Mass spectrometry Flashcards
How does Mass spectrometry work??
- The vaporised organic sample passes into ionisation chamber of mass spectrometer, then bombarded by stream of electrons. These electrons have enough energy to knock an electron off of the organic compound and make a positive ion, called the molecular or parent ion.
- The X+ ion travels through the mass spectrometer, and created a line on the abundance graph.
- Because the original ion can fragment in any place, you get more than one line with different heights, depending on the mass and abundance of that particular ion.
How do you work out the total Mr of the whole positive molecule (no fragmentation)
The last Mr line on the graph is the mr of the whole positive molecule.
What does the tallest fragment line represent??
The tallest line is called the base peak, and is the most common ion that forms after fragmentation, either because there are several ways to produce this fragment, or it is a particularly stable ion.
Why do you only get positive ions??
Because the fragmentation only produces a positive ion because you only lose electrons.
If asked what the fragment is for a certain peak, you must show it with a positive charge, because otherwise it wouldn’t be detected
You just have to play around with numbers until you have a fragment of the molecular/parent ion that has an Mr that matches the number of the peak you are working out.
Whats the difference between the mass spectrometry of an element, and the mass spectrometry of a compound??
The mass spec of an element- the lines represent a different isotope of that element
The mass spec of a compound- each line represents a different fragment produced when the molecular ion breaks up.
Why might there not be a peak at 46 for the mass spec of ethanol, even though the Mr of ethanol is 46??
The molecular ion of C2H5OH+ is too unstable, so fragments further
