Mass spectrometry

Question 1

what is mass spectrometry?

Answer 1

a technique used to measure the relative mass of molecules

Question 2

what does mass spectrometry measure?

Answer 2

the relative mass of molecules

Question 3

can it be applied to both small organic molecules and biomolecules?

Answer 3

yes

Question 4

what is the use of mass spectrometry beneficial for in pharmacy and medicine related areas?

Answer 4

• it helps to verify the identity of a drug substance
• confirms the presence of a particular drug in formulated species
• verifies the presence of drugs and drug metabolites in clinical samples
• identifies unknown drug metabolites

Question 5

in a schematic presentation of a mass spectrum, what % is the most abundant ion given?

Answer 5

100%

Question 6

what is plotted in a schematic presentation of a mass spectrum?

Answer 6

the abundance of each ion (Y axis) against M/Z (x axis)

Question 7

what has to happen to the molecules of the samples before they are identified according to their mass?

Answer 7

they have to be ionised

Question 8

what is a mass spectrometer?

Answer 8

a device used to produce and weigh ions

Question 9

what are the major stages of MS?

Answer 9

sample vaporisation, ion generation, ion seperation, ion detection

Question 10

what is the purpose of sample vaporisation?

Answer 10

to distribute molecular ions within the chamber.

Question 11

what is the most important stage in MS?

Answer 11

ion generation

Question 12

what are the ions seperated according to?

Answer 12

m/z

Question 13

what do the electric plates do in a MS?

Answer 13

they accelerate the ions

Question 14

what is the molecular source in a MS?

Answer 14

vacuum chamber

Question 15

how is sample volatilisation achieved?

Answer 15

using heat, the sample is placed in a vacuum.

using fast bombardment (e.g. irridation of sample by beam of xenon atoms)

Question 16

what does sample volatilisation result in?

Answer 16

results in even distribution of the individual molecules within the vacuum chamber

Question 17

how is sample ionisation achieved?

Answer 17

by bombarding the volatilised molecules with electrons from an electron gun.

Question 18

name the 6 ionisation techniques

Answer 18

• Electron Impact ionisation (EI) (oldest+cheapest method)
• Chemical ionisation (CI)
• Matrix Assisted laser desorption ionisation (MALDI)
• Fast atom bombardment (FAB)
• Atmospheric pressure chemical ionisation (APCI)
• Electrospray ionisation (ESI)

Question 19

what happens to the positive fragments produced after ionisation in EI?

Answer 19

they are accelerated under vacuum through a magnetic field into the deflection chamber

Question 20

how does the electron gun cause ionisation?

Answer 20

the rapidly moving electrons knock an electron out of the molecule

Question 21

how is ionisation in Chemical ionisation achieved?

Answer 21

Ci uses a stream of electrons to ionise a reagent gas (ammonia or methane)
- the ionisation of the gas results in the formation of the strong acid (e.g. NH4+ OR CH5+).
The volatilised analyte molecules are then ionised by the strong acid via protonation
-This results in the generation of ions

Question 22

what are the 2 gasses used for ionisation in CI?

Answer 22

ammonia or methane

Question 23

what are the acids formed due to ionisation of ammonia/ methane in CI?

Answer 23

NH4+ or CH5+

Question 24

are EI and CI gentle techniques?

Answer 24

no, they produce lots of fragments, EI is more harsh than CI, they are cheap and easy

Question 25

What type of molecules is EI and CI most suitable for?

Answer 25

small molecules

Question 26

what are the steps of FAB?

Answer 26

the analyte sample is suspended in a viscous matrix, it is bombarded with a beam of fast moving Xe atoms - Xe atoms transfer their energy to the matrix - intermolecular bonds break and ionisation occurs - desorption (release) of analyte ions into the gas phase

Question 27

why is a matrix used?

Answer 27

to protect sample from fragmentation. | if the matrix was absent, the direct bombardment of the analyte by the fast atoms would lead to extensive fragmentation

Question 28

can FAB be used to generate negatively charged ions?

Answer 28

YES

Question 29

is FAB a gentle techniques?

Answer 29

yes, produces few fragments

Question 30

does FAB require the sample to be volatile?

Answer 30

nah

Question 31

what types of analytes are ESI usually used for?

Answer 31

applied for polar analytes with MW up to 100,000 Da

Question 32

what is the process of ESI?

Answer 32

the analyte is dissolved in a mixture of an organic solvent (AN or MeOH). A pH modifier (e.g. acetic acid) is used to produce ions. ionisation proceeds via protonation or deprotonation (depends on the pH modifier used). the sample is sprayed from a capillary at high voltage, which results in dispersion of the droplets into aerosols so that they're easier to ionise later on. ions evaporate from the surface of the droplets.

Question 33

what is the difference between ESI and APCI?

Answer 33

the gas phase ionisation in APCI is more effective than ESI for analysing less-polar species. eveyrthing else is the same. they are complimentary methods

Question 34

what are the benefits of APCI?

Answer 34

they are good for less polar analytes, excellent

Question 35

what is liquid chromotography MS dependant on?

Answer 35

ionisation and ion vaporisation

Question 36

what ionisation methods are usually used in LC MS?

Answer 36

ESI or APCI

Question 37

where is fragmentation often carried out in MS-MS tandem MS?

Answer 37

mass analysis device. this gives superior fragmentation results and thus better structural information

Question 38

advantages and disadvantages of MS-MS tandem mass spectrometry?

Answer 38

Very expensive, allows you to do fragmentations at different stages

Question 39

how do alkanes undergo fragmentation? and what do they end up forming?

Answer 39

simple alkanes tend to undergo fragmentation by the initial loss of a methyl group to form a (m-15) species.

Question 40

what peaks dominate the mass spectrum?

Answer 40

secondary and tertiary carbocations

Question 41

in branched chains alkanes where does fragmentation occur?

Answer 41

next to the branch site

Question 42

why is a high vacuum applied in ion separation?

Answer 42

applied to minimse the interaction of analyte ions with molecules in the air

Question 43

what bends the pathway of the charged molecules?

Answer 43

an applied magnetic field