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MASS SPECTROMETRY

UWICAPE WORKSHOP 2011

Dr. K.Wilson

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CAPE Objectives

Explain the principles of mass spectrometry.

Use mass spectral data to:Determine relative isotopic masses and relative

isotopic abundance.Distinguish between molecules of similar relative

molecular mass.

Predict possible identities of simple organic

molecules based on their fragmentation pattern. Explain the significance of the (M+1) peak in a

mass spectrum.

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Introduction

Mass spectrometry (MS) is an analyticaltechnique that measures the mass to charge(m/z) ratios of charged particles.

Mass Spectroscopy can be used to determine:

Relative atomic mass of an element

Molecular mass of a molecule Structure elucidation of organic compounds

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Schematic of a MassSpectrometer

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Ionisation

The first step in the operation of thisinstrument is to IONISE the sample.

A beam of the vapourised sample is subjected to

an electric discharge. This causes an electron to be dislodged from the

sample and so produce a positive ion.

eMM gg

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Acceleration

The positive ions are repelled away from thevery positive ionisation chamber.

As the ions pass through the acceleration

chamber, they are made to accelerate with thesame velocity.

The ions are accelerated into a finely focused

beam.

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Deflection

At this stage, the ions with thesame mass/charge (m/z) ratio,travel in the same path but in a different path to ions with a

different m/z ratio.

The ions are deflected by amagnetic field according to theirmasses. The lighter they are, themore they are deflected.

The strength of the magnetic fieldincreases until all the ions ofinterest get detected.

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Detection

The beam of ions passing through the machine isdetected electrically.

Ions with a particular mass reach the detector.Different masses can be collected by changingthe magnetic or electric field.

The electrometer measures thecharge carried by each beam,

which is the recorded graphically.

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Isotopes and Relative AtomicMass

Isotopes Atoms of an element that have same atomic number

but different mass number.

Examples:

1

H,

2

H,

3

H &

12

C,

13

C

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Mass Spectrum of Isotopes of

chlorine

Based on the massspectrum results,chlorine has 2isotopes of atomicmasses 35 amu and37 amu.

The relativeabundances ofthese isotopes are75% and 25%

respectively.

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Isotopes and Relative Atomic Mass

(RAM)

Based on the mass spectra of chlorine, therelative atomic mass of Cl can be calculatedas:

RAM =

An elements atomic weight is a weighted

average of the isotopic masses of the elementsnaturally occurring isotopes.

amu5.35)37(100

25)35(

100

75

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Mass spectra of organiccompounds

The mass spectrum of an organic sample isobtained in the same way described earlier:

Ionisation, acceleration, deflection and detection.

The positive organic ion formed is called themolecular ion or the parent ion.

The molecular ion is often given the symbolM+.

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Mass spectra of organiccompounds

The vaporised organic molecules areenergetically unstable, and break up intosmaller charged fragments.

The charged fragments will travel through themass spectrometer like any other positive ion

until it gets to the detector.

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Typical Mass Spectrum

MolecularIon Peak..M+ peak

Base Peak(Most Intense/stable

peak)

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Mass spectrum of pentane

m/z Fragment

72+

CH3CH2CH2CH2CH3

57 +CH2CH2CH2CH3

43 +CH2CH2CH3

29 +CH2CH3

3222332223 CHCHCHCHCH]CHCHCHCH[CH

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erence e ween mass spec rumof a compound and that of anelement. In the mass spectrum of an element, each line

represents a different isotope of that element.

For a compound, each line represents adifferent fragment produced when themolecular ion breaks up.

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Peak heights and the stability ofions

The most intense peak on the mass spectrumis the most stable ion formed.

The positive fragments formed from organiccompounds are called carbocations

A carbocation is a carbon atom with a +vecharge.

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Carbocations

Primary

Secondary

Tertiary

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The (M+1) peak

A complete (un-simplified) massspectrum, will most certainly have asmall line one (1) m/z unit to the rightof the main molecular ion peak.

This small peak is called the M+1

peak.

The (M+1) peak is caused by thepresence of the 13C isotope in themolecule.

13C is a stable isotope of carbon and

not to be confused with the radioactive14C.

Carbon-13 accounts forapproximately1% of all carbon atoms.

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Some common fragments and their

(m/z ratio)

Difference in mass of fragmentand mass of molecular ion

Fragment lost

14 .CH2

17.

OH26 .CN

31 .OCH3

35 .Cl

45 .OCH2CH3

m/z Fragment

15 +CH3

29 +CH2CH3 or+CHO

43 +CH2CH2CH3NB or

C-C single bondfragments withm/z ratio of 43depends on the

intensity of thispeak, since at thispoint, thefragment can beisomeric.

C+

CH3

O

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Where are mass spectrometersused? Mass spectrometers are used in industry and academia for both routine and

research purposes. The following list is just a brief summary of some of the majormass spectrometric applications:

Biotechnology: The analysis of proteins, peptides, oligonucleotides

Pharmaceutical: Drug discovery, combinatorial chemistry, pharmacokinetics, drugmetabolism

Clinical: Haemoglobin analysis, drug testing

Environmental: Water quality, food contamination

Geological: Oil composition

Forensic Science:

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Question

Compound Q has the molecular formula C4H7ClOand does not produce misty fumes when added towater.

The mass spectrum of Q contains two molecularion peaks at m/z = 106 and m/z = 108. It also hasa major peak at m/z = 43.

Suggest why there are two molecular ion peaks.

A fragment ion produced from Q has m/z = 43 andcontains atoms of three different elements.

Identify this fragment ion and write an equationshowing its formation from the molecular ion of Q.

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The m/z = 43 should cause three possiblefragments should come to mind.

or or

Can rule out the first two because the questiontold you, the fragment has 3 different atoms.

CH3 CH2

CH2+

CH3 CH+

CH3

CH3 C+

O

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Molecular ion is therefore:

CH C

CH3

CH3

O

Cl

CH C

CH2

CH3

O

CH3

Cl

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Question

A low resolution mass spectrum of2-methybutan-2-ol shows peaks at m/z values of73, 71 and 59. The molecular ion peak is absent.Propose a structural formula for EACH of the

given peaks. m/z = 73

m/z = 71

m/z = 59.

The mass spectrum of pentan-2-ol which isisomeric to 2-methybutan-2-ol shows a molecularion peak. Account for this difference between thetwo isomers.

Give TWO other uses of mass spectrometry as an

analytical tool.

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Answer

2-methybutan-2-ol

m/z = 73 or

m/z = 71

m/z = 59

CH3 C CH2

CH3OH

CH3

CH3 C CH2

+

OH

CH3

C+

CH2

CH3OH

CH3

CH3 C+

CH2

CH3

CH3

CH3 C+

OH

CH3

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Questions????????

The End