Mass number and isotopes Flashcards
What is mass number?
Number of protons and neutrons
What is atomic number?
Number of protons
Define isotopes
The same number of protons but different number of neutrons.
What are the similarities and differences of isotopes?
Similarities: They have the same chemical properties (they have the same number of electrons in their outer shell)
Differences: They have different physical properties (they have different masses)
Explain why mass spectrometry is kept under a vacuum?
To prevent the ions that are produced colliding with molecules from the air
State the 4 stages in the mass spectrometer
Ionisation
Acceleration
Ion drift
Detector
What is the mass spectrometer used for?
To identify unknown purified compounds by composing mass spectrum obtained in data space.
What are the two types of ionisation for a mass spectrometer?
Electron impact
Electron spray ionisation
Describe the stage of electron impact.
The sample being analysed is vaporised and then high energy electrons are fired at it which comes from an electron gun which is hot filament wire with a current running through it that emits electrons. This usually knocks off one electron from each particle forming +1 ion
Describe the stages of electrospray ionisation.
- The sample is dissolved in a volatile solvent (e.g. methanol or water) and injected through a fine hypodermic needle as a spray into the ionisation chamber.
- The needle is attached to the positive terminal of a high voltage power supply.
- As the spray emerges from the end of the needle, the particles gain a proton from the positive charge of the supply.
- The solvent evaporates, leaving the 1+ ions.
Explain how ions are accelrated in mass psectrometer
The positive ions are accelerated using an electrical field so that they all have the same kinetic energy.
Lighter particles have a faster velocity and heavier particles have a lower velocity
Describe the flight tube
The positive ions travel through a hole in the negatively charged plate into a tube. The time of flight tube depends on its velocity which then also depends on the mass.
Describe the detective ion stage
The positive ions hit a negatively charged electric plate. Then the positive ions discharged by gaining electrons from the plate. This generates a movement of electrons so the electric current is measured.
Why do some ions travel faster?
- lighter in mass
- more highly charged
This means they have a higher velocity
What does the size of the current give to the plate?
Number of ions hitting the plate