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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 -24 g Avogadro’s Number (AN): 6.0254 * 10 23

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slide2

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?

slide3

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

slide5

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 X

C4H8 - 2CH4 = C2O2 ;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

slide6

Element Exact Mass

12C 12.0000

1H 1.00783

14N 14.0031

16O 15.9949

19F 18.9984

28Si 27.9769

31P 30.9738

32S 31.9721

35Cl 34.9689

79Br 78.9183

127I 126.9045

slide7

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 56.0124

CN2O 12.00+2*14.0031+ 15.9949 56.0011

CH2N3 … 56.0249

C2O2 55.9898

C2H2NO 56.0136

C2H4N2 56.0375

C3H4O 56.0262

C3H6N 56.0501

C4H8 56.0626

slide9

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)91 n =0

1 1 n = 1

1 2 1 n = 2

1 3 3 1 n = 3

1 4 6 4 1 n = 4

1 5 10 10 5 1 n = 5

1 6 15 20 15 6 1 n = 6

1 7 21 35 35 n = 7

1 8 28 56 56 n = 8

1 9 36 84 n = 9

(12C)9 + 9(12C)8(13C) +36(12C)7(13C)2

All 12C 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

slide10

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

slide11

What about

other elements?

slide17

+ -

- +

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 axiallybetween 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.

slide18

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.

slide19

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.

slide20

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.

slide25

CH5+

P = 143

P +H+

P+C3H6+

slide26

CH5+

P = 69

P+H+

P+C3H5+

slide27

P = 390

Different energetics associated with different ionization methods

slide28

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

slide29

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.

slide31

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

slide32

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.
  • Electrons in C-C bonds have lower ionization energies than C-H bonds.
  • Electrons in  bonds are easier to lose than sigma bonds.
  • Non-bonded electrons on heteroatoms are lost the easiest.
  • Conventions used in mass spectrometry means movement of 2 electrons;
  •  means movement of one electron
slide33

m/e 121: P-CH3

m/e 93: P – C3H7

68

m/e 68: P- C4H8

93

C10H16

136

slide34

CH2=CH-CH2-CH3

m/e 41: P –CH3

P+

slide35

57

m/e 57: P – C4H9

m/e 114 parent

m/e 99: P-CH3

slide36

m/e 91 P - H

m/e 92 parent

slide37

m/e 91 P – C2H5

m/e 120 parent

slide38

m/e 45 P – C2H5; CHO

m/e 74 Parent

m/e 59 P – CH3

slide39

m/e 77: P – CHO; C2H5

m/e 106 parent

m/e 105 P - H

slide40

43

m/e 58: P – C3H6

P-C2H2O

m/e 43 P- C4H9

P- C3H5O

58

m/e 100 parent

m/e 85: P – CH3

85

slide41

Mw 88

m/e 60: P – C2H4

P - CO

73

slide43

m/e 83

m/e 69

MW 140

125

P-CH3

slide47

m/e 43

P-C4H9O

P-C3H5O2

m/e 87

P-C2H4O

m/e 116

P-C2H5

m/e 56

P – CH3

m/e 101