Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd : YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented. Received: 25 January 1999 / Revised: 7 April 1999 / Accepted: 30 April 1999     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd?:?YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented.     

Optical far- and near-field femtosecond laser ablation of Si for nanoscale chemical analysis

Extending spatial resolution in laser-based chemical analysis to the nanoscale becomes increasingly important as nanoscience and nanotechnology develop. Implementation of femtosecond laser pulses arises as a basic strategy for increasing resolution since it is associated with spatially localized material damage. In this work we study femtosecond laser far- and near-field processing of silicon (Si) at two distinct wavelengths (400 and 800 nm), for nanoscale chemical analysis. By tightly focusing femtosecond laser beams in the far-field, we were able to produce sub-micrometer craters. In order to further reduce the crater size, similar experiments were performed in the near-field through sub-wavelength apertures, resulting in the formation of sub-30-nm craters. Laser-induced breakdown spectroscopy (LIBS) was used for chemical analysis with a goal to identify the minimum crater size from which spectral emission could be measured. Emission from sub-micrometer craters (full width at half maximum) was possible, which are among the smallest ever reported for femtosecond LIBS.     

On-line atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry

A new technique is presented for the coupling of atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) mass spectrometry with liquid delivery systems. Mass measurements of polymers and peptides are demonstrated using a co-dissolved matrix, e.g. alpha-cyano-4-hydroxycinnamic acid (HCCA). Improvements in terms of sensitivity are achieved by optimizing the shape und control of the exit capillary and by using a laser (355 nm) at a 1 kHz repetition rate. Two calibration experiments promise a good applicability of the presented coupling method for quantitative measurements. The limit of detection achieved so far is 500 nM for peptides in methanol solution containing 25 mM HCCA.     

Direct liquid ablation: a new calibration strategy for laser ablation-ICP-MS microanalysis of solids and liquids

Trace elements in microliter quantities of aqueous solutions were analysed by direct liquid ablation using an 193 nm excimer with an inductively coupled plasma mass spectrometer (ICP-MS). Fractionation resulting from splashing and evaporation can be minimised by covering the liquid surface with a thin plastic film, through which a 20 μm hole is drilled with the laser. Particle-size distribution and oxide formation in the plasma resulting from the direct liquid ablation are similar to those generated by solid ablation. The ICP-MS response in cps/ppm is approximately 100 × higher for the direct ablation, but is proportional to the response from solid ablation, within an accuracy < 15% for most trace elements in NIST 610 and NIST 612 glass standards. A matrix load up to 2.5 wt.-% NaCl in the solution does not affect the proportionality of trace element responses. Thus, direct liquid ablation is not only suited for analysing small volumes of complex aqueous solutions (e.g., the quantitative microanalysis of fluid inclusions in minerals), but also provides a new approach for calibrating laser ablation ICP-MS microanalysis of solids. Received: 2 December 1996 / Revised: 3 March 1997 / Accepted: March 1997     

Coupling laser ablation/desorption electrospray ionization to atmospheric pressure drift tube ion mobility spectrometry for the screening of antimalarial drug quality

Significant developments in the field of ambient desorption/ionization mass spectrometry (MS) have led to high-throughput direct analysis and imaging capabilities. However, advances in coupling ambient ionization techniques with standalone drift tube ion mobility spectrometry (DTIMS) have been comparatively slower, despite the attractive ruggedness and simplicity of IMS. In this study, we have developed and characterized a laser ablation/desorption electrospray ionization (LADESI) DTIMS platform, and applied it to the detection of active pharmaceutical ingredients (APIs) in antimalarial tablets collected in developing countries. The overarching goal of this work was to perform an initial evaluation of LADESI DTIMS as a technique with the potential for constituting the core of a portable drug quality-testing platform. The set-up consisted of an IR laser for desorption and an electrospray ionizer for capturing the ablated plume coupled to a high-resolution monolithic resistive glass drift tube ion mobility spectrometer. For more confident API identification, tablet extracts were also investigated via electrospray IM MS to correlate LADESI DTIMS reduced mobility (K(0)) values to m/z values. Overall, it was found that the IR LADESI DTIMS platform provided distinct ion mobility spectral fingerprints that could be used to detect the presence of the expected APIs, helping to distinguish counterfeit drugs from their genuine counterparts.     

