Relative Mass and the Mass Spectrometer

Question 1

Define relative atomic mass, Ar

Answer 1

The relative atomic mass, Ar, is the average mass of an atom of an element on a scale where an atom of carbon-12 is 12

Question 2

Define relative isotopic mass

Answer 2

The relative isotopic mass is the mass of an atom of an isotope of an element on a scale where carbon-12 is 12

Question 3

Define relative molecular mass

Answer 3

Relative molecular mass, Mr, is the average mass of a molecule on a scale where an atom of carbon-12 is 12

Question 4

What can a mass spectrometer tell you?

Answer 4

It can tell you the relative atomic mass, relative molecular mass and relative isotopic abundance

Question 5

What happens in a mass spectrometer?

Answer 5

There are 4 things that happen in a time of flight (TOF) mass spectrometer:

1. Ionisation
2. Acceleration
3. Ion Drift
4. Detection

Question 6

Explain ionisation

Answer 6

There are two ways of ionising your sample:

1. Electrospray ionisation - the sample is dissolved and pushed through a small nozzle at high pressure. A high voltage is applied to it, causing each particle to gain a H+ ion. The sample is turned into a gas made up of positive ions
2. Electron impact ionisation - the sample is vapourised and an ‘electron gun’ is used to fire high energy electrons at it. This knocks of one electron off each particle, so they become +1 ions

Question 7

Explain acceleration

Answer 7

The positively charged ions are accelerated by an electric field so that they all have the same kinetic energy. This means the lighter ions will end up moving faster than the heavier ions

Question 8

Explain ion drift

Answer 8

The ions enter a region with no electric field,so they just drift through it. Lighter ions will drift through faster than heavier ions

Question 9

Explain detection

Answer 9

Because lighter ions travel faster at higher speeds in the drift region, they reach the detector in less time than heavier ions. The detector detects charged particle and a mass spectrum is produced

Question 10

What does a graph of a mass spectrum look like?

Answer 10

A mass spectrum is mass/charge plotted against abundance. The y-axis gives the abundance of ions, often as a percentage. The x-axis units are given as a mass/charge ratio (shortened to m/z)

Question 11

How do you calculate the relative atomic mass, Ar, from the mass spectrum?

Answer 11

1. For each peak, read the % relative isotopic abundance from the y-axis and the relative isotopic mass from the x-axis. Multiple them together to get the total mass for each isotope
2. Add up these totals
3. Divide by 100 (as percentages were used)

Question 12

What do you do if percentages aren’t used as relative abundance?

Answer 12

Do steps 1 and 2, then divide by the sum of the relative abundances

Question 13

How can mass spectrometry be used to identify molecules?

Answer 13

A molecular ion is formed in the mass spectrometer when one electron is removed from the molecule. This gives a peak in the spectrum with a m/z ratio equal to the relative molecular mass of the molecule. This can be used to identify an unknown compound

Question 14

What happens during electrospray ionisation?

Answer 14

Electrospray ionisation - the sample is dissolved and pushed through a small nozzle at high pressure. A high voltage is applied to it, causing each particle to gain a H+ ion. The sample is turned into a gas made up of positive ions

Question 15

What happens during electron impact ionisation?

Answer 15

Electron impact ionisation - the sample is vapourised and an ‘electron gun’ is used to fire high energy electrons at it. This knocks of one electron off each particle, so they become +1 ions