sentence: Briefly , DNA hydrolysates were spiked with a mixture of internal standards in a volumetric ratio 4:1 to form concentrations of 50 fmols / µ L of [ D ] -5 - hmdC , [ C , N ] -5 - formyl-2 ' - deoxycytidine ( 5-fdC ) , [ C , N ] -5 - carboxyl-2 ' - deoxycytidine ( 5-cadC ) , [ C , N ] -5 - hydroxymethyl-2 ' - deoxyuridine ( 5-hmdU ) and [ N ] -8 - oxodG .