Liquid supports for ultraviolet atmospheric pressure matrix-assisted laser desorption/ionization

Atmospheric pressure (AP) liquid matrices for ultraviolet (UV) matrix-assisted laser desorption/ionization (MALDI) are presented. Doping a known organic chromophore, alpha-cyano-4-hydroxycinnamic acid (CHCA), into liquid media yielded a homogenous sample system with simplified sample preparation, increased sample lifetime, and added utility for APMALDI ion sources. Compared with vacuum situations, AP matrices are not as limited by vapor pressure, so liquid matrix formulations can focus on desorption and ionization versus vacuum stability and source contamination. The parameters studied include chromophore concentration, liquid support variations, and quantitation capability. Chromophore concentration adjustments provided insight into the necessary absorbance for UV-APMALDI and demonstrated the importance of laser penetration depth. Liquid support variations allowed adjustments of sample lifetime and analyte solvents. Extended sample lifetime is beneficial for instrument tuning and source optimization; however, increased liquid viscosity lowers signal intensity. The shot-to-shot reproducibility, as examined with individual ion packets, suggests that the liquid matrix can alleviate some inconsistencies seen with solid MALDI, suggesting a possibility for better quantitation. The measurements for laser penetration depth, solution viscosity, and solvent additives could add to the information on MALDI mechanisms. The liquid matrix offers advantages that complement current MALDI methods.     

Probing thymine with laser ablation molecular beam Fourier transform microwave spectroscopy

Laser ablation in combination with molecular beam Fourier transform microwave spectroscopy has been used to establish unambiguously the presence of the diketo form of thymine in the gas phase and to obtain its structure.     

Low pressure laser ablation coupled to inductively coupled plasma mass spectrometry

The particle size distribution in laser ablation inductively coupled plasma mass spectrometry is known to be a critical parameter for complete vaporization of particles. Any strategy to reduce the particle size distribution of laser generated aerosols has the potential to increase the ion signal intensity and to reduce fractionation effects. Due to the fact that vapor generation, nucleation, condensation, and agglomeration take place within an extremely short period of time, ablation under atmospheric pressure might not allow influencing these processes while under reduced pressure condition the cooling of the aerosol and therefore the condensation is expected to be slower. In this study, a low pressure laser ablation cell for the generation of laser aerosols was coupled to an ICP-MS. In contrast to the previously developed trapped ablation mode, the newly designed cell allows the adjustment of the pressure in the ablation cell between 20 and 1400 mbar prior to the ablation.Ablation experiments carried out using this configuration showed a dependence of the aerosol properties (size distribution and particle structure) on the ablation cell pressure. The intensity ratio U/Th measured as a figure of merit for complete vaporization within the ICP indicated a change in the aerosol structure at approximately 500 mbar toward smaller particle size. A significant difference between low pressure and at ambient pressure ablated aerosol was observed. The intensity ratios (U/Th) of the ablated sample moves closer to the bulk composition at lower pressures at the expense of sensitivity. Therefore the decrease in the ICP-MS signal intensity in the low pressure cell can be attributed to vapor deposition within the ablation cell walls.Moreover, scanning electron microscope images of aerosols collected on filters after the low pressure ablation cell suggest the possibility of a slower cooling velocity of the aerosol, which was observed in the condensed material on the surface of ejected spherical particles. The expansion of the laser aerosol was also investigated using polished brass substrates in the expansion path-way for particle collection.     

Influence of polymer molecular weight on the chemical modifications induced by UV laser ablation

This paper investigates the influence of polymer molecular weight (M(W)) on the chemical modifications of poly(methyl methacrylate), PMMA, and polystyrene, PS, films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI), following irradiation at 248 nm (KrF excimer laser, 20 ns fwhm and hybrid excimer-dye laser, 500 fs fwhm) and at 308 nm (XeCl excimer laser, 30 ns fwhm). The changes of intensity and position of the polymer Raman bands upon irradiation provide information on cleavage of the polymer bonds. Degradation of PMMA, which is a weak absorbing system at 248 nm, occurs to a higher extent in the case of a larger M(W), giving rise to the creation of unsaturation centers and to degradation products. For highly absorbing PS, no degradation is observed upon irradiation with a KrF laser. Consistently irradiating doped PS at 308 nm, where the absorption is low, induces degradation of the polymer. Results provide direct support for the bulk photothermal model, according to which ejection requires a critical number of broken bonds. In the case of irradiation of doped PMMA with pulses of 248 nm and 500 fs, neither degradation nor dependence with polymer M(W) are observed, indicating that mechanisms involved in the femtosecond laser ablation differ from those operating in the case of nanosecond laser ablation. Participation of multiphoton/avalanche processes is proposed.     

Modeling of ion processes in atmospheric pressure matrix-assisted laser desorption/ionisation

Atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) has proven a convenient and rapid method for ion production in the mass spectrometric analysis of biomolecules. This technique, like other atmospheric pressure ionization methods, suffers from ion loss during ion transmission from the atmosphere into the vacuum of the mass spectrometer. In this work, a simple model describing ion formation and ion motion towards the inlet capillary of the mass spectrometer is described. Both the gas flow and electric field near the MALDI plate were numerically calculated using the boundary element method (BEM). The ions were moving along with the gas flow and drifting in the electric field in accordance with their ion mobility properties. The ion signal dependence on an electric field strength obtained in the proposed model correlates well with experimental results.     

