Time of Flight Mass Spectroscopy

Question 1

One method of ionising samples in time of flight mass spectrometry is by electron impact. Describe how this ionisation is done? [4]

Answer 1

high energy electrons 1
fired at sample 1
(using an electron gun ) 1
this knocks off one electron, (forms 1+ ions) 1

Question 2

A method of ionising samples in time of flight mass spectrometry is by electrospray ionisation. Describe how this ionisation is done? [4]

Answer 2

sample is dissolved in volatile solvent 1
sample (injected) through fine hypodermic needle, producing (fine mist) 1
tip of needle has a high voltage / positive charge 1
so each atom gains a proton 1

Question 3

Which method of ionisation is most likely to lead to the fragmentation of ions? [1]

Answer 3

Electrospray impact 1

Question 4

Define the term relative atomic mass? [1]

Answer 4

average mass of an atom
_________________________

1 / 12 mass of a carbon-12 atom 1

Question 5

The 1+ ions are accelerated using a negatively charged electric plate.
Why is a negatively charged plate used? [2]

Why an electric field is used to accelerate ions? [1]

Answer 5

to attract the positive ions to negative plate / opposite charges will attract 2

so they have the same kinetic energy 1

Question 6

Ionisation by electron impact causes removal of one electron from each particle. Write an equation for the ionisation of each of the following species by loss of one electron.

i) Ca ii) C2H6

Answer 6

i) Ca (g) ——–> Ca+ (g) + e- 1

ii) C2H6 (g) ——–> C2H6+ (g) + e- 1

Question 7

Electrospray ionisation creates an ion by protonation of a molecule. Write an equation for the formation of a positive ion by electrospray ionisation of lactic acid (C3H6O2)? [1]

Answer 7

C3H6O2+ H+ ——–> C3H7O2+ 1

ignore state symbols

Question 8

Explain why different ions take different times to travel through flight tube? [2]

Answer 8

difference in Mr / lighter ions will travel faster due to lower Mr OR converse for heavier ions 1

time of flight depends on mass of ions 1

Question 9

A sample of bromine is being analysed using mass spectroscopy
Deduce which ion will reach the detector first and suggest why? [2] 79 Br +
81 Br +

Define the term ‘isotope? [1]

Answer 9

79 Br + 1
it has a lower Mr / is a lighter ion so will travel faster 1

same number of protons, different number of neutrons 1

Question 10

the mass spectrum of gallium shows peaks at m/z 69 and 71 with a relative abundance of 61.4 % and 38.6 %

Deduce what isotopes of Ga are present? [1]

Answer 10

69 Ga , 71 Ga 1

both isotopes required for 1 mark

Question 11

the mass spectrum of gallium shows peaks at m/z 69 and 71 with a relative abundance of 61.4 % and 38.6 %

Calculate a value for the relative atomic mass of Ga using the data? [2}

Answer 11

(69 x 61.4) + (71 x 38.6)
_____________________ = 69.7 ( correct answer
scores 2 marks)
61.4 + 38.6 OR (100)

Question 12

the mass spectrum of rubidium contains the isotopes 72.2% 85 Rb , and 27.8% of an unknown isotope
Ar value = 85.5

Calculate an m/z value for the unknown isotope? [3]

Answer 12

85.5 = (85 x 72.2) +  27.8x 
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_           1 
                  100
8550 = 6137 + 27.8x
(8550 - 6137)  2413 = 27.8x          1

(2413 / 27.8) x = 86.8 ( correct answer 3 marks )