Fig. 7
a Mass spectra of indigo molecules detected in indigo standard solution; b Mass spectra of residual indigo molecules detected in indigo solution after reaction with O3; c Mass spectra of product isatin molecules detected in indigo solution after reaction with O3; d Mass spectra of product C8H6NO3 detected in indigo solution after reaction with O3; e Mass spectra of product molecules C16H10N2O3 detected in indigo solution after reaction with O3