Transcript: the Basics of Mass Spectrometry-Mass Analyzers Richard Vachet Transcribed by Emily Ott, MA, NIC Master, SC:L, Sr
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Transcript: The Basics of Mass Spectrometry-Mass Analyzers Richard Vachet Transcribed by Emily Ott, MA, NIC Master, SC:L, Sr. Interpreter & Transcriber Office of Student Life Disability Services, The Ohio State University Hello. So the goal of this video lecture is to talk a little bit about mass spectrometry as a technique with a specific emphasis on mass analyzers, um, as really the heart of mass spectrometers. Before I go on to talking about mass analyzers and specifics, I thought I would just give a general overview about mass spectrometry as a technique, to say something about how it works, and what it's used for. And so mass spectrometry is really a measurement tool, in its simplest form, that's able to provide multiple levels of information about analytes, or compounds of interest that we might be interested in measuring or analyzing. And really, these three levels that are commonly obtained by mass spectrometry are: 1. molecular weight 2. elemental composition 3. structural information Now, molecular weight is perhaps the simplest of them to understand. When we're talking about a "mass" spectrometer, because it can be linked to mass quite easily. Molecular weight by essentially adding up the masses of individual atoms that comprise a particular molecule. Molecular weight, though, is very characteristic of these molecules and therefore a very valuable piece of information because of course all matter has mass, and therefore in principle, all molecules can be measured by mass spectrometry. But we can also get elemental composition with mass spectrometers by recognizing the fact that atoms that comprise molecules are made up of different atoms that have different isotopes to them. The most common molecules that we like to measure often are things that have carbons, nitrogens, hydrogens in them, and carbon, hydrogen, and nitrogen have isotopes. These have different masses to them, they have different natural abundances as well, so when we measure a molecule that's comprised of different atoms and their different isotopes, that can result in a pattern that is reflected in the mass that we measure for the molecule, and we can use that to determine, often, the number of that atom, or the type of the atom as well, and therefore get empirical formula information and elemental composition about a particular molecule. Now, structural information is also something that mass spectrometry has more recently become quite good at getting, and typically this is obtained during a tandem mass spectrometry experiment or doing MSMS, and in an MSMS experiment, typically what is done is the molecule of interest is brought into the mass spectrometer, and as we'll see, it's ionized, but then we typically break it apart into pieces and measure the masses of the individual pieces, and sort of like a puzzle, piece that back together to get structural information. Now, in future video series, we'll talk more in detail about tandem mass spectrometry, but we're going to focus today really on the measurement of the masses of the molecules we're interested in. Now, mass spectrometers are made up of several components, and this is a convenient way to discuss how mass spectrometers work, but the key components of a mass spectrometer are an inlet system that we can use to bring the sample of interest into, and then we can make ions of that sample, and this is a critical part about mass spectrometry that we'll talk a little bit more about in a second, where we can make ions that then are passed to a mass analyzer, where they can be separated according to mass to charge ratio, we'll see, and then detected to provide a mass spectrum. Now, the mass spectrum is typically shown here as ion abundance, or sometimes ion intensity, as a function of mass to charge ratio, and of course mass to charge ratio is where we're gonna get the information about mass of the compounds that we are analyzing. Now, you'll see that I draw these components of a mass spectrometer in a vacuum system, in this cartoon, and that is because when we do mass spectrometry, in order to do it best, we do it inside a vacuum system. The reason for that is to keep the pressure very low and minimize interactions that ions have with other neutral molecules or atoms, and this is because ions can be very reactive. As we move ions from one component to another, say from the ion source to the mass analyzer, we want to avoid those ions bumping into things that could compromise their detection. But, we can actually do mass spectrometry in a way where we can make the ions not under a vacuum but at atmospheric pressure, or at another pressure, and then bring those ions into a vacuum system where they are mass analyzed and detected. Now, as I mentioned, the mass analyzer works best in a vacuum system, and so almost solely, in fact in all commercial mass spectrometers, the mass analysis is done under low pressure to be done optimally. So just to recap up to this point, what we do when we do mass spectrometry is we are making ions some way, and then we are analyzing them by the mass analyzer. So the key part of this, it's a key part of the whole experiment, is that we need to do some chemistry to make gas phase ions. Ions that eventually go into the mass analyzer in the vacuum system as ions. Now because we're making ions and measuring their mass, we're actually measuring mass to charge. And so mass spectrometry itself is a little bit of a misnomer. A better name is perhaps mass to charge spectrometry, but that's a mouthful and so we'll stick with mass spectrometry. Another important thing to just recap and remind you is we're doing this at low pressures, and one of the consequences of measuring ions at low pressures is that if we can do it well, we can actually bring ions to a mass analyzer, to a detector in a very efficient fashion, and if we can also move ions from where they're made to the mass analyzer to detector, we can measure compounds very sensitively, and in fact that is true about modern mass spectrometers. They are very sensitive, can measure very low levels of compounds of interest, and that is one thing that makes them so interesting and powerful. Now to dig in a little bit more deeply and understand mass spectrometry as a technique in greater depth, it's convenient to think about the three main components of a mass spectrometer: the ion source, mass analyzer, and detector. And this particular type of presentation, I'm gonna emphasize mostly the mass analyzer, and different ways that we are able to analyze and separate ions according to their mass to charge ratio so that we can detect them, but before I do that, I just want to say a few brief words about how an ion source works, and some of the basic types of ion sources. So we can think about ion sources and categorize them into two different types. So-called gas phase techniques, and what I will call desorption techniques. Now these gas phase techniques are ones in which we work with compounds that are naturally volatile and thermally stable. And so this ionization by these gas phase techniques involves first volatilizing the compound of interest, and then ionizing it. Usually this is by electron ionization or chemical ionization, and involves often shooting electrons at the sample to knock off other electrons, um, and then making them ions, and then they're passed on to the mass analyzer. But there have been more recently developed desorption ionization techniques, such as electrospray ionization, and Matrix Assisted Laser Disruption Ionization, or MALDI, that allow us to ionize things that aren't necessarily very volatile, in fact can be non-volatile and sometimes even thermally labile. And in fact this is a very powerful way to make ions of a wider range of molecules of interest because at the end of the day, there are more, it turns out, probably compounds on this planet that are non-volatile and thermally labile than there are ones that are volatile and thermally stable. And so electrospray ionization and MALDI have been very powerful tools for advancing mass spectrometry and being able to apply it to a wider range of types of molecules, for example, biomedical systems or other complicated molecules. And in fact these techniques of electrospray and MALDI were so important that they, the people who developed them, were awarded with Nobel Prizes back in 2002, John Fenn for electrospray ionization and Koichi Tanaka for his developments in MALDI. Now detectors I want to say also something briefly about. Detectors, the basic goal of a detector is to take the ions that have been separated by the mass analyzer and convert those ions into some electrical signal, into essentially electrons that can be processed as a current and then a voltage by some computer. And this is done in a variety of different ways, but there are two typical ways that this is done. The first is to use um something that takes ions directly and converts them into electrons and furthermore cascade of electrons that multiplies the signal and amplifies the signal, and this is done with things like electron multipliers or micro channel plates. And again, the idea is it converts ion signal into a current, and this current is achieved by the electrons that are produced from the initial ion hitting some surface.