Lecture 11 - Mass Spectrometry

Question 1

what does mass spectrometry NOT contain?

Answer 1

contains NO electromagnetic waves but instead ELECTRONS

Question 2

what is mass spectrometry used to determine?

Answer 2

used to determine molecular masses and some structural information (albeit normally limited)

Question 3

what instrument is used for mass spectrometry?

Answer 3

a mass spectrometer

Question 4

downsides to mass spectrometry:

Answer 4

unlike other techniques, mass spectrometry destroys the sample - you don’t get it back

Question 5

upsides to mass spectrometry:

Answer 5

very small amounts of a sample are sufficient (mg - pg quantities will suffice)

Question 6

how is a sample prepared for mass spectrometry:

Answer 6

• the sample is vapourised in a vacuum and subjected to an electron beam of high energy, the energy of the beam is typically ~70eV (6700 kJ/mol) - this easily exceeds that of chemical bonds
• a radical cation is produced

Question 7

fragmentation reaction:

Answer 7

in a fragmentation reaction the radical cation decomposes and literally falls apart (these reactions are more common the larger the molecule is)

Question 8

mass spectrometry can only detect…

Answer 8

charged species

Question 9

each of the fragments are seperated according to their:

Answer 9

mass-to-charge ratio (m/z)

Question 10

what is the spectrum called when ions are produced by electron impact?

Answer 10

EI mass spectrum

Question 11

molecular ion (M):

Answer 11

the ion derived from electron ejection only (no fragmentation): M•+

Question 12

base peak:

Answer 12

the ion of greatest relative abundance in the spectrum

Question 13

why are there many peaks in mass spectrometers when you are testing a pure sample?

Answer 13

this is due to varying isotopes and fragmentation reactions

Question 14

reasoning behind M+1 peak:

Answer 14

M+1 peak is due to the carbon-13 isotope

Question 15

how can the relative abundance of the M+1 peak be calculated?

Answer 15

relative abundance = (no. of carbons) x (natural abundance of 13C / natural abundance of 12C)

relative abundance = (number of carbons x (0.0110)

Question 16

probability a carbon will be 13C isotope:

Answer 16

1.1% chance

Question 17

why are there M+2 peaks (additional ones) for Br & Cl?

Answer 17

this is because of it being a molecular ion therefore it can contain non/one/two isotopes

Question 18

two fragmentation reactions that can occur:

Answer 18

•M•+ → radical + cation

•M•+ → neutral molecule + radical-cation

Question 19

Only the fragment that is an ___ will be detected by the spectrometer

Answer 19

ion

Question 20

other fragmentation pathways:

Answer 20

example M•+ (even m/z) → neutral molecule + radical-cation (even m/z) by intramolecular H atom abstraction

Question 21

Chemical-Ionization Mass Spectra Technique:

Answer 21

more useful than electron impact as electron impact frequently fails to show the M•+ peak

chemical ionisation (CI) is a “softer” technique that ionises the sample with gas-phase protons (a protic environment)

commonly a sample is analysed by both electron impact and chemical ionisation spectrometry

Question 22

how do you write out a fragmentation reaction:

Answer 22

look up the different species

see if its a radical cation or just a cation

draw relevant fragments

Question 23

what happens (simply) to your sample when ionised using chemical ionisation:

Answer 23

a proton is added which results in a singular positive charge and an additional mass unit

Question 24

what does mass spectrometers sort ions by?

Answer 24

mass spectrometers sort ions by mass

Question 25

nominal mass:

Answer 25

the mass of a compound to the nearest whole number

Question 26

exact mass:

Answer 26

the mass of a compound to four or more decimal places