Transcript Metastable Ions

Mass Spectrometry
Exact Mass Measurements
What is exact mass?
m
mass of a proton:
1.672623 * 10-24 g
mass of a neutron:
1.674927 * 10-24 g
mass of a deuteron:
3.3427 * 10-24g
Avogadro’s Number (AN):
6.0254 * 1023
Molar mass of 2D = AN* mD = 6.0254*1023* 3.3427 *10-24 g
= 2.0141 g mol-1
Carbon = 6 2D = 6*2.0141 = 12.0846;
Carbon = 6(P +N) = 6(1.672623+1.674927)*0.60254 =12.1022
Mass of carbon = 12.0 Why the discrepancy?
E = m C2
Where E is the energy given off from a mass
discrepancy of m and C is the speed of light.
E = 0.0846 g* (3*1010 cm sec-1)2
Suppose you determined the exact mass of an ion by mass
spectrometry to be 56.0377. Nominal mass 56
Using the rule of 13, the hydrocarbon formula is C4H8
Other possible molecular formulas are:
C4H8 - CH4 = C3H4O
C4H8 - CH2 = C3H6N
C4H8 - 2CH4 = C2O2 ;
X
C4H8 - 2CH4 = C2S ;
C4H8 - 2CH2 = C2H4N2
C4H8 - CH4, CH2 = C2H2NO
X
C4H8 - 3CH2 = CH2N3
X
C4H8 - C = C3H20
X
C4H8 - CH4, 2CH2 = CN2O
C4H8 - 4CH2 = N4
Element Exact Mass
12C
1H
14N
16O
19F
28Si
31P
32S
35Cl
79Br
127I
12.0000
1.00783
14.0031
15.9949
18.9984
27.9769
30.9738
31.9721
34.9689
78.9183
126.9045
The exact mass of an ion by mass spectrometry was
determined to be 56.0377 amu
Nominal mass 56
exact mass
N4
4*14.0031
CN2O
12.00+2*14.0031+ 15.9949
CH2N3
…
C2O2
C2H2NO
C2H4N2
C3H4O
C3H6N
C4H8
56.0124
56.0011
56.0249
55.9898
56.0136
56.0375
56.0262
56.0501
56.0626
What is the origin of the peak at 141; called the P+1 peak
For a molecular formula of C9H16O, what’s the probability of
having 1 13C?
Probability is (X+Y)n where X and Y is the probability of having
isotope 12C and 13C, respectively and n is the number of C
(12C +13C)9
1
1
1
1
1
1
1
2
3
4
1
3
6
1
4
10
10 5
1
1
6
15 20 15 6
1
1 7
21 35
35
1 8
28
56 56
1 9 36
84
(12C)9
All 12C
5
1
n =0
n=1
n=2
n=3
n=4
n=5
n=6
n=7
n=8
n=9
+ 9(12C)8(13C) +36(12C)7(13C)2
1 13C
2
13C
(0.989)9 = 0.905; 9(0.989)8(0.011)= 0.091; 36(0.989)7(0.011)2 = 0.004
On the basis of the molecule with only 12C = 100
Then (0.989)9 = 100(0.905/0.905) = 100 %
9(0.989)8(0.011)= 100(0.091/0.905) = 10.0 %
36(0.989)7(0.011)2 = 0.004/.905 = 0.45 %
Including 1oxygen: 17O = 0.04
18O
= 0.2
P = 100 %
P+1 = 10.04 %
P+2 = 0.65 %
The contribution of 2H is pretty small
What about
other elements?
Electron impact mass spectrum of CCl4
Single focusing instrument
+
-
-
+
The quadrupole mass spectrometer
consists of four precisely straight and
parallel rods so arranged that the beam
of ions from the ionization source of the
spectrometer is directed axially
between them. A voltage comprising a
a DC component and a radio frequency electric field is applied between adjacent
rods, reinforcing and then overwhelming the DC field. Once inside the
quadrupole, the ions will oscillate normal to the field as a result of the high
frequency electric field. The oscillations are only stable for a certain function of
frequency and the DC voltage; otherwise the ions will strike the rods and become
dissipated. The mass range of the oscillating ions is scanned by changing the
DC voltage and the frequency, keeping the ratio of the DC voltage to the
frequency constant. Typical operating parameters include rf voltages of several
thousand volts, frequencies in the 106 range and DC voltages of several hundred
volts. Unlike a magnetic sector instrument, the mass is linear as the DC and
frequency are scanned.
