Mass Spectometry Flashcards
What are Isoptopes?
Isotopes are atoms with the same number of protons, but a different number of neutrons
How do isotopes differ from one another?
They have similar chemical properties because same electronic structure but varying physical properties because different masses
What is a mass spectrometer used for?
Used to determine all the isotopes present in a sample and to therefore identify the element
Why does it have to happen in a vacuum?
Otherwise the accelerated particles would collide with the air particles and the air particles would become ionised and registered on the detector. Also if the particles collided with air it would mean they would slow down causing making the results invalid because the time would be affected
What are the 4 steps for a mass spectrometer?
- Ionisation
- Acceleration
- Flight Tube
- Detection
Why would electron impact ionisation be used?
Used for elemets and substances with a low RFM because it can cause larger organic molecules to fragment
Why would electron spary ionisation be used?
Used with larger organic molecules. The softer condition mean fragmentation does not occur
How does Electron impact work?
-A vaporised sample is injected at a low pressure
-An electron gun fires high energy electrons at the samoke
-Knocks out outer electron
- Forming positive ions
Ti –> Ti+ + e-
How does Electro spray ionisation work?
- Sample is dissolved in a volatile polar solvent
- Injected through a fine hypodermic needle giving a fine mist
- Needle tip has a high voltage
- Top of the needle with the sample molecule gains a proton H_ from the solvent to form MH+
- Solvent then moves towards the negative plate
Why are the particles are accelerated?
Since all particles will have the same KE the velocity of each particle depends on its mass. Lighter particles have a faster velocity and heavier particles have a slower velocity.
What’s the equation for KE?
KE = 1/2 m v2
What’s the flight tube for?
Heavier particles with a higher m/z value will take longer to move through the drift area.
Ions are distinguished from this
Why is a detector used?
When ions reach the detector they generate a small current. The current is produced by electrons transferring from the detector to the positive ions. The size of the current is proportional to the abundance of the species
