Research Group Prof. Dr. G. Friedrichs

Mass Spectrometry

Mass Spectrometry
 

Heatable Photolysis Flow Reactor

In many cases, a sensitive and time-resolved detection system capable of measuring various kinds of reacting species (atoms, radicals, molecules) is needed for investigating the kinetics of chemical reactions. With such an instrument the rate constant of a single elementary reaction as well as the product distribution of the reaction can be measured. This information is useful for developing - step by step - complex reaction mechanisms which are needed for the understanding of practically important chemical processes. Examples stem from the field of combustion (ignition, pollutant formation), atmospheric chemistry (ozone layer, pollutant degradation), chemical vapor deposition (CVD), and exhaust gas cleaning technologies (DeNOx, Reburning). With a detailed reaction mechanism it is possible to model and, where applicable, to optimize these processes.
 

ms-setup

ms-bildMass spectrometry (MS) is a quite universal detection method for measuring all different kinds of species by "weighing" its masses. The five basic parts of any mass spectrometer are: a vacuum system; a sample introduction device; an ionization source; a mass analyzer; and an ion detector. The sample, a mixture of different components in the gas phase, is ionized (and fragmented) in a high-vacuum chamber. The ionized, charged particles are directed electrostatically into a mass analyzer, separated according to their mass (m/e ratio) and are finally detected. The Figures illustrate our experimental setup that couples laser photolysis with time-resolved mass spectrometric detection. Through a pinhole in the heatable flow reactor part of the reaction gases continuously expand into a differentially pumped vacuum chamber and are analyzed by electron impact ionization and quadrupole mass filtering. Radicals are formed by excimer laser photolysis. Data sampling and spectrometer operation are automated and controlled by a PC. As apparent from the photo of the experimental setup, the test gas mixtures were prepared in a gas manipulation system made out of glass and were sampled through calibrated mass flow controllers.

Contributing researchers: G. Friedrichs, J. Gripp, F. Temps and (formerly) G. Eshenko, C. Kerst, T. Köcher

HO2 Kinetics

Reactions of hydroperoxy radicals (HO2) are known to play decisive roles in atmospheric chemistry, low-temperature combustion, flame propagation, and fuel self ignition, which causes engine knocking. However, the investigation of HO2 chemistry is rather challenging, partly due to a lack of reliable HO2 radical sources and partly due to the difficulties involved in a time-resolved HO2 detection.
We were able to measure the overall rate constant of the reaction HO2C2H5 => products for the first time. Forming C2H5O + OH as the main reaction products, this reaction constitutes an important chain branching source. For example, the reaction was shown to be among the most sensitive reactions for the observed total ignition delay in the NTC region of n-heptane/air ignition (Kazakov et al. Chemical and Physical Processes in Combustion (2005), 46).
hyperoxid1Concentration-vs.-time profiles of HO2 and C2H5 were directly measured by means of time-resolved mass spectrometry in a laser photolysis/flow reactor combination. H2O2 was produced by the thermal decomposition of a urea hydrogenperoxide adduct as a new and reliable H2O2 source. Excimer laser photolysis of reaction mixtures containing C2H6H2O2 and oxalyl chloride provided experimental conditions such the the reaction HO2C2H5 could be studied under nearly pseudo first-order conditions with HO2 as the excess component (see Figure).
The need for a simultaneous generation of two different radical species constitutes a major problem for measurements of rate constants of radical-radical cross reactions. Moreover the possibility of competing radical self-reactions or reactions of the radicals with the precursor or product molecules requires a subtle control of the experimental conditions. As already mentioned above, the radicals C2H52 were generated by excimer laser photolysis (λ = 193 nm) of mixtures of C2H6H2O2 and oxalyl chloride. The C2H5 and HO2 radicals were thus generated by the fast H abstraction reactions of the primarily formed Cl atoms and OH radicals with C2H6 and H2O2, respectively. Here, a kinetic separation of the C2H5 and HO2 formation pathways (see mechanism below) allowed for a straightforward control of the absolute radical concentrations by simply adjusting the relative initial concentrations of C2H6 and (COCl)2 versus that of H2O2

hydroperoxi2


[1] W. Ludwig, B. Brandt, G. Friedrichs, and F. Temps, "Kinetics of the Reaction C2H5 + HO2 by Time-Resolved Mass Spectrometry", J. Phys. Chem. A 110 (2006) 3330-3337.

Contributing researchers: G. Friedrichs, J. Gripp, F. Temps and (formerly) W. Ludwig, B. Brandt

PEPICO at Swiss Light Source SLS