 0.05%

8 31 45120

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Other MS techniques

What we have described is called electron ionization mass spectrometry (EI-MS).

Other mass spectrometry techniques include Fast atom bombardment (FAB). Matrix-assisted laser desorption ionization (MALDI). Chemical ionization (CI). Electrospray ionization (ESI).

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Resolution

Resolution: A measure of how well a mass spectrometer separates ions of different mass. Low resolution: Refers to instruments capable of

separating only ions that differ in nominal mass; that is ions that differ by at least 1 or more atomic mass units (amu).

High resolution: Refers to instruments capable of separating ions that differ in mass by as little as 0.0001 amu.

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Resolution

C3H6O and C3H8O have nominal masses of 58 and 60, and can be distinguished by low-resolution MS.

C3H8O and C2H4O2 both have nominal masses of 60.

distinguish between them by high-resolution MS.

C2H4O2

C3H8O

60.02112

60.05754

60

60

MolecularFormula

Nominal Mass

PreciseMass

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Molecular formula in general

Even Mass : can contain C, H, O , halogen, even # N

Molecular ion:

Odd Mass : can contain C, H, O , halogen, odd # N

Fragmentation ions:

Even Mass : From even mass M+ comes from rearrangement or 2 bond breaking

even Mass : from Odd Mass M+ comes from single bond breaking

Odd Mass : From even mass M+ comes from single bond breaking

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THE RULE OF 1313قانون

الجزيئية الصيغة الستنباط يستعملعليه المتحصل الجزيئي الوزن من

الكتلة مطياف منMS Basic Molecular formula

(CH) Use table of element

equivalent to convert the basic molecular formula into

molecular formula تتالف ---> 13قانون اساسية صيغة

ذرات ثم ----> CHمن فقطما ونحذف اخرى عناصر نضيف

الـــ ذرات من وزنا CHيكافئها االساسية الصيغة حساب يمكن

على الجزيئي الوزن بقسمة وذلكالقسمة 13 ناتج على نحصل حيثn القسمة يلي rوباقي كما

M

13= n +

r

13

االساسية = CnH(n+r)الصيغة

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الجزيئي( 1مثال وزنه عضوي مركب46g/mol : يلي كما الكتلي الطيف وصورة

والجزيئية؟ االساسية الصيغة هي ما

m/z

0

100

Relative intensity

28 46

M+.

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الجزيئي( 1مثال وزنه عضوي وصورة 46g/molمركب : الصيغة و االساسية الصيغة هي ما يلي كما الكتلي الطيف

الجزيئية؟

M

13= n +

r

13


m/z

0

100

Relative intensity

28 46

M+.

1

2

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الجزيئي( 1مثال وزنه عضوي 46g/molمركب : الصيغة هي ما يلي كما الكتلي الطيف وصورة

والجزيئية؟ االساسية

M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7

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والجزيئية؟ االساسيةM

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7


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والجزيئية؟ االساسيةM

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7


االساسية = C3H10الصيغة

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الجزيئي( 1مثال وزنه مركبعضوي46g/mol : يلي كما الكتلي الطيف وصورة

والجزيئية؟ االساسية الصيغة هي ما

M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7


االساسية = C3H10الصيغةالجزيئي 46الوزن

m/z

0

100

Relative intensity

28 46

M+.

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الجزيئي( 2مثال وزنه مركبعضوي46g/mol ذرة على هي: Oيحتوي ما

والجزيئية؟ االساسية الصيغة

M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7



m/z

0

100

Relative intensity

28 46

M+.

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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7



m/z

0

100

Relative intensity

28 46

M+.

O +اضافة CH4حذف -

الجزيئي + 16الوزنالجزيئي – 16الوزن

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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7



m/z

0

100

Relative intensity

28 46

M+.



الجزيئي 46الوزن C2H6O الجزيئية الصيغة

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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7



m/z

0

100

Relative intensity

28 46

M+.




IHD = 0CH3CH2OH

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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7






IHD = 0CH3CH2OH

m/z

0

100

Relative intensity

28 46

H3C OH...

+ .

H3C

+ .H

.

H3C OH....

M+.

M+. - H2O



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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7

الجزيئي( 3مثال وزنه مركبعضوي46g/mol ذرة على الصيغة: Sيحتوي هي ما


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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7





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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7



S +اضافة C2H8حذف -




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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7





الجزيئي 46الوزن CH2S الجزيئية الصيغة



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M

13= n +

r

13

46/13 = 3 + 7/13n = 3; r = 7





الجزيئي 46الوزن CH2S الجزيئية الصيغة

IHD = 1CH2S

H H

S



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الكتلة مطيافMS

الجزيئي الوزنMolecular

mass

13قانون االساسية الصيغة(CH)

M

13= n +

r

13

حذف جدولواضافة

ما وحذف عنصر اضافةذرات من وزنا يكافئه

CH

الجزيئية الصيغة(CHX)

1 2

3

4

5

IHD التكسير

6البنائية الصيغة

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Interpreting a mass spectrum

Nitrogen Rule: If a compound has zero or an even number of nitrogen atoms, its molecular

ion will have an even m/z value. an odd number of nitrogen atoms, its molecular ion will

have an odd m/z value.

MW OF CH5N = 31MW OF C2H8N2 = 60

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Molecular formula: RULE of 13

Consider CH unit 13 amu

If we divide the mass by 13 we can establish easily possible formula: Example: Molecular ion => 152

152 / 13 = # carbons = 11Mass = 12 * 11 = 132 therefore H = 152 – 132 = 20

Basic formula C11 H20

If Oxygen is present : mass = 16 remove CH4

If Nitrogen is present : mass = 14 remove CH2

If one oxygen : C11 H20 – CH4 + O = C10 H16 O

If second oxygen : C10 H16 O – CH4 + O = C9 H12 O2

If third oxygen : C9 H12 O2 – CH4 + O = C8 H8 O3

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Molecular formula: RULE of 13

After establishing the basic formula with only Carbon/hydrogen,Other element can be introduced by substracting the proper hydrocarbonvalue

16O => CH4

14N => CH2

19F => CH7 1H12 => C28Si => C2H4

31P => C2H7

32S => C2H8

35Cl => C2H11

79Br => C6H7

127I => C10H7

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Nitrogen rule

Molecular ion => 26

26 / 13 = # carbons = 2

Mass = 12 * 2 = 24 therefore H = 2

Basic formula C2 H2

C C HH

Molecular ion => 27 odd!

27 / 13 = # carbons = 2

Basic formula C2 H3

One Nitrogen C2 H3 - CH2 + NC NH

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Nitrogen rule

Molecular ion => 100

100 / 13 = # carbons = 7

Mass = 12 * 7 = 84 therefore H = 16


1 Oxygen: C7 H16 - CH4 + O

C6 H12 O

Molecular ion => 99 odd!


1 Nitrogen: C7 H16 – CH2 + N

C6 H14 N

OH NH2

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Good Luck

Documents

14-1 CH-4 Part 1 Mass Spectroscopy Prepared By Dr. Khalid Ahmad Shadid Islamic University in Madinah Department of Chemistry