OCR A Level Chemistry Unit 2

Composite ions + their charges

OH-
NO2-
NO3-
CO32-
SO42-
Cr2O72-
PO43-
NH4+ (ammonium)

What is an isotope?

Atoms of the same element with different numbers of neutrons + different masses

How does a mass spectrometer work?

1. Vaporisation - the sample is converted into a gas

2. Ionisation - the gas is bombarded with high energy electrons, knocking an electron off the molecules, forming positive ions

3. Positive ions are accelerated using an electric field + are all given the same kinetic energy

4. Deflection - ions pass through a magnetic field, heavier ions deflect less, whilst lighter ions deflect more depending on the m/z ratio

What are 3 functions of a mass spectrometer?

1. Determine relative formula mass by the M+ ion

2. Identify elements/isotopes present → isotopic patterns e.g. 3:1 ratio for Cl35 and Cl37, helps to identify elements

3. Identify unknown compounds (structural info) → fragment peaks provide clues about how the molecule breaks apart, indicating its structure

What is M+? Equation to show its formation?

The molecular ion which has the highest m/z ratio

What is M+1? Why does it exist?

Caused by molecules containing one heavier isotope (e.g. C13 over C12), giving a slightly higher m/z peak than the molecular ion peak.
It exists because 1.1% of carbon is present as the carbon-13 isotope - so a small proportion of the molecules of a hydrocarbon will contain an atom of C13 - higher molecular mass

What is fragmentation?

When the molecular ion breaks down into smaller pieces, producing smaller peaks on a mass spectrum known as fragment ions

What does the simplest fragmentation produce? Example equation?

Positively charged ion (detected by mass spec)+ a radical (uncharged + not detected)

What is the base peak?

The tallest peak aka the most abundant ion

What are some common fragment ions + their values?

CH3+ = 15

C2H5+ = 29

C3H7+ = 43
C4H9+ = 57

What is relative atomic mass + its formula?

Ar = (relative isotopic mass of isotope 1 x percent abundance of isotope 1) + (relative isotopic mass of isotope 2 x percent abundance of isotope 2)/100