An ion trap is a combination of electric or magnetic fields that
captures ions in a region of a vacuum system or tube.
A quadrupole ion trap exists in both linear and 3D varieties and
refers to an ion trap that uses constant DC and radiofrequency (RF)
oscillating AC electric fields to trap ions.
The motion of an ion is complex but it is clear that specific
frequencies are involved. The frequencies can be used to manipulate
the ion population in a mass selective fashion.
Time of Flight MS
A variety of ways can be used to create ions. Ions are not suitable for analysis for a
time of flight mass spectrometer unless they are all ejected from the ion source with the
same starting time. This is easy to do with a pulsed laser This results in a group of ions
which can be turned on and off during time “t” rapidly so that it only creates ions only
during time “t”. The ions once formed are accelerated by a negative grid of known
potential. Once accelerated, all ions have the same kinetic energy but different
velocities (1/2mv2). They reach the detector at different times.
Formation of Ions
CH5+
P = 143
P +H+
P+C3H6+
CH5+
P = 69
P+H+
P+C3H5+
P = 390
Different
energetics
associated
with
different
ionization
methods
Single focusing
instrument and
metastable ions
Some ions are relatively unstable and fall
apart shortly after being formed. If they
survive long enough to be accelerated as
m1+ but then fragment shortly in the field
free region to m2+before encountering the
magnetic field, then
Metastable Ions
Metastable ions: accelerated as mp+ but analyzed as md+
where mp+ > md+ , then a peak often broadened as a result of
energy release accompanying decomposition, can be found at:
(md+)2/mp+
The usefulness of metastable is that they permit you to
identify connectivity of fragmentation (i.e. which parent ion
gave rise to which daughter ion)
Metastables are lost in instruments that use a quadruple mass
filter such as in most GCMS instruments.
Metastables observed
at m/e:
136.2 = 1602/188
131.4 = 1452/160
108.9 = 1322/160
103.7 = 1172/132
94.4 = 1172/145
67.7 =
892/117
Fragmentation Patterns in EI MS
Electrons with 70 eV are used to bombard the sample. In addition to a
molecular ion formed by loss of an electron, the resulting ions
frequently have sufficient energy to fragment into daughter ions.
The easiest way to interpret fragmentation patterns is to focus on the
molecular ion formed. The electron with the lowest ionization
potential is lost first. Secondary reactions focus around this center.
1. Electrons in C-C bonds have lower ionization energies than C-H
bonds.
2. Electrons in  bonds are easier to lose than sigma bonds.
3. Non-bonded electrons on heteroatoms are lost the easiest.
Conventions used in mass spectrometry
2 electrons;

means movement of
means movement of one electron
m/e 121: P-CH3
m/e 93: P – C3H7
68
m/e 68: P- C4H8
93
C10H16
136
CH2=CH-CH2-CH3
m/e 41: P –CH3
P+
57
m/e 57: P – C4H9
m/e 114 parent
m/e 99: P-CH3
m/e 91 P - H
m/e 92 parent
m/e 91 P – C2H5
m/e 120 parent
m/e 45 P – C2H5; CHO
m/e 74 Parent
m/e 59 P – CH3
m/e 77: P – CHO;
C2H5
m/e 106 parent
m/e 105 P - H
43
m/e 58: P – C3H6
m/e 43 P- C4H9
P- C3H5O
P- C2H2O
58
m/e 100 parent
m/e 85: P – CH3
85
m/e 60: P – C2H4
Mw 88
P - CO
73
Loss of neutral
molecules is
frequently observed
m/e 83
O
m/e 69
MW 140
125
P-CH3
m/e 43
P-C4H9O
P-C3H5O2
O
O
P-C2H4O
m/e 116
m/e 87
P-C2H5
m/e 56
P – CH3
m/e 101