chemical ideas 6.5 mass spectrometry what’s inside? in the nucleus? protons and around the...
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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?
+