Weathering of Hydrocarbons

Hydrocarbons on living organisms such as insects will change over time as new compounds are synthesised each time through the insects' biosynthesis. Furthermore, as compounds are emitted, either as signalling compounds or used in nestmate recognition, or during trophilaxis some compounds will decrease over time. However, the question to be asked is, whether the hydrocarbon composition will change over time independently of any biosynthesis, a situation that might occur on the puparia. In this case, no new compounds can be added in time and hence the stability of hydrocarbons is important. The question is how likely will they degrade or change over time? A large number of papers have been published on this topic, but not in relation to insects but in relation to oil spills and gasoline releases (for reviews see Christensen and Tomasi 2007; Medeiros and Simoneit 2007; Morrison 2000).

However these studies can give valuable information to a forensic entomologist on the behaviour of hydrocarbons in more natural conditions. Oils and gasoline consist mainly of hydrocarbons (linear and cyclic, saturated and unsaturated) and depending on the origin of an oil sample the hydrocarbon profile or composition will be different. When an oil spill has been observed it is very important to establish the time when the actual spill or release occurred. The area of environmental forensics deals with this type of research and already several models have been investigated in order to determine the origin of an oil/gasoline (oil spill fingerprinting, e.g. Christensen and Tomasi 2007) and to estimate when certain oil has been released. An excellent review in the textbook Environmental Forensics presents these different models or attempts to establish the time of release (Morrison 1999).

This change of the hydrocarbon composition is referred to as weathering, which can be further divided into (1) physical weathering (e.g. evaporation, dissolution), (2) biological weathering (microbial degradation, see above) and (3) chemical weathering (e.g. photodegradation under influence of sunlight) (Malmquist et al. 2007). Of these three processes the physical weathering will transport compounds from one area to the other, whereas the latter two will degrade and/or chemically alter the individual compounds. Very stable and unreactive compounds will survive and will see a much smaller change over time than less stable compounds. In general the amount of straight chain alkanes (especially those with low molecular masses) will decrease first followed by branched chain alkanes, aromatics and lastly by cycloalkanes. Within these models (e.g. to estimate the time of release) quite often the ratio of certain compounds are used, as this ratio seem to change over time. In an attempt by Schmidt et al. (2002), the ratio of toluene (a cyclic aromatic hydrocarbon) against octane (a linear alkane) was used to identify the origin and time of a certain gasoline release. Studying several GC chromatograms over time, there seems to be a trend in this ratio over the last 30 years, with the ratio of toluene/octane increasing and interestingly a jump in 1994. However as the authors state in the article, this is only useful if less than 50% of the gasoline release has evaporated. This model of using the ratio between several important compounds (such as biomarkers, see review by Medeiros and Simoneit 2007) has been used by other researchers as well. Frequently a compound that is very stable (low volatility and low water solubility) will be used as a (bio)marker and ratios of this marker to other less stable hydrocarbons are calculated. Examples that have been used are the ratio of heptadecane/pristine (Christensen and Larsen 1993) or octade-cane/phytane (Sauer et al. 1998).

0 0

Post a comment