Rapid methods of polymer and polymer additives identification: Multi-sample solvent-free MALDI, pyrolysis at atmospheric pressure, and atmospheric solids analysis probe mass spectrometry

Polymer manufacturers add antioxidants, waxes, dyes, and other materials to enhance polymer utility or processing. Numerous analytical methods are available to characterize various chemical aspects of polymers including methods interfaced with mass spectrometry (MS) such as pyrolysis (Py), gas chromatography (GC), liquid chromatography (LC), and thermogravimetric analysis (TGA). Current methods work well, but because of the necessity of extraction, chromatography, or thermal methods, most are too time consuming for high throughput analyses which might be necessary in, for example, regulatory laboratories. Here we discuss three MS methods for rapid analysis of polymers; multi-sample MALDI MS which allows rapid analysis of low molecular weight polymers, atmospheric pressure (AP) solids analysis probe MS for direct ambient additives analysis, and APPy MS for polymer identification. The latter methods provide information regardless of the composition or molecular weight of the polymeric material.     

An atmospheric pressure matrix-assisted laser desorption/ionization ion trap with enhanced sensitivity

When atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) became commercially available, the technique generated a great deal of interest because ion production was decoupled from mass analysis. Mass accuracy and resolution were therefore dependent on parameters governing the mass analyzer rather than the matrix and sample preparation. Researchers have successfully used AP-MALDI sources with both orthogonal acceleration time-of-flight (oaTOFMS) and ion trap mass spectrometers. However, one limitation of the technique has been sensitivity, especially for mixtures of peptides generated from tryptic digests. In this work, data are presented documenting an increase in sensitivity of approximately two orders of magnitude as compared with results previously reported in the literature. The improvement in sensitivity is thought to derive primarily from the novel use of a countercurrent heated gas stream directed at the sample, although the target plate position and ion sampling configuration have also been optimized to reduce chemical noise from low molecular weight ions. A tryptic digest of BSA containing 125 attomoles on the plate was successfully identified in MS-only mode, while MS/MS analysis of 250 attomoles of the same digest provided product ion spectra with sufficient information to identify the protein. More complicated mixtures of standard proteins were used to model proteomics experiments, and preliminary data suggest a minimum working dynamic range of 20-fold for the analysis of mixtures of protein digests.     

Functionalized nanoparticles for liquid atmospheric pressure matrix-assisted laser desorption/ionization peptide analysis

Nanoparticles for the extraction of peptides and subsequent analysis using atmospheric pressure matrix-assisted laser desorption/ionization (APMALDI) have been evaluated. The atmospheric pressure source allows for particles to be directly introduced in the liquid matrix, minimizing sample loss and analysis time. Described in this work are two sample preparation procedures for liquid APMALDI analysis: a C18 functionalized silica nanoparticle for hydrophobic extractions, and an aptamer functionalized magnetite core nanoparticle for rapid, affinity extractions. The C18 particles provide a non-selective support for rapid profiling applications, while the aptamer particles are directed towards reducing the complexity in biological samples. The aptamer functionalized particles provide a more selective analyte-nanoparticle interaction whereby the tertiary structure of the analyte becomes more critical to the extraction. In both cases, the liquid APMALDI matrix provides a support for ionization, and acts as the releasing agent for the analyte-particle interaction. Additionally, analyte enrichment was possible due to the large surface-to-volume ratio of the particles. The experiments conducted with functionalized nanoparticles, in an atmospheric pressure liquid matrix, present a basis for further methodologies and utilities of silica nanoparticles to be developed.     

Mass spectrometry of N-linked oligosaccharides using atmospheric pressure infrared laser ionization from solution

An atmospheric pressure (AP) infrared (IR) laser ionization technique, implemented on a quadrupole ion trap mass spectrometer, was used to analyze underivatized, N-linked oligosaccharides in solution. Experiments were conducted on an atmospheric pressure infrared ionization from solution (AP-IRIS) ion source which differed from previous AP IR matrix-assisted laser desorption/ionization (MALDI) interfaces in that the ion source operated in the absence of an extraction electric field with a higher power 2.94 microm IR laser. The general term 'IRIS' is used as the mechanism of ionization differs from that of MALDI, and is yet to be fully elucidated. The AP-IRIS ion source demonstrated femtomole-level sensitivity for branched oligosaccharides. AP-IRIS showed approximately 16 times improved sensitivity for oligomannose-6 and the core-fucosylated glycan M3N2F over optimal results obtainable on a AP UV-MALDI with a 2,4,6-trihydroxyacetophenone matrix. Comparison between IR and UV cases also showed less fragmentation in the IR spectrum for a glycan with a conserved trimannosyl core, core-substituted with fucose. A mixture of complex, high-mannose and sialylated glycans resulted in positive ion mass spectra with molecular ion peaks for each sugar. Tandem mass spectrometry of the sodiated molecular ions in a mixture of glycans revealed primarily glycosidic (B, Y) cleavages. The reported results show the practical utility of AP-IRIS while the ionization mechanism is still under investigation.