We know what you are thinking: what is a mass spectrometer? What does mass spectrometry do? In this series of blogs we at ESS Ltd will explain everything you need to know about M.S., starting with the simple question: What is it?
Mass Spectrometry basically ionises gas molecules, filters them into their individual component parts of a gas mixture, and then measures the amount of the component parts.
It measures gases by their molecular weight. So, for example water has a molecular weight of 18. The chemical formula for water is H2O, meaning that there are two Hydrogens (with a weight of 1 each) and one Oxygen (with a weight of 16). Add this lot together and you get….18.
Almost all Mass Spectrometers require a vacuum in order to function correctly. The main reason for this is that we need to “pull” the sample gas molecules apart, so that they are around 30cm from each other. The scientific terminology for this is the mean free path. This is required to prevent collisions between sample gas molecules themselves, and because ions are much, much smaller and lighter than a molecule, and we don’t want a big heavy molecule knocking the sample gas ions we have just created out of the way.
At ESS, we use Quadrupole Mass Spectrometers in our equipment. The mass spectrometer itself consists of three main components, these being the ion source, the mass filter and the detector assembly. We will now take a look at each of these component parts in turn.
The ion source: All that you have to remember here is that positive attracts negative. A voltage of -70V is applied to a filament (just like an old-fashioned light bulb), and electrons are ejected from it. We want these to hit our sample gas molecules, so we apply a positive voltage (typically around 5V) to a central electrode. As the electrode is positively charged, the negatively charged electron is attracted to it. What we are trying to do here is get the electron to hit a sample gas molecule, and in doing so it will remove an electron from the outer shell of the molecule, and create a positive ion with an energy of around +5V. The more molecules that are present, the more ions are created.
Once this has been done we want to get the ions out of the ion source, into the mass filter assembly where we can separate them. This is performed by applying a high negative voltage to a focussing plate.
The mass filter: The next bit is a little trickier. At the top of the mass filter we have ions of all the molecules that have entered the mass spectrometer. A Quadrupole filter is four metal rods that are held together with lumps of ceramic (OK, all of this is very precisely machined). If we apply Radio-Frequency (or RF as we call it) and positive direct voltage (DC) to one pair of rods, this creates a Quadrupole field. The strength of the Quadrupole field determines which ion is allowed through. So, for example, if we want to let water through, we set the Quadrupole field to a specific value. If we want to let Oxygen through, we simply increase the strength of the Quadrupole field.
The detector: When the ions leave the mass filter, we need to detect them. The simplest form of detector is basically a polished metal plate, which the ejected ions hit. This generates a current, and the more ions that hit it, the bigger that current is. This is then amplified by the electronics.
Usually, we use a more effective type of detector, known as an electron multiplier. The operation of this is quite complex, so we won’t go into detail here other than to say it acts as a miniature amplifier of the ions that strike it’s surface, before they in turn strike a Faraday plate as described above.
Here is an example for you. It is just normal air that has been analysed. The mass spectrometer has broken down the air into its individual components, and measured them.