Malondialdehyde (MDA), a product of lipid peroxidation, causes mutations in bacterial and mammalian cells and cancer in rats. MDA reacts with deoxynucleosides in vitro and the monomeric adduct of MDA with deoxyguanosine (M1G-dR) is the major adduct formed. We have developed a sensitive analytical method to characterize and quantify M1G-dR from biological matrices using gas chromatography/electron capture negative chemical ionization mass spectrometry (GC/ECNCI MS). Reduction of M1G-dR with sodium borohydride produced a dihydro derivative (H2-M1G-dR). This more stable analog had improved high-performance liquid chromatographic characteristics which facilitated its isolation from biological fluids. H2-M1G-dR was converted to a monopentafluorobenzyl derivative with simultaneous depurination; it was then converted to the corresponding t-butyldimethylsilyl derivative and analyzed by GC/ECNCI MS. (2H2)H2-M1G was used as internal standard. Quantitative analysis was carried out using selected ion monitoring of m/z 302 and m/z 304 where the limit of detection was 10 pg (30 fmol) injected on-column. The level of M1G-dR in normal rat liver was 5.2 +/- 0.2 modified bases per 10(7) bases (n = 6 rats).