Internal energy transfer in laser desorption/ionization from silicon nanowires

Laser-induced desorption/ionization from silicon nanowires (SiNW) is an emerging method for mass spectrometry of small to medium-size molecules. In this new technique, we examined the internal energy transfer to seven benzylpyridinium thermometer ions and extracted the corresponding internal energy distributions. To explore the effect of the energy-deposition rate on the internal energy transfer, two lasers with significantly different pulse lengths (4 ns vs 22 ps) were utilized as excitation sources. A comparison of ion yields indicated that the SiNW substrates required 5-8 times less laser fluence for ion production than either matrix-assisted laser desorption/ionization (MALDI) or desorption/ionization on silicon (DIOS). In contrast however, the survival yield (SY) values showed that the internal energy transferred to the thermometer ions was more than (ps laser) or comparable to (ns laser) MALDI but it was significantly less than in DIOS. The internal energy transfer was only slightly dependent on laser fluence and on wire density. These effects were rationalized in terms of the confinement of thermal energy in the nanowires and of unimpeded three-dimensional plume expansion. Unlike in MALDI from CHCA and in perfluorophenyl-derivatized DIOS, for desorption from SiNWs the effect of laser pulse length on the internal energy transfer was found to be negligible.     

Molecular dynamics simulations of MALDI: laser fluence and pulse width dependence of plume characteristics and consequences for matrix and analyte ionization

Molecular dynamics simulations of matrix‐assisted laser desorption/ionization were carried out to investigate laser pulse width and fluence effects on primary and secondary ionization process. At the same fluence, short (35 or 350 ps) pulses lead to much higher initial pressures and ion concentrations than longer ones (3 ns), but these differences do not persist because the system relaxes toward local thermal equilibrium on a nanosecond timescale. Higher fluences accentuate the initial disparities, but downstream differences are not substantial. Axial velocities of ions and neutrals are found to span a wide range, and be fluence dependent. Total ion yield is only weakly dependent on pulse width, and consistent with experimental estimates. Secondary reactions of matrix cations with analyte neutrals are efficient even though analyte ions are ablated in clusters of matrix. Copyright © 2010 John Wiley & Sons, Ltd.     

Surface modification and laser pulse length effects on internal energy transfer in DIOS

Benzyl-substituted benzylpyridinium (BP) chloride salts were used as a source of thermometer ions to probe the internal energy (IE) transfer in desorption/ionization on porous silicon (DIOS). To modify their wetting properties and the interaction energies with the thermometer ions, the DIOS surfaces were silylated to produce trimethylsilyl- (TMS), amine- (NH2), perfluoroalkyl- (PFA), and perfluorophenyl-derivatized (PFP) surfaces. Two laser sources--a nitrogen laser with pulse length of 4 ns and a mode locked 3 x omega Nd:YAG laser with a pulse length of 22 ps--were utilized to induce desorption/ionization and fragmentation at various laser fluence levels. The corresponding survival yields were determined as indicators of the IE transfer and the IE distributions were extracted. In most cases, with increasing the laser fluence in a broad range (approximately 20 mJ/cm2), no change in IE transfer was observed. For ns excitation, this was in remarkable contrast with MALDI, where increasing the laser fluence resulted in sharply (within approximately 5 mJ/cm2) declining survival yields. Derivatization of the porous silicon surface did not affect the survival yields significantly but had a discernible effect on the threshold fluence for ion production. The IE distributions determined for DIOS and MALDI from alpha-cyano-4-hydroxycinnamic acid reveal that the mean IE value is always lower for the latter. Using the ps laser, the IE distribution is always narrower for DIOS, whereas for ns laser excitation the width depends on surface modification. Most of the differences between MALDI and DIOS described here are compatible with the different dimensionality of the plume expansion and the differences in the activation energy of desorption due to surface modifications.     

Laser spray: electric field-assisted matrix-assisted laser desorption/ionization

A new liquid chromatography/mass spectrometry interface, the laser spray, has been developed. Explosive vaporization and mist formation occur when an aqueous solution effusing out from the tip of the stainless-steel capillary is irradiated from the opposite side of the capillary by a 10.6 microm infrared laser. Weak ion signals could be detected when the plume was sampled through the ion sampling orifice. When a high voltage (3-4 kV) was applied to the stainless-steel capillary, strong ion signals appeared. The ion abundances were found to be orders of magnitude greater than those obtained by conventional electrospray ionization in the case of aqueous solutions. The present method is regarded as an electric-field assisted form of matrix-assisted laser desorption/ionization in which the liquid chromatographic solvent (water, etc.) acts as a liquid matrix. Laser spray ionization is expected to become a versatile method for biological mass spectrometry because this method is compatible with the natural solvent, water.     

