Jochen Wieser

Trace detection of organic compounds in complex sample matrixes by single photon ionization ion trap mass spectrometry: real-time detection of security-relevant compounds and online analysis of the coffee-roasting process.

An in-house-built ion trap mass spectrometer combined with a soft ionization source has been set up and tested. As ionization source, an electron beam pumped vacuum UV (VUV) excimer lamp (EBEL) was used for single-photon ionization. It was shown that soft ionization allows the reduction of fragmentation of the target analytes and the suppression of most matrix components. Therefore, the combination of photon ionization with the tandem mass spectrometry (MS/MS) capability of an ion trap yields a powerful tool for molecular ion peak detection and identification of organic trace compounds in complex matrixes. This setup was successfully tested for two different applications. The first one is the detection of security-relevant substances like explosives, narcotics, and chemical warfare agents. One test substance from each of these groups was chosen and detected successfully with single photon ionization ion trap mass spectrometry (SPI-ITMS) MS/MS measurements. Additionally, first tests were performed, demonstrating that this method is not influenced by matrix compounds. The second field of application is the detection of process gases. Here, exhaust gas from coffee roasting was analyzed in real time, and some of its compounds were identified using MS/MS studies.

Excimer formation using low-energy electron-beam excitation

Excimer formation in dense gas targets is studied using a 10 to 20 keV electron beam for the excitation of rare gases. A SiNx ceramic foil, only 300 nm thick but strong and completely vacuum tight is used as an entrance window. The electrons loose only 4 to 6% of their energy in this foil. The low electron energy allows a table top setup without high voltage or x-ray radiation hazards. First applications of the system for excimer physics research and the development of novel vacuum ultraviolet light sources are described.

Electron Beam Pumped Ultraviolet Light Sources

Low energy electron beam excitation of dense gases is used for developing UV and VUV light sources. Emission spectra for various gases are shown and a first application for single photoionization in a time of flight mass spectrometer is presented.

New interpretation of the third rare-gas excimer continua

Time correlated optical emission spectra of argon, krypton and xenon in the wavelength range from 110 to 450 nm, and 0 to 6000 ns time interval, recorded at the Munich Tandem accelerator using heavy ion beam excitation with 2 ns beam pulses, were measured in order to clarify the origin of the so called third rare gas excimer continua. Experiments were performed at xenon and krypton pressures between 50 and 250 mbar, and argon pressures between 230 and 1500 mbar. All spectra clearly show different distinct peaks, emerging at different time delays after excitation. These spectral maxima are interpreted as arising from excimer emissions by separate radiating species, formed by gas kinetic processes. While the spectral shape of the components, forming the third continuum radiation in the heavy rare gases krypton and xenon turned out to be of complicate structure, in the case of argon all wavelength spectra could be reproduced by fitting a limited number of Gaussian functions with fixed center wavelengths and fixed widths to the data. Hereby, six distinct maxima, appearing at four different times after the excitation pulse, could be identified.

Light Sources using Energy Transfer from Excimer to Line Radiation

Efficient energy transfer between neon excimer molecules and hydrogen has been found. A small, high gap density light source has been developed, emitting entirely on the hydrogen 2p-1s transition at 121.567 nm (Lyman-(alpha) ). Light output densities of 10 W/cm2 are obtained. Electron beam energy conversion efficiencies of approximately equals 10% have been measured.

Low-energy electron-beam-pumped lasers

Low energy electron beams with particle energies of typically 10 to 20 keV are used for pumping gas lasers. Extremely thin (300 nm) ceramic (SiNx) membranes are used as entrance foils for the electron beam. Laser gas pressure up to several atmospheres is possible using this technique if the dimension of one side of the foil is restricted to about 1 mm. Energy loss of the electrons in the foil is less than 10%. The short range of the low energy electrons in the laser medium leads to a high specific power deposition. In transverse geometry the beam pumped volume is cylindrical with typically 1 to 3 mm diameter. This is well matched with the diameter of optical modes in stable optical cavities. The new pumping method is demonstrated using the 1.73 micrometers 5d[3/2]1-6p[5/2]2 XeI laser line in Ar-Xe laser gas mixtures at pressures between 130 and 650 mbar. Laser effect was observed for Xe concentrations between 0.1 and 1%. A low threshold pumping power of 5.5 W and a maximum output power of 6 mW at 13 W pumping power were measured. Scaling to higher power and shorter wavelength laser systems is discussed.

