Please enable JavaScript.
Coggle requires JavaScript to display documents.
Module 4 - Chapter 17 - Spectroscopy - Coggle Diagram
Module 4 - Chapter 17 - Spectroscopy
Mass spectrometry
An organic compound placed in the mass spectrometer loses an electron and forms a positive ion (molecular ion)
Mass spectrometer detects the mass-charge ratio of the molecular ion and gives the molecular mass of the compound
Molecular mass
Molecular ion peak is the clear peak at the highest m/z value on the right hand side of the mass spectrum
You'll usually see a very small peak one unit after the M+ peak, this is referred to as the M+1 peak and its because some carbon-13 isotope exists
Fragmentation
Some molecular ions break into smaller pieces
Causes other peaks in the mass spectrum
Simplest fragmentation breaks a molecular ion into two species - a positively charged fragment ion and a radical
Positive ions are detected by the mass spectrometer, but the uncarged radicals aren't detected
Identify organic molecules
Molecules will all fragment in slightly depending on their structures
Two molecules with the same molecular mass and peak may fragment differently
Process
1) Ionisation - sample is vapourised where a high voltage is apssed over it, electron shot at sample knowcks off an electron from the atom
2) Acceleration - Positively charged ions are accelerated to a negatively charged detection plate
3) Ion drift - Ions are deflected by a magnetic field
4) Detection - when positive ions hit the detection plate, they gain an electron, and a flow of charged is produced. Greater charge means greater abundance
5) Analysis - current values are used with flight times to produce a spectra print out
Infrared spectroscopy
Covalent bonds
Covalent bonds possess energy and vibrate about a central point
Increased temperature increases vibration
Bonds can absorb infrared radiation, which makes them bend/ stretch
Vibrations
Stretch
Rhythmic movement along the line between the atoms
Distance between atom centres increases/ decreases
Bend
Change in bond angle
Amount of bend
Mass of atoms - heavier atoms vibrate slower
Strength of bond - stronger bonds vibrate faster than weaker bond
Any particualr bond can only absorb radiation that has the same frequency as the natural frequency of the bond
Because frequency values are very high, wavenumber is used instead, is proportional to frequency
Atmostpheric gases
Most of the sun's visibile and IR radiaiton is unaffected by atmostpheric gases, and passes through the atmostphere
Some is e-emitted from the Earth's surface in the form of longer wavelegth IR radiaiton
Greenhouse gases (water vapour/ CO2/ CH4) absorb longer wavelength IR radiation,as it has the same frequency as the natural frequency of their bonds
Vibrating bonds in these molecules re-emit this energy as radiaiton that increases the temperature of the atmostphere
Organic molecules
Process
Sample under investigations is placed inside an IR spectrometer
Beam of UR radiaiton in the range 200-4000 cm^-1 is passed through the sample
Molecules absorbs some of the IR frequencies, and the emerging beam of radiaiton is analysed to identify absorbedfrequencies
IR spectrometer is usually connected to a computer that plots a graph of transmittance against wavenumber
Dips in graph are called peaks, each peak is a wavenumber than can be related to a particular bond
There are many peaks in the fingerprint region, these can be used to identify the particulr molecule
Peaks are clearer outisde the fingerprint region
Applications of infrared spectroscopy
Pollutants can be identified by their IR spectral fingerprints
Sensors analyse the IR spectra to detect and measure CO, CO2 and hydrocarbons
IR-based breathalysers pass a beam of IR radiation through the breath and detect the IR absorbance of compounds
More IR radiaiton absorbed, higher the reading, more ethanol in the breath
Determining structures
Elemental analysis - use of percentage compostion data to determine the empirical formula
Mass spectrometry - use of molecular ion peak from mass spectrum to determin the molecular mas (fragment ions identify sections)
Infrared spectroscopy - use of absorbtion peaks from an IR spectrum to identify bonds and functional groups
