Development of a Jet-REMPI (Resonantly Enhanced Multiphoton Ionization) Continuous Monitor for Environmental Applications

Resonance-enhanced multiphoton ionization mass spectrometry (REMPI-MS): applications for process analysis

Process analysis is an emerging discipline in analytical sciences that poses special requirements on analytical techniques, especially when conducted in an online manner. Mass spectrometric methods seem exceedingly suitable for this task, particularly if a soft ionization method is applied. Resonance-enhanced multiphoton ionization (REMPI) in combination with time-of-flight mass spectrometry (TOFMS) provides a selective and sensitive means for monitoring (poly)aromatic compounds in process flows. The properties of REMPI and various variations of the ionization process are presented. The potential of REMPI for process analysis is highlighted with several examples, and drawbacks of the method are also noted. Applications of REMPI-TOFMS for the detection and monitoring of aromatic species in a large variety of combustion processes comprising flames, vehicle exhaust, and incinerators are discussed. New trends in technical development and combination with other analytical methods are brought forward.

Combination of Laser- and Mass-Spectroscopic Techniques for the Investigation of Fuel-Rich Flames

Soot is one of the most important pollutants originating from combustion. Despite recent advances in the measurement of size and composition of soot particles, their actual formation mechanism is still under debate. It depends on fuel, stoichiometry, temperature, flow conditions and the concentration of a large number of intermediate species. An adequate characterization of this complex reaction system generally requires the use of several complementary techniques.

In this article, we present measurements aiming to study reactions in fuel-rich flames using several complementary techniques. Only with a combination of optical and mass-spectrometric measurements, important features of the early polyaromatic hydrocarbon (PAH) and soot formation chemistry are accessible in detail. Three different techniques are combined to investigate one-dimensional laboratory flames on the same low-pressure burner and their respective merits are discussed: (i) cavity ring-down spectroscopy (CRDS) for the detection of small radicals and measurement of the temperature, (ii) mass spectrometry with electron-impact (EI) ionization in order to measure species with molecular weights up to m/e = 90, and (iii) mass spectrometry with resonantly-enhanced multi-photon ionization (REMPI) to distinguish isomers with masses up to m/e = 178. Measurements of this type may prove a valuable input to improve kinetic and combustion models.

Membrane introduction/laser photoionization time-of-flight mass spectrometry

Two-photon resonance enhanced multiphoton ionization (REMPI) has been shown to be a unique ionization method for mass spectrometry, exhibiting both high sensitivity and chemical selectivity. Because REMPI is a gas-phase method, its applications have been limited either to direct analysis of vapor phase samples, or in conjunction with an initial laser desorption or other vaporization step. We describe here for the first time a combination of membrane introduction mass spectrometry (MIMS) and REMPI with time-of-flight mass spectrometry (TOF-MS), which allows for the direct analysis of trace amounts of organic compounds in water samples. The objective of our research was the detection of very low levels of aromatic contaminants, particularly benzene, toluene, and xylene (BTX), in aqueous solutions without interference due to the water. We have measured limits of detection (LOD) for selected aromatics in water below 1 part-per-trillion with an averaging time of less than 10 s using a continuous sample flow.

Detection of Explosives and Explosives-Related Compounds by Single Photon Laser Ionization Time-of-Flight Mass Spectrometry

The application of single photon ionization in combination with mass-selective detection by time-of-flight mass spectrometry is described for the rapid detection of the nitro-containing explosives and explosives-related compounds nitrobenzene, 1,3-dinitrobenzene, o-nitrotoluene, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene, as well as the peroxide-based explosive triacetone triperoxide in the gas phase. The technique is demonstrated to be a plausible approach for laser-based rapid detection of explosives. The limits of detection for nitrobenzene and 2,4-dinitrotoluene using SPI were also measured and determined to be 17-24 (S/N approximately 2:1) and approximately 40 ppb (S/N approximately 2:1), respectively.

Development of medium pressure laser ionization, MPLI. Description of the MPLI ion source

A novel pulsed valve/ion source combination capable of time-resolved sampling from atmospheric pressure has been developed for use with laser ionization time of flight mass spectrometry. The source allows ionization extremely close to the nozzle of the pulsed valve, enabling ultra-sensitive detection of a number of compounds, e.g., NO, at mixing ratios <1 pptV. Furthermore, at analyte mixing ratios in the ppbV range, the temporal resolution of the system is in the sub-second regime, allowing time-resolved monitoring of highly dynamic and complex mixtures, e.g., human breath or reacting chemical mixtures in atmospheric smog chamber experiments. Rotational temperatures of approximately 50 K have been observed for analytes seeded in the supersonic jet expansion at a distance of 1 mm downstream of the nozzle orifice. The refinement of the original ion source has drastically reduced the impact of reflected laser light and the resultant electron impact signals previously observed. The general applicability of this technique is demonstrated here by coupling the source to commercially available as well as home-built time-of-flight mass spectrometers. Finally, we discuss the MPLI technique in view of the very recently introduced atmospheric pressure laser ionization (APLI) as well as the traditional jet-REMPI approach.

