Why perticles ionised before being analysed in a mass spectrometer?

Answer 1

It's because of the way a mass spectrometer works.

It works by accelerating the particles into a magnetic field. In order to accelerate them, they need to be ionized, otherwise they won't be affected by the electric fields used to accelerate them.

As the particles move into the magnetic field, their trajectory curves; charged particles moving perpendicular to magnetic fields feel a force on them that is perpendicular to both their path and to the magnetic field.

Different particles have different masses, though, and so they curve by different amounts with the same force applied. By the amount the trajectory changes, you can calculate the mass.

And, as i hope that explananation makes clear, none of that would work with neutrally charged particles.

Answer 2

mass sensitivity. All particles like life have an essence that is aware of their surroundings.