code: def get_cantera_mass_fraction(self, species_conversion=None):
          
          if self.composition_type in ['mole fraction', 'mole percent']:
              raise ValueError('Cannot get mass fractions from the given composition.\n'
                               '{}'.format(self.composition)
                               )
          else:
              return self.get_cantera_composition_string(species_conversion)
docs: Get the mass fractions in a string format suitable for input to Cantera.
  
          Arguments:
              species_conversion (`dict`, optional): Mapping of species identifier to a
                  species name. This argument should be supplied when the name of the
                  species in the ChemKED YAML file does not match the name of the same
                  species in a chemical kinetic mechanism. The species identifier (the key
                  of the mapping) can be the name, InChI, or SMILES provided in the ChemKED
                  file, while the value associated with a key should be the desired name in
                  the Cantera format output string.
  
          Returns:
              `str`: String of mass fractions in the ``SPEC:AMT, SPEC:AMT`` format
  
          Raises:
              `ValueError`: If the composition type is ``'mole fraction'`` or
                  ``'mole percent'``, the conversion cannot be done because no molecular
                  weight information is known
  
          Examples:
              >>> dp = DataPoint(properties)
              >>> dp.get_cantera_mass_fraction()
              'H2:2.2525e-04, O2:4.4775e-03, Ar:9.9530e-01'
              >>> species_conversion = {'H2': 'h2', 'O2': 'o2'}
              >>> dp.get_cantera_mass_fraction(species_conversion)
              'h2:2.2525e-04, o2:4.4775e-03, Ar:9.9530e-01'
              >>> species_conversion = {'1S/H2/h1H': 'h2', '1S/O2/c1-2': 'o2'}
              >>> dp.get_cantera_mass_fraction(species_conversion)
              'h2:2.2525e-04, o2:4.4775e-03, Ar:9.9530e-01'