@article{MichalikOnichimowskaBeitzPanneetal.2017,
  author    = {Michalik-Onichimowska, Aleksandra and Beitz, Toralf and Panne, Ulrich and L{\"o}hmannsr{\"o}ben, Hans-Gerd and Riedel, Jens},
  title     = {Microsecond mid-infrared laser pulses for atmospheric pressure laser ablation/ionization of liquid samples},
  series = {Sensors and actuators : B, Chemical},
  volume    = {238},
  journal   = {Sensors and actuators : B, Chemical},
  publisher = {Elsevier},
  address   = {Lausanne},
  issn      = {0925-4005},
  doi       = {10.1016/j.snb.2016.06.155},
  pages     = {298 -- 305},
  year      = {2017},
  abstract  = {In many laser based ionization techniques with a subsequent drift time separation, the laser pulse generating the ions is considered as the start time to. Therefore, an accurate temporal definition of this event is crucial for the resolution of the experiments. In this contribution, the laser induced plume dynamics of liquids evaporating into atmospheric pressure are visualized for two distinctively different laser pulse widths, Delta t = 6 nanoseconds and Delta tau = 280 microseconds. For ns-pulses the expansion of the generated vapour against atmospheric pressure is found to lead to turbulences inside the gas phase. This results in spatial and temporal broadening of the nascent clouds. A more equilibrated expansion, without artificial smearing of the temporal resolution can, in contrast, be observed to follow mu s-pulse excitation. This leads to the counterintuitive finding that longer laser pulses results in an increased temporal vapour formation definition. To examine if this fume expansion also eventually results in a better definition of ion formation, the nascent vapour plumes were expanded into a linear drift tube ion mobility spectrometer (IMS). This time resolved detection of ion formation corroborates the temporal broadening caused by collisional impeding of the supersonic expansion at atmospheric pressure and the overall better defined ion formation by evaporation with long laser pulses. A direct comparison of the observed results strongly suggests the coexistence of two individual ion formation mechanisms that can be specifically addressed by the use of appropriate laser sources.},
  language  = {en}
}
@article{ZuehlkeSassRiebeetal.2017,
  author    = {Z{\"u}hlke, Martin and Sass, Stephan and Riebe, Daniel and Beitz, Toralf and L{\"o}hmannsr{\"o}ben, Hans-Gerd},
  title     = {Real-Time Reaction Monitoring of an Organic Multistep Reaction by Electrospray Ionization-Ion Mobility Spectrometry},
  series = {ChemPlusChem},
  volume    = {82},
  journal   = {ChemPlusChem},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2192-6506},
  doi       = {10.1002/cplu.201700296},
  pages     = {1266 -- 1273},
  year      = {2017},
  abstract  = {The capability of electrospray ionization (ESI)-ion mobility (IM) spectrometry for reaction monitoring is assessed both as a stand-alone real-time technique and in combination with HPLC. A three-step chemical reaction, consisting of a Williamson ether synthesis followed by a hydrogenation and an N-alkylation step, is chosen for demonstration. Intermediates and products are determined with a drift time to mass-per-charge correlation. Addition of an HPLC column to the setup increases the separation power and allows the determination of further species. Monitoring of the intensities of the various species over the reaction time allows the detection of the end of reaction, determination of the rate-limiting step, observation of the system response in discontinuous processes, and optimization of the mass ratios of the starting materials. However, charge competition in ESI influences the quantitative detection of substances in the reaction mixture. Therefore, two different methods are investigated, which allow the quantification and investigation of reaction kinetics. The first method is based on the pre-separation of the compounds on an HPLC column and their subsequent individual detection in the ESI-IM spectrometer. The second method involves an extended calibration procedure, which considers charge competition effects and facilitates nearly real-time quantification.},
  language  = {en}
}
@article{ZuehlkeZenichowskiRiebeetal.2017,
  author    = {Z{\"u}hlke, Martin and Zenichowski, Karl and Riebe, Daniel and Beitz, Toralf and L{\"o}hmannsr{\"o}ben, Hans-Gerd},
  title     = {Subambient pressure electrospray ionization ion mobility spectrometry},
  series = {International journal for ion mobility spectrometry : official publication of the International Society for Ion Mobility Spectrometry},
  volume    = {20},
  journal   = {International journal for ion mobility spectrometry : official publication of the International Society for Ion Mobility Spectrometry},
  publisher = {Springer},
  address   = {Heidelberg},
  issn      = {1435-6163},
  doi       = {10.1007/s12127-017-0215-x},
  pages     = {47 -- 56},
  year      = {2017},
  abstract  = {The pressure dependence of sheath gas assisted electrospray ionization (ESI) was investigated based on two complementary experimental setups, namely an ESI-ion mobility (IM) spectrometer and an ESI capillary - Faraday plate setup housed in an optically accessible vacuum chamber. The ESI-IM spectrometer is capable of working in the pressure range between 300 and 1000 mbar. Another aim was the assessment of the analytical capabilities of a subambient pressure ESI-IM spectrometer. The pressure dependence of ESI was characterized by imaging the electrospray and recording current-voltage (I-U) curves. Qualitatively different behavior was observed in both setups. While the current rises continuously with the voltage in the capillary-plate setup, a sharp increase of the current was measured in the IM spectrometer above a pressure-dependent threshold voltage. The different character can be attributed to the detection of different species in both experiments. In the capillary-plate experiment, a multitude of charged species are detected while only desolvated ions attribute to the IM spectrometer signal. This finding demonstrates the utility of IM spectrometry for the characterization of ESI, since in contrast to the capillary-plate setup, the release of ions from the electrospray droplets can be observed. The I-U curves change significantly with pressure. An important result is the reduction of the maximum current with decreasing pressure. The connected loss of ionization efficiency can be compensated by a more efficient transfer of ions in the IM spectrometer at increased E/N. Thus, similar limits of detection could be obtained at 500 mbar and 1 bar.},
  language  = {en}
}