Only a few years after the invention of the laser, the concept of laser microprobe mass spectrometry (LMMS), a technique which employed intense laser radiation for ion generation, was introduced. In these early studies at excessive irradiation microplasma formation could be observed to be an effective channel for ion formation. However, this plasma generation in vacuum led to undesired distortions of the mass analyzers and, thus, was discarded as an analytical ion source.
Under ambient conditions, the surrounding air effectively cools the plasma cloud, making the plasma more controllable. The resulting laser induced plasma is nowadays commonly used in laser induced breakdown spectroscopy (LIBS) applications as excitation source for optical emission spectroscopy experiments. However, little effort has been made to introduce a LIBS plasma as a promising ion source for ambient mass spectrometry. The main hindrance is the transient character of laser induced plasmas that typically only has a lifetime on the order of several microseconds. This drastically reduces the duty cycle of these plasma sources. After these microseconds, the generated ions recombinate to uncharged
atoms and even newly bound molecules, making them inaccessible to mass-to-charge analyzers. The advent of high repetition lasers together with the ever growing knowledge about manipulation of charged species at atmospheric pressures allow overcoming these obstacles. This presentation will introduce an ionization scheme using a laser induced plasma as the primary ion source. We believe that this novel ionization strategy will pave the way for future applications in ambient mass spectrometry.
Laser microprobe mass analysis (LMMS) employs local ionization by a focused laser and subsequent mass analysis. At excessive irradiation microplasmas led to undesired distortions. Thus, LMMS was discarded as promising ion source. Effective cooling under ambient conditions resulted in more controllable plasmas and development of laser induced breakdown spectroscopy (LIBS). However, little effort has been made to combine LIBS and ambient MS, since these plasmas only provide microsecond lifetimes. After these, recombination yields uncharged and newly bound species, making them inaccessible for MS. The combination of high repetition rate lasers together with growing knowledge about manipulation of charged species at atmospheric pressure allow overcoming these obstacles.
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.
Recently, a novel ionization scheme for ambient MS has been introduced. It is based on a quasi-continuous laser induced plasma (LIP), ignited in front of the MS inlet. This setup comprises the advantages of an ambient probe, electro neutrality, a sufficient duty cycle, a ubiquitous plasma medium, low power consumption, the absence of solvents and high sensitivity. To assess its future applicability for the detection of volatile organic compounds, plasma properties and operating conditions are investigated to understand the processes, that lead to the unexpected formation of intact molecular ions. Comprehensive studies include optical Emission spectroscopy, shadowgraphic shockwave visualization and time-of-flight mass spectrometry.