1 Spanners: Helium-5 decays to helium-4 almost instantly. Your “investment” would have been worthless before you even handed over the money.
2 Serron: No, I did a mass spectrometer test on it after it was loaded. It read helium-5.
3 Spanners: Where did you get a mass spectrometer?
4 Serron: The guy who sold me the helium threw it in for free!

A mass spectrometer is a machine that measures the ratio of mass to charge of particles. It does this by ionising the particles and accelerating them in a magnetic field, where their paths are deflected by an amount that depends on the electric charge and the mass, according to Maxwell's equations and good old Isaac Newton's second law. You can use this to measure the ratios of different elements and isotopes in samples of material.

Mass spectrometers are commonly used in applications requiring the identification of unknown compounds, for scientific studies related to Earth and atmospheric sciences, astronomical samples, forensics, medicine, and also for detection of hazardous materials.

And in the not-so-distant future, as with every invention ever invented, for ripping people off.