Direct injection of plasma samples after ultrafiltration into programmed temperature vaporiser-gas chromatography-mass spectrometry with packed liner

Significance of the Topic

Direct injection of ultrafiltered plasma into GC-MS offers a streamlined approach for bioanalysis in drug development, reducing sample preparation time and solvent use while preserving sensitivity and selectivity for local anesthetics such as ropivacaine.

Objectives and Overview

This study aimed to evaluate the feasibility of injecting plasma ultrafiltrate directly into a gas chromatograph with a packed injector liner. Ropivacaine and its primary metabolite PPX were selected as model compounds to assess method performance.

Methodology and Instrumentation

Sample preparation involved acidification of thawed plasma with phosphoric acid followed by ultrafiltration using a 20 kDa cutoff device. A 50 µL aliquot of filtrate was injected into a GC-MS fitted with a programmed temperature vaporizer (PTV) and an ATAS “A” packed liner. Chromatographic separation employed a BPX35 column with helium carrier gas and a temperature gradient from 80 °C to 330 °C at 30 °C/min. Mass spectrometric detection used electron impact ionization (70 eV), scanning from 70 to 300 amu.

Main Results and Discussion

The method exhibited linear calibration ranges up to 1140 nmol/L for ropivacaine and 1490 nmol/L for PPX (R²>0.996). Limits of quantification were 1.1 nM (301 pg/mL) for ropivacaine and 1.4 nM (325 pg/mL) for PPX. Accuracy (102–117%) and precision (CV ≤16%) met bioanalytical criteria. No significant matrix interferences or carry-over (<2%) were observed. The PTV liner maintained consistent response for approximately 20 injections before requiring replacement.

Benefits and Practical Applications

• Elimination of liquid–liquid and solid-phase extraction steps
• Rapid turnaround and reduced operational costs
• High sensitivity and specificity for pharmacokinetic studies
• Applicability to QA/QC and metabolic profiling in preclinical development

Future Trends and Applications

Advances in PTV design, back-flash techniques and membrane materials are expected to extend liner life and further minimize matrix effects. Coupling this approach with tandem MS and automated platforms will broaden its utility in high-throughput bioanalysis.

Conclusion

The validated direct ultrafiltration–GC-MS method provides a simple, robust and sensitive solution for quantifying ropivacaine and PPX in plasma, offering significant advantages over conventional extraction procedures.

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