Mass Spectrometry

Question 1

What is the definition of relative atomic mass (Ar)?

Answer 1

The average mass of an atom of an element compared to 1/12 of the mass of an atom of Carbon-12

Question 2

What is the definition of relative molecular mass (Mr)?

Answer 2

The average mass of a molecule compared to 1/12 of the mass of an atom of Carbon-12

Question 3

What is the definition of relative isotopic mass?

Answer 3

The mass of an isotope of an element compared to 1/12 of the mass of an atom of Carbon-12

Question 4

What are the 4 steps of ToF MS?

Answer 4

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

Question 5

What happens in electrospray ionisation?

Answer 5

The sample is dissolved and pushed through a small nozzle at a high pressure. A very high voltage is applied and the sample gains a H+ ion. The sample becomes a gas made up of positive ions.

Question 6

What happens in electron impact ionisation?

Answer 6

The sample is vaporised and an electron gun fires high energy electrons at it. This knocks of one electron from each particle and they become 1+ ions.

Question 7

What happens during acceleration in ToF MS?

Answer 7

The positively charged ions are accelerated by an electron field so they all have the same kinetic energy.

Question 8

What happens during ion drift in ToF MS?

Answer 8

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

Question 9

What happens during detection in ToF MS?

Answer 9

The ions which travel the fastest (the lightest ions) will reach the detector before the slowest (heaviest) ions do. The detector detects charged particles and creates a mass spectrum.

Question 10

How does the detector work in ToF MS?

Answer 10

An electrical current is produced in the detector when a charged particle hits it.

Question 11

What is plotted on a mass spectrum?

Answer 11

Mass/charge (m/z) against abundance

Question 12

What is displayed on the y-axis of a mass spectrum?

Answer 12

The abundance of ions, either as a percentage or a relative number

Question 13

What does the height of each peak on a mass spectrum of an element show?

Answer 13

The relative isotopic abundance

Question 14

What does each line represent on a mass spectrum of an element?

Answer 14

A different isotope of the element

Question 15

What does the x-axis of a mass spectrum display?

Answer 15

The mass/charge (m/z)

Question 16

How do you calculate the Ar from a mass spectrum?

Answer 16

(∑(m/z x abundance))/(∑(abundance))