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Mass Spectrometry
sample
ion
plot intensity (I) vs. mass-to-charge ratio (m/e)
mass spectrometer ––– ion source, analyzer, detector
ion source
M
[M‧]+
neutral and positively
molecular ion
charged fragments
volatile samples – vaporization
nonvolatile or thermally fragile samples –––
desorption : condensed phase
ion
characteristic: give high concentration of the
molecular ion
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common ion sources
name (abbreviation)
electron ionization (EI)
field ionization (FI)
chemical ionization (CI)
fast atom bombardment
(FAB)
field desorption (FD)
laser desorption (LD)
plasma desorption (PD)
type
gas phase
gas phase
gas phase
ionizing agent
energetic electrons
high-potential electrode
reagent positive ion or
electron capture
desorption* highly energetic neutral atoms
desorption† high-potential electrode
(10-8 – 10-9 V/m)
desorption† laser beam
desorption† high-energy fission fragment
from 252Cf
desorption† 1~20 keV ions
secondary ion mass
spectrometry (SIMS)
thermal desorption (TIMS) desorption† heat
* sample as solids or solutions
† sample as solids, gases or solutions
electron impact (EI) M + eM+˙ + 2eM + neMn+˙ + (n+1)efast atom bombardment (FAB)
charged Ar or Xe atom
higher chance of observing the parent ion
matrix – glycerol
chemical ionization (CI)
CH5+ NH4+
AH+ + M
A + MH+
m/e M + 1 M + 17 or M + 18 ([M+AH]+)
thermospray mass spectrometer
injection of an electrolyte solution through a
heated capillary into a vacuumevaporation of
solvent leave single positive and negative ion
even dication can be detected
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e.g. [Me3N(CH2) 3NMe3]2+
analyzer
electrostatic
cause the ions to move in circle
magnetic
radii depend on m/e ratio of the ions
kinetic energy
eV = 1/2mv2
curved trajectory Bev = mv2/r
m
B2r2
==>
―― = ―――
e
2V
spectrum
plot of ion current vs. m/e ratio
majority ― single positive charge ion integral
mass values
small number of doubly, triply charged ions
molecular ion ― loss of an electron from the
sample molecule, has the same mass as
the parent molecule
e.g. manganese carbonyl m/e = 390
===> Mn2(CO)10
molecular ion
(i) not necessary the strongest peak
(ii) may not be visible
(iii) can be increased by reducing the energy of
the electron beam
(iv) with ionization methods other than electron
impact (particular laser desorption), the
molecular ions are much prominent
==> measuring molecular weight
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(v) with FAB ionization, sample is mixed with a
mulling agent, usually glycerol positive ions
are formed by protonation and appear with a
mass of (M.W. + 1) anions are formed with
mass of (M.W. –1)
(vi) determining the mass of an ion to the nearest
integer may not be good enough
ex. an iron carbonyl complex with a molecular ion at
504 amu possible formula – Fe(CO)16, Fe2(CO)14,
Fe3(CO)12, Fe4(CO)10, Fe5(CO)8
with high resolution instruments the atomic
masses are not exact integers, it is possible to
determine the masses of ions within a few ppm the
exact masses for
Fe3(CO)12 503.7438 amu and
Fe4(CO)10 503.6889 amu
isotope abundance pattern
many elements have more than one isotopes, and if
such elements are present in a compound ==>
there will be not just one molecular ion, but a whole
series of the pattern can be distinctive and diagnostic
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isotopic abundances of natural occurring isotopes
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ex.
