Answer the following question given this paragraph: 

Mass spectrometry today is used extensively in chemistry and biology laboratories to identify chemical and biological substances according to their mass-to-charge ratios. In medicine, mass spectrometers are used to measure the concentration of isotopes used as tracers. Usually, biological molecules such as proteins are very large, so they are broken down into smaller fragments before analyzing. Recently, large virus particles have been analyzed as a whole on mass spectrometers. Sometimes a gas chromatograph or high-performance liquid chromatograph provides an initial separation of the large molecules, which are then input into the mass spectrometer.


Q: Mass spectrometry today is used extensively in chemistry and biology laboratories to identify chemical and biological substances according to their ratios of what?

 Choices:

- ph to charge

- volume to charge

- mass to volume

- mass-to-charge

A:
mass-to-charge
