Forensic, definition

Informed debate and decisions on such important matters as the depletion of the ozone layer, acid rain and the quality of waterways all depend on the data provided by analytical chemists. Forensic evidence also often depends on chemical measurements. National and international trade are critically dependent on analytical results. Chemical composition is often the basis for the definition of the nature of goods and tariff classification. In all of these areas not only is it important to get the right answer but it is essential that the user of the results is confident and assured that the data are truly representative of the sample and that the results are defendable, traceable and mutually acceptable by all laboratories. 

A comprehensive approach to a states response to a chemical terrorism includes having a plan not only for the crisis and consequence management phases of the incident, but also for all elements required for complete resolution of the event. This may include the necessity to definitively establish whether chemical agents were used, to provide supporting evidence to confirm other analyses, or to provide the forensic proof required to support a criminal prosecution. The collection and analysis of biomedical samples - blood, urine or other tissue from affected humans or animals - is one of the means for providing such information. Although current capabilities such as urinary thiodyglycol excretion or plasma cholinesterase activity can be performed, there is scope for far more sensitive and specific assessments that overcome the limitations of these approaches. 

The first forensic application of the method occurred in 1989 and utilized 14C activity as an isotopic tracer to assign human bone samples to one of three periods a nonmodern period (prior to A.D. 1650), a premodern period (A.D. 1650-1950), and a modern period (A.D. 1950 to present) (Taylor et al. 1989). A bone dated in the nonmodern period was considered to be of no forensic interest if dated in the premodern period it was considered to be potentially of forensic interest, and a sample dated in the modern period was considered to be of definite forensic interest. The technique was successful in identifying two of the five samples analyzed as being of forensic interest. The advantages of the 14C method were its relative accuracy for the period in question and, more importantly, the fact that it was not affected by environmental variables. However, disadvantages of the method included the cost and amount of time taken to analyze the samples as well as the size of the sample required and the destructive nature of the technique. 

Non-Polymeric Materials. Waxes, soaps, greases, asphalts, oils. Complex mixtures of hydrocarbons of different molecular weight when treated to a common thermal history (such as slow program-cooling from above the melt) give complex but highly repeatable characteristic DSC melting profiles (16,17). In the absence of any other forensic evidence, these thermal profiles should be sufficiently specific and repeat-able to be definitive evidence. 

TLC is not quantitative and while it is a rapid and inexpensive means of obtaining exclusion of any relationship between heroin samples, it cannot be used with any reliability to confirm that two samples were once part of the same larger batch. HPLC is quantitative but does suffer from lack of definitive identification of each (chromatographic) peak. It is for these reasons that many forensic science laboratories use GC-MS for the comparison of heroin samples. 

Acceptable forensic practice recommends confirmation of the detection of an analyte (drug) by a second or definitive method. For VOCs, two acceptable GC confirmation procedures are re-sampling and re-analysis using a different column or use of mass spectrometry for absolute identification. 

---