Mass/ Infrared spec Flashcards
What is mass spectrometery used for?
- Mass spectrometery is used to find the relative molecular mass (Mr) of a compound.
What do all the peaks on a mass spectrometer trace show, what does the last peak/ peak(s) show?
- Peaks show fragments of original molecule.
- Last peak = M+1 peak OR molecular ion peak.
- Molecular ion peak is the same as the relative molecular mass of original molecule.
When is high resolution mass spectrometery useful?
- Useful when identifying different molecules that have the same molecular mass, rounded to the nearest whole no.
What do high resolution mass spectrometers do compared to low resolution mass spectrometers?
- Measure the relative atomic/ molecular mass to several decimal places unlike standard low resolution MS –> may only be able to measure relative mass to nearest whole no.
What does IR spectroscopy do?
- IR spectrosopy uses infrared radiation to increase the vibrational energy of covalent bonds in a sample.
True or False
Every covalent bond will absorb the same frequency of infrared radiation.
- False!
- Each covalent bond will absorb infrared radiation at characteristic wavenumbers, specific to the bond. (frequency of infrared.)
() - extra info for clarification.
What does the frequency of infrared radiation absorbed by a covalent bond depend on?
- The atoms that are either side of the bond.
- The position of bond in a molecule.
What is the “fingerprint region” of a spectra from infrared spectroscopy?
- Finger print region is the part of the spectra that allows us to identify a specific molecule.
- Fingerprint spectra lies between 500cm⁻¹ and 1500cm⁻¹.
- The size and position of the peaks in fingerprint region are specific to specific molecule.
What can you do when you have determined the fingerprint region of a molecule?
- Compare the fingerprint region generated against a known library of spectra to identify the molecule.
You have identified fingerprint region for unknown molecule, you compare this region to your library of spectra and find that it is similar to ethanoic acid but there are a few extra peaks, why are these extra peaks present in your spectra?
- Extra peaks in fingerprint region indicates impurities in the sample.
What does “fingerprinting” in infrared spectroscopy allow for?
- ‘Fingerprinting’ allows identification of a molecule by comparison of spectra.
Describe the greenhouse effect.
- EM radiation is absorbed by Earth. Some of this radiation is re-emitted as infrared.
- Greenhouse gases will absorb infrared radiation- trapping heat.
- It is the covalent bonds in the greenhouse gases that actually absorbs the infrared radiation.
Give the 3 main greenhouse gases.
CH4
CO2
H20
What leads to global warming?
- Human activities ie. burning fossil fuels , lanfills = larger volume of greenhouse gases entering atmosphere.
- More infrared radiation/ heat trapped in atmosphere.
- Gradual warming of Earth = global warming.
What is fragmentation during mass spectrometery?
- Fragmentation is the breaking of 1 covalent bond in an ionised molecule, forming smaller ions and radicals.
What is the difference between peaks due to fragments/ molecular ion peaks?
- Molecular ion peaks = FULL POSITIVE molecule has been detected.
- Fragments= positive PARTS of molecule have been detected (but not full molecule.)
What different peaks would you find on mass spectrometer trace for Cl2?
- Molecular ion peaks: 70,72, 74.
- Peaks due to fragmentation: 35, 37
What is the OVERALL equation for fragmentation during mass spec? What will be detected by the mass spectrometer?
- M⁺⋅ –> X⁺ + Y⋅
⋅ –> shows unpaired electron - Mass spectrometer will detect M⁺⋅ and X⁺ (the molecular ion peak and your fragments.)
Why do the peaks furthest to right indicate Mr of molecular ion? How about if you have isotopes?
- Without isotopes, single peak furthest to right = Mr of molecular ion (because this is greatest peak detcted/ FULL molecular ion ALWAYS has to be detected!)
- With isotopes, it would be more than 1 peak, so you would calculate the Mr by calculating an average.
Determine the m/z values for the MOELCULAR ION PEAKS (not fragments) of CH₃Cl. What are the relative abundances?
(CH₃³⁵Cl) ⁺ : m/z = 50
(CH₃³⁷Cl) ⁺: m/z = 52
Relative abundances –> 3:1 respectively
Mass spec using CH₃Cl: Write fragmentation equation for peak at m/z 15.
Refer to Gautum notes (29/01/25) if needs be!
(CH₃Cl)⁺⋅ –> (CH₃)⁺ + Cl⋅
Mass spec using CH₃Cl: Write fragmentation equation for peak at m/z 37.
(CH₃³⁷Cl)⁺⋅ –> CH₃⋅ + ³⁷Cl⁺
C = 12.001
O = 15.998
S = 32.0025
A substance has precise Mr of 44.0035, determine its molecular formula.
precise masses here are just made up!
- CS
If you are given an infrared spectrum of a molecule, and the table for wavenumber of bonds, where would you look to find the bonds present in the molecule?
- Left hand side of the spectrum.
- The “troughs” (ie. bottom parts) are considered as peaks, look at wavenumber for particular peaks and see what bond range they fall into.