Irradiation effects in MALDI,ablation, ion production,and surface modifications. Part II. 2,5-dihydroxybenzoic acid monocrystals

Irradiation effects at low and high laser fluence on 2,5-dihydroxybenzoic acid large crystals were investigated. Contrary to what was observed for matrices as cinnamic acid derivatives, no chemical degradation of matrix is evidenced and continuous ablation as well as ion production resulted of extended irradiation in all the fluence range corresponding to classical matrix-assisted laser desorption /ionization. Ripples are formed on the base of the crater for a limited number of laser shots under moderate fluence. For extended irradiation, conical shape craters are formed with the axis of the crater oriented along the incident direction of the laser beam. A study of the craters showed that ablation through the ablated volume slowly varied with the laser fluence when a strong increase of ion production (matrix and analyte) was recorded. Ablation volume was found to vary non-linearly with the number of laser shots. On a same spot, the ablated volume and the ion production were measured as a function of the laser energy. With an increasing laser energy (or fluence), the ablated volume slowly increases when the ion production strongly increases. This gives evidence of a decoupling between ablation and ionization. Interaction of the plume with the incoming beam is thus probable.     

Fragmentation of leucine enkephalin as a function of laser fluence in a MALDI TOF-TOF

The effects of laser fluence on ion formation in MALDI were studied using a tandem TOF mass spectrometer with a Nd-YAG laser and alpha-cyano hydrocinnamic acid matrix. Leucine enkephalin ionization and fragmentation were followed as a function of laser fluence ranging from the threshold of ion formation to the maximum available, that is, about 280-930 mJ/mm2. The most notable finding was the appearance of immonium ions at fluence values close to threshold, increasing rapidly and then tapering in intensity with the appearance of typical backbone fragment ions. The data suggest the presence of two distinct environments for ion formation. One is associated with molecular desorption at low values of laser fluence that leads to extensive immonium ion formation. The second becomes dominant at higher fluences, is associated initially with backbone type fragments, but, at the highest values of fluence, progresses to immonium fragments. This second environment is suggestive of ion desorption from large pieces of material ablated from the surface. Arrhenius rate law considerations were used to estimate temperatures associated with the onset of these two processes.     

Infrared laser post-ionization of large biomolecules from an IR-MALD(I) plume

A two-infrared laser desorption/ionization method is described. A first laser, which was either an Er:YAG laser or an optical parametric oscillator (OPO), served for ablation/vaporization of small volumes of analyte/matrix sample at fluences below the ion detection threshold for direct matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). A second IR-laser, whose beam intersected the expanding ablation plume at a variable distance and time delay, was used to generate biomolecular ions out of the matrix-assisted laser desorption (MALD) plume. Either one of the two above lasers or an Er:YSGG laser was used for post-ionization. Glycerol was used as IR-MALDI matrix, and mass spectra of peptides, proteins, as well as nucleic acids, some of which in excess of 10(5) u in molecular weight, were recorded with a time-of-flight mass spectrometer. A mass spectrum of cytochrome c from a water ice matrix is also presented. The MALD plume expansion was investigated by varying the position of the post-ionization laser beam above the glycerol sample surface and its delay time relative to the desorption laser. Comparison between the OPO (pulse duration, tau(L) = 6 ns) and the Er:YAG laser (tau(L) approximately 120 ns) as primary excitation laser demonstrates a significant effect of the laser pulse duration on the MALD process.     

Influence of the laser intensity and spot size on the desorption of molecules and ions in matrix-assisted laser desorption/ionization with a uniform beam profile

The dependence of the number of desorbed particles on laser fluence has been investigated for matrix-assisted laser desorption/ionization (MALDI) of analyte and matrix ions as well as for (photoionized) neutral matrix molecules using a homogeneous “flat-top” laser profile. Laser spot diameters ranging from 10 to 200 μm in size have been used. 2,5-Dihydroxybenzoic acid (DHB) and 3,5-dimethoxy-4-hydroxycinnamic acid (sinapic acid) have been tested as matrices. The threshold (for ion detection) is higher and the dependence of the ion signal upon higher-than-threshold fluences is stronger for directly desorbed ions than for photoionized neutral molecules. Directly desorbed analyte ions exhibit the same dependence on fluence as the matrix ions with only minor differences between the two matrices tested, so both have approximately the same detection threshold. For both ions and photoionized neutral molecules, the fluence threshold increases with decreasing spot size while the slope of the intensity/fluence curves decreases. A quasi-thermal, sublimation/desportion model was found to describe the experimental results with excellent precision. For a complete explanation, non-equilibrium effects had to be taken into account.     

Phenomenological models for matrix-assisted laser desorption ion yields near the threshold fluence

Phenomenological models were proposed to explain the experimentally observed dependence of protein ion yields with laser fluence in matrix-assisted laser desorption. Assuming that the illuminating laser had a Gaussian intensity profile at the sample being examined, it was possible to fit the experimental points with a model that only assumes a fluence threshold for ion production. No additional dependence of protein ion yield on illuminating fluence above the threshold value was necessary to explain the data.     

