Filtern
Dokumenttyp
- Zeitschriftenartikel (10)
- Posterpräsentation (7)
- Vortrag (6)
Schlagworte
- Ambient mass spectrometry (5)
- Laser-induced plasma (5)
- DPSS laser (4)
- High repetition rate (3)
- Ionization (3)
- Laser induced plasma (3)
- Laser-spark (3)
- Ambient ionization (2)
- Dielectric barrier discharge (2)
- Mass spectrometry (2)
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (6)
Raman spectroscopy is becoming a commonly used, powerful tool for structural elucidation and species identification of small liquid samples, e.g. in droplet-based digital microfluidic devices. Due to the low scattering cross sections and the temporal restrictions dictated by the droplet flow, however, it depends on amplification strategies which often come at a cost. In the case of surface-enhanced Raman scattering (SERS), this can be an enhanced susceptibility towards memory effects and cross talk, whereas resonant and/or stimulated Raman techniques require higher instrumental sophistication, such as tunable lasers or the high electromagnetic field strengths which are typically provided by femtosecond lasers. Here, an alternative instrumental approach is discussed, in which stimulated Raman scattering (SRS) is achieved using the single fixed wavelength output of an inexpensive diode-pumped solid-state (DPSS) nanosecond laser. The required field strengths are realized by an effective light trapping in a resonator mode inside the interrogated droplets, while the resonant light required for the stimulation is provided by the fluorescence signal of an admixed laser dye. To elucidate the underlying optical processes, proof-of-concept experiments are conducted on acoustically levitated droplets, mimicking a highly reproducible and stable digital fluidic system. By using isotope-labeled compounds, the assignment of the emitted radiation as Raman scattering is firmly corroborated. A direct comparison reveals an amplification of the usually weak spontaneous Stokes emission by up to five orders of magnitude. Further investigation of the optical power dependence reveals the resulting gain to depend on the intensity of both, the input laser fluence and the concentration of the admixed fluorophore, leaving SRS as the only feasible amplification mechanism. While in this study stable large droplets have been studied, the underlying principles also hold true for smaller droplets, in which case significantly lower laser pulse energy is required. Since DPSS lasers are readily available with high repetition rates, the presented detection strategy bears a huge potential for fast online identification and characterization routines in digital microfluidic devices.
Advances in laser-induced plasmas have enabled various rapid and simple analytical applications. Especially, their uses in the analyses of condensed-phase samples have drawn significant attention in the past few decades. Depending on the laser energy per pulse, various analytical goals can be achieved. Laser-induced airborne plasmas allow direct analysis of species in ambient air. Importantly,
all of these applications are based on a fundamental understanding of the laser–medium interaction. Recent developments of diode-pumped solid-state lasers offer an alternative to conventional powerful, yet bulky lasers, which can specifically operate at high Repetition rates. Although these lasers deliver much lower power per pulse (mJ compared to mJ), the outstanding repetition rates offer significant improvement to meet statistical needs in some cases. In the present work, a mJ-laserinduced airborne plasma was characterized through optical emission analysis. By using a ns-timegated image detector coupled with specific bandpass filters, spatially, temporally, and spectrally resolved plasma images were recorded. Compared to conventional mJ-laser-induced plasmas, the one induced by mJ-lasers demonstrated unique features during its evolution. Specifically,
measurements of the distribution of ionic and atomic species revealed distinctive energy/matter transfer processes during early ignition of the plasma. Meanwhile, dynamic investigations suggested subsequent matter transport in the later stage.
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.
A versatile ionization scheme for atmospheric pressure MS is presented. It is based on a quasi-continuous laser-induced plasma (LIP), generated by a 26 kHz pulsed DPSS-laser, which is ignited in front of the MS inlet. Analytes are determined with different sampling regimes, comprising either an ambient desorption/ionization mechanism, a liquid-phase or gas-phase sample introduction.
The MS signal closely resembles the ionization behavior of APCI-like plasma-based sources, such as DBD or DART. Though LIPs are known to efficiently atomize/ionize any sample material, mass spectra of intact molecular ions are recorded, exhibiting low fragment-ion content. To understand this contradictory behavior, the plasma properties are investigated that lead to the formation of molecular ions. Comprehensive studies include optical emission spectroscopy, shadowgraph imaging and mass spectrometry diagnostics.
The results show that the ionization of analyte does not occur in the plasma itself, but in the cold adjacent gas layer. The pulsed character of LIPs induces an expanding shockwave, which concentrically expands around the plasma core and sweeps the molecules toward the plasma edges, where they are ionized either directly by the self-emission of the hot core or via interaction with secondary reactants. However, this unidirectional transport causes a rarefaction inside the plasma center, which leads to a decrease in plasma intensity and number density. Thus, a restoration of the former gaseous medium by other dynamically equilibrated diffusion processes would be favorable. Besides gas replenishing, we demonstrate the beneficial use of an acoustical standing wave inside an ultrasonic resonator on the performance of the LIP.
Optical elemental analysis in the gas phase typically relies on electrically driven plasmas. As an alternative approach, laser-induced plasmas (LIPs) have been suggested but have so far been only scarcely used.
