mass spectrometry Flashcards
what does a mass spectrometer do
separates molecules according to their charge and mass
what type of molecules can mass spectrometry separate
ions of slightly different masses and can easily distinguish between different isotopes for a given element
how can we identify a sample chemical
by measuring its molecular weight (formula mass)
what is relative mass
the mass of one atom compared to the mass of a different atom (carbon-12)
what is relative atomic mass
the average mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12
what is relative isotopic mass
mass of an isotope of an atom compared with 1/12 of the mass of an atom of carbon-12
what is relative molecular mass
the average mass of a molecule or formula on a scale where an atom of carbon-12 is 12
what can you use mass spectrometry for
it can tell you the relative atomic mass, relative molecular mass, relative isotopic abundance and molecular structure of an atom
what are the three main parts of a mass spectrometer
the ionisation source
the analyser
the detector
ionisation source..
..the sample if ionised, forming positively charged ions
analyser..
..the ions produced are separated according to their mass
detector..
..the ions cause the detector to produce an electric current that computer system converts into a mass spectrum
describe the process of mass spectrometry (time of flight mass spectrometry)
- liquids and gases are injected but solids are heated to vaporise them into a gas
- a heated element produces high energy electrons which bombard atoms or molecules and knockjed electrons out to form cations
- an electric field accelerates the ions to give them the same KE
- placed in a vacuum so the ions don’t collide with any air molecules
- lighter ions will reach detector first because they all have the same KE
- computer detects the lighter ions first and converts them into a mass spectrum
what is a mass spectrum
the mass/charge (m/z) ratios of the ions present in a sample plotted against their intensities (% abundance)
what does the y-axis of the mass spectrum tell us
the abundance of ions often as a %.
what does the height of each peak tell us
the relative isotopic abundance
what does each line represent if the sample is an element
each line will represent a different isotope of the element
what does the x-axis of the mass spectrum represent
the mass/charge ratio or simply the relative mass
what can we calculate from the mass spectrum
the relative atomic mass
how do we calculate relative atomic mass from a mass spectrum
1) multiply the relative %relative isotopic abundance from the y-axis and the relative isotopic mass from the x axis
2) add up the totals
3) divide by 100
what happens if relative abundance in the mass spectrum is not given as a %
you would divide by the total relative abundance instead of 100
why are there many peaks in a mass spectrum
fragments of the original molecule ions are formed in the ionisation chamber. if the fragments have a positive charge it will be accelerated by an electric field and detected.
what does the highest/intense peak refer to
the base peak which means its the most stable fragment that is made the most often and the most detectable
what is the name for the ion with the higest m/z ratio
molecular ion/original parent molecule.
