A Typical Mass Spectrometer
Mass spectrometer
measures the masses of isotopes, molecules and compounds. It is the most
accurate instrument for getting the mass of compounds and can also easily
differentiate various elements from their respective isotopes.  Often the purity of a substance is dependent
on its boiling and melting point, thus if it is a solid (melting point) and if
it is a liquid (boiling point). 

In a typical mass spectrometer, a pure
substance will give one exact value for the molar mass of the substance.
In a mass
spectrometer, atomic mass unit is usually used, and atomic mass unit is the amount
of matter that has a mass of one-twelfth that of carbon -12. From this
explanation and definition, one carbon atom has a mass of 16 amu, and one hydrogen
atom has a mass of 1 amu.
The mole is the amount
of a substance which contains as many elementary entities such as (molecule,
ions, and atom) as contain in 12 grams of carbon-12. Note the mole of a
substance is represented in mol.