Electron-beam-sustained discharge revisited - light emission from combined electron beam and microwave excited argon at atmospheric pressure

A novel kind of electron beam sustained discharge is presented in which a 12 keV electron beam is combined with a 2.45 GHz microwave power to excite argon gas at atmospheric pressure in a continuous mode of operation. Optical emission spectroscopy is performed over a wide wavelength range from the vacuum ultraviolet (VUV) to the near infrared (NIR). Several effects which modify the emission spectra compared to sole electron beam excitation are observed and interpreted by the changing plasma parameters such as electron density, electron temperature and gas temperature.

Fast detection of narcotics by single photon ionization mass spectrometry and laser ion mobility spectrometry

In this contribution two analytical devices for the fast detection of security-relevant substances like narcotics and explosives are presented. One system is based on an ion trap mass spectrometer (ITMS) with single photon ionization (SPI). This soft ionization technique, unlike electron impact ionization (EI), reduces unwanted fragment ions in the mass spectra allowing the clear determination of characteristic (usually molecular) ions. Their enrichment in the ion trap and identification by tandem MS investigations (MS/MS) enables the detection of the target substances in complex matrices at low concentrations without time-consuming sample preparation. For SPI an electron beam pumped excimer light source of own fabrication (E-Lux) is used. The SPI-ITMS system was characterized by the analytical study of different drugs like cannabis, heroin, cocaine, amphetamines, and some precursors. Additionally, it was successfully tested on-site in a closed illegal drug laboratory, where low quantities of MDMA could be directly detected in samples from floors, walls and lab equipments. The second analytical system is based on an ion mobility (IM) spectrometer with resonant multiphoton ionization (REMPI). With the frequency quadrupled Nd:YAG laser (266 nm), used for ionization, a selective and sensitive detection of aromatic compounds is possible. By application of suited aromatic dopants, in addition, also non-aromatic polar compounds are accessible by ion molecule reactions like proton transfer or complex formation. Selected drug precursors could be successfully detected with this device as well, qualifying it to a lower-priced alternative or useful supplement of the SPI-ITMS system for security analysis.

Application and field test of a mobile thermal desorption - single photon ionization - ion trap mass spectrometer (TD-SPI-ITMS) for trace detection of security relevant substances

The objective of this accomplished project funded by the German BMBF was to develop a single photon ionization ion trap mass spectrometer (SPI-ITMS) for detection of security relevant substances in complex matrices at low concentrations. The advantage of such a soft ionization technique is a reduction of target ion fragmentation allowing identification of signals from complex matrices and enabling MS/MS capability. To obtain low detection limits, the applied photon energy has to be below the ionization potential (IP) of the bulk matrix components. Therefore, photon energies between 8 eV (155 nm) and 12 eV (103 nm) are necessary which was achieved with newly developed electron beam excimer lamps (EBEL). They generate light at different wavelengths depending on the selected rare gas emitting wavelengths adapted to the analyzed substances. So, e.g. with a krypton-EBEL with 8.4 eV photon energy most narcotics can be ionized without notable fragmentation. Due to their higher IPs, EBEL with higher photon energy have to be used for most explosives. Very low false-positive and false-negative rates have been achieved using MS/MS studies. First field tests of a demonstrator provided the proof of principle.

Detection of security relevant substances within the cooperative project SAFE XUV

The objective of this project funded by the German BMBF was to show that security relevant substances can be detected in complex matrices at low concentrations using single photon ionization ion trap mass spectrometry (SPI-ITMS). The advantage of such a soft ionization technique is a reduction of unwanted fragment ions in mass spectra allowing identification of signals from complex matrices and enabling MS/MS capability. The MS/MS studies permit low false-positive and false-negative rates. Additionally, the accumulation of the ions in the ion trap decreases the detection limit. To obtain low detection limits the ionization potentials (IPs) of the relevant substances have to be below the IPs of the bulk matrix components. That enables the utilization of a photon energy unaffecting the matrix components resulting in increased sensitivity due to essentially non-existent background signals. As literature values for many ionization potentials are unavailable, IPs of several security relevant substances were determined using monochromatized synchrotron radiation from BESSY, Germany. All analyzed substances exhibited IPs significantly below the IPs of common matrix molecules such as water, nitrogen and oxygen. First measurements with a pre-demonstrator show that it is possible to shield matrix substances using a well chosen photon energy for soft ionization.