ATOMIC STRUCTURE

 find the abundance and mass of each isotope in an element allowing us to determine its relative atomic mass  find the relative molecular mass of substances made of molecules.

particles of the substance are ionised to form 1+ ions which are accelerated so that they all have the same kinetic energy. The time taken to travel a fixed distance is then used to find the mass of each ion in the sample.

• electron spray - electron gun

1- The sample being analysed is vaporised and then high energy electrons are fired at it. 2- The high energy electrons come from an ‘electron gun’ which is a hot wire filament with a current running through it that emits electrons. 3- This usually knocks off one electron from each particle forming a 1+ ion. 4-The 1+ ions are then attracted towards a negative electric plate where they are accelerated.

X(g) –> X+(g) + e–

This technique is used for elements and substances with low formula mass (that can be inorganic or organic molecules).

When molecules are ionised in this way, the 1+ ion formed is known as a molecular ion

The molecular ion often breaks down into smaller fragments some of which are also detected in the mass spectrum.

1- The sample X is dissolved in a volatile solvent (eg water or methanol) and injected through a fine hypodermic needle to give a fine mist (aerosol). 2- The tip of the needle is attached to the positive terminal of a high-voltage power supply. 3- The particles are ionised by gaining a proton (ie an H+ ion which is simply one proton) from the solvent as they leave the needle producing XH+ ions (ions with a single positive charge and a mass of Mr + 1). 4- The solvent evaporates away while the XH+ ions are attracted towards a negative plate where they are accelerated.

X(g) + H+ –> XH+(g)

This technique is used for many substances with higher molecular mass including many biological molecules such as proteins. This is known as a ‘soft’ ionisation technique and fragmentation rarely takes place.

1- ionisation 2-acceleration 3-flight tube 4-detection

using an electric field?

so that they all have the same kinetic energy.

KE = kinetic energy of particle (J) m = mass of the particle (kg) 𝑣 = velocity of the particle (m s–1)

𝒗 = √(𝟐𝑲𝑬)/m

its mass

Lighter particles have a faster velocity, and heavier particles have a slower velocity.

The positive ions travel through a hole in the negatively charged plate into a tube.

The time of flight of each particle through this flight tube depends on its velocity which in turn depends on its mass.

t=d/v t=𝒅√(𝒎/𝟐𝑲E) t = time of flight (s) d = length of flight tube (m) 𝑣 = velocity of the particle (m s–1) m = mass of the particle (kg) KE = kinetic energy of particle (J)

This shows that the time of flight is proportional to the square root of the mass of the ions. Therefore lighter ions travel faster and reach the detector in less time than the heavier particles that move slower and take longer to reach the detector

1=24Mg+ 2= 25Mg+ 3=26Mg+ in order of mass, lighter= faster

The positive ions hit a negatively charged electric plate. When they hit the detector plate, the positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and hence an electric current that is measured. The size of the current gives a measure of the number of ions hitting the plate.