What determines MALDI ion yields? A molecular dynamics study of ion loss mechanisms

Ion recombination in matrix-assisted laser desorption/ionization (MALDI) is as important as any ion formation process in determining the quantity of ions observed but has received comparatively little attention. Molecular dynamics simulations are used here to investigate some models for recombination, including a Langevin-type model, a soft threshold model and a tunneling model. The latter was found to be superior due to its foundations in a widespread physical phenomenon, and its lack of excessive sensitivity to parameter choice. Tunneling recombination in the Marcus inverted region may be a major reason why MALDI is a viable analytical method, by allowing ion formation to exceed ion loss on the time scale of the plume expansion. Ion velocities, photoacoustic transients and pump-probe measurements might be used to investigate the role of recombination in different MALDI matrices, and to select new matrices.     

Influence of the laser fluence in infrared matrix-assisted laser desorption/ionization with a 2.94 microm Er : YAG laser and a flat-top beam profile

The dependence of the signal intensity of analyte and matrix ions on laser fluence was investigated for infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry using a flat-top laser beam profile. The beam of an Er : YAG laser (wavelength, 2.94 microm; pulse width, 90 ns) was coupled into a sapphire fiber and the homogeneously illuminated end surface of the fiber imaged on to the sample by a telescope. Three different laser spot sizes of 175, 350 and 700 microm diameter were realized. Threshold fluences of common IR matrices were determined to range from about 1000 to a few thousand J m(-2), depending on the matrix and the size of the irradiated area. In the MALDI-typical fluence range, above the detection threshold ion signals increase strongly with fluence for all matrices, with a dependence similar to that for UV-MALDI. Despite the strongly different absorption coefficients of the tested matrices, varying by more than an order of magnitude at the excitation laser wavelength, threshold fluences for equal spot sizes were found to be comparable within a factor of two. With the additional dependence of fluence on spot size, the deposited energy per volume of matrix at threshold fluence ranged from about 1 kJ mol(-1) for succinic acid to about 100 kJ mol(-1) for glycerol.     

The effect of laser profile, fluence, and spot size on sensitivity in orthogonal-injection matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The influence of incident laser parameters on sensitivity in matrix-assisted laser desorption/ionization (MALDI) has been investigated using orthogonal-injection time-of-flight (TOF) instruments. A qualitative comparison was first made between the beam profiles obtained with a N(2) laser and a Nd:YAG laser using 2-m long optical fibers. The N(2) laser gives better sensitivity, consistent with a more uniform fluence distribution and therefore better coverage of the N(2) laser profile. Most of the difference disappears when a 30-m long fiber is used or when the fibers are twisted during irradiation to smooth out the fluence distribution. In more systematic measurements, the total integrated ion yield from a single spot (a measure of sensitivity) was found to increase rapidly with fluence to a maximum, and then saturate or decrease slightly. Thus, the optimum sensitivity is achieved at high fluence. For a fluence near threshold, the integrated yield has a steep (cubic) dependence on the spot size, but the yield saturates at higher fluence for smaller spots. The area dependence is much weaker (close to linear) for fluence values above saturation, with the result that the highest integrated yields per unit area are obtained with the smallest spot sizes. The results have particular relevance for imaging MALDI, where sensitivity and spatial resolution are important figures of merit. Copyright (c) 2008 John Wiley & Sons, Ltd.     

The role of the laser pulse duration in infrared matrix-assisted laser desorption/ionization mass spectrometry

The role of the laser pulse duration in matrix-assisted laser desorption/ionization mass spectrometry with infrared lasers (IR-MALDI-MS) emitting in the 3 microm wavelength range has been evaluated. Mass spectrometric performance and characteristics of the IR-MALDI process were examined by comparing a wavelength-tuneable mid-infrared optical parametric oscillator (OPO) laser of 6 ns pulse duration, tuned to wavelengths of 2.79 and 2.94 microm, with an Er:YAG laser (lambda = 2.94 microm) with two pulse durations of 100 and 185 ns, and an Er:YSGG laser (lambda = 2.79 microm) with a pulse duration of 75 ns. Threshold fluences for the desorption of cytochrome C ions were determined as a function of the laser pulse duration for various common IR-MALDI matrices. For the majority of these matrices a reduction in threshold fluence by a factor of 1.2-1.9 was found by going from the 75-100 ns long pulses of the Erbium lasers to the short 6 ns OPO pulse. Within the experimental accuracy threshold fluences were equal for the 100 and the 185 ns pulse duration of the Er:YAG laser. Some pronounced pulse duration effects related to the ion formation from a glycerol matrix were also observed. The effect of the laser pulse length on the duration of ion emission was furthermore investigated.     

