Mass Spectrums Flashcards
What is a mass spectrum?
A graph compiled by a computer from the data from a mass spectrometer
Why is the mass/charge ratio effectively just mass?
- because ions ionised by electrospray ionisation have a 1+ charge, as do most that have been ionised by electron impact
- if not, a mass spectrometer can be set up to ignore ions with a charge greater than +1
What is the difference between high resolution mass spectrometry and low resolution mass spectrometry?
The accuracy. High resolution can measure Ar up to 5 d.p, whereas low resolution is done to 1 d.p
What does 1 vertical line on a mass spectra represent?
A certain isotope.
How do you work out Ar from a mass spectrum?
combined mass of all isotopes / combined abundance of all isotopes
How do you work out the combined mass of all of the isotopes?
Multiply each mass by its relative abundance or percentage and add these values together
When a sample has been ionised by electron impact how do you find out the relative atomic mass of the molecular ion on the mass spectra?
It is the value with the greatest m/z ratio
What is the cause of other small peaks present around the molecular ion?
These are due to some molecular ions containing different isotopes
Peaks present at much lower m/z values are caused by what?
Fragments of the molecular ion
How do you work out the mass of an isotope from a sample ionised by protonation? explain
subtract one from the m/z value since a proton weighs 1
How would you approach a question asking you to predict abundance peaks from a ratio
- list all possible isotopes
- work out probability of the isotopes occurrence from the ratio
- convert to a ratio
Why might the value in the periodic table vary from a value from a mass spectra for the same element?
- abundance may vary
- different mass spectrometers (high res/low res) detect to different levels of accuracy