PFBr2
ReBr
ReBr2
fragmentation
excess energy
(i) excited state
molecular ion ――――→
(ii) new ion + neutral part
the pattern of breakdown ==>
measure parameters such as bond dissociation
energy
e.g. triatomic molecule ABC
mass spectrum include [ABC]+, [AB]+, [BC]+,
[A]+, [B]+, [C]+
==> miss [AB]+ ==> the compound is A–B–C
e.g. Re2Cl2(CO)8
a series of peaks corresponding to the molecular
ion and ions showing successive loss of all 8 CO
groups
==> Re2Cl2 hold together strongly
the structure is
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e.g. part of mass spectrum of Re(CO)5Br
e.g. thio-ether
ion [SnMe3O]+ is found in the mass spectrum
==> the structure should be
not
e.g. B2H6
e.g. TiCl4 + NaCp ―→ product
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ion reactions
it is possible to obtained direct information about
ion reactions by observing the peaks associated
with metastable ions
metastable ions – the ions have such short lifetimes
that they dissociate while moving through the
spectrometer one ion (of mass m1) is accelerated
after the initial ionization, but different ion (of
mass m2) passes through the magnetic analyzer.
the resulting peak comes at m* in the spectrum
m22
m* = ——
m1
the ions are formed during 10-5 seconds or so that
they spend between the electrostatic and magnetic
analyzers
==> they give quite broad spectral peaks
ex. 4 normal ions and 1 peak arising from a
metastable ion
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ex. the mass spectrum of P(OPF2)3
5 weak peaks attributed to metastable ions
the peak at 143.3 amu was attributed to
[P(OPF2)3]+ (286 amu) and [P(OPF2)2]+ (201 amu)
thermodynamic data
ion [AB]+ decomposes to give [A]+ and B, the
appearance potential of [A]+ is the sum of the
ionization potential (IP) of A and the bond
dissociation energy (BDE) of AB
if the IP of A is known, BDE of AB may be derived
BDE
IP
AB ―――→ A∙+ B∙――→ A+ + B∙
appearance potential of [A]+
= IP of [A]+ + BDE of [AB]
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ex. dissociation of complex [Fe(h5-C5H5)(CO)L(MX3)]
[MX]+ appearance MX3 IP
potential (eV)
(eV)
[Fe(C5H5)(CO)2(SiMe3)]
9.22
7.25
[Fe(C5H5)(CO)2(SnMe3)]
9.12
6.81
[Fe(C5H5)(CO)2(SnPh3)]
8.87
6.29
[Fe(C5H5)(CO)(PPh3)(SiMe3)
9.48
7.25
ex.
Fe-M BDE
(eV) (kJ/mol)
1.97 190
2.31 223
2.58 249
2.23 215
D
ClCH2PH2
CH2=PH + HCl
C + H2 + P
CH2=PH
D H fo = ?
IP of CH2=PH : 10.3  0.2 eV
AP of [CH2=PH]+ from ClCH2PH2: 11.0  0.2 eV
DHfo (HCl) = -92 kJ/mol
DHfo (ClCH2PH2) = -44  4 kJ/mol
standard heat of atomization:
H, 218; C, 717; P, 315 kJ/mol
standard single bond energy:
C—H, 413; P—H, 321 kJ/mol
estimate C=P bond energy
ClCH2PH2 [CH2=PH]+ + e- + HCl
DH = 11.0  0.2 eV (1061  20 kJ/mol)
1061 = DHfo([CH2=PH]+) + DHfo(HCl) - DHfo (ClCH2PH2)
= DHfo ([CH2=PH]+) + (-92) - (-44)
DHfo ([CH2=PH]+) = 1061 + 92 – 44 = 1109  20 kJ/mol
(10.3  0.2 eV)
DHfo (CH2=PH) = 1109 – 994 = 115  30 kJ/mol
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C + 3/2 H2 + 1/4 P4
CH2=PH
(1 C=P + 2 C—H + 1 P—H)
C+3H+P
(218 x 3 + 717 + 315) – (2 x 413 + 321) – (BDE of C=P)
= DHfo = 115
BD of C=P
= -115 + (218 x 3 + 717 + 315) – (2 x 413 + 321)
= 424 kJ/mol
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mass-analyzed ion kinetic energy (MIKES)
example
―→
―→ C6F5PCl+ + C6F5S
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tandem mass spectrometry
multistage mass spectrometry (MS/MS)
two major applications
(i) it is a very powerful analytical tool for
mixtures, working on picogram quantities, it
is possible to distinguish between isomers
(ii) it is a means of studying the
(iii) decomposition of ion
example MS/MS spectra for [(Me3P)2BH]+
(a) with 11B and
(b) with 10B
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