The sorting heart of the machine
We have made ions and we know how to read the final picture. The missing middle step is the sorting: something inside the instrument must take the jumbled crowd of ions and separate them by their m/z. That something is the mass analyzer, and it is the true heart of a mass spectrometer. Think of a postal machine that routes parcels into bins purely by weight, so that by the end everything of the same weight has landed together — except this machine sorts ions by m/z, and it does so using nothing but electric and magnetic fields.
The analyzer fixes two qualities of the whole instrument that you will hear about constantly. First, resolution: how finely it can tell two close masses apart — can it separate a peak at 200 from one at 200.1, or does it blur them into one? Second, mass range: the heaviest and lightest ions it can sort at all. No single design wins on everything, so choosing an analyzer is always a trade-off between resolution, speed, mass range, and cost.
The quadrupole: a tunable gate
The most common analyzer on everyday benchtop instruments is the quadrupole. Picture four parallel metal rods arranged in a square, with the ion beam flying straight down the middle. The rods carry voltages that flip back and forth very fast, creating an oscillating field that makes every ion wobble as it flies through. Here is the magic: for any given setting of the voltages, only ions of *one particular m/z* wobble in a stable, surviving path; all others swing too wildly, crash into the rods, and are lost.
So a quadrupole is really a tunable gate that lets exactly one mass through at a time. To build a whole spectrum, it sweeps its voltages rapidly across a range, opening the gate to m/z 50, then 51, then 52, and so on, recording how many ions come through at each setting. A full sweep takes only a fraction of a second, so it can scan again and again, which is exactly what you want when compounds are flowing past one after another from a separation.
Time-of-flight: a race down a tube
A completely different idea powers the time-of-flight analyzer, or TOF. Recall the runners from the first guide: fire a starting pistol, give every runner the same shove, and let them race down a hallway. Light runners reach the far wall first; heavy ones arrive last. Just by timing each arrival, you can rank everyone by weight. A TOF analyzer does exactly this — it gives every ion the same electric shove, so they all carry the same energy, then sends them flying down a long, empty tube.
Because they all start with the same energy, lighter ions (lower m/z) fly faster and strike the detector sooner, while heavier ions arrive later. The instrument times each arrival with extraordinary precision — we are talking millionths of a second — and converts flight time directly into m/z. Unlike the quadrupole, which looks at one mass at a time, a single TOF push captures the *entire* mass range at once, and it handles very heavy ions with ease, which is why it is a workhorse for large molecules.
TOF marries beautifully to MALDI, the laser method from the ionization guide. Each laser pulse launches a fresh batch of ions and gives a clean, sharp starting time for the clock — exactly what a flight-time measurement needs. That is why 'MALDI-TOF' is one of the most famous pairings in the whole field, especially for weighing big biomolecules. The honest caveat is that TOF's accuracy depends on every ion truly starting alike; tiny spreads in starting position or energy smear the arrival times, so high resolution demands careful design (modern instruments add an ion 'mirror' to sharpen the timing).
Picking the right tool
How does a chemist choose? It comes down to the question being asked. If you are hunting for a few known compounds at very low levels and want a rugged, affordable instrument that can park on chosen masses, a quadrupole is hard to beat. If you need to weigh huge molecules, capture a wide mass range in a single shot, or chase high resolution, a time-of-flight earns its keep. And there is no rule that you must pick just one — many of the most capable instruments chain analyzers together, a theme the final guide explores.