Here are some good explanations of how HPLC and ESI-MS work in the scientific field.

HPLC: https://www.chemguide.co.uk/analysis/chromatography/hplc.html

ESI MS: https://chem.libretexts.org/Textbook_Maps/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Instrumental_Analysis/Mass_Spectrometry/Mass_Spectrometers_(Instrumentation)/Electrospray_Ionization_Mass_Spectrometry

Scintillation Counter Diagram

From http://www.ntanet.net/how-do-sodium-iodide-scintillation-detectors-work