The mass spectrometer - Chemical Ideas 6.5
A mass spectrometer is a way of taking a sample of an element and then finding the relative atomic mass off the sample by measuring how long it takes for the ions of the isotopes to reach to detector.
These are the verious parts of the mass spectrometer:
Sample inlet - sample is injected into the vacuum chamber
Ionisation area - a heated filament produces high-energy electrons. These electrons bombard any atoms or molecules in the sample and removes electrons to change atoms into positively charged ion
Acceleration area - an electronic field is used to accelerate the positively charged ions so that they have the same ammount of kinetic energy
Drift region- there is a vacuum so that traveling ions do not collide with air particles (this would change direction of flight). As kinetic energy = mass x velocity2 all ions have the same kinetic energy, this causes heavier ions to more slower through vacuum.
Ion detector- as lighter ions have a lower mass they reach detector faster than heavier ions. A computer then converts these results into a mass spectrum. Only positive ions are detected.
These are the verious parts of the mass spectrometer:
Sample inlet - sample is injected into the vacuum chamber
Ionisation area - a heated filament produces high-energy electrons. These electrons bombard any atoms or molecules in the sample and removes electrons to change atoms into positively charged ion
Acceleration area - an electronic field is used to accelerate the positively charged ions so that they have the same ammount of kinetic energy
Drift region- there is a vacuum so that traveling ions do not collide with air particles (this would change direction of flight). As kinetic energy = mass x velocity2 all ions have the same kinetic energy, this causes heavier ions to more slower through vacuum.
Ion detector- as lighter ions have a lower mass they reach detector faster than heavier ions. A computer then converts these results into a mass spectrum. Only positive ions are detected.
The mass spectrometer and isotopes
relative atomic mass is very simple to calulate all you have to be able to do is substitute the correct values into the correct equasion.
Example
Example
If there is 75% of Chlorine-35 and 25% of Chlorine- 37
relative atomic mass = average mass of 100 atoms = (35 x 75) + ( 37 x 25) = 35.5
100
relative atomic mass = average mass of 100 atoms = (35 x 75) + ( 37 x 25) = 35.5
100
The mass spectrometer and molecules
A mass spectrometer is much more difficult for a compound than an element. The ion with the greater mass corresponds to the molecular mass of the sample compound (the original compound injected into vacuum chamber). This is called the molecular ion, and corresponds to the parent molecule minus an electron. The most intense peak is called the base peak. There are many peaks because fragments are formed in the ionisation chamber.
The molecular ion breaks down into fragments. If the fragment has a positive charge then it will be accelerated by the electrical field and detected later in the mass specrometer.
The molecular ion breaks down into fragments. If the fragment has a positive charge then it will be accelerated by the electrical field and detected later in the mass specrometer.