Mass spectrometry Flashcards
4 stages of mass spectrometry
Ionisation
Acceleration
Deflection
Detection
Acceleration
The positively charged ions are attracted to a negative plate causing them to accelerate
They have same kinetic energy so now their speed is dependant on the mass
Detection
Ions hit a negatively charged plate
Causes a current and the size of it gives a measure of how many ions hit the plate
To give the abundance of the molecule
Mass spectrum
Produced as a result of ions being detected
X axis has mass to charge ratio
Y axis has % abundnace
Molecular ion peak
The peak with the largest mass to charge ratio aka furthest right
Represents the mass of the molecule we are analysing
Peaks clustered around certain mass to charge ratio
Isotopes of the molecule/ element
Have different masses so different mass/charge ratio
Fragments
Smaller and lighter peaks that occur as a result of ionisation of the sample breaking it off into smaller molecules
And larger molecules
How to use the spectrum to view different isotopes and their relative abundance
Isotopic mass is along the x axis
Isotopic abundance is along the y axis
Molecular/ parent ion
When one electron is knocked off a molecule forming a positive ion
Represented as M+ and is situated the furthest right on the mass spectrum graph with highest mass/charge ratio
Why does the molecular/ parent ion fragment?
Because it is energetically unstable due to having electron knocked off during bombardment of electrons
Why are there so many peaks on a mass spectrum of a molecule?
Because many fragments are possible with the atoms of a parent ion
Radical
A fragment that isn’t charged so won’t be detected and as a result lost in the machine
Mass spectrum of an organic molecule
Peaks all along the x axis (different mass numbers)
Because they are fragments of the molecular ion so can be varying masses
Mass spectrum of a pure element
Peaks represent different isotopes of that element
In a diatomic molecule the peaks will be at the molecule not individual atoms eg around 72 for chlorine
And the different masses of chlorine possible due to isotopes
How to work out the ratio peaks will appear in a mass spectrum for diatomic molecules
Turn the percent abundance of each isotope into a fraction
Write out each possible combination of individual atoms can form the diatomic molecule
Multiply fractions together giving ratios peaks will appear in
Base peak in a mass spectrum
Most common fragment or multiple ways for fragmentation to result in this molecule
How to identify the identity of the molecular ion peak for an organic molecule
Consider any functional groups told about such as alcohol and take away the masses of those elements
Divide by 12 to see how many carbons present
Remainder = number of hydrogen = identified organic molecule
M+1 peak
When analysing the fragments of organic compounds, there may be a peak at the end
Which has a m/z ratio one more of molecular ion peak
What is the M+1 peak?
When organic compounds have traces of the isotope carbon-13 rather than carbon-12 which increases the mass number of it so it’s more than the molecular ion peak
What is the point of mass spectrometry?
A machine that produces positive ions and deflects them in a magnetic field depending on their relative masses
So we can determine the masses of atoms and molecules
What could a m/z ratio of 15 indicate
That a CH3+ fragment/ion is present
What could a m/z ratio of 17 indicate
That an alcohol group such as OH is present
How can we use the mass spectrum of an element provided in calculation?
The y axis shows the exact isotopic abundance
And the m/z charge ratio shows the relative isotopic masses
So we can put that in the calculation to find the relative atomic mass of an element
Finding the relative peak heights of a mass spectra for diatomic molecules
For the singular isotopes of the element, work out the ratio of isotopic abundance
Eg Cl-35 to Cl-37 is 1:3
Put as fractions and for each diatomic molecule find the fraction probability eg Cl-70 is 3/4x3/4 = 9/16
(The numerator is the ratio of relative peak heights for diatomic molecule)
How to find the relative molecular mass of poly atomic molecules?
Just use the molecular ion peak
CAREFUL NOT TO USE THE M+1 PEAK
