A Mobile Mass Spectrometer for Comprehensive On-Line Analysis of Trace and Bulk Components of Complex Gas Mixtures:  Parallel Application of the Laser-Based Ionization Methods VUV Single- Photon Ionization, Resonant Multiphoton Ioniza- tion, and Laser-Induced Electron Impact Ionization

Abstract

A newly developed compact and mobile time-of-flight mass spectrometer (TOFMS) for on-line analysis and monitoring of complex gas mixtures is presented. The instrument is designed for a (quasi-)simultaneous application of three ionization techniques that exhibit different ionization selectivities. The highly selective resonance-enhanced multiphoton ionization (REMPI) technique, using 266-nm UV laser pulses, is applied for selective and fragmentationless ionization of aromatic compounds at trace levels (parts-per-billion volume range). Mass spectra obtained using this technique show the chemical signature solely of monocyclic (benzene, phenols, etc.) and polycyclic (naphthalene, phenathrene, indol, etc.) aromatic species. Furthermore, the less selective but still fragmentationless single photon ionization (SPI) technique with 118-nm VUV laser pulses allows the ionization of compounds with an ionization potential below 10.5 eV. Mass spectra obtained using this technique show the profile of most organic compounds (aliphatic and aromatic species, like nonane, acetaldehyde, or pyrrol) and some inorganic compounds (e.g., ammonia, nitrogen monoxide). Finally, the nonselective ionization technique laser-induced electron-impact ionization (LEI) is applied. However, the sensitivity of the LEI technique is adjusted to be fairly low. Thus, the LEI signal in the mass spectra gives information on the inorganic bulk constituents of the sample (i.e., compounds such as water, oxygen, nitrogen, and carbon dioxide). Because the three ionization methods (REMPI, SPI, LEI) exhibit largely different ionization selectivities, the isolated application of each method alone solely provides specific mass spectrometric information about the sample composition. Special techniques have been developed and applied which allow the quasi-parallel use of all three ionization techniques for on-line monitoring purposes. Thus, a comprehensive characterization of complex samples is feasible jointly using the characteristic advantages of the three ionization techniques. Laboratory applications show results on rapid overview characterization of mineral oil-based fuels and coffee headspace. The first reported field applications include timely resolved on-line monitoring results on automobile exhausts and of waste incineration flue gas.

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

“…4 with the associated vehicle speeds in the JC08 mode. We focused on the emission profiles of benzene and toluene since no other hydrocarbons with m/z = 78 and 92 were observed by GC/MS in this study and in previous exhaust measurements by Butcher et al 19 and Mühlberger et al 21 Two test runs in the JC08 mode were performed to confirm the reproducibility of the temporal emission profiles of benzene and toluene. In the gasoline vehicle exhaust, high concentrations of benzene and toluene were measured within the first few minutes, shown in Figs.…”

“…Although N2 is not ionized by 10.5 eV SPI, it can be ionized by LEI; i.e., collisions with electrons emitted by the photoelectric effect and accelerated by the ion optics. 21,28 Broad signals at m/z = 32 and 44 in each figure are from O2 and CO2, respectively, which are also ionized by LEI.…”

“…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.…”

“…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.…”

Rapid Analysis of Gasoline-Contaminated Soil Using Multiphoton Ionization/Time-of-Flight Mass Spectrometry

“…[5][6][7] In TOFMS, several ionization sources are in use: electron ionization (EI), 8 single photon ionization (SPI) with vacuum ultraviolet (VUV) photons, [9][10][11][12][13] and multiphoton ionization (MPI) with ultraviolet (UV) photons. [13][14][15][16] MPI/TOFMS combines a high selectivity for aromatic compounds with a high tolerance to contamination. 17,18 In environmental analysis, both content and concentration in real samples are, of course, unknown before measurement.…”