def get_cantera_mass_fraction(self, species_conversion=None):
        """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'
        """
        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)