The Evolving Role of GC×GC in Forensic Analysis

The application of oil spill forensic investigations could —and has — benefited greatly from the additional peak capacity offered by comprehensive two-dimensional gas chromatography (GC×GC). Petroleum and other hydrocarbon heavy applications benefit so greatly from GC×GC because of the complexity of the sample types across the application and the number of analytes within one sample.^1–3Analyzing these samples using one-dimensional gas chromatography (GC) typically results in an “unresolved complex mixture,” or hump, in part of the chromatogram where a large portion of the analytes are unresolved and consequently unidentified. Other forensic applications discussed in this interview such as toxicology or fingerprint chemistry might not have the same number of analytes in each sample; however, these samples benefit greatly from the additional peak capacity of GC×GC when performing analysis of traces within a biological matrix, such as blood, plasma, fingerprint residue, etc.^4–6

Many forensic GC×GC studies focus on non-targeted analysis. What are the principal challenges of validating non-targeted workflows for use in accredited forensic laboratories?

Validating non-targeted workflows are a challenge because of the lack of standardization that currently exists compared to targeted or quantitative analyses. Without standardized protocols, nontargeted analysis has traditionally involved a large component of manual curation that can be subjective to the analyst. However, sampling, sample preparation, and chromatographic methodologies can be validated through a variety of figures of merit to provide a level of standardization and performance criteria across laboratories, the same as they are already done in one-dimensional gas chromatography mass spectrometry (1D GC–MS). Data processing methods for nontargeted workflows can also be standardized to a degree, but ultimately includes some level of interpretation from an analyst. With proper training and documentation, forensic analysts can demonstrate their expertise in a courtroom.

Your review emphasizes analytical readiness and legal admissibility. What additional validation data would you consider essential before introducing a GC×GC method into routine forensic casework?

There are many criteria that must be met for analytical methods to be considered reliable within a forensic laboratory. However, the final validation steps that are essential include intra- and inter-laboratory validation studies and standardization of chromatographic and data processing methods. In the paper, we discuss a 4-level technology readiness level (TRL) scale used in the field of forensic chemistry.^7–9 These two criteria are included in the Level 4 TRL before implementation or adoption into routine casework. Intra- and inter-laboratory validation ensures a method is rugged and can reproduce acceptable results over a variety of conditions. Standardization of sampling, chromatographic, and data processing methods would increase confidence in the reliability of a forensic method.

How do the requirements of legal frameworks such as the Frye Standard, Daubert Standard, Federal Rule of Evidence 702, or the Mohan Criteria influence the development and implementation of new chromatographic methods?

Requirements of legal frameworks for admissibility of expert witness testimony (e.g. Frye and Daubert Standards, Federal Rule of Evidence 702, or Mohan Criteria) as well as the expert witness evaluation based on educational and training background (voir dire) are essential to development and implementation of new chromatographic methods within forensic laboratories. The criteria that guard expert witness testimony in court are guidelines for researchers to follow to ensure new methods are developed to be rapid, reliable, and reviewable for active casework. They also help to guide researchers to focus on which studies will be the most useful next steps towards routine adoption of new methods, helping to keep forensic research on track with practical needs.

The authors of the review identify inter-laboratory validation, error rate determination, and standardization as priorities. Which of these do you consider the greatest barrier to widespread forensic adoption of GC×GC, and why?

The greatest barrier to widespread forensic adoption of GC×GC could be standardization. Inter-laboratory validation and error rate determination are both key parts of the standardization of methods from the Organization of Scientific Area Committees for Forensic Science (OSAC), the American Society for Testing and Materials (ASTM) International, or other standards developing organizations. These organizations publish standardized methods for forensic evidence analysis. However, the publishing of a standardized method for a specific application, a hallmark of TRL 4, includes many of the other requirements of previous technology readiness levels.

Several forensic application areas—including illicit drugs, toxicology, fingerprint residue, CBRN materials, arson debris, and oil spill analysis—are reviewed. Which application do you think is closest to routine implementation, and what evidence supports your assessment?

Although both oil spill forensics and decomposition odor both are categorized as a TRL 3 due to the number of studies that have been conducted, sophisticated data processing guidelines, and lack of inter-laboratory studies and known error rates, the application of oil spill forensics is probably closest to routine implementation. This application has a greater number of studies conducted on real forensic samples (nineteen oil spills) compared to odor decomposition (fifteen human cadavers). In addition, there are existing ASTM methods, for example ASTM D3328-00 or ASTM D5739-06, for comparison of petroleum oils for source identification whereas no such standardized methods exist for the analysis of odor. Existing 1D GC standardized methods can guide researchers in methodology, necessary figures of merit, and adequate data processing strategy for acceptable standardized methods, easing the way for development of GC×GC methods and implementation to casework.

The paper uses a technology readiness scale (TRL 1–4) to evaluate forensic GC×GC applications. How useful is a technology readiness framework for assessing whether an analytical method is suitable for operational forensic laboratories?

A technology readiness scale is extremely useful for quantifying the readiness of a technology for forensic laboratories. Each of the four levels includes benchmark criteria for method development that build on the previous level. Having clear criteria for technology readiness at each level, and a clear progression from level to level, directs researchers to design methodology that is rapid, reliable, and reviewable by record for courtroom presentation.

As GC×GC instrumentation and data processing become increasingly sophisticated, how should forensic laboratories balance the benefits of enhanced analytical capability against increased method complexity, cost, training requirements, and explainability in court?

As with any new technology within a research or routine laboratory, forensic laboratories must invest the initial cost for acquiring new instrumentation, validating new instrumentation that has been acquired, and time for training personnel on new equipment in order to be able to better serve the courts and the justice system. This initial cost of instrumentation and training would be worth the reward if it can be used routinely in specialized forensic laboratories. Routine use would require finalization of standardized methods for multiple sample types such as ignitable liquid residue or oil spill investigations that have existing one-dimensional standardized methods. Explainability in court should have no more difficulty than current methods; one published study found that non-experts rated GC and GC×GC output plot comparisons to be no harder to distinguish than two photographs, indicating that forensic analysts should have no more difficulty in those comparisons as well as lay-person juries.¹⁰ Implementation of sophisticated instrumentation into forensics laboratories depends highly on its usability.

References

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6. Ladislavová, N.; Pojmanová, P.; Vrbka, P.; Šnupárková, J.; Urban, Š. Human Scent Signature on Cartridge Case Survives Gun Being Fired: A Preliminary Study on a Potential of Scent Residues as an Identification Tool. PLoS ONE 2023, 18 (3), e0283259. DOI: https://doi.org/10.1371/journal.pone.0283259
7. Federal Rule of Evidence 702 1975.
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