4.2.4 Analytical technique

Definitions

Term	Definition
Fragmentation	The process in mass spectrometry that causes a positive ion to split into smaller pieces, one of which is a positive fragment ion
Fragment ions	Ions formed from the breakdown of the molecular ion in a mass spectrometer

Bonds vibrate at a particular frequency
- Stretch: moving along the line between atoms so the distance between them changes
- Bend: results in change in bond angle
Bonds only absorb radiation with the same frequency as the natural frequency of the bond
- The frequency of the light depends on bond strength, bond length and atomic masses at both ends of the bond
- Most bonds absorb at a frequency of 300 - 4000 \(cm^{-1}\), i.e. IR radiation
Absorbing IR radiation causes covalent bonds to absorb energy and vibrate more

Most of the Sun's radiation is short wave and is relatively unaffected by atmospheric gases
They pass through the atmosphere to the Earth's surface and some is reflected as long wave radiation
\(C=O\), \(O-H\) and \(C-H\) bonds absorb radiation in the IR range which causes bond in gas molecules to vibrate
- e.g. \(CO_2\), \(H_2O\) and \(CH_4\) molecules
The vibrating bonds eventually re-emit the energy as radiation that increases the temperature of the atmosphere close to the Earth's surface
This creates incentives to reduce \(CO_2\) emission to reduce global warming

Determine the functional groups present
Sample placed in IR spectrometer
IR radiation beams with wavenumber 200-4000 \(cm^{-1}\) is passed through the sample
Molecules absorb some IR + emerging beam is analysed to identify frequencies absorbed
IR spectroscopy is usually connected to a computer that plots a graph of transmittance against wavenumber
The computer uses the fingerprint region to identify the compound
- Fingerprint region: region below 1500 \(cm^{-1}\) with unique peaks to identify particular molecule
All organic compounds produce a peak 2850-3100 from \(C-H\) bond
Look at other peaks to identify other bonds present

Remote sensors analyse IR spectra of vehicle emissions to detect pollutants
IR-based breathalysers pass beams of IR through breathed out gas + detect IR absorbance
- Detecting \(C-O\) bonds in alcohol molecules
- \(O-H\) bond is present in water vapour breathed out so it is not used
- Blood test taken if the result suggests that the person is too drunk to drive safely

\(Mr\) = the m/z value of the rightmost peak
There might be a small peak after M+ peak called the M+1 peak due to the presence of carbon-13 isotope
Other peaks are due to fragment ions

m/z value	Ion (remember to include the + charge)
15	\(CH_3^+\)
29	\(CH_3CH_2^+\)
31	\(CH_2OH^+\)
41	\(C_3H_5^+, C_2H_3N^+\)
43	\(CH_3CH_2CH_2^+ / CH_3CO / C_3H_7\)
45	\(CH_3CH_2O^+\)
49	\(CH_3CH_2CH_2CH_2^+\)