Influence of structural features of isomeric hydrocarbons on ion formation at atmospheric pressure

Ion mobility spectrometry (IMS) in combination with different techniques of atmospheric pressure ionization (⁶³Ni ionization, photoionization, Corona discharge ionization) was applied to determine the influence of structural features of aromatic and cyclic hydrocarbons on ion mobility spectra. For this purpose, different sets of isomeric hydrocarbons were investigated using the above-mentioned ionization techniques. We found different structural features of these isomeric non-polar compounds which cause distinct differences in ion mobility spectra. These differences result from the formation of different product ions or a different relative abundance of ions formed depending on the occurrence of certain structural features (position of the double bond, arrangement of double bonds within the carbon ring, configuration of aliphatic side chain in the space, position of aliphatic side chain on the carbon ring and the number of carbon atoms in the aliphatic side chain). The nature of product ions formed was determined using a coupling of IMS with mass spectrometry (MS).     

Atmospheric pressure chemical ionization studies of non-polar isomeric hydrocarbons using ion mobility spectrometry and mass spectrometry with different ionization techniques

The ionization pathways were determined for sets of isomeric non-polar hydrocarbons (structural isomers, cis/trans isomers) using ion mobility spectrometry and mass spectrometry with different techniques of atmospheric pressure chemical ionization to assess the influence of structural features on ion formation. Depending on the structural features, different ions were observed using mass spectrometry. Unsaturated hydrocarbons formed mostly [M - 1]+ and [(M - 1)2H]+ ions while mainly [M - 3]+ and [(M - 3)H2O]+ ions were found for saturated cis/trans isomers using photoionization and 63Ni ionization. These ionization methods and corona discharge ionization were used for ion mobility measurements of these compounds. Different ions were detected for compounds with different structural features. 63Ni ionization and photoionization provide comparable ions for every set of isomers. The product ions formed can be clearly attributed to the structures identified. However, differences in relative abundance of product ions were found. Although corona discharge ionization permits the most sensitive detection of non-polar hydrocarbons, the spectra detected are complex and differ from those obtained with 63Ni ionization and photoionization.     

NMR spectroscopic determination of enantiomers via their triphenyltin derivatives: A new application of chiral solvating agents

A method is described for the NMR determination of the enantiomeric composition of chiral carboxylic acids and alcohols using their triphenyltin derivatives together with the chiral solvating agents quinine hydrochloride and phenylethylamine.     

Ion mobility spectrometry of laser desorbed pesticides from fruit surfaces

The applicability of a technique combining laser desorption and ion mobility spectrometry for rapid detection of pesticides on fruit surfaces is explored. A selection of 20 fungicides, herbicides and insecticides with very different chemical and spectroscopic characteristics was used to study its features. For a variety of fruits the maximum tolerable laser irradiance was determined, at which no signals are generated by the clean surface. The spectra recorded for each substance on fruits were compared to those obtained for the same compound using laser desorption from a well defined surface. Furthermore, the influence of a dew layer on the sample was investigated. Limits of detection were estimated to be in the range of some tens of ng/kg for apples.     

Zur Stereoisomerie der 2,6-Dichlor-4-methoxyheptane

2,6-Dichloro-4-methoxy-heptanes may exist as 2 optical active isomers and 2 meso compounds. By double addition of ,-dichloro-dimethylether to propene a mixture of these stereoisomers was obtained. They were separated by HPLC. Using¹H- and¹³C-NMR-spectroscopy their configuration could be established.

     

Response of halogenated compounds in ion mobility spectrometry depending on their structural features

Ion mobility spectrometry (IMS) with handheld and transportable devices permits the sensitive detection of chlorinated compounds which are important in environmental monitoring. The ion mobility spectra in negative measuring modus mostly show one product ion peak [(H₂O)_nCl⁻] due to dissociative electron attachments. In this paper, we investigated relevant chlorinated compounds (R-Cl) where R represents allyl-, benzyl-, phenyl-, alkyl- and vinyl-groups. These groups cause differences in the R-Cl bond strength and differences in the cleavage of chlorine can therefore be expected. All chlorinated substances investigated provide the same product ion peak at 2.75 cm² Vs⁻¹ independent on the different C-Cl bond strength. However, distinct influences of structural features on the peak intensities of the (H₂O)_nCl⁻ product ion peak were established. Generally, increasing sensitivities were obtained in the order chlorobenzenes < vinyl- < allyl- < alkyl compounds < benzylchlorides. Sensitivities and limits of detection (LODs) of aromatic compounds depend on the nature and position of second substituent. Electron-withdrawing substituents (chlorine, fluorine, nitrile) enhance sensitivity while electron-repelling substituents decrease it. A dependence of sensitivity on the chain length or ring size can be observed for alkyl compounds. Additional influences of intramolecular interactions on the sensitivity were found for di-halogenated compounds. Therefore, the quantification of negative product ion peaks of chlorinated compounds requires a consideration of structural features of analytes.     

Ion mobility spectra of cyclic and aliphatic hydrocarbons with different substituents

We investigated the influence of structural differences on the ionization pathways and drift behavior in ion mobility spectrometry for cyclic and aliphatic hydrocarbons with different functional groups. The sets of cyclic and aliphatic compounds had an identical mass or a mass difference of 2 Da. Therefore, mass effects can be neglected during the investigation of these compounds. Depending on the functional group, considerable differences were found in the detectable concentration ranges and in the number and position of product ion peaks in ion mobility spectra. The spectra of chlorinated compounds and hydrocarbons show no correlation to their calculated collisional cross sections. Differences in collisional cross section between cyclic and aliphatic substances investigated were only found to influence the drift times detected for amines and aliphatic aldehydes while complex ion chemistry was observed for the other substances.     

Bestimmung der integralen molaren Extinktionskoeffizienten im IR für äquatoriale und axiale C—Halogen-Valenzschwingungen

Using the population ratios of the different conformations measured by NMR peak area method at low temperatures we determined the integrated molar extinction coefficients of the C-X stretching vibrations in the IR spectra of cyclohexyl-chloride, -bromide and -iodide under the same conditions. The obtained values are compared with the integrated molar extinction coefficients of the conformationally homogeneouscis- andtrans-4-tert-butylcyclohexyl derivatives, which are assumed to reflect the spectroscopic properties of the axial and equatorial conformations of the monosubstituted cyclohexanes.