Mass Spec Cape Workshop 2011(1)
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Transcript of Mass Spec Cape Workshop 2011(1)
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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