7 - Modern analytical techniques Ⅰ
Mass spectrometry
● Determines the masses of atoms/(fragments of) molecules
● Mass spectrometres detect positively charged species + presence\distribution of
isotopes
● Stages (VIASD):
1. Vaporisation - substances must be in gas phase
2. Ionisation - high energy electron bombardment, causes sample to lose
electrons (ie form + charge)
e- (fast-moving electron) + X (atom in sample vapour) → X+ (positive ion) + e- (electron knocked out of X) + e- (slower-moving electron)
3. Acceleration - ions accelerated by an electr(ostat)ic field (plates of reducing
voltage)
4. Separation (by deflection) - by a magnetic field/time of flight (within mass
spectrometre)
5. Detection
● Look out for isotopes (of individual elements):
(mass-to-charge ratio; z is normally (+)1)
35
Cl+ (m/z = 35)
37
Cl+ (m/z = 37)
—————————————————————————————————————
35
Cl2+ (m/z = 70) {relative abundance: 75%2 = 56.25%, relative peak height: 9}
35
Cl37Cl+/37Cl35Cl+ (m/z = 72) {relative abundance: (75% x 25%) x 2 = 37.5%, r.p.h.: 6}
37
Cl2+ (m/z = 74) {relative abundance: 25%2 = 6.25%, relative peak height: 1}
● ***Higher m/z peak = Mr of compound
○ Other peaks - fragmentation of molecule
1
Mass spectrometry
● Determines the masses of atoms/(fragments of) molecules
● Mass spectrometres detect positively charged species + presence\distribution of
isotopes
● Stages (VIASD):
1. Vaporisation - substances must be in gas phase
2. Ionisation - high energy electron bombardment, causes sample to lose
electrons (ie form + charge)
e- (fast-moving electron) + X (atom in sample vapour) → X+ (positive ion) + e- (electron knocked out of X) + e- (slower-moving electron)
3. Acceleration - ions accelerated by an electr(ostat)ic field (plates of reducing
voltage)
4. Separation (by deflection) - by a magnetic field/time of flight (within mass
spectrometre)
5. Detection
● Look out for isotopes (of individual elements):
(mass-to-charge ratio; z is normally (+)1)
35
Cl+ (m/z = 35)
37
Cl+ (m/z = 37)
—————————————————————————————————————
35
Cl2+ (m/z = 70) {relative abundance: 75%2 = 56.25%, relative peak height: 9}
35
Cl37Cl+/37Cl35Cl+ (m/z = 72) {relative abundance: (75% x 25%) x 2 = 37.5%, r.p.h.: 6}
37
Cl2+ (m/z = 74) {relative abundance: 25%2 = 6.25%, relative peak height: 1}
● ***Higher m/z peak = Mr of compound
○ Other peaks - fragmentation of molecule
1
