Analytical Techniques Flashcards
(10 cards)
How do you use a mass spectra to determine the relative atomic mass of an element?
(% abundance 1 x m/z)+ ( % abundance 2 x m/z) / 100
How do you use a mass spectra to identify the molecular mass of a molecule?
The molecular ion (M+) produces peak furthest to the right in mass spectra.
Other peaks in spectra are a result of fragmentation.
always write the molecules as ions as they are ionised (lose electron) when placed in mass spectra (e.g. ethanol C2H5OH+)
E.g. The mass spectra has a M+ peak at m/z= 46 so the mr of molecule X is 46.
How does fragmentation occur in a mass spectra and how would you present it in a question?
Excess energy from ionisation process can be transferred to molecular ion, causing it to vibrate, making the bonds weaken and split into pieces.
Fragmentation results in positive fragment ion and a neutral species.
E.g. possible fragmentation ethanol is : [C2H5OH]+ —> [CH2OH]+ + [CH3]+ You would see peaks at m/z 31 and 15.
Always write as + ions.
On alcohols, there would be a m/z peak at 17 due to the OH+ ion.
What does an infra red spectroscopy show and what do all molecules absorb?
All molecules absorb infrared-red radiation which makes bonds vibrate causing them to stretch or bend.
An infrared red spectroscopy tells us what functional groups are present.
What are the uses of infra-red spectroscopy in everyday life?
Pollutants can be identified by their IR fingerprint.
Air pollution (e.g. CO2, CO, NO) from vehicle exhausts can be detected and measured by remote IR sensor.
Used in breathalyser to monitor breath of drunk drivers.
How would you identify a functional group using an infra-red spectroscopy?
Use the data sheet in the sheet given.
Ignore peaks below 1500cm-1 as don’t have to analyse.
Most organic compounds produce peak at around 3000cm-1 due to absorption by C-H bond but don’t need to anylse just make sure not to mix it up with an O-H bond.
How would you answer a question if there was a key peak at 3200-3600cm-1?
The IR spectra shows a key peak between 3200-3600cm-1 indicating an O-H group of an alcohol is present. This tells us that compound X is an alcohol.
How would you answer a question if there was a key peak at 1630-1820cm-1?
The IR spectra shows a key peak between 1630-1820cm-1 indicating a C=O bond is present from a carboxyl. This tells us that compound X is either a ketone or an aldehyde.
How would you answer a question if there was a key peak at 2500-3300cm-1(which is very broad) and at 1630-1820cm-1?
The IR spectra shows 2 key peaks- The IR spectra shows a key peak between 1630-1820cm-1 indicating a C=O bond is present. Peak between 2500-3300cm-1 very broad indicating the O-H of a carboxylic acid. This tells us that compound X is a carboxylic acid.
What does analysis of a mass spectra provide info about?
- number of naturally occurring isotopes present in an element.
- the relative abundance of each isotope in sample.
