What Is Mass Spectrometry, in Plain English?

Mass spectrometry is a way of weighing individual molecules. A tiny amount of a sample is given an electric charge, then flung through a space where electric or magnetic forces push it sideways. Light molecules are pushed off course easily. Heavy ones barely budge. By watching where each one ends up, the machine works out how heavy it was. That weight is often enough to confirm what the molecule is.

It is the natural partner of the article before this one. HPLC, the slow race through a packed tube, is very good at separating a mixture and timing each arrival, but it cannot tell you what arrived. Mass spectrometry answers exactly that question. It does not time things. It weighs them.

Weighing something by flinging it

You cannot put a single molecule on a kitchen scale. It is far too small, and it would never sit still. So mass spectrometry weighs molecules indirectly, by watching how they move.

Here is the picture. Stand in a strong, steady crosswind and throw a table-tennis ball and a cricket ball with exactly the same push. The table-tennis ball is blown well off course. The cricket ball flies almost straight. If you did not know which ball was which, you could tell them apart just by where they landed. The heavier the ball, the less the wind bends it.

A mass spectrometer does the same thing with molecules. The crosswind is an electric or magnetic force. The throw is a push from an electric field. And the landing spot, or in some machines the time taken to arrive, tells you the weight. There are several designs, and they differ a great deal in engineering, but every one of them is some version of throwing things and seeing how their weight changes the flight 1.

Step one: giving a molecule a charge

The crosswind only works on things that carry an electric charge. An ordinary molecule has no overall charge, so forces of this kind simply pass it by. The first job, then, is to give each molecule a charge. A molecule carrying a charge is called an ion, and the part of the machine that makes them is called the ion source 1.

For a long time this step was the obstacle for anything as large and delicate as a peptide. The older ways of charging a molecule were rough enough to shatter big molecules into fragments before they could be weighed. The breakthrough came from a technique called electrospray. The sample, dissolved in a liquid, is sprayed out of a very fine needle held at a high voltage. The spray breaks into a mist of tiny charged droplets. As the liquid evaporates, the droplets shrink until the molecules inside are left floating on their own, gently charged and still in one piece.

The chemist who developed it for large molecules shared a Nobel Prize for it, and titled his prize lecture after the problem it solved: giving wings to molecular elephants 3. Getting something as big as a protein to fly intact had seemed about as likely as a flying elephant. Electrospray made it routine.

Step two: the fling

Once the molecules are charged, they are pushed into the part of the machine called the mass analyser. This is the crosswind. Electric or magnetic fields bend, filter or time the ions so that they separate according to their weight 1.

There is one wrinkle, and it is worth understanding because it shows up on every printout. How much a force bends a moving ion depends on two things: how heavy it is, and how much charge it carries. More charge means a stronger shove from the same field. So the machine does not measure weight on its own. It measures weight divided by charge. Scientists write this as m/z, where m stands for mass and z stands for the number of charges, and it is the formal quantity a mass spectrum is plotted against 2.

For a small molecule carrying a single charge, dividing by one changes nothing, so m/z is simply the weight. Peptides are different. Electrospray tends to give a peptide several charges at once, so the same molecule can turn up in more than one place on the printout: once carrying two charges, again carrying three, and so on 1. It sounds like a nuisance. In practice it is a gift, because the pattern of those related signals lets the software work backwards to a single, very precise weight for the whole molecule.

Step three: counting the landings

At the end sits the detector, which counts ions as they arrive and amplifies each tiny signal into something measurable 1. The result is drawn as a mass spectrum: a chart with m/z running along the bottom and the number of ions detected running up the side. Each vertical line is a peak, and each peak says that ions of that particular m/z arrived.

The unit chemists use for molecular weight is the dalton, named after the chemist John Dalton. One dalton is roughly the weight of a single hydrogen atom. A short peptide weighs somewhere in the thousands of daltons. A modern instrument can pin that down precisely enough to tell apart two molecules whose weights differ by far less than a single hydrogen atom.

Why weight confirms identity

Here is the reason all this effort is worth it. A peptide is a chain of amino acids, the small building blocks the body strings together. There are twenty or so common kinds, and each has its own exact weight. When they join into a chain, a tiny piece is lost at each link, and that loss is known too.

So if you know which building blocks a peptide is supposed to contain, you can add up their weights on paper and predict what the whole chain should weigh, to several decimal places, before the sample ever goes near the machine. Then you weigh the real thing and compare.

If the two numbers agree closely, that is strong evidence the molecule is the one intended. If they do not, something is different. A building block might be missing. There might be an extra one. Something might have attached where it should not, or part of the molecule might have changed during storage. Weight is such a sensitive fingerprint that even small unintended changes usually show up as a shift. This is why a certificate that reports an identity result usually names mass spectrometry beside it.

Breaking it into pieces to read the order

Some instruments go one step further. They pick out ions of one particular weight, deliberately break them into fragments inside the machine, and then weigh the fragments. This is called tandem mass spectrometry, and it is how laboratories study the building blocks of proteins in bulk 4.

A peptide tends to break at the links between its building blocks. So the fragments form a ladder: one building block, then two, then three, and so on from each end. The differences in weight between the rungs of the ladder reveal which building block sits at each step. It is a little like working out the order of beads on a necklace by snapping it at every link and weighing each piece.

What mass spectrometry cannot tell you

Weighing is powerful, but it has edges, and they are worth knowing precisely.

• The order of the building blocks, from weight alone. Two chains with the same building blocks in a different order weigh exactly the same. Only the fragment ladder can tell them apart.
• Mirror-image forms. Most amino acids come in a left-handed and a right-handed version with identical weights. A plain weighing cannot see the difference.
• How much is in a container. The height of a peak depends on how easily each molecule picks up a charge, so it is a poor guide to amount unless the method has been specially set up and calibrated for that.
• Purity, on its own. A mass spectrum shows what weights are present, but different molecules respond so differently that the peak sizes do not translate directly into shares.
• Anything about living contamination. Bacteria and their leftovers need separate tests.
• Anything about safety in a person. Knowing what a molecule is tells you nothing about what it would do.

That list explains why the two instruments in this set are so often used together. HPLC is good at proportions and poor at names. Mass spectrometry is good at names and poor at proportions. Joined into one instrument, called LC-MS, the race separates the mixture and the scale weighs each arrival as it leaves the tube. Each covers the other's blind spot 4.

The one picture to keep

Keep the crosswind. Give a molecule a charge so the wind can catch it. Throw it. Watch how far it is blown off course. Light things bend a lot, heavy things bend a little, and where they land tells you their weight.

Because the building blocks of a peptide each weigh a known amount, that weight can be predicted in advance and checked, which is why mass spectrometry is the usual answer to the question of identity. It sees the runner's face that the HPLC race could only glimpse through fog. It still cannot see everything: order, mirror images and amounts all need something more. But when a laboratory wants to know whether a molecule is what it says it is, weighing it is almost always where the answer starts.