A two-color three-photon ionization scheme for the efficient and selective ionization of a chlorinated aromatic hydrocarbon

A two-color three-photon ionization scheme, for the efficient and selective ionization of a chlorinated aromatic hydrocarbon that has an ionization potential higher than the two-photon energy of the laser used for excitation, is described. In this technique, an ultraviolet (UV) laser, i.e., the second harmonic emission of a fundamental (VIS) laser, is used for excitation and a UV and VIS laser for the subsequent two-photon ionization from the electronic excited state. A sample of o-chlorophenol was used as a model compound to demonstrate the advantage of this technique. The signal in supersonic jet/resonance-enhanced multiphoton ionization/mass spectrometry was increased approximately 4 times by the introduction of the VIS beam, when the polarization was adjusted to be parallel to the UV beam. Thus, the two-color three-photon (2UV+VIS) ionization scheme is more sensitive than one-color three-photon (3UV) ionization. The merits of this method over other ionization schemes such as two-color two-photon (UV(1)+UV(2)) ionization are discussed in terms of sensitivity and selectivity in spectrometric analysis.

Real-Time, On-Line Characterization of Diesel Generator Air Toxic Emissions by Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry

The laser-based resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) technique has been applied to the exhaust gas stream of a diesel generator to measure, in real time, concentration levels of aromatic air toxics. Volatile organic compounds, as well as several polycyclic aromatic hydrocarbons were detected in the concentration range of 10-200 ppb in the steady-state diesel generator exhaust. The results were verified and compared with conventional extractive sampling and analytical techniques using gas chromatography/mass spectrometry (GC/MS). The high isomer selectivity of the REMPI-TOFMS instrument provided data for individual xylene isomers that are otherwise (partially) coeluting in standard GC/MS analyses. Good agreement was observed between results for volatile and semivolatile organic compounds obtained with REMPI-TOFMS and conventional extractive sampling. Transient events, such as cold start-ups of the diesel generator, resulted in sharp (less than 15 s) peak emissions that were, for benzene, up to a factor of 90 higher than the predominately constant concentrations observed during steady-state operation; warm restarts resulted in lower peak concentrations by a factor of 2.5. These fast transient emissions are only detectable using a real-time approach (1-s resolution) as demonstrated here using REMPI-TOFMS.

Multiphoton ionization mass spectrometry of chlorophenols as indicators for dioxins

Mono-, di- and trichlorophenols were measured using resonance-enhanced multiphoton ionization mass spectrometry (MPI-MS) combined with supersonic jet (SSJ) or effusive molecular beam (EMB) spectrometry. All mono- and dichlorophenols, except 2,6-dichlorophenol, provided sharp and structured MPI spectra for the S1 <-- S0 transition. Selectivity and sensitivity were both enhanced when SSJ spectrometry was used, compared with EMB spectrometry, because of a narrower linewidth in the MPI spectrum, given by molecular cooling by supersonic jet expansion. The ionization efficiency decreased with increasing number of chlorine substituents for the chlorophenols, since they have shorter excited-state lifetimes and require three photons for ionization. Some of the chlorophenols, which are toxic themselves, have the potential for use as indicators for analysis of polychlorinated dibenzo-p-dioxin/dibenzofurans in flue gases emitted from an incinerator.

Resonance-enhanced multiphoton ionization mass spectrometry using a two-color laser beam generated by stimulated Raman scattering

Two-color (1+1') resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (REMPI/TOF-MS) combined with supersonic jet (SSJ) spectrometry has been demonstrated. The methodology uses a two-color laser beam consisting of a Stokes beam generated by stimulated Raman scattering (SRS) and a fundamental beam for the excitation and subsequent ionization of p-dichlorobenzene. The MPI signal was found to be substantially increased when a two-color laser beam was employed. This approach greatly simplifies the analytical system and reduces the cost of the instrumentation required for two-color ionization. The potential advantage of applying this method for the analysis of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) emitted from an incinerator is discussed.

Development of a compact supersonic jet/multiphoton ionization/time-of-flight mass spectrometer for the on-site analysis of dioxin, part I: Evaluation of basic performance

A new type of compact supersonic jet/resonance-enhanced multiphoton ionization/time-of-flight mass spectrometer is described. The analytical instrument, consisting of a single turbo molecular pump equipped with a rotary pump, was maintained at < 2 x 10(-3) Pa when a 0.3-atm sample was injected into a vacuum at 10-Hz using a 200-micros pulse valve. The diameters of the extraction and ground skimmer electrodes were expanded to 30 mm in order to avoid strong focusing and defocusing of the ion, and the optimum conditions for the system were investigated. The mass spectrometer functioned as expected: (1) no defocusing of the ion beam was observed even when the potential of the einzel lens was adjusted to zero; (2) the direction of the ion beam to an assembly of microchannel plates deviated in the expected manner when the potential of the defection electrode was changed from 0 to 30 V.