Single Photon Ionization (SPI) via Incoherent VUV-Excimer Light:  Robust and Compact Time-of-Flight Mass Spectrometer for On-Line, Real-Time Process Gas Analysis

Abstract

Fast on-line detection of organic compounds from complex mixtures, such as industrial process gas streams, require selective and sensitive analytical methods. One feasible approach for this purpose is the use of mass spectrometry (MS) with a selective and soft (fragment-free) ionization technique, such as chemical ionization (CI) or photo ionization (PI). Single photon ionization (SPI) with vacuum ultraviolet (VUV) light is a particularly sof tionization technique, well-suited for detection of both aromatic and aliphatic species. Problematic, however, is the generation of the VUV light. In general, the vacuum ultraviolet (VUV) light sources for SPI-MS are based either on lasers (e.g., 118-nm radiation generated by frequency-tripling of the third harmonic of a Nd:YAG laser) or on conventional VUV lamps, such as deuterium lamps. Althoughthe laser-based techniques are very sophisticated and expensive, the conventional lamps have serious drawbacks regarding their optical parameters, such as low-output power, low spectral power density, and broad emission bands. In this work, a novel excimer VUV light source, in which an electron beam is used to form rare gas excimer species, is used. The excimer VUV light sourceproduces brilliant and intense VUV light. The novel VUV light source was coupled to a compact and mobile time-of-flight mass spectrometer (TOFMS). A special interface design, including optical (VUV optics) as well as electronic measures (e.g., pulsed ion extraction) was realized. The use of the excimer VUV lamp for SPI will allow the realization of very compact, rugged, and sensitive SPI-TOFMS devices, which preferably will be adapted for process analytical application or monitoring issues (e.g., chemical warfare detection). The excimer VUV-lamp technology delivers VUV light with a good beam quality and high-output power at low costs. Furthermore, it allows changing the emitted wavelength as well as the bandwidth of the excimer VUV lamp in t he 100-200-nm region by changing the gas filling. Consequently, SPI-TOFMS with an excimer light source is a fast detection technique that can be used for online monitoring, for example, in environmental studies or industrial manufacturing processes. In this paper, technology and characteristics of the new excimer light source, as well as the combination with the TOFMS, are presented. Furthermore, a first characterization of the SPI-TOFMS instrument, regarding analytical parameters such as detection limits and selectivity, is given. This includes a discussion of potential improvements that probably will be achievable within a future prototype genertation. Finally, first applications of the system for on-line measurement of organic trace species in a complex gas mixture (here, motorcycle exhaust gas) are presented.

Electron beam induced light emission

“…Brilliant VUV excimer light sources have been developed. They are for example very successfully used in the ion sources of mass spectrometers for so called single photon ionization (SPI) which leads to mass spectra which are essentially free of fragmentation of the analyte molecules [22][23][24]. In test experiments the electron beam has also been used to induce chemical reactions (so called gas reformation), for example for hydrogen enrichment of fuel gases [25].…”

Measurement of Volatile Organic Compounds in Vehicle Exhaust Using Single-Photon Ionization Time-of-Flight Mass Spectrometry

“…16,[19][20][21] Butcher et al measured oxygenated organics and aromatics in automobile exhaust using a SPI ion source and an ion-trap mass spectrometer. 19 Their ion source was 118 nm VUV generated by the frequency tripling of 355 nm third harmonic Nd:YAG laser pulses in a Xe gas cell.…”

“…16 They also developed a TOFMS that ionized hydrocarbons using resonance-enhanced multi-photon ionization (REMPI), laser-induced electron impact ionization (LEI) or SPI. 20 Measurements on automotive exhaust using these instruments have been reported. 16,20,21 Tanaka et al measured polyaromatic hydrocarbons (PAHs) in diesel exhaust using non-resonant multi-photon ionization (MPI) TOFMS equipped with a supersonic jet sample introduction system to investigate temporal changes in the PAH concentrations under different driving conditions.…”

“…20 Measurements on automotive exhaust using these instruments have been reported. 16,20,21 Tanaka et al measured polyaromatic hydrocarbons (PAHs) in diesel exhaust using non-resonant multi-photon ionization (MPI) TOFMS equipped with a supersonic jet sample introduction system to investigate temporal changes in the PAH concentrations under different driving conditions. 22 This system was used for the measurement of aromatic hydrocarbons in both gasoline and diesel exhaust gases before and after the catalyst.…”

Tunable single-photon ionization TOF mass spectrometry using laser-produced plasma as the table-top VUV light source

“…The concentrations of VOCs in the exhaust are usually measured with traditional gas chromatographic methods [13]. Different analysis methods for monitoring the exhaust composition have proposed the use of TOF mass spectrometry with VUV single photon ionization at a fixed wavelength produced with coherent [14] or incoherent sources [15]. However, the use of an ionization source with a fixed wavelength only allows partial speciation of the compounds due to the presence of isomers.…”