Polarization dependent fragmentation of ions produced by laser desorption from nanopost arrays

Tailored silicon nanopost arrays (NAPA) enable controlled and resonant ion production in laser desorption ionization experiments and have been termed nanophotonic ion sources (Walker et al., J. Phys. Chem. C, 2010, 114, 4835-4840). As the post dimensions are comparable to or smaller than the laser wavelength, near-field effects and localized electromagnetic fields are present in their vicinity. In this contribution, we explore the desorption and ionization mechanism by studying how surface derivatization affects ion yields and fragmentation. We demonstrate that by increasing the laser fluence on derivatized NAPA with less polar surfaces that have decreased interaction energy between the structured silicon substrate and the adsorbate, the spectrum changes from exhibiting primarily molecular ions to showing a growing variety and abundance of fragments. The polarization angle of the laser beam had been shown to dramatically affect the ion yields of adsorbates. For the first time, we report that by rotating the plane of polarization of the desorption laser, the internal energy of the adsorbate can also be modulated resulting in polarization dependent fragmentation. This polarization effect also resulted in selective fragmentation of vitamin B(12). To explore the internal energy of NAPA generated ions, the effect of the post aspect ratios on the laser desorption thresholds and on the internal energy of a preformed ion was studied. Elevated surface temperatures and enhanced near fields in the vicinity of high aspect ratio posts are thought to contribute to desorption and ionization from NAPA. Comparison of the fluence dependence of the internal energies of ions produced from nanoporous silicon and NAPA substrates indicates that surface restructuring or transient melting by the desorption laser is a prerequisite for the former but not for the latter.     

Equilibrium conditions in laser-desorbed plumes: thermodynamic properties of alpha-cyano-4-hydroxycinnamic acid and protonation of amino acids

The equilibrium nature of a plume of laser desorbed material is explored through the application of a simple equilibrium model to the ion signals observed in 355 nm laser desorption/ionization mass spectra of mixtures of the MALDI matrix alpha-cyano-4-hydroxycinnamic acid (alphaCHCA) with the amino acids glycine, alanine, valine, isoleucine, and phenylalanine. In these studies it is found that there are systematic and predictable increases in the relative yield of protonated amino acid with increases in amino acid gas-phase basicity. In addition, the thermodynamic values extracted from the equilibrium plot are shown to be in good agreement with values obtained from computational investigation of plausible alphaCHCA proton donor species. These results are supportive of a picture wherein the laser-desorbed material is viewed as a dense plume in which facile charge transfer occurs leading, ultimately, to a thermodynamically equilibrated distribution of proton donor and proton acceptor species.     

Theoretical analysis of correlation between ionization threshold fluence in IR‐MALDI and IR absorption spectrum of matrix molecules

To investigate the correlation between the wavelength dependence of ionization threshold fluence of target molecule in matrix‐assisted laser desorption/ionization by infrared (IR) laser and the IR absorption spectrum of matrix molecule, we have analyzed the IR absorption spectra of four matrix molecules using density functional theory and correlated ab initio molecular orbital method. The calculated IR absorption spectra of the isolated molecules showed more qualitative correlation with the wavelength dependence of ionization threshold fluence than those of the solid state structures. We can consider that a portion of matrix molecules lost the ordered crystal structure and that the transition to the diluted or isolated state occurred at the early process of IR laser irradiation. © 2012 Wiley Periodicals, Inc.     

Analysis of MALDI FT-ICR mass spectrometry data: A time series approach

Matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry is a technique for high mass-resolution analysis of substances that is rapidly gaining popularity as an analytic tool. Extracting signal from the background noise, however, poses significant challenges. In this article, we model the noise part of a spectrum as an autoregressive, moving average (ARMA) time series with innovations given by a generalized gamma distribution with varying scale parameter but constant shape parameter and exponent. This enables us to classify peaks found in actual spectra as either noise or signal using a reasonable criterion that outperforms a standard threshold criterion.     

Two-photon ionization thresholds of matrix-assisted laser desorption/ionization matrix clusters

Direct two-photon ionization of the matrix has been considered a likely primary ionization mechanism in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. This mechanism requires that the vertical ionization threshold of matrix materials be below twice the laser photon energy. Because dimers and larger aggregates may be numerous in the early stages of the MALDI plume expansion, their ionization thresholds are important as well. We have used two-color two-photon ionization to determine the ionization thresholds of jet cooled clusters of an important matrix, 2,5-dihydroxy benzoic acid (DHB), and mixed clusters with the thermal decomposition product of DHB, hydroquinone. The thresholds of the clusters were reduced by only a few tenths of an eV compared to the monomers, to an apparent limit of 7.82 eV for pure DHB clusters. None of the investigated clusters can be directly ionized by two nitrogen laser photons (7.36 eV), and the ionization efficiency at the thresholds is low.