Vacuum Laser Photoionization inside the C-trap of an Orbitrap Mass Spectrometer: Resonance-Enhanced Multiphoton Ionization High-Resolution Mass Spectrometry

State-of-the-art mass spectrometry with ultraviolet (UV) photo-ionization is mostly limited to time-of-flight (ToF) mass spectrometers with 1000-10000 m/Delta m mass resolution. However, higher resolution and higher spectral dynamic range mass spectrometry may be indispensable in complex mixture characterization. Here, we present the concept, implementation, and initial evaluation of a compact ultrahigh-resolution mass spectrometer with gas-phase laser ionization. The concept is based on direct laser photoionization in the ion accumulation and ejection trap (C-trap) of an Orbitrap mass spectrometer. Resonance-enhanced multiphoton ionization (REMPI) using 266 nm UV pulses from a frequency-quadrupled Nd:YAG laser was applied for selective and efficient ionization of monocyclic and polycyclic aromatic hydrocarbons. The system is equipped with a gas inlet for volatile compounds and a heated gas chromatography coupling. The former can be employed for rapid system m/z-calibration and performance evaluation, whereas the latter enables analysis of semivolatile and higher-molecular-weight compounds. The capability to evaluate complex mixtures is demonstrated for selected petrochemical materials. In these experiments, several hundred to over a thousand compounds could be attributed with a root-mean-square mass error generally below 1 ppm and a mass resolution of over 140 000 at 200 m/z. Isobaric interferences could be resolved, and narrow mass splits, such as 3.4 mDa (SH4/C-3), are determined. Single laser shots provided limits of detection in the 20-ppb range for p-xylene and 1,2,4-trimethylbenzene, similar to compact vacuum REMPI-ToF systems.

Automated summary

A novel compact ultrahigh-resolution mass spectrometer with gas-phase laser ionization has been developed and evaluated, showing exceptional mass resolution and analytical capabilities for complex mixture characterization. The system, with direct laser photoionization in an Orbitrap mass spectrometer, demonstrates high selectivity and efficiency in ionization of aromatic hydrocarbons, allowing for rapid system calibration and analysis of volatile and semivolatile compounds. The capability to accurately evaluate complex mixtures is highlighted, with hundreds of compounds identified with high mass accuracy and resolution, even detecting narrow mass splits and isobaric interferences. Detection limits similar to compact vacuum REMPI-ToF systems were achieved for specific compounds.