Miniature time-of-flight mass analyzer for use in combination with a compact highly-repetitive femtosecond laser ionization source

Abstract

In most cases, a molecular ion is observed in femtosecond laser ionization mass spectrometry, which provides information concerning the molecular weight of the analyte. However, the Ti:sapphire laser currently used as the ionization source is costly and involves special skills for operation and maintenance, which prevents its practical use in many applications. In this study, we report on the development of a miniature time-of-flight mass analyzer with a flight tube length of 65 mm for use in combination with a compact highly-repetitive (120-560 kHz) femtosecond Yb laser and a time-correlated single ion counting system. The fundamental beam (1030 nm) was converted into ultraviolet beams emitting at 343, 257, and 206 nm, which was utilized as an efficient two-photon ionization source. A mass resolution of 670 was achieved for the molecular ion of chlorobenzene, the minimum time for measuring a mass spectrum being 0.1 s. This mass spectrometer was used in the on-site real-time monitoring of products appeared by the combustion of plastic, some nerve agent analogs, and an explosive in the air. The interference arising from nitrogen and oxygen in the air was suppressed, since they require nonresonant four- and three-photon ionizations, respectively. The mass spectrometer was combined with a gas chromatograph and used for the comprehensive analysis of polycyclic aromatic hydrocarbons, suggesting its potential advantage for use in the practical trace analysis of organic compounds in the environmental and forensic sciences.

Keywords: Femtosecond ionization; Gas chromatograph detector; Mass spectrometry; Miniature mass analyzer; Time-correlated single ion counting.