Chemical Ideas 6.5

Mass spectrometry

What’s inside?In the

nucleus?

ProtonsAnd around the nucleus?

ElectronsNeutrons

How are they

different?Mass

Charge

Location

Mass on Ar scale

Charge

p 1 +1

n 1 0

e- 0.00055 -1

Nuclear symbols

Na23

11

Mass number =?

Atomic number =?

p + n

p ( = e-)p?

e-?

n?

•11

•11

•12

What are isotopes?

Cl35

17Cl

37

17

p?

e-?

n?

p?

e-?

n?

17

17

18 20

17

17

•Atoms of the same element with:

•The same atomic number but different mass numbers

•Same number of protons but different numbers of neutrons

Why is 35Cl O.K.?

Cl35

17Cl

37

17

Relative isotopic mass

Cl-35 Cl-37

Chlorine always has an atomic number of 17!

So what is relative atomic mass?

Cl-35

Cl-37

75%

25%

Relative atomic mass is the average of the relative isotopic

masses

Cl-35

Cl-37

75%

25%

How do we get a relative atomic mass of 35.5?

What 2 things did we need to know?

Relative isotopic masses.

The relative abundances.

How do we find them out?

Use a mass spectromete

r

Mass spectrometry

sample inlet

vacuum chamber

Mg

e-

e-

Mg+

sample inlet

vacuum chamber

electric field

sample inlet

vacuum chamber

electric field

flight path

time measurement

Ion detector

intensity

m/z

KE = ½ mv2

• Ion 1

• KE = 16

• m = 2

• v = 4

• Ion 2

• KE = 16

• m = 8

• v = 2

•Greater the mass, the less the velocity

•Greater the mass, greater the time of flight

•From time of flight, mass is calculated

sample inlet

vacuum chamber

electric field

flight path

time measurement

Ion detector

intensity

m/z

A mass spectrum …

35 3736

20

40

60

80

100

%

m/z

Relative atomic mass is the average of the relative isotopic

masses

Cl-35

Cl-37

75%

25%

How do we get a relative atomic mass of 35.5?

CH3COCH3e-

e-

e-CH3COCH3+CH3CO+ CH3

Other fragments from propanone - CH3COCH3?

CH3COCH3+

CH3CO+

•CH3+

•CH3CO

•O

•O+

•CH3C

•CH3C+

CH3

Which give a peak on a mass spectrum?

Which is the molecular ion?

CH3

Name it.

CH3 CH2 CH CH2 CH3O

2-ethoxybutane

This is the mass spectrum for 2-ethoxybutane;

15CH3

15CH3

14CH2

13CH

16O

14CH2

15CH3

Molecular ion ?

Base peak?

3?

2?

1?

+