Chemometrics in Forensic Chemistry - Part III: Quality assessment and interpretation of chemometric output.

The number of samples sent to forensic laboratories as well as the complexity of the drug situation has increased tremendously during recent years. At the same time the amount of data gathered from chemical measurements has been mounting. This creates challenges for forensic chemists: how to handle the data, how to reliably answer the questions asked, and how to examine the data to find new properties or how to disclose connections with respect to source attribution of samples within a case or retrospective to past cases, stored in a database. Previously published articles Chemometrics in Forensic Chemistry - Part I and II discussed where in the forensic workflow of routine casework chemometrics is applied, and presented examples of chemometric methods used in cases of illicit drugs. This article explains through examples that the chemometric results must never stand-alone. Before such results are reported, quality assessment steps, which may consist of operational, chemical, and forensic assessments are required. In each case a forensic chemist needs to consider the suitability of chemometric methods, based on their strengths, weaknesses, opportunities and threats (SWOT). This is because while chemometric methods are powerful tools managing complex data, they are to some extent chemically blind.

The influence of impact velocity on bullet trajectory deflection through ballistic gelatine.

This paper presents the results of a study on bullet trajectory deflection, for 9 mm Luger Full Metal Jacket Round Nose (FMJ-RN) bullets fired through 23-24 cm of ballistic gelatine. The bullets were fired at different velocities. Impact velocity, energy transfer and bullet trajectory deflection after gelatine perforation were measured and calculated. As was expected, energy transfer to the gelatine blocks generally increased with increasing impact velocity, indicating an altering bullet/gelatine interaction with altering velocity. This alteration did not result in a discernible alteration of bullet trajectory deflection. Deflection angles fell between 5.7° and 7.4° for 136 of the 140 fired shots, with four outliers below 5.7°.

A combined cowhide/gelatine soft tissue simulant for ballistic studies.

The ballistic resistance of a combined soft tissue simulant was studied, consisting of gelatine as a simulant for human muscle tissue and tanned cowhide (leather) as a simulant for human skin. The simulant was manufactured by applying cowhide to liquid ballistic gelatine, as the gelatine solidified in its mould. Combining a skin and muscle tissue simulant in this adhered way opens the possibility to produce purpose-built proxies for human body parts in ballistic studies or for forensic shooting incident reconstructions. Ballistic resistance of adhered cowhide - (bonded) to solidifying ballistic gelatine - was compared to that of the same material applied on gelatine blocks in loose condition. Ballistic resistance of tanned cowhide was found to be more consistent in adhered condition. This enhanced consistency is a benefit, increasing reproducibility of results in ballistic studies. Additionally, two ways to assess ballistic resistance of a skin simulant were described and compared. Logistic regression, from a number of measured velocities and associated (non)perforations is recommended for testing ballistic resistance.

Chemometrics in forensic chemistry - Part II: Standardized applications - Three examples involving illicit drugs.

In the recently published article "Chemometrics in forensic chemistry - Part I: Implications to the forensic workflow" the application of chemometric methods in forensic casework was described. The steps to facilitate standardized chemometric procedures and the availability of chemometric tools such as software and a guideline are under development. Three examples of typical illicit drugs casework, wherein chemometric methods were applied, are presented in the current paper. The kind of questions presented in these examples cover identification, classification, comparison and quantification of illicit drugs. The examples include several types of data (low- or high-dimensional), pre-processing and chemometric analyses that are applied to answer the questions presented. The performance measures for the chemometric methods are described based on separate datasets for training and testing (validation) purposes. In this way it is illustrated how a chemometric method is set up and data analysis may be performed. The presented methods are intended to be easily translatable to questions in forensic chemistry that are not drug-related.

Chemometrics in forensic chemistry - Part I: Implications to the forensic workflow.

The forensic literature shows a clear trend towards increasing use of chemometrics (i.e. multivariate analysis and other statistical methods). This can be seen in different disciplines such as drug profiling, arson debris analysis, spectral imaging, glass analysis, age determination, and more. In particular, current chemometric applications cover low-dimensional (e.g. drug impurity profiles) and high-dimensional data (e.g. Infrared and Raman spectra) and are therefore useful in many forensic disciplines. There is a dominant and increasing need in forensic chemistry for reliable and structured processing and interpretation of analytical data. This is especially true when classification (grouping) or profiling (batch comparison) is of interest. Chemometrics can provide additional information in complex crime cases and enhance productivity by improving the processes of data handling and interpretation in various applications. However, the use of chemometrics in everyday work tasks is often considered demanding by forensic scientists and, consequently, they are only reluctantly used. This article and following planned contributions are dedicated to those forensic chemists, interested in applying chemometrics but for any reasons are limited in the proper application of statistical tools - usually made for professionals - or the direct support of statisticians. Without claiming to be comprehensive, the literature reviewed revealed a sufficient overview towards the preferably used data handling and chemometric methods used to answer the forensic question. With this basis, a software tool will be designed (part of the EU project STEFA-G02) and handed out to forensic chemist with all necessary elements of data handling and evaluation. Because practical casework is less and less accompanied from the beginning to the end out of the same hand, more and more interfaces are built in through specialization of individuals. This article presents key influencing elements in the forensic workflow related to the most meaningful chemometric application and evaluation.

Pistol bullet deflection through soft tissue simulants.

Trajectory deflections of pistol bullets from four different firearms, fired through soft tissue simulants under two different incidence and exit angles were studied. The data from this study can be used in reconstructions of shooting incidents where human soft tissues (not bones) were perforated with pistol bullets and assumptions must be made about bullet deflection in order to correctly reconstruct trajectories. The results demonstrate that deflection was influenced by the length of the "wound channel" through the simulants. In short, deflection was small to virtually absent with 5 and 10cm channel lengths. With channel lengths of 15, 20 and 25cm, there was a clear increase in deflection and/or a more erratic deflection behaviour with most shots. The data also suggest an influence of the angle of incidence and/or exit on both the direction and the magnitude of the deflection.

Preservation and storage of prepared ballistic gelatine.

The use of ballistic gelatine, generally accepted as a human muscle tissue simulant in wound ballistic studies, might be improved by adding a preservative (Methyl 4-hydroxybenzoate) which inhibits microbial growth. This study shows that replacing a part of the gelatine powder by the preservative does not significantly alter the penetration depth of projectiles. Storing prepared blocks of ballistic gelatine over time decreased the penetration depth of projectiles. Storage of prepared gelatine for 4 week already showed a significant effect on the penetration depth of projectiles.