Isotope in ms (OAN MUHAMMAD SAHITO)
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Transcript of Isotope in ms (OAN MUHAMMAD SAHITO)
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OUTLINES
INTRODUCTION BASIS OF SEPARARION IONIZER, ANALYZER AND DETECTOR TYPES ISOTOPES AND OBSERVATION OF ISOTOPES CLASSIFICATION OF ISOTOPES SPECTRAS ISOTOPIC ABUNDANCE CONCLUSION REFERENCES ACKNOWLEDGEMENTS
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It subjects vaporized molecules to bombardment by a stream of high-energy electrons, converting these
molecules to ions
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INTRODUCTION
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These ions are then accelerated in an electric fieldThe accelerated ions are then separated according to their mass-to-charge ratio in a magnetic or electric
field
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BASIS OF SEPARATION
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INSTRUMENTAL DIAGRAM
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IonizerSample introduction/ionization method:
Ionizationmethod
Typical Analytes
Sample Introduction
Mass Range
Method Highlights
Electron Impact (EI)
Relatively small volatile
GC or liquid/solid probe
to1,000
Daltons
Hard method versatile provides structure
info
Chemical Ionization (CI)
Relatively smallvolatile
GC orliquid/solid
probe
to1,000
Daltons
Soft method molecular ion peak
[M+H]+
Electrospray (ESI)PeptidesProteins
non-volatile
LiquidChromatography
or syringe
to200,000 Daltons
Soft method ions often multiply charged
Fast Atom Bombardment (FAB)
Carbohydrates Organometallics
Peptides nonvolatile
Sample mixed in viscous
matrix
to6,000
Daltons
Soft method but harder than ESI or
MALDI
Matrix Assisted Laser Desorption(MALDI)
Peptides Proteins
Nucleotides
Sample mixed in solid matrix
to 500,000 Daltons
Soft method very high
mass
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Ion Analyzer
Analyzer System Highlights
QuadrupoleUnit mass resolution, fast scan, low cost
Sector (Magnetic and/or Electrostatic) High resolution, exact mass
Time-of-Flight (TOF)Theoretically, no limitation for m/z maximum, high throughput
Ion Cyclotron Resonance (ICR)Very high resolution, exact mass, perform ion chemistry
Tandem Mass Spectrometry (MS/MS) Molecular structure determination
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Detector, Vacuum
DetectorConvert the beam of ions in an electrical signal that can be processed, stored, displayed and recorded in many ways.
VacuumSystem
MS require the high vacuum is maintained in all spectrometer components (except signal processing)
Electron Multiplier (most commonly used)
Faraday cupPhotographic platesScintillation type
Other:
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Mass spectrometers analyze gas-phase ions, not
neutral moleculesNeutrals don’t respond to electric and
magnetic fields If a molecule cannot ionize, MS cannot help
MS is not a “magic bullet” techniqueMS can describe atomic composition of an ionConnectivity of the atoms is much more
challenging
Although MS requires a vacuum, it cannot be performed in a vacuum of informationDeriving useful information from MS data
often requires some knowledge of the system under investigation
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IMPORTANT POINTS TO REMEMBER
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Francis William Aston
"For his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his
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1922 Nobel Prize
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Isotopes can be classified asMono-isotopeDi-isotopePoly-isotope
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Isotopic Classification of the Elements
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Monoisotopic Elements.
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Among 82 naturally occurring stable elements, 20 elements do exist in the form of only one single naturally occurring stable isotope.
Among the monoisotopic elements,Fluorine (19F), Sodium (23Na), Phosphorus (31P), and Iodine (127I)belong to the more prominent examples in organic mass spectrometry.
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Methyl Floride
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Nonetheless,there are many more such as9Be, 27Al, 55Mn, 59Co, 75As, 93Nb, 103Rh, 133Cs, and 197Au
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The monoisotopic elements are also referred to as
M, A or X elements.If radioactive isotopes were also taken into account, not a single monoisotopic element
would remain.
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Di-isotopic Elements
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Several elements exist naturally in two isotopes.These elements can even be sub-classified into those havingONE isotope that is 1 u heavier than the most abundant isotope. The first group has been termed M+1 elements.
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Prominent examples of M+1 elements areHydrogen (1H, 2H = D),Carbon (12C, 13C),andNitrogen (14N, 15N).
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Those having one isotope that is 2 u heavier than the most abundant isotope
They have been termed M+2 elements, respectively
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Elements such as chlorine, bromine, oxygen, sulfur, and silicon, can be dealt with as X+2 elements
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Silicon
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Among the X+2 elements,Chlorine (35Cl, 37Cl),Bromine (79Br, 81Br) are relatively common but Copper (63Cu, 65Cu),Gallium (69Ga, 71Ga),Silver (107Ag, 109Ag),Indium (113In, 115In),and antimony (121Sb, 123Sb) also belong to this group.
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84-49=35 49-14=35
A+2 peak with fragment of 35 indicates Cl presence.
