No ion, no signal
Recall the central rule from the first guide: a mass spectrometer can only detect things that carry a charge. A plain, neutral molecule drifts through the instrument unseen, like a person with no badge slipping past a sensor. So the very first job, before any weighing can happen, is ionization: giving each molecule that electric 'badge'. This single step deserves a whole guide because the way you do it shapes everything that follows — the choice of ionization method is one of the most consequential decisions in the whole experiment.
There is more than one way to charge a molecule. You can rip an electron out of it; you can hand it an extra proton, so it becomes positive; you can pull a proton off, so it becomes negative; or you can stick it to an already-charged partner. All of these turn a silent neutral molecule into something the instrument can grab. The big practical split is between gentle methods that leave the molecule whole and rough methods that tend to break it.
Electrospray: lifting fragile giants gently
Picture a perfume atomizer, but instead of ordinary mist it sprays a fine cloud of electrically charged droplets. As each tiny droplet flies through the air it dries and shrinks, until the crowded charges packed inside burst it into still-smaller droplets — again and again — until at last bare charged molecules float free in the gas. That delicate cascade is electrospray ionization, or ESI. It is the gentlest method in common use: the molecule is coaxed into the gas phase rather than blasted, so even large, fragile species survive intact.
ESI changed biology forever, because it made it routine to weigh proteins, peptides, and DNA — delicate molecules that older harsh methods would simply have destroyed. It has a charming quirk: a big molecule sprayed this way often picks up several protons at once, so it appears carrying many charges. Far from a problem, this is a gift — recall that m/z is mass divided by charge, so a huge molecule with many charges shows up at a modest m/z value that ordinary instruments can handle.
MALDI: a laser and a protective crowd
There is a second gentle method for big, fragile molecules, and it works in a completely different way. Imagine you must launch a delicate egg into the air without cracking it. Throwing it directly would smash it — so instead you bury it in a crowd of tough rubber balls and fire a blast at the crowd; the rubber balls absorb the shock and carry the egg up unharmed. That is the idea behind MALDI — matrix-assisted laser desorption/ionization. The fragile molecule is mixed into a large excess of a small, sturdy 'matrix' compound, and a pulse of laser light strikes the mixture.
The matrix soaks up the laser's energy and bursts into a tiny gas plume, gently carrying the big molecule along with it and handing it a charge in the chaos. The molecule survives, now ionized and airborne. Two differences from electrospray are worth noting: MALDI usually makes singly charged ions (so m/z reads straight as mass), and it fires in pulses, one laser shot at a time — a feature that, as the analyzer guide will show, marries beautifully to a particular kind of mass analyzer.
Electron ionization: the deliberate hard knock
Now the rough method — and, surprisingly, its roughness is exactly why it is loved. In electron ionization (EI), a beam of fast-moving electrons is fired at gas-phase molecules. An electron slams into a molecule and knocks one of its own electrons clean out, leaving the molecule positively charged. But the collision delivers so much extra energy that the freshly made ion is left trembling — far too energetic to stay whole — and it shatters into smaller charged pieces.
Why would anyone want their molecule smashed? Because a given molecule breaks in characteristic, repeatable places, and electron ionization is so standardized that the *same* compound gives the *same* shattering pattern on any EI instrument in the world. That makes giant searchable libraries possible: shatter your unknown, compare its pattern to millions of reference patterns, and get an identification in seconds. The trade-off is that the intact-molecule peak, the molecular ion, can be faint or even missing — which is precisely why the gentle methods exist alongside it.
The fairness catch, revisited
Here is the honest subtlety that every analyst lives with. Different molecules ionize with wildly different ease — one compound may give charged ions readily while another resists, even at the same amount present. So the height of a peak reflects two things tangled together: how much of the substance is there, *and* how willingly it ionizes. This is why you cannot simply read peak heights as 'how much of each' without calibration, and why matching ionization method to sample is not a detail but a craft.