Open Access

Analyzing complex mixtures with mass spectrometry (MS) usually requires extensive sample preprocessing and separation techniques such as high-pressure iquid chromatography (HPLC). This approach allows for subsequent detection of one analyte at a time but at a cost of throughput. Recently, increasing attention has been put on thesimultaneous ionization of all analytes in complex mixtures without extensive preprocessing, aiming to streamline the analysis process while accepting a compromise in identification. Such methods are commonly referred to as direct mass-spectrometric analysis. Among those, pulsed laser-based ablation, allows for contactless chemical investigation and reaction monitoring in situ, with minimal sample pretreatments. Specifically, the mid-infrared lasers encompass the fundamental vibrational resonances of typical biological matrices and solvents; their use results in efficient material removal from the bulk phase. Interestingly, the resulting analyte chronograms during the ablation process showed that the analytes were not detected concomitantly. Instead, chronograms offered time-domain information by being detected at different timestamps. It is certain that chemical information is encoded in the time domain. However, the analyte-specific chronograms could be difficult to interpret with conventional workflows as they mostly overlap. In this study, we aim exploiting the commonly discarded information in the time domain as an orthogonal dimension to the m/z domain, achieving ategorization and grouping mass-spectral peaks according to their chemical origins. Here, we applied a pulsed diode-pumped solid-state (DPSS) laser at 3-μm wavelength to 10 μL hanging droplets of analyte. A secondary electrospray ionization (SESI) source was built in-house to ionize ablated analytes. With a model sample, i.e. a mixture of two antibiotics, remarkable differences in their chronograms were observed. In fact, the time-domain information reflects the chemical signature of the analytes and can provide an additional data dimension for accurate interpretation. With a phase-sensitive algorithm, the modified cross-correlation (mXcorr), the similarity between chronograms can be gauged. Consequently, mass-spectral peaks that shared the same chronogram features were grouped together, yielding analyte-specific mass spectra that can aid analyte identification. Lastly, the applicability for biological samples, such as peptides, will be discussed.

Laser-induced breakdown spectroscopy (LIBS) is becoming a more mature technology every year with new variants such as laser ablation molecular isotopic spectrometry, reheating by various discharge techniques, and multiple pulse excitation schemes, in which sometimes lasers of different pulse lengths are used. However, lasers with inherent parameters like pulse length and repetition rate are still almost exclusively employed. Recent years have witnessed the advent of novel high-repetition-rate laser concepts for machining processes, like welding, milling, and engraving. Here, a comprehensive study of single-pulse LIBS spectra of a single aluminum target is presented to showcase the applicability of flexible high duty-cycle master oscillator power amplifier (MOPA) lasers. Although traditional flashlamp-pumped Fabry–Pérot lasers only permit a variation in the pulse energy and are operated at very low duty-cycles, MOPA lasers add repetition rate and pulse length as variable parameters. A thorough analysis of the temporal plasma behavior revealed the emission dynamic to closely match the excitation laser pulse pattern. An aluminum sample’s spectral response was shown to be significantly impacted by variations in both rate and length. Although the spectral emission strength of the elemental lines of Al, Sr, and Ca all peaked at slightly different parameter settings, the strongest impact was found on the relative abundance of molecular AlO bands. Unlike in previous laser ablation molecular isotopic spectrometry (LAMIS) publications, the latter could be readily detected with a good intensity and well-resolved spectral features without any temporal gating of the detector. This finding, together with the fact that MOPA lasers are both inexpensive and dependable, makes for a promising combination for future studies including the detection of diatomic band structures.

In ion-based spectrometry techniques the possibility to manipulate ions is fundamentally important. Currently used ion optics mostly rely on magnetic or electric fields. The electromagnetic forces compete with ion diffusion and are therefore most effective under low-pressure conditions. Therefore, high-pressure ion optics pose challenges related to elevated powers and potentials as well as complex structures and electrode contamination. Acoustic Ion Manipulation (AIM) is a recently discovered phenomenon that relies on the unique interactions between gas-phase ions and acoustic waves. This presentation shows selected patterns of ion manipulation, highlights the flexibility of AIM and discusses its current limitations and potentials.

The coupling of electrospray ionization (ESI) with the newly discovered acoustic ion manipulation (AIM) phenomenon is demonstrated for biopolymeric ions, including peptides, proteins, and oligonucleotides. It is shown that large(i.e.,up to ca.29 kDa) and multiply charged Ions are influenced by the presence of acoustic fields. Specifically, we demonstrate the ability to gate, modulate, redirect, and focus These biomolecular Ions with an acoustic field. To ensure effective acoustic control of ESI-generated ions, it was necessary to desolvate the electrospray droplets and ions prior to the AIM field through optimization of drying tube temperature and spray gas pressure. In general, small and highly charged Ions were more readily deflected from the unstable antinode Region toward the acoustic nodes.This trend Extended to acharge-state-dependent behavior for proteinions, where higher-charge Ions of the same molecule werere directed more easily than their lower-charge counterparts. This work laysa foundation for the implementation of AIMin the study of biomolecules aswell as other analytes introducedbyESI to ion-based spectroscopic techniques, such as massspectrometry and ion mobility spectrometry.