Chemical Ideas 6.5 Mass spectrometry
What’s inside? In the nucleus? Protons And around the nucleus? Neutrons Electrons
Location How are they different? Mass Charge
Mass on Ar scale Charge p 1 +1 n e- 0.00055 -1
Na Nuclear symbols p + n p ( = e-) 11 p? 12 e-? n? 23 11 Mass number =? Na 23 11 p ( = e-) 11 12 Atomic number =? p? e-? n?
Cl Cl What are isotopes? p? p? e-? e-? n? n? 17 17 17 17 18 20 35 37 e-? e-? 17 17 18 20 n? n? 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.? Cl Cl Cl-37 Cl-35 Relative isotopic mass Chlorine always has an atomic number of 17! Relative isotopic mass Cl Cl 37 35 17 17 Cl-37 Cl-35
So what is relative atomic mass? Cl-35 75% 25% Cl-37
Relative atomic mass is the average of the relative isotopic masses Cl-35 75% Cl-37 25% How do we get a relative atomic mass of 35.5?
Relative isotopic masses. The relative abundances. What 2 things did we need to know? Relative isotopic masses. The relative abundances.
Use a mass spectrometer How do we find them out? Use a mass spectrometer
Mass spectrometry
sample inlet vacuum chamber
Mg+ Mg e- e-
electric field sample inlet vacuum chamber
electric field flight path sample inlet Ion detector vacuum chamber intensity vacuum chamber time measurement m/z
KE = ½ mv2 Ion 1 Ion 2 KE = 16 KE = 16 m = 2 m = 8 v = 4 v = 2 Greater the mass, the less the velocity Greater the mass, greater the time of flight From time of flight, mass is calculated
electric field flight path sample inlet Ion detector vacuum chamber intensity vacuum chamber time measurement m/z
A mass spectrum … 100 80 60 % 40 20 m/z 35 36 37
Relative atomic mass is the average of the relative isotopic masses Cl-35 75% Cl-37 25% How do we get a relative atomic mass of 35.5?
CH3CO+ CH3COCH3+ CH3COCH3 CH3 e- e- e-
Other fragments from propanone - CH3COCH3? Which is the molecular ion? CH3CO+ CH3 CH3+ CH3CO O O+ CH3C CH3C+ Which give a peak on a mass spectrum?
Name it. CH3 CH2 CH O CH2 CH3 CH3 2-ethoxybutane
+ Base peak? 2? Molecular ion ? 3? 1?