In a mass spectrometer, the analytes are separated as charged molecules (in some cases also fragments / aggregates) in an electric field. They need to be introduced in the gaseous state. The "gas" has to be very highly diluted so that the ions can be analyzed on individual trajectories. Otherwise collisions with other ions could happen, thereby passing their charge forming hybrids or influencing the trajectories of each other. Coupled to an HPLC system, the ion source must be able to separate and ionize the analytes as effectively as possible from the vast excess of eluent / solvent, so that the ions can be analyzed in the quadrupole.