What is a mass spectrometer? I was just asked this question. It gave me pause.
You know, most of us forget that what is completely familiar to us in our daily lives might be utterly foreign to 99% of the world. For example, I’m a vertebrate paleontologist and know many, many paleontologists. Sometimes, I think the whole world is teeming with paleontologists. But when I think about it, there’s maybe 5-10 thousand people in the world that can call themselves a vertebrate paleontologist. Still a big number, but when compared to the world’s population of ~7 billion, or the population of New York City at ~8 million, it’s quite possible that I may be the only paleontologist that many of my non-paleontology friends know and may ever meet.
I suspect that there are more mass spectrometer technicians in the world than paleontologists, just because there are so many different kinds of mass spectrometers and zillions of applications for mass spectrometry. Nevertheless, most people’s exposure to mass spectrometry comes from watching episodes of CSI, where (naturally) the television show gets it mostly wrong. (Seriously, you don’t just turn on a mass spectrometer in the morning and expect to get results in a few hours. I switched ours on yesterday and I’m extremely hopeful that I can start running analyses tomorrow!)
So, then, what’s a mass spectrometer? Breaking down the name itself is a good start.
Mass: This is the science kind of mass, not a religious ceremony. Mass is generally equated with ‘weight’ or ‘size.’ ‘Mass’ in science-ese is actually more specific than that, but this works. We’re basically considering something in terms of its size or weight.
Spectrometer: Well, the ‘spectro-’ part is the same as spectrum – a range. Just like a spectrum of colors: red, orange, yellow, green, blue, violet. The ‘meter’ part just says that a measurement is being made. We’re measuring a range of something.
Since it’s a MASS spectrometer, we’re measuring a range of sizes or weights.
OK, but measuring weights of what?
Now here’s the fun part, and why I say there are so many kinds of mass spectrometers. We’re usually looking at the weights of components of some material. It may be an unknown material, and we want to know what it is. Or it may be a known material, but we want to look for impurities or (potentially) for its origin.
Some mass spectrometers are set up to look for heavy elements like strontium or uranium and measure their abundance. Others look at organic compounds like fats or waxes to determine, for example, how much unsaturated fat versus saturated fat there is. The one I work with is highly sensitive and can only be used with ‘light’ elements like carbon, oxygen, and nitrogen (and occasionally hydrogen – but I hate hydrogen… we won’t go there!)
All mass spectrometers have the same general components: A means to get the sample into the instrument (an inlet system or peripheral device), a means to separate the masses, and a means to measure the different masses.
A common instrument you might see on a TV show like CSI is a gas-chromatograph mass spectrometer (or GC-MS, seriously, that’s a mouthful!). The inlet system is a combustion chamber (a furnace) where the samples are burned, causing the original molecules to break down into smaller molecules that are now gaseous (rather than a solid). These molecules are separated, by mass, using a chromatographic column, which is essentially just a really, really long narrow tube. The smaller molecules flow faster down this tube than the bigger ones. At the end of the tube is a collector of sorts, which basically counts how many molecules of each size pass through the tube and we get a spectrum of the different sizes of molecular fragments that came from our original sample. The pattern of molecule sizes and amounts is characteristic of a particular material.
Another common mass spectrometer is a quadrupole mass spectrometer. This is used for the heavy elements, and makes measurements atom by atom (not whole or fragmentary molecules). We had one running here at the University of Rochester for a while. The inlet system on it was experimental, but fun. A laser was shot at the sample, forming a fine dust which was then carried into the inlet system in Argon gas. There it went into a plasma torch and was burned up and the gas went into the mass spectrometer. This system has the fancy name of Laser Ablation Inductively Coupled Plasma Mass Spectrometry, or LA ICP-MS. Changing voltages on the four metal rods for which the quadrupole instrument gets its name is how the different masses are selected. A collector is at the end of these rods, which measures how many of our specified atoms got through.
The instrument that I manage is called an isotope ratio mass spectrometer (IRMS). There are several different peripheral devices attached to ours, one of which has a furnace like on the GC-MS, and another that has a series of vials with a moving needle. The peripheral devices are where the solid samples are converted to gas. In the first, samples are burned up and converted only to carbon dioxide and nitrogen gasses. In the other, the solid samples placed in the vials are reacted with phosphoric acid to make carbon dioxide gas, which is what we measure. (And I inject that acid drop-by-drop, vial-by-vial. So when you see me say that I’m dropping acid, that’s what I’m doing!). These gasses go into the mass spectrometer and are ionized by an electron beam (3000 Volts!!) after which they fly away from the electron beam toward the collectors. The different masses are separated by a strong magnet and a voltage is measured by the collector cups.
What’s different about what I do is that I’m only looking at one molecule at a time, usually carbon dioxide. But I’m looking at isotopes. Not radioactive isotopes, but stable ones. Isotopes are atoms of the same element, but with different masses (or weights). Every atom is an isotope. Some are just unstable.
Carbon dioxide has carbon and oxygen. Carbon has two stable isotopes: Carbon-12 and Carbon-13 (and one unstable, radioactive isotope, Carbon-14). Oxygen has two important isotopes: Oxygen-16 and Oxygen-18.
Some math: carbon dioxide = one carbon plus two oxygens. Most carbon dioxide is composed only of carbon-12 and oxygen-16. Take those numbers and add them up: 12 (the carbon) plus 16 (one oxygen) plus 16 (the other oxygen) equals 44 – the total mass of ‘light’ carbon dioxide.
Let’s say the carbon is the ‘heavy’ isotope instead (Carbon-13). Math again: 13 (the ‘heavy’ carbon) plus 16 (one oxygen) plus 16 (the other oxygen) equals 45 – the mass of carbon dioxide with heavy carbon.
What if one of the oxygens are heavy? 12 (the carbon) plus 16 (one oxygen) plus 18 (the ‘heavy’ oxygen) equals 46 – the mass of carbon dioxide with one heavy oxygen.
Obviously, there are other combinations possible, but these are rare and we don’t worry about them. What’s important is that carbon dioxide comes in three masses: 44, 45, and 46. An IRMS can separate these out and we can measure them.
Subtle differences between the relative amounts of heavy and light isotopes of oxygen and carbon (and nitrogen and hydrogen) can tell us a lot about the origins and history of the sample that we’re analyzing. For some examples from my own research, look in my blog under “stable isotopes”