mass spectrometry steps- time of flight mass spectrometer

Stage 1- ionisation type a

Electron impact- sample vaporised, high energy electrons fired at it with electron gun (hot wire filament with current running through that emits beam of high energy electrons) electron knocked off from each particle in the sample forming +1 ion. Technique used for elements and substances with a low formula mass.

Stage 1- ionisation type b

Electrospray ionisation- sample dissolved in volatile solvent (water or methanol) and sprayed through a fine hypodermic needle to give fine mist- aerosol. Tip of needle attached to positive terminal of high voltage power supply. Particles ionised by gaining a proton form solvent ie H+ ion which is just one proton. X(g) + H+ —> XH+. Solvent evaporates, leaving single positively charged ions which are attracted to negative plate where they are accelerated. Used for high Mr compounds like proteins

Stage 2- acceleration

Positive ions accelerated using electric field so they all have same KE. Kinetic energy= ½ mv². Given that all particles have same KE, velocity of each particle depends on its mass. Lighter mass= faster velocity.

Stage 3- ion drift

Positive ions travel through hole in negatively charged plate into a tube. Time of flight of each particle depends on its velocity which depends on its mass. T=d/v. T=d root m/2KE. Time of flight proportional to square root of mass of ions. Lighter ions move faster

Stage 4- detection

Detector= negatively charged plate. When positive ions hit plate they are discharged by gaining electrons form plate. This generates a movement of electrons hence an electric current that is measured. Size of current proportional to number of ions- more ions= bigger current.