1 - Mass number, isotopes and TOF mass spec

Question 1

Which letter is used to represent the atomic (proton) number of an atom?

Answer 1

Z

Question 2

What does the atomic number tell us about an element?

Answer 2

The number of protons in an atom

Question 3

What letter represents mass number?

Answer 3

A

Question 4

How is the mass number calculated?

Answer 4

Mass number = number of protons + number of neutrons

Question 5

Define relative atomic mass (Ar)

Answer 5

The average mass of an atom of an element relative to 1/12 of the mass of a carbon-12 atom

Question 6

Define relative isotopic mass

Answer 6

The mass of an atom of an isotope of an element relative to 1/12 of the mass of a carbon-12 atom

Question 7

What is an isotope?

Answer 7

Atoms of the same element that contain the same number of protons and electrons but a different number of neutrons

- results in a different mass numbers

Question 8

What is an ion?

Answer 8

An ion is an atom with more or less electrons than protons resulting in either a positive or negative charge

Question 9

How is an ion formed?

Answer 9

Ions are formed when an atom loses or gains an electron/s

Question 10

What happens at stage 1 ionisation (electrospray) in TOF mass spectrometry?

Answer 10

• Sample is dissolved in a volatile and polar solvent
• Injected at high pressures through a fine hollow needle
• Tip of the needle has high voltage
• At tip of the needle the sample molecule (X) gains a proton (H⁺) from the solvent, forming XH⁺
• Solvent evaporates into the vacuum, XH⁺ ions move towards a negative plate

Question 11

What is the equation of electrospray ionisation?

Answer 11

X_(g) + H⁺ → XH⁺_(g)

Question 12

When do we use electrospray ionisation?

Answer 12

When analysing large organic molecules

- This method prevents fragmentation

Question 13

What happens at stage 1 ionisation (electron impact) in TOF mass spectrometry?

Answer 13

• Sample is vapourised and injected at low pressures
• A heated coil “electron gun” fires high energy electrons at the sample
• Knocks out an outer electron
• Forms positive ions with different charges

Question 14

What is the equation of electron impact ionisation?

Answer 14

X_(g) → X⁺_(g) + e^-

Question 15

When do we use electron impact ionisation?

Answer 15

When analysing elements and substances with low formula mass

- This method cannot be used for large molecules

Question 16

What happens at stage 2 acceleration in TOF mass spectrometry?

Answer 16

• Positive ions are accelerated by an electric field to a constant kinetic energy (KE) towards a negatively charged plate

Question 17

What happens at stage 3 ion drift in TOF mass spectrometry?

Answer 17

• Ions enter the flight tube (length d)
• Ions with different masses (m) have a different time of flight

Question 18

What happens at stage 4 ion detection in TOF mass spectrometry?

Answer 18

• Detector is a negatively charged plate
• A current is produced when ions hit the plate
• More ions more current

Question 19

What happens at stage 5 data analysis in TOF mass spectrometry?

Answer 19

• Current produced and the flight tube times are taken
• Produces a mass spectrum
• Y axis shows the no of particles (abundance) of each mass that hit detector
• X axis shows mass/charge ratio

Question 20

What two equations are needed for TOF mass spectrometry calculations?

Answer 20

KE = 1/2mv²
KE = kinetic energy / J - joules
m = mass / kg - kilograms
v = velocity (speed) / ms^-1 - metres per second

t = d/v
t = time of flight / s - seconds
d = distance / m - metres
v = velocity (speed) / ms^-1 - metres per second