Revealing Fine Isotope Structure with Resolution Enhancement Mode on the SELECT SERIES™ MRT

Importance of the Topic

High-resolution mass spectrometry (HRMS) plays a pivotal role in pharmaceutical research, environmental monitoring and metabolomics by enabling confident identification of unknown compounds in complex matrices. Interrogation of fine isotope patterns further strengthens empirical formula assignment, reducing false positives and improving data reliability.

Objectives and Study Overview

This application brief evaluates a novel Resolution Enhancement Mode (REM) on the SELECT SERIES™ MRT, a hybrid Q-Tof instrument featuring a multi-reflecting time-of-flight analyzer. The study aims to demonstrate how extending the ion flight path via a second analyzer pass boosts resolving power beyond 300 000 FWHM at chromatographic scan rates. Non-targeted LC-MS screening of common drug metabolites in diluted human urine serves as the test case.

Instrumentation

• Mass Spectrometer: Waters SELECT SERIES™ MRT with multi-reflecting TOF analyzer
• Acquisition Mode: Resolution Enhancement Mode (REM) enabling a ~92 m flight path
• LC System: Waters ACQUITY™ UPLC™ I-Class Premier
• Column: ACQUITY UPLC HSS T3 C18, 100 mm × 2.1 mm, 1.8 μm

Methodology

Urine from a healthy volunteer was collected at 2, 4 and 6 hours after dosing with carbamazepine, acetaminophen and naproxen. Samples were diluted 1:10 in water and injected (5 μL) onto the UPLC at 0.5 mL/min, 40 °C column temperature, with a water/acetonitrile gradient (both containing 0.1% formic acid). REM synchronized the ion packet release to maintain duty cycle while selectively triggering a second analyzer pass for the m/z window of interest.

Main Results and Discussion

Base peak chromatograms at the four-hour time point revealed numerous overlapping features. Two co-eluting metabolites at 4.8 min, desmethyl naproxen sulfate (m/z 295.03, C13H10O6S) and carbamazepine hydroxy sulfate (m/z 331.04, C15H10N2O5S), were isolated via extracted ion chromatograms. Mass spectra exhibited resolving power >300 000 FWHM, achieving mass accuracy within ±0.6 ppm. Fine isotope structures clearly resolved minor isotopologues such as 18O, 33S and 15N, unambiguously confirming elemental composition.

Benefits and Practical Applications

By leveraging REM, analysts can:

• Confirm empirical formulas through direct observation of multiple isotopes
• Improve confidence in non-targeted metabolic profiling
• Maintain chromatographic throughput while achieving ultrahigh resolution
• Reduce compound misidentification in drug development and QA/QC workflows

Future Trends and Applications

As ultrahigh resolving power becomes routine at chromatographic timescales, potential applications include untargeted proteomics, lipidomics and environmental screening. Integration with real-time data processing and machine learning may further accelerate structural annotation and isotopic pattern recognition in increasingly complex samples.

Conclusion

Resolution Enhancement Mode on the SELECT SERIES™ MRT extends the ion flight path to deliver >300 000 FWHM, enabling detailed isotope pattern analysis without sacrificing throughput. This advance significantly elevates confidence in empirical formula assignments and broadens the scope of high-resolution LC-MS applications.

Reference

1. Description of the SELECT SERIES™ MRT multi-reflecting time-of-flight analyzer, Waters Corporation.