Topic 7 - Modern Analytical Techniques I Flashcards
What is a mass spectrum?
A bar chart of relative abudnace (y) against m/z (x), used to show the relative abundance of different ions in a sample.
What is used to produce a mass spectrum?
Mass spectrometer.
What is on the y axis of a mass spectrum?
Relative abundance (%)
What is on the x axis of a mass spectrum?
m/z (Mass to charge ratio)
What two things can a mass spectrum be produced for?
- Relative isotopic abundances of one element
* Molecular samples of a compound
What is the molecular ion on a mass spectrum?
- The ion of the entire molecule, with a +1 charge.
* Formed when 1 electron is removed from the molecule.
What is the symbol for the molecular ion on a mass spectrum?
M+ (g)
How do you find the relative molecular mass of a compound from its mass spectrum?
- Find the peak with the highest m/z value (ignoring any small M+1 peaks due to C-13)
- The m/z value is the Mr (assuming the ion has a +1 charge).
What can cause a small peak to the right of the molecular peak on a mass spectrum?
A M+1 peak may be present due to the existence of C-13 isotopes, which increase the m/z by 1.
What is the pattern of fragments on a mass spectrum called?
Fragmentation pattern
What can the fragmentation pattern on a mass spectrum be used for?
Identifying molecules and their structure.
On a mass spectrum, what causes the peaks at smaller m/z values than the molecular peak?
Bombarding electrons make some of the molecular ions to break up into fragments.
On a mass spectrum, when the molecular ion fragments, do both products show up on the mass spectrum?
- No, becauses the ion will break up into an ion and a free radical.
- Only the ion will show up, not the radical.
How might CH3CH2CH3+ fragment?
CH3CH2CH3+ -> CH3CH2• (radical) + CH3+ (ion)
OR
CH3CH2CH3+ -> CH3CH2+ (ion) + •CH3 (radical)
(Only the ions will show up on the mass spectrum)
Explain how you can use a mass spectrum to work out the structure of a molecule.
1) Identify the fragments -> Look at the m/z values and determine what ions could have produced them
2) Piece together the fragments to give a molecule with the correct Mr -> Make sure the m/z values add up to the Mr
Practise using the mass spectrum on pg 100 to work out the structure of the molecule.
Blue box, botom of pg 100 of revision guide.
In mass spectrometry, what fragment is likely to give an m/z of 15?
CH3+
In mass spectrometry, what fragment is likely to give an m/z of 29?
C2H5+
In mass spectrometry, what fragment is likely to give an m/z of 43?
CH3CH2CH2+
OR
CH3CHCH3+
In mass spectrometry, what fragment is likely to give an m/z of 17?
OH+
How can you tell apart two molecules with the same Mr using their mass spectrum?
Look at the different fragments produced -> One might produce a certain fragment that the other doesn’t
Can two molecules have the same mass spectrum?
- No, because each will produce different fragments
* An element’s spectrum is like a fingerprint
Compare the mass spectrum of propanal and propanone.
Propanal will have a peak for a C2H5+ fragment, while propanone won’t.
(See pg 101 of revision guide)
What are the two ways a compound can be analysed to work out what it is?
- Mass spectrometry
* Infrared spectroscopy
How does infrared spectroscopy work?
- A beam of IR radiation is passed through a sample of a chemical
- IR radiation is absorbed by the covalent bonds in their molecules, increaasing their vibrational energy
- Different bonds absorb different frequencies of IR radiation
- When this happens, there is a dip in the % transmittance, so there is a peak on the IR spectrum
What two things determine the frequency of IR radiation that a bond will absorb in IR spectroscopy?
- The atoms the bond is between
* The type of molecule the bond is in (e.g. carboxylic acid)
What are the two types of bond vibration in IR spectroscopy?
- Stretching
* Bending
Do you need to memorise the IR spectroscopy absorption values?
No, they’re given in the exam.
What is on the x and y axis of an IR spectrum?
- x -> Wavenumber (cm^-1)
* y -> % Transmittance
What is wavenumber on the x-axis of an IR spectrum?
It is just another measure of frequency (1/wavelength).
What is used to produce an IR spectrum?
Infrared spectrometer
What can an IR spectrum be used to do?
Identify the functional groups in a molecule.
What do the peaks on an IR spectrum show?
Where IR radiation is being absorbed.
NOTE: The peaks on IR spectra are upside down.
What is the only broad peak on an IR spectrum?
The OH peak -> It is smooth and wide.
What part of an IR spectrum should you look at?
Above 1500cm^-1.
What is the fingerprint region of an IR spectrum and what can it be used for?
- The complex region below 1500cm^-1 which contains many singl bond stretching and bending vibrations.
- It is unique to each molecule -> Can be used to identify the molecule.
When looking at an IR spectrum and you are unsure about a certain peak, what is it a good idea to do?
Look instead for other peaks that might be missing.
How can computers be used along with IR spectra to identify an compound?
The fingerprint region of an IR spectrum can be compared with a database of fingerprint regions to identify the compound.
Apart from identifying compounds, what can IR spectroscopy be used for?
Checking if a reaction has happene, by looking at the functional groups present.
How can mass spectra and IR spectra be used together?
• Mass spectrum -> Used to find Mr and possible fragments
• IR spectrum -> Used to find functional groups present
Together, the two can be used to identify a compound exactly
Remember to revise interpreting mass spectra and IR spectra.
Pgs 100-103 + Find some examples online
