[throwawaymaths]

Mass specs can detect down to the ten thousands? IIRC or so molecules.

[londons_explore]

But I don't think they can do that at the same time as having trillions of some other uninteresting molecule like water in them.

It's basically a dynamic range problem.

[sseagull]

Mass specs would have this problem, yes. That's why they are usually paired with chromatography of some sort (usually GC/MS or LC/MS). The GC or LC more or less reduce a mixture into components, which are fed into the mass spectrometer to analyze what those components are.

[throwawaymaths]

Yep the Ms that does this is tethered to chromatography first (sometimes even 2 or 3 different separation schemes in tandem)