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Chlorine
35Cl (P= .75)37Cl (P= .25) Ratio 3:1
Probability : [M+] / [M + 2] = 0.75 / 0.25 = 3 / 1
100 : 33
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Methyl Bromide: An example of A+2 isotopes
The ratio of peaks containing 79Br and its isotope 81Br (100/98) confirms the presence of bromine in the compound.
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A-1 Peak
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If we do not restrict our view to the elements, one should add the class of A–1 elements with one minor isotope of 1 u lower mass than the most abundant one
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The elementsLithium (6Li, 7Li),Boron (10B, 11B),and Vanadium (50V, 51V)come along with a lighter isotope of lower abundance than the heavier one and thus, they can be grouped together as X–1 elements.
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BORON
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Poly-isotopic Elements
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The majority of elements are grouped as poly-isotopic elements because they consist
of three or more isotopes showing a wide variety of isotopic distributions.
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Tungsten
Index MS-fragmentation04/12/2023 35
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Isotopic abundances are listed either as their sum being 100% or with the abundance of the most abundant isotope normalized to 100%. The custom of reporting mass spectra normalized to the base peak.
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Representation of Isotopic Abundance
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Isotopic abundances for Carbon containing compounds
Relative ratio: [M+1]+ / [M+ ] = n(0.989)n-1 (0.011) / (0.989)n
= n (0.011) / (0.989)
= n (0.0111)
In percentage: n x 1.1 %
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Isotopic abundances for other common nuclei
For 15N: [M+1]+ / [M]+ => n x 0.36%
For 33S: [M+1]+ / [M]+ => n x 0.80%
For 18O: [M+2]+ / [M]+ => n x 0.20%
For 34S: [M+2]+ / [M]+ => n x 4.42%
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Have seen that for Cl and Br, having two common
isotopes, two radical cation peaks produced. What about other elements having more than one isotope?
We know what the isotopes are and their natural occurrence.
For the M+1 peak, one atom must be using an isotope heavier by one.
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Molecular Peaks, M+1
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We will use isotopic occurrence data for H, C, O for the M + 1 peak.
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Technique to obtain molecular formula using intensities of M, M+1, M+2 peaks.
Consider the M+1 peak, nominal mass + 1. If we know the formula we should be able to calculate the relative intensity of that peak due to the contributions from each of the atoms present. Here are the major contributors to M+1.
Example. Given the data.
Peak Intensity
150 (M) 100
151 (M+1)
10.2
152 (M+2)
0.88Looking at M+2 there is no Br, Cl or S. There could be oxygen.Even mass for M means there could only be even number of Nitrogen
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Technique to obtain molecular formula using intensities of M, M+1, M+2 peaks.
Example. Given the data.
Peak Intensity
150 (M) 100
151 (M+1)
10.2
152 (M+2)
0.88EquationsM+1: (1.11% x # of C) + (0.38 x # of N+ small contributions from OM+2: (0.20 x # of O) + (1.1 x # of C)2/200
We can have 0 or 2 nitrogens. Even number.
We can have 0,1,2,3,4 oxygens. 0.88/0.2 < 5Can have 0,1,2,3,4,5,6,7,8,9 carbons. 10.2/1.11 <10
Find molecular formulas having reasonable M+1 peaks
M+1 M+2
C7H10N4 9.25 0.38
C8H10N2O 9.61 0.61
C9H10O2 9.96 0.84
C9H14N2 10.71
0.52
Examine reasonable formulae. Calculate M+1, M+2 peaks
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Example. Identify this molecule
m/e Abundance
1 <0.1
16 1.0
17 21
18 100
19 0.15
20 0.22Due to heavier isotopes
Molecular radical ionEjection of an H
H2O
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Example 2
m/e Abundance
12 3.3
13 4.3
14 4.4
15 0.07
16 1.7
28 31
29303132
100891.30.21
Heavier isotopes
parent
H ejection
Oxygen
carbon
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The “exact mass” feature in ChemDraw will give
you a monoisotopic mass Not always correct for complex isotope patterns
Two freeware apps are available from MSF website “Links” page These can be used to predict the entire isotopic pattern
as an exportable image
MS-Search program on GC-MS computer can be used to retrieve mass spectra from NIST’02 library
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Some useful software tools
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Mass spectrometry is very useful technique for identification of ISOTOPES, through different ionizaion sources for different molecules
It is observed through study that isotope of some elements are 1unit heavier while others are 2unit heavier than most abundant isotope hence shown as M+1 and M+2 peaks respectively in Mass Spectra.
Isotopes 1unit lighter observed as M-1 peak in Mass Spectra. Observation of poly-isotopes as M+3 and so on.
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REFERENCES
1. MASS SPECTROMETRY BY JURGEN H GROSS, SPRINGER INTERNATIONAL EDITION
2. NIST WEB BOOK http://webbook.nist.gov3. LECTURE SLIDES OF DR SHAHABUDDIN MEMON4. WIKIPEDIA5. SPECTROMETRIC IDENTIFICATION OF OGANIC COMPOUNDS
BY ROBERT M.SILVERSTEIN 7th EDITION 20056. http//www.chemistry.ccsu/glagovich/teaching/316/ms/7. GOOGLE IMAGES FORM www.google.com
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