Robert Laudien

Laser-based ion mobility spectrometry for sensing of aromatic compounds

The drift time spectra of polycyclic aromatic hydrocarbons (PAH), alkylbenzenes and alkylphenylethers were recorded with a laser-based ion mobility (IM) spectrometer. The ion mobilities of all compounds were determined in helium as drift gas. This allows the calculation of the diffusion cross sections (Ωcalc) on the basis of the exact hard sphere scattering model (EHSSM) and their comparison with the experimentally determined diffusion cross sections (Ωexp). These Ωexp/Ωcalc-correlations are presented for molecules with a rigid structure like PAH and prove the reliability of the theoretical model and experimental method. The increase of the selectivity of IM spectrometry is demonstrated using resonance enhanced multiphoton ionisation (REMPI) at atmospheric pressure, realized by tuneable lasers. The REMPI spectra of nine alkylbenzenes and alkylphenylethers are investigated. On the basis of these spectra, the complete qualitative distinction of eight compounds in a mixture is shown. These experiments are extended to alkylbenzene isomer mixtures.

Phenylureas. Part 1. Mechanism of the basic hydrolysis of phenylureas

The mechanism of the hydrolytic decomposition of phenylureas in basic media in the pH range 12 to 14 is investigated. In this pH range a levelling of the rate–pH curve is observed as well as a change of the substituent influence on the hydrolysis rate. These experimental findings suggest the formation of an unreactive side product of the phenylurea in a parasitic side equilibrium at sufficiently high pH. The urea dissociates at the aryl–NH group to give its conjugate base. For the hydrolytic decomposition of phenylureas an addition–elimination mechanism is proposed as has been established for the alkaline hydrolysis of carboxylic acid esters and amides.

Phenylureas. Part 2. Mechanism of the acid hydrolysis of phenylureas

The mechanism of the hydrolytic decomposition of phenylureas in acid media is investigated. It includes, in part, knowledge already present in the literature. Over the investigated pH range the occurrence of a rate maximum in the pH curves due to the strongly reduced water activity at higher acid strengths is observed. An addition–elimination mechanism with rate-determining attack of water at the N-protonated substrate is proposed. The reversion of the substituent influence on the reaction rate with increasing acidity of the reaction medium points to a change of the hydrolytic decomposition mechanism in strongly acidic media.

Kinetic investigations of proton transfer and complex formation reactions by laser ion mobility spectrometry

The detection of polar molecules, like ketones and ethers, in a laser ion mobility spectrometer was investigated. Because the direct multiphoton ionization (MPI) for these compounds shows too high limits of detection (LOD) and intensive fragmentation of the molecular ions, alternative ionization methods based on ion-molecule-reactions (IMR) were investigated. These ionization methods should retain the advantages of the laser ionization. As examples for IMR two reaction classes, proton transfer reactions (PTR) and complex formation reactions (CFR), were studied. The PTR are based in a first step on the proton transfer from toluene radical cations to polar molecules. In a second step protonated dimers are formed. The CFR are characterized by the complex formation between aniline or phenol radical cations and polar molecules. All products are formed at atmospheric pressure and are characterized by transfer into a time-of-flight mass spectrometer. In both IMR the ionic reactants are formed selectively by 1+1 REMPI. The rates of the following IMR are near to the collision limit. Therefore the reactions are very efficient. The LOD for the analysis of selected ketones and ethers by IMR are in the low ppb-range, much lower than the corresponding LOD for direct MPI. The required laser intensities for the IMR are up to a factor of 1000 lower than the laser intensities for direct MPI. The fragmentation of the product ions is much lower for both IMR in comparison to direct MPI. The IMR allow the quantitative analysis of substance mixtures.

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.

Detection of explosive related nitroaromatic compounds (ERNC) by laser-based ion mobility spectrometry

In this study two issues are addressed, namely laser ionisation of selected nitroaromatic compounds (NAC) and the characterisation of their anions by photodetachment (PD) spectroscopy. Laser ionisation of the NAC at λ = 226.75 nm is investigated by ion mobility (IM) spectrometry at atmospheric pressure. The main product after laser ionisation is the reactive NO+ ion formed in a sequence of photofragmentation and multiphoton ionisation processes. NO+ is trapped by specific ion molecule reactions (IMR). Alternatively, NO, added as laser dopant, can directly be ionised. The formed NO+ reacts with the NAC under complex formation. This allows fragmentless NAC detection. The combination of IM spectrometry and PD spectroscopy provides real-time characterisation of the anions in the IM spectrum. This is useful to differentiate between NAC and interfering substances and, thus, to reduce false-positive detections of NAC. The electrons detached by the PD laser at λ = 532 nm are detected in the same spectrum as the anions. The potential of PD-IM spectrometry in terms of cross section determination, analytical improvements, tomographic mapping, spatial hole burning etc., is outlined.