Negative Ion Chemical Ionization (NCI) studies using the Thermo Scientific ISQ 7000 single quadrupole GC-MS system

Significance of the Topic

Negative ion chemical ionization offers enhanced sensitivity and selectivity for electrophilic compounds such as environmental pollutants, pesticides, and derivatized steroids and drugs. This soft ionization mode produces abundant molecular ions with minimal fragmentation, enabling trace-level detection in complex matrices.

Objectives and Study Overview

This study demonstrates the NCI performance of the Thermo Scientific ISQ 7000 single quadrupole GC-MS system using methane reagent gas. The goals include determining instrument detection limits with octafluoronaphthalene and assessing the influence of ion source temperature and methane flow rate on mass spectral patterns and chromatographic peak shapes for TMS-derivatized benzo[a]pyrene tetrol and PFPA-HFIP-derivatized 11-nor-9-carboxy-THC.

Methodology

Standard solutions of octafluoronaphthalene at attogram levels were prepared in isooctane. Benzo[a]pyrene tetrol was silylated with MSTFA+1% TMCS at 60 °C for 40 h. 11-nor-9-carboxy-THC was acylated with perfluoropropionic anhydride and hexafluoroisopropanol to yield a highly fluorinated derivative. Data were acquired in SIM and scan modes and processed using Chromeleon CDS software with the Cobra peak detection algorithm.

Used Instrumentation

• TRACE 1310 GC with AI/AS 1310 autosampler
• ISQ 7000 single quadrupole GC-MS with ExtractaBrite source
• Methane as NCI reagent gas at 70 eV, flow 1–3 mL/min
• SIM monitoring at m/z 272 and full-scan acquisition

Main Results and Discussion

• An IDL of 0.1 fg OFN on-column (100 ag) was achieved with S/N>3 and 4% RSD over 100 injections, outperforming EI mode (0.64 fg).
• Lower source temperatures (200–250 °C) favored high-mass ion abundance and reduced low-mass fragmentation but increased peak tailing for silylated analytes.
• Optimal methane flow rates (1–2 mL/min) balanced soft ionization and robustness; higher flows enhanced dissociative capture and low-mass fragmentation.

Benefits and Practical Applications

The ISQ 7000 NCI configuration delivers attogram-level sensitivity, robust operation without an external electron collector, and streamlined analysis of environmental contaminants, steroid metabolites, and drug biomarkers in biological and environmental samples.

Future Trends and Possibilities

Future developments may include integration with MS/MS for enhanced selectivity, automated sample derivatization, high-throughput QA/QC workflows, miniaturized GC-MS platforms, and AI-driven data interpretation to broaden the scope of trace analyte detection.

Conclusion

The Thermo Scientific ISQ 7000 single quadrupole GC-MS demonstrates exceptional NCI performance with 100 ag detection limits. Careful tuning of ion source temperature and reagent gas flow yields an optimal balance between sensitivity, selectivity, and chromatographic performance, making it a powerful tool for diverse trace analysis applications.

Reference

1. Rosenfelder N; Vetter W Rapid Commun Mass Spectrom 2009 23 3807-3812
2. Tagami T; Kajimura K; Takagi S et al Yakugaku Zasshi 2007 127 1167-1171
3. Xiao XY; McCalley D; McEvoy J J Chromatogr A 2001 923 195-204
4. Sparkman OD; Penton ZE; Kitson FG Gas Chromatography and Mass Spectrometry A Practical Guide Academic Press 2011
5. Day B; Naylor S; Gan LS et al Cancer Research 1990 50 4611-4618
6. Huestis MA; Barnes A; Smith ML Clin Chem 2005 51 2289-2295
7. Ciceri E; Albertini T; Caruso A AN 10493 Thermo Fisher Scientific
8. Harrison AG Chemical Ionization Mass Spectrometry CRC Press 1992
9. McLafferty FW; Turecek F Interpretation of Mass Spectra University Science Books 1993
10. Gujar A; Anderson T; Cavagnino D; Patel A TN 10597 Thermo Fisher Scientific