Here, a novel signal enhancement strategy for laser-based airborne plasma optical Emission spectroscopy for gas phase analytics is presented. In contrast to an electrically driven plasma, in the laser-induced analogue dynamic matter transport equilibrium builds up. The latter results in a rarefied density regime in the plasma core itself, surrounded by an area of compressed matter. The central rarefaction leads to a decrease in plasma intensity and analyte number density, both of which are detrimental for analytical purposes. Since the repetitive ignition of LIPs is a transient process, a restoration of the former gaseous medium by other dynamically equilibrated diffusion processes would be favourable. The presented combination of an airborne LIP and an ultrasonic acoustic resonator yields a fourfold signal enhancement while the Background contribution of ubiquitous air is at the same time effectively suppressed. Since the entire enhancement effect occurs without contact, no additional sources for abrasive sample contamination are introduced.
An airborne laser plasma is suggested as an ambient ion source for mass spectrometry. Its fundamental physical properties, such as an excellent spatial and temporal definition, high electron and ion densities and a high effective cross section in maintaining the plasma, make it a promising candidate for future applications. For deeper insights into the plasma properties, the optical plasma emission is examined and compared to mass spectra. The results show a seemingly contradictory behavior, since the emitted light reports the plasma to almost entirely consist of hot elemental ions, while the corresponding mass spectra exhibit the formation of intact molecular species. Further experiments, including time- resolved shadowgraphy, spatially resolved mass spectrometry, as well as flow-dependent emission spectroscopy and mass spectrometry, suggest the analyte molecules to be formed in the cold plasma vicinity upon interaction with reactive species formed inside the hot plasma center. Spatial separation is maintained by concentrically expanding pressure waves, inducing a strong unidirectional diffusion. The accompanying rarefaction inside the plasma center can be compensated by a gas stream application. This replenishing results in a strong increase in emission brightness, in local reactive species concentration, and eventually in direct mass spectrometric sensitivity. To determine the analytical performance of the new technique, a comparison with an atmospheric pressure chemical ionization (APCI) source was conducted. Two kitchen herbs, namely, spearmint and basil, were analyzed without any sample pretreatment. The presented results demonstrate a considerably higher sensitivity of the presented laser-spark ionization technique.
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.
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.
A novel direct sampling ionization scheme for ambient mass spectrometry is presented. Desorption and ionization is achieved by a quasi-continuous laser induced plasma in air. Since there are no solid or liquid electrodes involved the ion source does not suffer from chemical interferences or fatigue originating from erosive burning or from electrode consumption. A laser plasma was ignited under ambient conditions in front of a modified TOF MS atmospheric pressure interface, using a high repetition rate DPSS laser operating at 532 nm and 26 kHz and an aspherical lens with a focal length of 8 mm. Emission spectroscopy (40-1100 nm) and time resolved studies on specific plasma parameters revealed insight into the physical and chemical plasma properties. Plasma ignition can be performed in rare gases and under ambient conditions. The hot plasma zone was kept at a certain distance from the sample region. Thus, effective collisional cooling seemed to prevent thermal fragmentation. Every single spark generates a shockwave, providing new reactive species, which expands concentrically from the hot region. Under ambient conditions primary charge carriers (ions and electrons) as well as VUV radiation initialize reaction cascades equivalent to other ambient ionization methods, such as DART or DBD. Mass spectra of polar/nonpolar hydrocarbons, sugars, pharmaceuticals and natural biomolecules in food were observed. Comprehensive emission spectroscopic measurements and time resolved electron current studies revealed insight into some plasma properties, such as the emitted high energetic radiation and the time evolution of the expanding plume.
A novel direct sampling ionisation scheme for ambient mass spectrometry is presented. Desorption and ionisation are achieved by a quasi-continuous laser induced plasma in air. Since there are no solid or liquid electrodes involved the ion source does not suffer from chemical interferences or fatigue originating from erosive burning or from electrode consumption. The overall plasma maintains electro-neutrality, minimising charge effects and accompanying long term drift of the charged particles trajectories. In the airborne plasma approach the ambient air not only serves as the plasma medium but at the same time also slows down the nascent ions via collisional cooling. Ionisation of the analyte molecules does not occur in the plasma itself but is induced by interaction with nascent ionic fragments, electrons and/or far ultraviolet photons in the plasma vicinity. At each individual air-spark an audible shockwave is formed, providing new reactive species, which expands concentrically and, thus, prevents direct contact of the analyte with the hot region inside the plasma itself. As a consequence the interaction volume between plasma and analyte does not exceed the threshold temperature for thermal dissociation or fragmentation. Experimentally this indirect ionisation scheme is demonstrated to be widely unspecific to the chemical nature of the analyte and to hardly result in any fragmentation of the studied molecules. A vast ensemble of different test analytes including polar and non-polar hydrocarbons, sugars, low mass active ingredients of pharmaceuticals as well as natural biomolecules in food samples directly out of their complex matrices could be shown to yield easily accessible yet meaningful spectra. Since the plasma medium is humid air, the chemical reaction mechanism of the ionisation is likely to be similar to other ambient ionisation techniques.