Suppose you had a bowling ball traveling past you and you wanted to deflect it as it went by. All you've got is a jet of water from a hose that you can squirt at it. Because the bowling ball is so heavy, it will hardly be deflected at all from its path.   But suppose instead you tried to deflect a ping pong ball traveling at the same speed, using the same jet of water. Because this ball is so light, you will get a huge deflection. This is the principle behind how a mass spectrometer works to identify atoms by their mass. If something is moving and you subject it to a sideways force, instead of moving in a straight line, it will move in a curve - deflected out of its original path by the sideways force. The amount of deflection you will get for a given sideways force depends on the mass of the object. If you knew the speed of the object and the size of the force, you could calculate the mass of the object as you observed the curved path it was deflected through. The less the deflection, the heavier the object. You can apply this exact same principle to atomic sized particles. Atoms can be deflected by magnetic fields, provided the atom is first turned into an ion. Ions are electrically charged particles and are affected by a magnetic field. Electrically neutral ones aren't. There are four stages to how a mass specrometer identifies atoms in a sample. All this happens in a vacuum. Stage 1: Ionization The atom is ionised by knocking one or more electrons off its outer shell, to give it a net positive charge. Mass spectrometers always work with positive ions. The vaporized sample passes into an ionization chamber where an electrically heated metal coil gives off electrons that bombard the sample and knock out electrons from its atoms, ionizing them. Stage 2: Acceleration The ions are accelerated by electrically charged plates so that they all have the same kinetic energy, and form a narrowly focused beam. Stage 3: Deflection The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected. These two factors are combined into the mass/charge ratio. Mass/charge ratio is given the symbol m/z. For example, if an ion had a mass of 28 and a charge of +1, its mass/charge ratio would be 28. An ion with a mass of 56 and a charge of +2 would also have a mass/charge ratio of 28. Stage 4: Detection The beam of ions passing through the machine is detected electrically. In the diagram above, ion stream A is most deflected - it will contain ions with the smallest mass/charge ratio. Ion stream C is the least deflected - it contains ions with the greatest mass/charge ratio. It makes it simpler to talk about this if we assume that the charge on all the ions is 1+. Most of the ions passing through the mass spectrometer will have a charge of 1+, so that the mass/charge ratio will be the same as the mass of the ion. Assuming all the ions in the stream have a charge of +1, stream A has the lightest ions, stream B the next lightest and stream C the heaviest. Lighter ions are going to be more deflected than heavy ones. In this simplified example, only ion stream B makes it right through the machine to the ion detector. The other ions collide with the walls where they will pick up electrons and be neutralised. Eventually, they get removed from the mass spectrometer by a vacuum pump. When an ion hits the detector, its charge is neutralized by an electron jumping from the metal onto the ion. That leaves a space amongst the electrons in the metal detector, and the electrons move to fill it. A flow of electrons in the is detected as an electric current which can be amplified and recorded. The more ions arriving, the greater the current. The output is usually simplified into a 'stick diagram'. This shows the relative current produced by ions of varying mass/charge ratio. The stick diagram for molybdenum is at the right. The commonest ion has a mass/charge ratio of 98. Other ions have mass/charge ratios of 92, 94, 95, 96, 97 and 100. That means that molybdenum consists of 7 different isotopes (atoms with differing numbers of neutrons in their nuclei). Resources HTML, graphics & design by Bill Willis 2024