Method Development of Mixed-Mode Solid Phase Extraction for Forensics Applications

Significance of the Topic

Sample preparation is a critical step in forensic toxicology workflows, directly impacting sensitivity, selectivity and reliability of downstream chromatographic analyses. Mixed-mode solid phase extraction (SPE) offers combined hydrophobic and ion-exchange interactions that are especially well suited to isolate basic, lipophilic drugs such as phencyclidine (PCP) from complex biological matrices. Optimizing mixed-mode SPE can reduce method development time, lower detection limits and improve data quality in both LC/MS/MS and GC/MS applications.

Objectives and Study Overview

This study presents a complete SPE method development workflow for extracting PCP from human urine using Agilent Bond Elut Certify mixed-mode cartridges. Key goals included:

• Developing an optimized SPE procedure that balances removal of interferences with high PCP recovery
• Comparing wash and elution solvents to identify conditions yielding clean extracts
• Evaluating sample solvent composition and injection volume effects on LC/MS/MS performance

Methodology

Standards of PCP and PCP-D5 were spiked into control urine (19 ng/mL). Bond Elut Certify cartridges (130 mg, 3 mL) were conditioned with methanol and phosphate buffer, equilibrated, then loaded with the sample. Wash steps were split into acidic aqueous rinse and organic washes. Elution was tested using acetonitrile (ACN) + 2% NH₄OH, methanol + 2% NH₄OH and a DCM:IPA:NH₄OH mixture. All fractions were collected, evaporated and reconstituted for chromatographic analysis. A secondary study varied sample solvent (5–100% ACN with/without formic acid) and injection volumes (2–10 µL) to assess peak shape and response.

Instrumentation

The study employed LC/MS/MS with tandem quadrupole detection and GC/MS with pulsed splitless/split injection modes. Detailed instrument parameters followed published SAMHSA-compliant protocols. High-purity reagents and LC/MS-grade solvents were used to minimize background noise.

Main Results and Discussion

The loading and early washing steps showed no PCP breakthrough, confirming strong sorbent retention. MeOH-based elution produced dirty extracts and lower recoveries, whereas ACN + 2% NH₄OH and DCM:IPA:NH₄OH yielded visually clear eluates and high peak areas. Full-scan chromatograms demonstrated minimal background with ACN-based elution. Injection studies indicated that at small injection volumes (2 µL), sample solvent had limited impact. For larger volumes, matching sample composition to the initial mobile phase improved peak shape and intensity.

Benefits and Practical Applications

Optimized mixed-mode SPE delivers:

• Efficient removal of urine interferences while preserving PCP analyte
• Reduced method development time through systematic wash and elution screening
• Enhanced sensitivity meeting forensic screening guidelines
• Compatibility with both LC/MS/MS and GC/MS platforms

These improvements support robust toxicology workflows in forensic and clinical laboratories.

Future Trends and Opportunities

Emerging directions in SPE for forensic analysis include:

• Automation and online SPE coupling to liquid chromatography systems
• Use of greener, alternative extraction solvents and sorbent materials
• Miniaturized SPE formats for low-volume or high-throughput applications
• Integration with high-resolution mass spectrometry for broader screening panels

Adoption of these innovations will further streamline workflows and enhance analytical performance.

Conclusion

This application note demonstrates that systematic optimization of mixed-mode SPE—through judicious choice of wash and elution conditions and careful evaluation of injection parameters—can yield high-quality, interference-free extracts for PCP analysis. The optimized protocol meets regulatory sensitivity requirements and is readily adaptable to other analytes and matrices.

Reference

Mike Chang. Analysis of Phencyclidine in Urine to U.S. SAMHSA Guidelines with LC/MS/MS and GC/MS. Agilent Technologies Application Note, 5991-4575EN (2014).