Week 6

Question 1

what is the basic principle of mas spectrometry ? What kind of result de we get?

Answer 1

generate ions from inorganic or organic compounds, separate them by their m/z ratio -> qualitative and quantitative detection

Question 2

what is the isotopic mass? what are the mass nb and the atomic nb ?

Answer 2

exact mass of an isotope, very close to the nominal mass of the isotope (carbon 12 is th exception)

mass nb = protons + neutrons
atomic nb = nb of protons = nb of electrons

Question 3

give formulas for absolute and relative mass accuracy

Answer 3

absolute: m/z_exp - m/z_theoretical

relative: absolute/(m/z_theoretical)-> given in ppm

Question 4

what is the resolution? how is it calculated ?

Answer 4

smallest difference in m/z that can be separated for a given signal.
R = m / delta_m = m/z / delta_m/z

the delta_m is measured halfway up a peak

Question 5

what are the 3 main components of mass spectrometers?

Answer 5

1) ion source
2) mass analyzer
3) detector

Question 6

what are the two different ways of generating ions ? (explain)

Answer 6

MALDI: mix the proteins with a matrix, shoot laser, matrix will explode and transfer protons to proteins.
- the matrix is a proton donor, has an aromatic ring to absorb energy, and is hydrophilic for good interaction.
- downside: difficult to automate

ESI: narrow capillary under electrical potential, at the end creates proteins with multiple charges on them
- can measure heavier species (resolve charge state of multiply-charged ions with high resolution)
- can be automated and combined with chromatography

Question 7

give a list of mass analysers (without explaining yet)

Answer 7

• time of flight
• ion trap
• quadrupole
• fourier transform ion cyclotron resonance (FT-ICR)
• orbitrap

Question 8

how does time of flight work ?

Answer 8

potential difference transferred into kinetic energy for the masses -> the smallest mass travels the fastest

m/z = 2eU*t^2/s^2

Question 9

how does the ion trap work? how is the resolution ?

Answer 9

switching between positive and negative polarity of electric field to trap the ions -> by controlling the period of the switching, we can let some ions escape and detect them : lighter to heavier

Resolution is worse than time of flight

Question 10

what are quadrupoles ? two advantages and one disadvantage

Answer 10

only a specific m/z ratio will have a stable trajectory -> it is an ion filter. Low voltages recquired. High scan speed.

Resolution is not very high, can’t resolve charge state of a protein.

Question 11

How does the FT-ICR work ?

Answer 11

cyclotron movement of the ions under a magnetic field -> we record the frequency of the rotation of the ion -> this is the inverse of the m/z ratio

extremely high resolution, but huge machines because high M fields are needed

Question 12

how does the orbitrap work ?

Answer 12

same principle as FT-ICR but electric field instead. Very high resolution as well.

w = sqrt(1/ m/z)

Question 13

what is tandem mass spectrometry ?

Answer 13

ions go through first quadrupole, they are then fragmented using collisio gas in a second quadrupole, then the fragments go through a third quadrupole and are detected.

With the fragments, we can reconstruct the original molecule

Question 14

how do you estimate the intensity of the 13C peak?

Answer 14

multiply the number of carbon atoms by 1.1% -> gives you the relative abundance of the 13C peak in %

Question 15

multiply charged ions: how is the m/z scale compressed?

Answer 15

divided by the value of the charge -> reduces the distance between the isotopic peaks