Mass Spec Lecture

5/20/2018 Mass Spec Lecture

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MassSpectrometry


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Background

Mass spectrometry (Mass Spec or MS)uses high energy electrons to break amolecule into fragments.

Separation and analysis of the fragmentsprovides information about: Molecular weight

Structure


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Background

The impact of a stream of high energyelectrons causes the molecule to lose anelectron forming a radical cation.

A species with a positive charge andone unpaired electron

+ e-C H

H

HH H

H

H HC

+ 2 e

-

Molecular ion (M+)m/z = 16


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Background

The impact of the stream of high energyelectrons can also break the molecule orthe radical cation into fragments.

(not detected by MS)

m/z = 29

molecular ion (M+) m/z = 30

+ C

H

H

H

+ H

HH C

H

H

C

H

H

H C

H

H

C

H

H

H CH

H

+ e-

H C

H

H

C

H

H

H

m/z = 15


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Background

Molecular ion (parent ion): The radical cation corresponding to the

mass of the original molecule

The molecular ion is usually the highestmass in the spectrum Some exceptions w/specific isotopes Some molecular ion peaks are absent.

H

H

H

HC H C

H

H

C

H

H

H


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Background

Mass spectrum of ethanol (MW = 46)

SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/1/09)

M+


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Background

The cations that are formed areseparated by magnetic deflection.


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Background

Only cations are detected. Radicals are invisible in MS.

The amount of deflection observeddepends on the mass to charge ratio(m/z). Most cations formed have a charge of

+1 so the amount of deflectionobserved is usually dependent on themass of the ion.


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Background

The resulting mass spectrumis a graphof the mass of each cation vs. itsrelative abundance.

The peaks are assigned an abundance asa percentage of the base peak. the most intense peak in the spectrum

The base peak is not necessarily thesame as the parent ion peak.


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Background


M+base peak

The mass spectrum of ethanol


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Background

Most elements occur naturally as amixture of isotopes. The presence of significant amounts of

heavier isotopes leads to small peaksthat have masses that are higher thanthe 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+


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Easily Recognized Elements in MS

Nitrogen: Odd number of N = odd MW

CH3CN

M+= 41



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Bromine: M+~ M+2 (50.5% 79Br/49.5% 81Br)

2-bromopropane

M+~ M+2

SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced IndustrialScience and Technology, 11/1/09)


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Chlorine: M+2 is ~ 1/3 as large as M+

Cl


M+2

M+


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Sulfur: M+2 larger than usual (4% of M+)


M+

Unusually

large M+2

S



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Iodine I+at 127 Large gap

Large gap

I+

M+


ICH2CN


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Fragmentation Patterns

The impact of the stream of high energyelectrons often breaks the molecule intofragments, commonly a cation and aradical. Bonds break to give the most stable

cation. Stability of the radical is less

important.


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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 fragmentforming the most stable carbocations.


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Mass spectrum of 2-methylpentane


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Alkenes: Fragmentation typically forms

resonance stabilized allylic carbocations


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Aromatics: Fragment at the benzylic carbon, forming aresonance stabilized benzylic carbocation(which rearranges to the tropylium ion)

M+

CH

H

CH Br

HC

H

H

or


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

NO2

77

M+= 123

77


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Alcohols Fragment easily resulting in very small or

missing parent ion peak May lose hydroxyl radical or water M+- 17or M+- 18

Commonly lose an alkyl group attached tothe carbinol carbon forming an oxonium

ion. 1oalcohol usually has prominent peak atm/z = 31corresponding to H2C=OH+


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MS for 1-propanol

M+M+-18

CH3CH2CH2OH

H2C OH



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Amines Odd M+(assuming an odd number of

nitrogens are present)

-cleavage dominates forming animinium ion

CH3CH2 CH2 N

H

CH2 CH2CH2CH3 CH3CH2CH2N CH2

Hm/z =72

iminium ion


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Fragmentation Patterns86

CH3CH2 CH2 N

H

CH2 CH2CH2CH3

72


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Ethers -cleavage forming oxonium ion

Loss of alkyl group forming oxonium ion

Loss of alkyl group forming acarbocation


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H O CHCH3

MS of diethylether (CH3CH2OCH2CH3)

CH3CH2O CH2H O CH2


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Aldehydes (RCHO) Fragmentation may form acylium ion

Common fragments:

M+- 1for

M+- 29for

RC O

R (i.e. RCHO - CHO)

RC O


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MS for hydrocinnamaldehyde

M+= 134C C C H

H

H

H

H

O

133

105

91

105

91



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Ketones Fragmentation leads to formation of

acylium ion:

Loss of R forming

Loss of R forming RC O

R'C O

RCR'O


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MS for 2-pentanoneCH3CCH2CH2CH3

O

M+

CH3CH2CH2C O

CH3C O



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Esters (RCO2R) Common fragmentation patterns

include:

Loss of OR peak at M+- OR

Loss of R

peak at M+- R


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Frgamentation Patterns

M+= 136

C

O

O CH3

105

77 105

77



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Rule of Thirteen

The Rule of Thirteen can be used toidentify possible molecular formulas for anunknown hydrocarbon, CnHm.

Step 1: n = M+/13 (integer only, useremainder in step 2)

Step 2: m = n + remainder from step 1


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Rule of Thirteen

Example: The formula for a hydrocarbonwith M+ =106 can be found:

Step 1: n = 106/13 = 8 (R = 2)

Step 2: m = 8 + 2 = 10

Formula: C8H10


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Rule of Thirteen

If a heteroatom is present, Subtract the mass of each heteroatom

from the MW

Calculate the formula for thecorresponding hydrocarbon Add the heteroatoms to the formula


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Rule of Thirteen

Example: A compound with a molecular ionpeak at m/z = 102 has a strong peak at1739 cm-1in its IR spectrum. Determineits molecular formula.


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GC-Mass Spec: Experiment 23

Mass Spec can be combined with gaschromatography to analyze mixtures ofcompounds. GC separates the components of the

mixture. Each component is analyzed by the

Mass Spectrometer.


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Assignment: Observe the GC-mass spec experiment Record experimental conditions

Analyze the mass spectrum of eachcomponent of your mixture: Parent ion peak?Heteroatoms apparent from

spectrum?A minimum of 1 or two significant

fragments and their structures


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Assignment (cont.): Using the Mass Spec data, retention

times, and boiling points, identify thecomponents of your mixture.

Write three paragraphs (one percompound) summarizing and interpreting

alldata. See your data sheet formore details.

Documents

Mass Spec Lecture