infrared spectroscopy and mass spectrometry

How are is the percentage abundance of an isotope found

Experimentally using a mass spectrometer

How does a mass spectrometer work

1.sample is vaporised and then ionised to form positive ions

2. The ions are accelerated. Heavier ions move slower and are harder to deflect than lighter ions so the ions of each isotope are separated

3. The ions are detected in a mass spectrum as a mass to charge ratio. Each ion reaching the detector adds to the signal so the greater the abundance the larger the signal

What is a molecular ion

The positive ion formed in mass spectrometry when a molecule loses an electron.

What is a fragment ion

Ions formed from the breakdown of the molecular ion in the mass spectrometer

What is the M+ peak and how is it used

Used to find the molecular mass
It is the clear peak at the highest m/z value on the furthest to the right of the mass spectrum

What is the M + 1 peak

Very small peak after the M+ peak and it exists because 1.1% of carbon is present at the carbon - 13 isotope

How do you write a fragment ion

• you remove the thing that is lost (don't write the thing that is lost)
• add a plus to show it is a positive ion

What is the functional group for an aldehyde

CHO

What is the functional group of a ketone

-C(CO)C-

What is the functional group of an ester

coo

What is the functional group of actors chloride

-COCl

What is the functional group of nitrates

-CN

What happens when a covalent bond absorbs IR

The energy from the IR radiation causes the bond to vibrate more intensely.
The vibration involved stretching and bending

What does stretching and bending involve

Stretching - bond length increasing and decreasing rhythmically
Bending - where the bond angle changes

When does absorption of IR occur

Only occurs if the frequency of the IR matches the natural frequency of vibration of the bond, as a result the bond moves to a higher vibrational state

What are the two factors effecting the natural frequency of the bond

Mass of the atoms involved - heavier atoms vibrate more slowly leading to lower natural frequencies
Strength of the bond - stronger bonds like double bonds vibrate at higher frequencies

What are the three most abundant greenhouse gases

H2O, CO2, methane

Why are H2O, CO2 and CH4 classified as greenhouse gases

They all absorb IR very effectively so they are able to absorb the IR emitted by the earths surface

What happens when the concentration of greenhouse gases increase

-more IR emmited from the earths surface is absorbs by the atmosphere rather than disappearing into space
-the absorbed energy is re emoted in all directions, including back towards the earths surface causing the lower atmosphere and surface to warm up

• greenhouse effect leads to an overall increase in global temperatures

What does a peak look like

A dip on the graph

What are the applications of IR spectrometry

1. Identifying pollutants
2. IR based breathalysers

How do you answer one of the questions when it asks you to identify an unknown compound using mass spectrum and IR spectrum

1. Calculate the empirical formula for the %compostion
2. Use the mass spectrum to get the Mr from the M+ peak and compare with EF to deduce Mr
   3.use the IR to identify bonds referring to the data sheet
3. Use fragment peak from mass spectrum to indenify groups of atoms within the molecule