Statistics and performance under combined impairments induced by polarization-dependent-loss in polarization-division-multiplexing digital coherent transmission.

Two-dimensional statistics and Q-penalty performance under the combination of two major impairments induced by polarization-dependent loss, namely level imbalance and loss of orthogonality between polarization-multiplexed tributaries, for a polarization-division-multiplexing digital coherent transmission at over 100-Gb/s are presented for the first time to estimate the outage probability needed for designing the system.

Phase-transparent flexible waveband conversion of 43 Gb/s RZ-DQPSK signals using multiple-QPM-LN waveguides.

We report phase-transparent waveband conversion with polarization insensitivity based on second harmonic (SH) wave pumped difference frequency generation (DFG) using multiple-quasi-phase-matched LiNbO(3) (QPM-LN) waveguides. Flexible waveband conversion is demonstrated over the entire C-band using a tunable DFB-LD array (TLA) as a pump source for a multiple-QPM-LN waveguide. The penalty free waveband conversion of 43 Gb/s return-to-zero differential quadrature phase-shift-keying (RZ-DQPSK) waveband signals is successfully achieved.

A comparison of EDI with solvent-free MALDI and LDI for the analysis of organic pigments.

To evaluate the applicability of EDI to material analysis as a new ionization method, a comparison of EDI with solvent-free matrix-assisted laser desorption ionization (MALDI) and laser desorption ionization (LDI) was made for the analysis of organic pigments, e.g. Pigment Yellow 93, Pigment Yellow 180, and Pigment Green 36, as test samples, which are poorly soluble in standard solvents. In EDI, the samples were prepared in two ways: deposition of suspended samples in appropriate solvents and dried on the substrate, and the direct deposition of the powder samples on the substrate. No matrices were used. Both sample preparation methods gave similar mass spectra. Equally strong signals of [M + H](+) and [M - H](-) ions were observed with some fragment ions for azo pigments in the respective positive or negative mode of operation. For the powder sample of the phthalocyanine pigment PG36, M(+*) and [M + H](+) in the positive mode and M(-*) in the negative mode of operation were observed as major ions. Positive-mode, solvent-free MALDI gave M(+), [M + H](+) and [M + Na](+) and negative mode gave [M - H](-) depending on the sample preparation. As solvent-free MALDI, EDI was also found to be an easy-to-operate, versatile method for the samples as received.

Characteristics of probe electrospray generated from a solid needle.

Probe electrospray ionization (PESI) has recently been developed, in which the electrospray was generated from a solid needle instead of by using a capillary. In this paper, the characteristics of probe electrospray ionization were studied based on the measurement of spray current, optical microscopy, and PESI mass spectrometry. In the experiment, the solid needle was moved up and down a vertical axis, and a small amount of sample was repeatedly loaded to the needle when the tip of the needle touched the surface of the liquid sample at the lowest position. After the application of high voltage, a liquid droplet was formed on the tip of the solid needle probe, with its size was determined by the size of the needle tip. The liquid flow rate to the tip, as indicated by the spray current, depends on the voltage applied to the needle as well as the loaded liquid amount. Stable electrospray can be maintained until the total consumption of liquid sample. The kilohertz current pulsation takes place in the case of overloading the sample to the needle. The influences of the applied voltage and the liquid flow rate on the PESI mass spectra were also examined.

Fundamental aspects of electrospray droplet impact/SIMS.

A new ionization method, electrospray droplet impact ionization (EDI), has been developed for matrix-free secondary-ion mass spectrometry (SIMS). The charged droplets formed by electrospraying 1 M acetic acid aqueous solution are sampled through an orifice with a diameter of 400 microm into the first vacuum chamber, transported into a quadrupole ion guide, and accelerated by 10 kV after exiting the ion guide. The droplets impact on a dry solid sample (no matrix used) deposited on a stainless steel substrate. The secondary ions formed by the impact are transported to a second quadrupole ion guide and mass-analyzed by an orthogonal time-of-flight mass spectrometer (TOF-MS). Ten pmol of gramicidin S could be detected with the presence of as much as 10 nmol of NaCl. The ion signal for arginine disappeared with decrease in the substrate temperature below 150 K owing to the formation of ice film over the sample surface. While 10 fmol of gramicidin S could be detected for 30 min, the ionization/desorption efficiency for EDI becomes smaller with an increase in the molecular weight (MW) of a biological sample. The largest protein samples detected to date are cytochrome c and lysozyme. The high sensitivity for EDI is due to the fact that samples only a few monolayers thick are subject to desorption/ionization by EDI, with little fragmentation. A coherent phonon excitation may be the main mechanism for the desorption/ionization of the solid sample.

Ambient imaging mass spectrometry by electrospray ionization using solid needle as sampling probe.

