主题：【求助】这段该怎么翻译？

谁能帮忙翻译下面文字？拜谢了。本人翻译过，只是感觉很拗口。
In recent years, the technique of compound specificisotope analysis (CSIA) has found applications for a variety of biogeochemical questions. Because the stable isotopic signature of individual compounds is determined, added resolution to traditional molecular analyses is gained. The stable isotopic signature of each compound of interest gives an indication of the history of the compound. In most cases, the details that researchers are interested in are centered on the reactants that went into producing a particular compound or what reservoir it came from. Owing to the utilization of different reactants and production processes, two compounds with identical elemental compositions and atomic arrangements would have unique isotopic signatures. For contaminants that were derived from a certain company, combustion process or natural reservoir, unique stable isotopic signatures would serve as a fingerprint for tracing that compound in the environment. This approach to studying isotopic signatures of contaminants has aided in the apportioning of sources of these compounds. Fundamental to this research is the theory that CSIA approaches are less subject to chemical and biological alterations than traditional molecular techniques. Because each compound is investigated separately, the technique does not rely on havingnumerous compounds or compound classes remain unaltered for their entire history. Compound specific isotope analysis thus has the potential for being very robust in light of the effects of chemical or biological alteration because each compound can be treated and interpreted independently rather than being part of an interpretation based on the interdependence of several compounds.
With respect to PAHs, compound specific isotope analysis first was used to apportion sources of anthropogenic compounds, such as those found in various soots and oils transported by urban runoff (O’Malley etal., 1994). Distinct isotopic signatures were observed for many PAHs and were utilized in apportioning sources of the contaminants extracted from harbor sediments. Owing to different isotope fractionations in C3 and C4 vegetation, insight has also been gained on determining the sources of PAHs derived from biomass burning (O’Malley et al., 1997) and between anthropogenic and biomassderived compounds (Ballentine et al., 1996). CSIA has also been used to differentiate between coal-derived PAHs that were produced under varying processing conditions (McRae et al., 1999). In most of the above applications of CSIA, PAH isotopic compositions were determined for compounds that had not been exposed to possible biodegradation. However, chemical or biological alterations may affect the isotopic compositions of the compounds of interest. If there were relatively small changes in concentration of a few compounds because of biodegradation, the concentration ratios would potentially be more affected than the isotopic compositions. However, it has not been established whether the isotopic compositions of some, none, or many PAHs would be affected by extensive biodegradation. Regardless of the concentration changes, if the isotopic compositions of only some of the PAHs were affected,source apportionment could still be achieved by CSIA.Furthermore, fractionation due to biodegradation should be predictable, as the microbes would tend to attack the isotopically depleted (containing less 13C) molecules. Knowledge of fractionations associated with biodegradation would aid greatly in source apportionment studies when systems have experienced complex histories.
It has never been conclusively shown whether biodegradation alters the isotopic signature of PAHs, and if so, the magnitude of this effect and the trend with the extent of degradation. There is some evidence that for low molecular weight PAHs, the effect is minimal (O’Malley et al., 1994; Meckenstock etal., 2001; Lollar, et al., 1999; Ahad et al., 2000) but the effects for higher molecular weight molecules has not been investigated. If CSIA is to be used effectively for source apportionment of PAHs, knowledge of the possible effects of biodegradation is vital, especially if the PAHs have resided in areas, such as sediment, that contain populations of PAH-degrading microbes. The extent and general trends of any isotope fractionations will aid in not only assessing the general applicability of CSIA for sites of possible biodegradation, but will also aid future studies that monitor the fates of PAH in the environment. The overall purpose of this study, therefore, was to monitor in a controlled experiment, the isotopic composition of a suite of PAHs as they were exposed to varying conditions of degradation over time.