3.1.1.2 Mass number and Isotopes

Mass number

A

Number of protons and neutrons

Atomic number

Z

Number of protons+neutrons

Define isotope

Atoms with the same number of protons, but different number of neutrons

Isotopes chemical properties

They have similar chemical properties because they have same electronic structure

May have slightly varying physical properties because they have different masses.

5 steps of TOF mass spectrometer

Ionisation

Acceleration

Flight tube/ion drift

Ion detection

Data analysis

Ionisation in TOF

Two techniques, electron gun and electro spray ionisation

Principle of electron gun

Vaporised sample is injected at low pressure

Electron gun fires high energy electrons at sample

This knocks out electron forming positive ions.

What type of elements/substances is electron gun used for

Ones with low formula mass

Can cause larger organic molecules to fragment

Principle of electro spray ionisation

Sample is dissolved in volatile, polar solvent

Then injected though a fine needle, tip has a high voltage

At tip of needle sample gains a proton from the solvent forming MH+

Solvent evaporates away while MH+ ions move towards a negative plate

Type of molecules electro spray ionisation is used for

larger organic molecules

Softer conditions of technique means fragmentation doesn’t occur

Acceleration in TOF

Positive ions accelerated by an electric field giving all particles same KE

Light particles move faster, heavier particles move slower

Flight tube/ion drift in TOF

Ions leave field with constant velocity and KE into flight tube where there’s no electrical field so they drift

Ions with smaller m/z value will move faster than ion with higher m/z value

Detection in TOF

Ions reach detector and generate small current which is fed into computer for analysis

Current produced by electrons transferring from the detector to the positive ions

Purpose of mass spectrometer

Identify elements by determining the isotopes present in a sample of an element .