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Components of Mass Spectrometer
Sample inlet : bring the sample to the lower pressure of MS.
Ion source : the sample molecules are transformed into gas phase ion
which are accelerated by the electromagnetic field.
Mass analyzer : sepatate the sample ions based on their m/z ratio.
Detector : count the ions.
Data system : the signal is recorded by PC (mass spectrum)
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Sample Introduction
Gas sample : use of a simple inlet system (larger reservoir)
Less-volatile materials : the system can be designed to fit within an oven
which can heat the sample to increase the vapor pressure
Nonvolatile samples : use of direct probe method. The sample is placed
on a thin wire loop or pin on the tip of the probe, which is
then inserted through a vacuum lock into the chamber and
positioned close to the ion source.
Gas chromatography – mass spectrometry (GC-MS)
High-performance liquid chromatography – mass spectrometry (LC-MS)
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Electron Ionization (EI)
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Mass Analysis
1. The magnetic sector mass analyzer
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The greater the value of m/z, the larger the radius of the
curved path. The analyzer tube of the instrument is
constructed to have a fixed radius of curvature. Thus,
The magnetic field strength is continuously varied.
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2. Double-focusing mass analyzers
The particles leaving the ionization chamber do not all
have precisely the same velocity, so the beam of ions
passes through an electric field region before or after
the magnetic sector. In the presence of an electric field,
the particles all travel at the same velocity.
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3. Quadrupole mass analyzers
A set of four solid rods is arranged parallel to the direction
of the ion beam. A direct-current (DC) votage and a RF is
applied to the rods, generating an oscillating electrostatic
field in the region between the rods. Ions of the correct m/z
ratio undergo a stable oscillation of constant amplitude.
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Detection and Quantitation : The Mass Spectrum
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Base peak : The most abundant ion peak
Molecular ion (M+) : largest m/z value
Isotope peak also appear (M+1, M+2)
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Metastable ion peak
Ions with lifetimes on the order of 10-6 may disintegrate into
fragments while they are passing into the analyzer region of
the mass spectrometer. The fragment ion does not follow
an abnormal flight path on its way to the detector. This ion
appears at an m/z ratio that depends on its own mass as
well as the mass of the original ion from which it formed.
M1 is the mass of the original ion and m2 is the mass of new one
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Determination of Molecular Weight
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Determination of Molecular Weight
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Determination of Molecular Weight
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Determination of Molecular Formulas
1. Precise mass determination
High-resolution mass spectrometer can be used to determine the
precise molecular weights of substances with four or five decimal
places of an ion’s m/z value and also allows one to know the
exact molecular formula.
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2. Isotope ratio data
Another method of determining molecular formulas is to examine
the relative intensities of the peaks
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Example (mass 42) C3H6 and CH2N2
For propene
M+1 peak : (3 x 1.08) + (6 x 0.016) = 3.34%
M+2 peak : 0.05%
For diazomethane
M+1 peak : 1.08 + (2 x 0.016) + (2 x 0.36) = 1.87%
M+2 peak : 0.006%
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3. Calculation of halogen isotope
For Cl2 :
35Cl x 35Cl = 1 x 1 = 1
M
M+2 (35Cl x 37Cl) + (37Cl x 35Cl) = (1 x 0.33) + (0.33 x 1) = 0.66
M+4 37Cl x 37Cl = 0.33 x 0.33 = 0.11
For Br2 :
79Br x 79Br = 1 x 1 = 1
M
M+2 (79Br x 81Br) + (81Br x 79Br) = (1 x 0.98) + (0.98 x 1) =1.96
M+4 81Br x 81Br = 0.98 x 0.98 = 0.96
For BrCl :
79Br x 35Cl = 1 x 1 = 1
M
M+2 (79Br x 37Cl) + (81Br x 35Cl) = (1 x 0.33) + (0.98 x 1) =1.31
M+4 81Br x 37Cl = 0.98 x 0.33 = 0.32
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Structural Analysis and Fragmentation Patterns
1. Stevenson’s rule
The most probable fragmentation is the one that leaves
the positive charge on the fragment with the lowest
ionization energy. Fragmentation processes that lead
to the formation of more stable ions are favored over
processes that lead to less-stable ions.
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Alkanes
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Alkenes
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2. The initial ionization event
It is easier to remove an electron from a nonbonding orbital n than
it is to strip an electron from a p orbital. Similarly, it is much easier
to eject an electron from a p orbital in comparison to a s orbital.
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3. Radical-site-initiated cleavage : a-Cleavage
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4. Charge-site-initiated cleavage : Inductive cleavage
Attraction of an electron pair by an electronegative heteroatom that
ends up as a closed-shell neutral molecule.
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5. Two-bond cleavage
Elimination occurs and the odd-electron molecular ion yields an OE+
and an even-electron neutral fragment N : H2O, HX, alkene
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6. Retro Diels-Alder cleavage
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7. McLafferty rearrangements
A hydrogen atom on a carbon 3 atom away from the radical cation of
an alkene, arene, carbonyl, or imine is transferred to the charge site
via a six-membered transition state, with concurrent cleavage of the
sigma bond between the a and b position of the tether.
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Aromatic Hydrocarbons
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