public static boolean isBackboneFragmentIonType(FragmentIonType ionType) {
        return (ionType.equals(FragmentIonType.A_ION) || ionType.equals(FragmentIonType.B_ION) || ionType.equals(FragmentIonType.C_ION) ||
                ionType.equals(FragmentIonType.X_ION) || ionType.equals(FragmentIonType.Y_ION) || ionType.equals(FragmentIonType.Z_ION));
    }