Although being an atmospheric pressure ion source, electrospray ionization (ESI) has rarely been used directly for ambient imaging mass spectrometry because the sample has to be introduced as liquid solution through the capillary. Instead of capillary, probe electrospray ionization (PESI), which has been developed recently, uses a solid needle as the sampling probe, as well as the electrospray emitter, and has been applied not only for liquid solutions but also for the direct sampling on wet samples. Biological tissues are composed of cells that contain 70-90% water, and when the surface is probed by the needle tip, the biological fluid adhering to the needle can be electrosprayed directly or assisted by additional solvent added onto the needle surface. Here, we demonstrate ambient imaging mass spectrometry of mouse brain section using PESI, incorporated with an auxiliary heated capillary sprayer. The solvent vapor generated from the sprayer condensed on the needle tip, re-dissolving the adhered sample, and at the same time, providing an indirect means for needle cleaning. The histological sections were prepared by fixation using paraformaldehyde, and the spatial analysis was automated by maintaining an equal sampling depth into the sample in addition to raster scan. Phospholipids and galactosylceramides were readily detected from the mouse brain section in the positive ion mode, and were mapped with 60 microm lateral resolution to form mass spectrometric images.

Application of probe electrospray to direct ambient analysis of biological samples.

Recently, we have developed probe electrospray ionization (PESI) that uses a solid needle. In this system, the probe needle moves up and down along the vertical axis by a motor-driven system. At the highest position of the probe needle, electrospray is generated by applying a high voltage. In this study, we applied PESI directly to biological samples such as urine, mouse brain, mouse liver, salmon egg, and fruits (orange, banana, etc.). Strong ion signals for almost all the samples were obtained. The amount of liquid sample picked up by the needle is as small as pL or less, making PESI a promising non-invasive technique for detecting biomolecules in living systems such as cells. Therefore, PESI may be useful as a versatile and ready-to-use semi-online analytical tool in the fields of medicine, pharmaceuticals, agriculture, food science, etc.

Development of probe electrospray using a solid needle.

Electrospray using a solid needle probe for the analysis of liquid samples was developed. The needle was moved up and down a vertical axis by a motor-driven system. At the lowest position of the needle, the tip of the needle touched the surface of the liquid sample and a small amount of liquid was picked up by the needle. At the highest position, a high voltage of approximately 3 kV was applied to the needle and the sample loaded on the tip of the needle was electrosprayed. The ions formed were measured by an orthogonal time-of-flight mass spectrometer. A single sample loading by the needle gave reasonably strong ion signals for amino acids, peptides, proteins and polyethylene glycol (PEG) in aqueous solution. The addition of salts or acids to aqueous solution led to dramatic enhancement of the ion signals.

Study on ion formation in electrospray droplet impact secondary ion mass spectrometry.

A new type of cluster secondary ion mass spectrometry (SIMS), named electrospray droplet impact (EDI), has been developed in our laboratory. In general, rather strong negative ions as well as positive ions can be generated by EDI compared with conventional SIMS. In this work, various aspects of ion formation in EDI are investigated. The Brønsted bases (proton acceptor) and acids (proton donor) mixed in the analyte samples enhanced the signal intensities of deprotonated molecules (negative ions) and protonated molecules (positive ions), respectively, for analytes. This suggests the occurrence of heterogeneous proton transfer reactions (i.e. M + M' --> [M+H](+) + [M'-H](-)) in the shockwave-heated selvedge of the colliding interface between the water droplet and the solid sample deposited on the metal substrate. EDI-SIMS shows a remarkable tolerance to the large excess of salts present in samples. The mechanism for desorption/ionization in EDI is much simpler than those for MALDI and SIMS because only very thin sample layers take part in the shockwave-heated selvedge and complicated higher-order reactions are largely suppressed.

Electrospray droplet impact/secondary ion mass spectrometry: cluster ion formation.

The electrospray droplets that are sampled through an orifice into the vacuum chamber are accelerated by 10 kV and impact on the stainless steel substrate. The mass and the kinetic energy of electrospray droplets are roughly estimated to be a few 10(6) u and approximately 10(6) eV, respectively. The molecular ion M(+.) and the protonated molecule [M+H](+) are observed as secondary ions for chrysene and coronene deposited on the metal substrate (no matrix used). The ionization may take place in the shock wave generated by the high-momentum coherent collision between the droplet projectile and the solid sample. Cluster ions of H(+)(H(2)O)(n) and CF(3)COO(-)(H(2)O)(n), with n up to approximately 150, were observed as secondary ions formed by the electrospray droplet impact ionization (EDI) for 10(-2) M trifluoroacetic acid (TFA) aqueous solution. This indicates that the charged droplets that collide with the metal substrate with the kinetic energy of approximately 10(6) eV do not vaporize completely but are disintegrated into many tiny microdroplets. The ion signal intensity anomalies (i.e. magic numbers) were observed for the cluster ions of H(3)O(+)(H(2)O)(n) and CF(3)COO(-)(H(2)O)(n) for 10(-2) M TFA aqueous solution and of Cs(+)(H(2)O)(n), I(-)(H(2)O)(n), Cs(+)(CsI)(n), and I(-)(CsI)(n) for 10(-2) M CsI aqueous solution.