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One major drawback of matrix-assisted laser desorption/ionization (MALDI) is still the relatively poor pulse-to-pulse reproducibility of the signal intensity. This problem, caused by insufficient homogeneity in the matrix/analyte co-crystallization, is usually circumvented by averaging the detected ion intensity over several shots. However, during the consecutive laser pulses, the applied matrix gets depleted and only a number of subsequent experiments can be done on the same sample spot. In order to achieve the desired long-term stability in combination with a sufficient pulse-to-pulse reproducibility, recently liquid MALDI matrices have been introduced. This contribution demonstrates the promising combination of liquid matrices with high repetition rate lasers for atmospheric pressure MALDI (AP-MALDI). To demonstrate the robustness of the new approach, two different kinds of liquid matrices were used in combination with both a typical flashlamp pumped 15 Hz laser and a diode pumped solid state laser operated at 5 kHz. The latter showed a stable ion signal over more than 3,500,000 consecutive laser pulses.
Most ambient sample introduction and ionization techniques for native mass spectrometry are highly selective for polar agents. To achieve a more general sensitivity for a wider range of target analytes, a novel laser ablation dielectric barrier discharge (LA DBD) ionization scheme was developed. The Approach employs a two-step mechanism with subsequent sample desorption and post-ionization. Effective Ablation was achieved by the second harmonic output (λ = 532 nm) of a diode pumped Nd:YVO₄ laser operating at a high-repetition rate of several kHz and pulse energies below 100 μJ. The ejected analytecontaining aerosol was consecutively vaporized and ionized in the afterglow of a DBD plasma jet.
Depending on their proton affinity the superexcited Helium species in this afterglow produced analyte ions as protonated and ammoniated species, as well as radical cations. The optimization procedure could corroborate underlying conceptual consideration on the ablation, desorption and ionization mechanisms.
A successful detection of a variety of target molecules could be shown from the pharmaceutical ibuprofen, urea, the amino acids L-arginine, L-lysine, the polymer polyethylene glycol, the organometallic compound ferrocene and the technical mixture wild mint oil. For a reliable evaluation of the introduced detection procedure spectra from the naturally abundant alkaloid capsaicin in dried capsicum fruits were recorded.
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.
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.
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.
An airborne high repetition rate laser-induced plasma was applied as a versatile ambient ionization source for mass-spectrometric determinations of polar and nonpolar analytes in solution. The laser plasma was sustained between a home-built pneumatic nebulizer and the inlet capillary of an Orbitrap mass spectrometer. To maintain stable conditions in the droplet-rich spray environment, the plasma was directly fed by the fundamental output (λ = 1064 nm) of a current state-of-the-art diode-pumped solid-state laser. Ionization by the laser-driven plasma resulted in signals of intact analyte ions of several chemical categories. The analyte ions were found to be fully desolvated since no further increase in ion signal was observed upon heating of the inlet capillary. Due to the electroneutrality of the plasma, both positive and negative analyte ions could be formed simultaneously without altering the operational parameters of the ion source. While, typically, polar analytes with pronounced gas phase basicities worked best, nonpolar and amphoteric compounds were also detected. The latter were detected with lower ion signals and were prone to a certain degree of fragmentation induced during the ionization process. All the described attests the laser-induced microplasma by a good performance in terms of stability, robustness, sensitivity, and general applicability as a self-contained ion source for the liquid sample introduction.