Spectroscopy and Chromatography
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What does the mass spectrometer detect?
The mass to charge ratio of the molecular ion, which gives the molecular mass of the compound.
How are molecular ions formed in mass spectrometry?
Electron is lost to form a positive ion
How is molecular mass found from a mass spectrum?
The molecular ion peak (M+ peak) has to be located.
What is the cause of the M + 1 peak?
Some carbon is present as the carbon-13 isotope.
What is fragmentation?
The molecular ions break down into smaller pieces called fragments.
What causes the other peaks in a mass spectrum?
Fragment ions
How can a mass spectrum differentiate between structural isomers?
They will break apart differently and thus will have different mass spectra.
What is the effect on a bond when it absorbs IR?
Bend or stretch more as they gain energy.
What does the amount a bond streches or bends depend on?
Mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms Bond strength - Stronger bonds vibrate faster than weaker bonds
What is unique about each bond with regards to IR absorption?
Can only absorb radiation that has the same frequency as the natural frequency of the bond.
What scale is used for the frequency values on ir spectroscopy?
Wavenumber 200/cm to 4000/cm
How does infrared spectroscopy work?
Sample placed inside an IR spectrometer, A beam of IR radiation in range 200/cm to 4000/cm passed through the sample Molecule absorbs some of the IR frequencies and the emerging beam is analysed to find the absorbed frequencies.
What is the term given to the dips in the graph?
Peaks
What do peaks help us predict?
The bond types in the molecules.
What is below 1500/cm known as on the IR spectrum and why?
Fingerprint region The peaks in this region are unique to each molecule.
In an exam question what can be used to determine the molecule in question?
IR absorptions on formula sheet
What is the typical sequence for identification of an unknown substance?
Elemental analysis - determine empirical formula Mass spectrometry - Determine molecular mass and identify sections in molecule from fragment ions Infrared spectroscopy - identify bonds and functional groups present Identity of unknown compound can be worked out now.
What are the two phases for all forms of chromatography?
Stationary phase Mobile phase
What is the stationary phase?
Does not movewhere a solid or a liquid supported on a solid
What is the mobile phase?
Does move its Normally a liquid or a gas
What is a TLC plate made of?
a TLC plate is made of a Plastic/glass sheet Coated with thin layer of a solid adsorbent substance such as silica
Outline adsorption in reference to TLC
Process by which the solid silica holds the different substances in the mixture to its surface
How is the retention factor calculated?
Rf= Distance moved by the component/distance moved by the solvent front
What is gas chromatography useful for?
Separating and identifying volatile organic compounds present in a mixture