Peptide analysis of mammalian decomposition fluid in relation to the post-mortem interval

We report the results of a semi-quantitative peptide analysis of decomposition fluid under field-based conditions in the absence of a soil matrix. Sixteen domestic pig (Sus scrofa domesticus) cadavers were used to model human decomposition in trials conducted in the summer and winter months in Western Australia. Physical characteristics were recorded and targeted peptide components of decomposition fluid were analysed using high performance liquid chromatography-triple quadrupole mass spectrometry. Principal component analysis identified 29 peptides, originating from haemoglobin subunits alpha and beta, creatine kinase, beta-enolase and lactate dehydrogenase, that contributed to differences in the mean peak areas of samples collected during the early period of decomposition (days 6-12 and day 2 in winter and summer, respectively) and during the later period (days 24-34 and days 8-10 in winter and summer, respectively). Fold changes for 8 peptides between these periods were significantly different. Three peptides derived from haemoglobin subunit beta, one from beta-enolase and two from lactate dehydrogenase displayed consistent trends, in that a notable increase in mean peak area was followed by a marked decrease in both the summer and winter samples. When temperature was accounted for, these trends occurred at different time points in summer and winter, indicating that factors other than temperature had impacted the rate of degradation of the proteins involved. The single peptides derived from haemoglobin subunit alpha and creatine kinase displayed consistent increases in mean peak area for the summer samples, suggesting that temperature played the most significant role in their degradation. Further analyses revealed that 7 peptides (one originating from haemoglobin subunit alpha, three from haemoglobin subunit beta and three from lactate dehydrogenase) displayed consistent trends that could be correlated with total body score and with the early stages of decomposition. The consistent trends (mean peak area versus time) for peptides derived from several proteins during decomposition trials conducted under different temperature regimes further emphasised the potential of peptide analysis in time since death estimation.