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Among the large number of new ambient ionization schemes in the last few years, dielectric barrier discharge (DBD) has witnessed special attention. In this contribution a versatile dual mode DBD is introduced and characterized by means of optical emission spectroscopy and time-of-flight mass spectrometry. A direct comparison of the individual results from spectroscopy, spectrometry and transient current/voltage consumption gives evidence for the existence of two individual operational mechanisms. The first is driven by rapid transient changes in the potential difference between the two electrodes over time (usually denoted as the homogeneous mode), while the second is caused at high static potential differences (leading to filamentary discharges). The transient versus steady-state characteristics of the individual discharge origin suggest the driving force for the current flow to be inductive and capacitive, respectively. In most cases of dielectric barrier plasmas both discharge types coexist as competitive ion formation channels, however, detailed plasma characteristics of DBDs operated under different conditions allow for a clear distinction of the individual contributions. In this way, two characteristic product channels for the ionization of ambient water could be observed resulting in the generation of either preferentially protonated water clusters or ammonium water clusters. Careful tuning of the operation parameters of the discharge device allows an operation predominated by either of the two modes. As a consequence, facile switching into the desired operational mode results in either protonated molecules or ammoniated molecules of the analyte. Plasma characteristics for both moieties were evaluated and cross-correlated on the basis of several factors including: the production of reagent ions, the individual appearance of current/voltage profiles, UV/Vis spectroscopy, voltage and flux dependence and the individual response to test compounds. Although the filamentary mode has been already discussed in the literature to induce fragmentation processes, no experimental evidence for analyte dissociation could be found in the case of the test compounds used
Ambient mass spectrometry on a molecular level has become an indispensable analytical technique for the detection and characterization of organic molecules of different type, composition and size. Novel strategies, as well as fundamental and mechanistic research, has been successfully employed to obtain new hyphenated interrogation schemes, including laser ablation dielectric barrier discharge ionization (LA-DBDI) and laser ablation droplet ionization mass spectrometry (LDI-MS).
To approach more complex analytical problems, development yielded in a hyphenated instrument using one shared high repetition rate laser for laser desorption mass spectrometry coupled to dielectric barrier discharge postionization and Raman scattering. Raman spectroscopy displays structural information. Mass spectrometry allows for an accurate determination of the molecular mass. Thus, a combination of LA-MS and optical spectroscopy could be highly beneficial for an unambiguous identification of complicated analytical samples.
Acoustic levitation of droplets has matured to a powerful tool for containerless handling of microliter samples. In microfluidic systems the absence of confining walls is greatly beneficial because it can effectively suppress agglomeration and contamination of the sample originating at the liquid/solid interfaces. Here we present a set-up, utilizing the ?? = 2.94 µm output of a diode pumped Er:YAG laser for excitation, opening laser spray ionization to any OH-group containing solvents.
To obtain deeper insights into ongoing laser desorption processes mechanistic studies have been carried out utilizing shadowgraphy experiments. Those nicely show the evaporation plume and its spatial distribution during a time period of up to 1 ms. Achieved results allow for further sensitivity improvement via a better ionization efficiency and a better subsequent ion transmission.