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year 2 med sem 1 > Mass spectrometry > Flashcards

Mass spectrometry Flashcards

1
Q

what is mass spectrometry?

A

a technique used to measure the relative mass of molecules

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2
Q

what does mass spectrometry measure?

A

the relative mass of molecules

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3
Q

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

A

yes

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4
Q

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

A
  • 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
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5
Q

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

A

100%

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6
Q

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

A

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

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7
Q

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

A

they have to be ionised

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8
Q

what is a mass spectrometer?

A

a device used to produce and weigh ions

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9
Q

what are the major stages of MS?

A

sample vaporisation, ion generation, ion seperation, ion detection

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10
Q

what is the purpose of sample vaporisation?

A

to distribute molecular ions within the chamber.

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11
Q

what is the most important stage in MS?

A

ion generation

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12
Q

what are the ions seperated according to?

A

m/z

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13
Q

what do the electric plates do in a MS?

A

they accelerate the ions

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14
Q

what is the molecular source in a MS?

A

vacuum chamber

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15
Q

how is sample volatilisation achieved?

A

using heat, the sample is placed in a vacuum.

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

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16
Q

what does sample volatilisation result in?

A

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

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17
Q

how is sample ionisation achieved?

A

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

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18
Q

name the 6 ionisation techniques

A
  • 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)
19
Q

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

A

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

20
Q

how does the electron gun cause ionisation?

A

the rapidly moving electrons knock an electron out of the molecule

21
Q

how is ionisation in Chemical ionisation achieved?

A

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

22
Q

what are the 2 gasses used for ionisation in CI?

A

ammonia or methane

23
Q

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

A

NH4+ or CH5+

24
Q

are EI and CI gentle techniques?

A

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

25
Q

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

A

small molecules

26
Q

what are the steps of FAB?

A

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
27
Q

why is a matrix used?

A

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

28
Q

can FAB be used to generate negatively charged ions?

A

YES

29
Q

is FAB a gentle techniques?

A

yes, produces few fragments

30
Q

does FAB require the sample to be volatile?

A

nah

31
Q

what types of analytes are ESI usually used for?

A

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

32
Q

what is the process of ESI?

A

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.

33
Q

what is the difference between ESI and APCI?

A

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

34
Q

what are the benefits of APCI?

A

they are good for less polar analytes, excellent

35
Q

what is liquid chromotography MS dependant on?

A

ionisation and ion vaporisation

36
Q

what ionisation methods are usually used in LC MS?

A

ESI or APCI

37
Q

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

A

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

38
Q

advantages and disadvantages of MS-MS tandem mass spectrometry?

A

Very expensive, allows you to do fragmentations at different stages

39
Q

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

A

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

40
Q

what peaks dominate the mass spectrum?

A

secondary and tertiary carbocations

41
Q

in branched chains alkanes where does fragmentation occur?

A

next to the branch site

42
Q

why is a high vacuum applied in ion separation?

A

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

43
Q

what bends the pathway of the charged molecules?

A

an applied magnetic field

year 2 med sem 1 (3 decks)

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