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 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

Element Exact Mass

12C 12.00001H 1.0078314N 14.003116O 15.994919F 18.998428Si 27.976931P 30.973832S 31.972135Cl 34.968979Br 78.9183127I 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 56.0124CN2O 12.00+2*14.0031+ 15.9949 56.0011CH2N3 … 56.0249C2O2 55.9898C2H2NO 56.0136C2H4N2 56.0375C3H4O 56.0262C3H6N 56.0501C4H8 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 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 = 61 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

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

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.

+ -

- +

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

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 MSA 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

P = 143

P +H+

P+C3H6+

CH5+

P = 69

P+H+

P+C3H5+

CH5+

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 means movement of 2 electrons;

means movement of one electron

136

93

68

m/e 121: P-CH3

m/e 93: P – C3H7

m/e 68: P- C4H8

C10H16

CH2=CH-CH2-CH3

P+

m/e 41: P –CH3

m/e 114 parent

57

m/e 99: P-CH3

m/e 57: P – C4H9

m/e 92 parent

m/e 91 P - H

m/e 120 parent

m/e 91 P – C2H5

m/e 74 Parent

m/e 59 P – CH3

m/e 45 P – C2H5; CHO

m/e 106 parent

m/e 105 P - H

m/e 77: P – CHO; C2H5

85

58

43

m/e 100 parent

m/e 85: P – CH3

m/e 58: P – C3H6

P- C2H2Om/e 43 P- C4H9

P- C3H5O

Mw 88

73

m/e 60: P – C2H4

P - CO

Loss of neutral molecules is frequently observed

125P-CH3

m/e 83

O

MW 140

m/e 69

P – CH3

m/e 116

m/e 87

m/e 43

m/e 56P-C2H5

O

O

P-C2H4O

P-C4H9OP-C3H5O2

m/e 101