Mass Spectrometery

Question 1

What is mass spectrometery?

Answer 1

• Mass spectrometery is an accurate, instrumental technique used to determine the relative isotopic mass and the relative abundance for each isotope. From this, relative atomic mass can be calculated.

Question 2

What is relative isotopic mass?

Answer 2

• Mass of each atom of isotope, relative to 1/12th mass of one atom of carbon-12.

Question 3

What are the 4 key stages in a time of flight spectrometer?

Answer 3

• Ionisation.
• Acceleration.
• Ion Drift
• Ion detection.

Question 4

What happens in the ionisation stage of time of flight

Answer 4

• Sample X is dissolved in polar solvent and injected into inlet/ needle.
• Tip of inlet/ needle has high voltage and (high voltage applied to sample X, so sample gains a proton- forming XH+. )

Question 5

What type of ionisation will you be asked about in Exam Q’s?

Answer 5

• Electrospray ionisation.

Question 6

Equation for sample gaining proton…..

Answer 6

X(g) + H⁺ —> XH⁺ (g)

Question 7

Why is electrospray ionisation more used than firing electrons at the sample so that they lose one electron and become positively charged?

Answer 7

• Used for large molecules (ensures fragmentation of molecule doesn’t occur.)

Question 8

What happens in the acceleration stage?

Answer 8

• Positively charged ions attracted to negatively charged plate (at end f acceleration area.)
• All ions accelerated to constant kinetic energy.
• Attracted by an electric field so all have same kinetic energy/ speed in the acceleration area AND when entering the flight tube.

Question 9

Why does time of flight mass spectrometery have to occur in a vaccum?

Answer 9

• Prevents air colliding with sample (would perhaps not allow the sample particles through the spectrometer.)

Question 10

What happens at the flight tube/ ion drift stage?

Answer 10

• All ions (of sample) enter flight tube at same time.
• Lighter ions travel faster/ reach detector in less time.
• Flight path: separates ions depending on mass (giving values of mass for different isotopes.)

Question 11

Why do all ions have to enter the flight tube at the same time?

Answer 11

• So that only the mass affects the separation in the flight tube (not the kinetic energy of the ions.)

Question 12

What happens at the detection stage?

Answer 12

• Detector plate is negatively charged (at end of the ion-detector.)
• Positive ions hit detector/ negatively-charged plate: positive ions gain electrons from negatively chared plate.
• Movement/ flow of electrons –> produces electric curent (current is measured.)
• More ions that hit detector, bigger the current.
• Computer converts the current into the abundance (current is proportional to the abundance.)

Question 13

What forms the ions into a beam?

Answer 13

• Ions are forced through narrow holes - this forms a beam of ions.

Question 14

What two things can mass spectrometery be used for?

Answer 14

• Can be used to identify elements.
• Can be used to determine relative molecular/ atomic mass.

Question 15

What is the name of the type of ionisation you will be asked about?

Answer 15

• Electrospray ionisation.

Question 16

What does m/z mean on x axis of spectra?

Answer 16

• Ratio between isotopes’ mass and its charge (essentially the relative isotopic mass of an isotope when the charge is +1. )

Question 17

If a question says: “there is a peak at m/z 113”- (for instance), what does this mean?

Answer 17

• That the isotope with relative isotopic mass (113) was detected by the detector (and a certain value for abundance was given - as seen on the spectra.)

Question 18

EXAMPLE

I have (Br₂)⁺ **molecules **and two isotopes Br-79 and Br-81, where would the 3 peaks be on the mass spectra be and where would each one peak to?

Answer 18

Molecule possibilties:
⁷⁹Br - ⁷⁹Br = 158 –> 25%
⁸¹Br - ⁸¹Br = 162 –> 25%
⁷⁹Br - ⁸¹Br OR ⁸¹Br- ⁷⁹Br –> 50% (two possibilities to chose from.)

Question 19

I have (Br₂)⁺ atoms (not molecules) and two isotopes Br-79 and Br-81, where would the 2 peaks be on the mass spectra and where would each one peak to?

Answer 19

-⁷⁹Br (50%)
- ⁸¹Br (50%)

Question 20

Q.)/ KLP

Explain why it is necessary to ionise samples when measuring their mass in a TOF mass spectrometer

2 main reasons

Answer 20

• Ions, not molecules, will interact with and be accelerated by an electric field (2 key words!)
• Only ions will create a current when hitting the detector’s (negatively-charged plate.) (You missed this point.)

Question 21

What ratio do the isotopes Br-79 and Br-81 exist in?

Answer 21

• 1:1 ratio

Question 22

What ratio do the isotopes Chlorine - 35 and Chlorine-37 exist in?

Answer 22

• 3:1

Question 23

Detrmine the relative abundance for the 3 molecular ion peaks at m/z 70, 72, 74 for chlorine.

Answer 23

• 9:6:1