def check_availability(self, isos):
        '''
        This routine checks if the requested set of isotopes is
        available in the dataset.

        Parameters
        ----------
        isos : list
            set of isotopes in format ['Si-28','Si-30'].

        Returns
        -------
        list
           [index, delta_b, ratio_b].
           index: where is it.
           delta_b: is it a delta value or not?
           ratio_ib:  True if ratio is inverted, false if not

        '''
        # make names
        iso1name =  iso_name_converter(isos[0])
        iso2name =  iso_name_converter(isos[1])

        ratio = iso1name + '/' + iso2name
        ratio_inv = iso2name + '/' + iso1name
        delta = 'd(' + iso1name + '/' + iso2name + ')'
        delta_inv = 'd(' + iso2name + '/' + iso1name + ')'

        index = -1
        # search for data entry
        try:
            index = self.datadict[ratio]
            delta_b = False
            ratio_b = False
        except KeyError:
            try:
                index = self.datadict[ratio_inv]
                delta_b = False
                ratio_b = True
            except KeyError:
                try:
                    index = self.datadict[delta]
                    delta_b = True
                    ratio_b = False
                except KeyError:
                    try:
                        index = self.datadict[delta_inv]
                        delta_b = True
                        ratio_b = True
                    except KeyError:
                        index = -1
                        delta_b = None
                        ratio_b = None
        return index, delta_b, ratio_b
This routine checks if the requested set of isotopes is
        available in the dataset.

        Parameters
        ----------
        isos : list
            set of isotopes in format ['Si-28','Si-30'].

        Returns
        -------
        list
           [index, delta_b, ratio_b].
           index: where is it.
           delta_b: is it a delta value or not?
           ratio_ib:  True if ratio is inverted, false if not