A mass spectrometer is used to measure the atomic or molecular mass of different particles in a sample.
In a mass spectrometer, atoms are converted to positive ions; these are then deflected by electric and magnetic fields before being detected. The mass of the elements individual isotopes are found as well as their abundance.
Ionisation: the sample has to be ionised to form cations (positive ions) --> ionised sample passes through the ionisation chamber in which a electronically heated metal coil gives of e- (electrons) which are attracted to the charged plate which is positively charged.
- How it occurs : A stream of high energy electrons from the heated filament bombard the sample of the element. this bombardment knocks an electron from the outermost energy level of the atom thus producing positive ions.
x(g)+e- ---> x+ (g) +2e- ( this is the equation you must remember the x can be any element or ion )
Deflection and Detection
Deflection: different ions are deflected by the magnetic field the amount of deflection depends on.
- The mass of the ion, meaning that lighter ions are deflected more than heavier ones
- The charge of the ion , ions with 2+ positive charges are deflected more than the ones with one charge.
THESE TWO MAKE THE m/z ( mass charge ratio)
Detector: Produces an electric current when hit by ions a computer system than converts this into mass spectrum which shows both the mass of the ionised atoms and their relative abundances. The Mass of each ion being detected is related to the size of the magnetic field being used to bring the ion to the detector.
Summary of the stages of Mass spectrometry and key
1. Sample is ionised
2. Sample is bombarded with high energy electrons and this ionises the sample producing cations or in other words Electron Impact
3. Ions are accelerated by an Electric Field
4. Ions are Deflected by a Magnetic Field
5. The detector indicates the Mass of the ionised atoms and their relative abundances
Exam Questions on MASS SPECTROMETRY
What is the purpose of the vacuum chamber ?
The vacuum chamber is there so that the ions produced can run freely through the machine without hitting any Oxygen molecules.
Who was Thomson ?
He was the scientist who discovered the Electron by studying the behaviour of Cathode rays in a gas filled vacuum tube. He was also known for deflecting rays with both electric and magnetic fields in which he discovered that he could determine the particle mass by measuring how far the ion deflected.
what happens in the Ionisation chamber?
One grid is positively charged and repels the ions; the other is negatively charged and attracts them. Because the repulsion and attraction act in the same direction, the ions move rapidly toward the negatively charged grid, which has many tiny holes. The ions pass through the holes at varying speeds. Lighter ions travel faster than heavier ones.
More exam styled Questions and answers
In a time of flight mass spectrometer
1.) What causes Lead atoms to lose electrons and become cations in the ionisation area?
A stream of electrons (idea of moving electrons) or laser pulse
1b.) What causes the acceleration of lead ions in the acceleration area?
Negative plates or electric field or electrostatic attraction
1c.)What properties of different isotopes cause their ions to take different times to cross the drift region?
(atomic / isotopic) Mass
Firstly , periodicity means the occurrence of periodic patterns.
- Mendeleev organised 61 known elements in accordance to increasing RAM. He left gaps for undiscovered elements to be discovered.
- Elements change from metallic to non metallic across a period. They become increasingly more metallic down a group and more non-metallic up a group.
Electronic Conductivity of Elements: The metallic elements on the Left are good conductors whereas the ones on the left are not as they are non-metals.
Density and Melting & Boiling points
Density: Of an Element is it MASS PER UNIT VOLUME
- Elements with high densities contain HEAVY particles which are closely packed together so they occupy a relatively small volume. ( vice versa for LIGHT particles)
Melting/Boiling points: When an element is melted or boiled, the bonds between their constituent particles must be overcome. The strengths of these bonds determine whether an element has high/low melting and boiling points.
Periodicity carried on
Atomic Size: Atoms become smaller Going Across a Period despite the increase in Electrons. This is because the successive electrons added on Crossing a period enter the same electron shell. Additionally, the number of protons in the nucleus steadily increases across a period and the increasing positive nuclear charge attracts the electrons more and more strongly as a result the atom becomes smaller despite the increase of electrons.
Periodicity in ionisation enthalpies: is the amount of energy needed to remove an electron from an atom. Each element as a different ionisation enthalpy that corresponds to the energy required to remove the outermost electron from the atom .
Periodicity Styled Questions
1.)(This is for Barium Carbonate )There is a trend in thermal stabilities of Group two carbonates. Describe a simple experiment you could conduct using lime water. How you would make this a fair test and what would you expect to see?
Firstly I would Heat the carbonate in a flask passing gas through lime water. To make it a fair test I would have the same heating conditions and use the same volume of lime water.
2.)Explain how the electronic structure of Barium is related to the group and period of the periodic table it is found?
In its outermost shell it has two electrons and therefore in group two also it has six shells therefore is in period 6.