Scientists in Germany have developed a mobile, resonant, multiphoton ionization time-of-flight mass spectrometer (REMPI-TOFMS) for environmental analysis and industrial process monitoring. The system was developed by Ulrich Boesl and Ralf Zimmermann with coworkers at the Institute for Physical and Theoretical Chemistry of the Technical University in Munich. Zimmermann is now with GSF Forschungszentrum, Oberschleissheim-Neuherberg near Munich, where a special laboratory for analytical laser mass spectroscopy has been set up.
The mobile instrument consists of a linear time-of-flight mass spectrometer, a heatable quartz pipe and capillary sampling system, and a compact 248-nm excimer laser that provides 10-ns pulses with pulse energies up to 10 mJ and a repetition rate up to 50 Hz.1 The laser, the mass spectrometer, and the electronics are mounted in a mobile rack on castors, with overall dimensions of 80 × 110 × 160 cm. The instrument is set up for sample inlets from effusive molecular beam, skimmed or free supersonic beam, or gas chromatography (GC).
Ionization at 248 nm is well suited for benzene and its alkylated derivatives and for many polycyclic aromatic hydrocarbons, whereas aliphatic hydrocarbons, carbonyl compounds, chlorinated aromatics, and many inorganic molecules are effectively excluded from ionization. The mass resolution can be set up to 1500 by operating the TOFMS in the reflection mode. Mass detection is performed either by single-particle counting or by gated integration, according to the peak intensity. The instrument was calibrated with a gas mixture containing about ten components in the parts-per-billion concentration range.
The REMPI-TOFMS system is installed in the exhaust line of a waste incineration pilot plant in Clausthal, Germany, where incineration concepts are being investigated. A spectrum taken at the end of the postcombustion chamber during disturbed combustion of contaminated wood, that is, with understoichiometric oxygen supply, showed the presence of benzene, naphthalene, and its methylated derivatives, anthracene, and pyrene—products typical of incomplete combustion. The spectrum also showed phenol, a product typical of wood gasification. Isomeric molecules were not discriminated because the REMPI-TOFMS worked with a fixed laser frequency and an effusive sample inlet.
The laser repetition rate was 50 Hz, and 25 spectra were averaged so that the time resolution amounted to 0.5 s. Intensity transients at four different mass peaks during a change of the combustion conditions were also recorded. Process parameters were varied from standard combustion to gasification.
Potential applications
This technique is not ideal for detection of aromatics containing several chlorine substitutes. Here the efficiency of REMPI ionization is reduced because of the reduction of the intermediate-state lifetimes caused by increased inter-system crossing rates. This is not a fundamental obstacle to application of REMPI for detection of chlorinated compounds in, for example, flue gases, however, because in such a case low-chlorinated indicators may be detected.2 An alternative route may be GC coupling equipped with a postcolumn catalytical dehalogenation device.3
The potential of REMPI-TOFMS is much greater than described here, however. For example, the group used a stationary frequency-quadrupled Nd:YAG laser providing an output at 266 nm to monitor the volatiles in the headspace of brewed coffee and in the coffee-roasting process gas.4 The goal was to detect flavor-active species by two-photon (each 266 nm) ionization. By studying the molecular data the investigators expected phenolic compounds to be preferentially detectable. Phenol derivatives are mainly responsible for the coffee flavor.
The concentration of the major compounds (phenol, guaiacol, 4-vinylguaiacol) turned out to be about 100 ppb. The small peak at mass 194 is attributed to caffeine. The spectrum also contains peaks not yet assigned. The re searchers found similar spectra during the simulation of the roasting of green coffee beans. Similarly, molecules in cigarette smoke within the mouth space are easily detectable.
Fragmentation can still spoil REMPI results in certain species, where inner molecular relaxation is very fast as compared to the duration of the laser pulse. Use of subpicosecond pulses may resolve this problem and is currently being investigated elsewhere.
An important aspect of REMPI-TOFMS analysis is the high optical selectivity during ionization, which means extensive sample preparation to remove unwanted contaminants in a sample—such as is currently required for conventional food-flavor analysis—can be eliminated. This selectivity has the potential to be more fully exploited by setting up broadly tunable, dual-color REMPI systems. Hence, the system described here may be just the beginning of REMPI applications in food, drug, and pollution analysis, as well as industrial process monitoring and control. And with the solid-state laser systems currently being developed, this may occur sooner than expected.
