Drug Imaging on a Model Hair Sample - Toward the Observation of Drug Use History -

Significance of the Topic

The use of imaging mass spectrometry to map drug deposits in hair provides a non-invasive historical record of substance intake. As hair grows at a steady rate and incorporates trace analytes from the bloodstream, detailed spatial visualization of drug distribution along its length can reveal timing and extent of exposure. This capability is highly valuable in forensic investigations, clinical compliance monitoring and anti-doping analysis.

Objectives and Study Overview

The study aimed to develop and validate a workflow for high-resolution imaging of a model drug (methoxyphenamine, MOP) in hair sections. Specific goals included:

• Preparation of hair samples spiked with controlled MOP levels.
• Optimization of matrix application and instrument settings for MS/MS imaging.
• Visualization and quantification of MOP distribution at different spatial resolutions.

Methodology and Instrumentation

Model Hair Preparation:

• Human hair from a drug-free donor was cut into 2 cm segments and soaked at 37 °C for 24 h in MOP solutions:
• Sample A: 100 μg/mL
• Sample B: 10 μg/mL
• Sample C: 0 μg/mL (negative control)

Quantitative Verification:

• Washed hairs were extracted and analyzed by LC–MS to confirm MOP levels: 83.1 ng/mg (A) and 20.0 ng/mg (B).

Imaging Protocol:

• Longitudinal sections (~half hair diameter) were prepared with a microtome and mounted on conductive tape.
• CHCA matrix was applied by sublimation (iMLayer) or by a two-step sublimation+spray method.
• Measurements were conducted on an iMScope TRIO system in positive ion mode using MS/MS transition m/z 180.14 > 149.10.
• Spatial resolutions of 50 μm and 10 μm were compared (laser diameters φ50 μm and φ10 μm).

Main Results and Discussion

MS Images at φ50 μm:

• Drug signals co-localize with hair regions in optical images.
• No MOP signals detected in the negative control.

High-Resolution Imaging at φ10 μm:

• Revealed MOP concentrated in peripheral layers (cuticle and cortex), with limited medullary deposition.

Enhanced Sensitivity via Two-Step Coating:

• Applying a spray layer after sublimation increased signal intensity by 3–6×, improving contrast and detection.

Benefits and Practical Applications

This method enables:

• Temporal reconstruction of drug intake based on hair growth rate (~1 cm/month).
• Non-destructive, label-free analysis preserving spatial context.
• High specificity via MS/MS transitions, reducing background noise.

Applications extend to forensic toxicology, therapeutic drug monitoring, doping control and hair care product evaluation.

Future Trends and Potential Applications

Advances may include:

• Multi-analyte imaging to profile drug cocktails or metabolites.
• Integration with software for automated timeline reconstruction.
• Adaptation to curved or multi-section hair arrays for higher throughput.
• Combination with other imaging modalities (e.g., Raman) for chemical and structural correlation.

Conclusion

The developed MS imaging workflow demonstrates reliable, high-resolution mapping of a model drug in hair. By optimizing matrix deposition and laser parameters, researchers achieved clear visualization of MOP distribution in hair cross-sections. This approach holds promise for detailed retrospection of substance use and broad applications across forensic and clinical fields.