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.

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.