Totaro Imasaka

A new approach for the generation of ultrashort optical pulses

Abstract A short optical pulse is essential in studies on ultrafast phenomena and nonlinear spectroscopy. Laser pulses as short as 6 fs were generated in 1987, but there has been no breakthrough since. In this paper we propose a new approach to generate a shorter optical pulse at terawatt levels based on a two-color stimulated Raman effect using a simple optical system.

Hadamard transform capillary electrophoresis

This paper reports the first demonstration of a multiplex sample injection technique in capillary electrophoresis. The sample was injected into a capillary (effective length, 4 cm) as a pseudorandam Hadamard sequence by a photodegradation technique using a high-power gating laser, and the fluorescence signal, which was measured using a probe excitation beam, was decoded by an inverse Hadamard transformation. The signal-to-noise ratio was improved by a factor of 8, which was in good agreement with the theoretically predicted value of 8.02. This approach is potentially useful for the enhancement of the sensitivity by 3 orders of magnitude in high-resolution capillary electrophoresis, combined with fluorescence detection.

Generation of high-order rotational lines in hydrogen by four-wave Raman mixing in the femtosecond regime

More than 40 rotational Raman lines are generated using an 800-fs Ti:sapphire laser. The spectral region extends from the near-infrared to the far-ultraviolet with a considerably flat intensity distribution. The effects of laser polarization, pulsewidth, hydrogen pressure, and focusing conditions on the efficiencies of stimulated Raman scattering, four-wave Raman mixing, self-phase modulation, self-focusing, and harmonic generation are investigated. A white light continuum, generated by self-phase modulation, acts as a seed beam for the generation of high-order rotational lines through four-wave Raman mixing. Strong self-phase modulation, however, suppresses the generation of the Raman emission, due to a line broadening of the pump beam. Thus, optimization of experimental conditions is necessary for the efficient generation of high-order rotational lines.

Determination of cyanine-labeled amino acid enantiomers by cyclodextrin-modified capillary gel electrophoresis combined with diode laser fluorescence detection

Enantiomeric amino acids were labeled with a cyanine derivative (Cy5) for selective determination by capillary electrophoresis combined with diode laser-induced fluorometry. Cyanine-labeled glutamic acid has a high negative charge so that it was difficult to detect by conventional capillary zone electrophoresis due to its electrophoretic mobility being greater than the electroosmotic mobility. Thus, cyclodextrin-modified capillary gel electrophoresis using replaceable poly(vinylpyrrolidone) (PVP), which effectively suppresses the electroosmotic flow, was employed to separate Cy5-labeled amino acid enantiomers. The separation of six amino acid enantiomers could be achieved by using 1% PVP solution containing 70 mM γ-cyclodextrin. It was also shown that PVP does not affect the selectivity of optical isomers.

Optical chromatography: Size determination by eluting particles

A new method for the determination of particle size was developed using optical chromatography. After separating polystyrene particles, the laser power was gradually reduced, permitting the elution of small to large particles. Particle size was calculated from the laser power when the particle was eluted with a medium flow. This approach is more accurate than the technique previously reported because there is no need to determine the position of the beam waist. Advantages of the new approach are discussed theoretically and experimentally. The precision in size determination was improved by a factor of 3.3, i.e. the standard deviation in the measurement was reduced from 10 to 3% for 1 mum beads by replacing optical chromatography with the present method.

Separation of polycyclic aromatic hydrocarbons by micellar electrokinetic chromatography with laser fluorescence detection

Abstract Polycyclic aromatic hydrocarbons (PAHs) were separated by micellar electrokinetic chromatography and were detected by laser fluorometry. The PAHs with large molecular weights were separated by using dimethylformamide as an organic modifier. A He-Cd laser (325 nm, 5 mW) or a blue semiconductor laser (415 nm, 50 μW) was used as an exciting light source. The detection limit ( S N = 3 ) for perylene was 1.4 × 10−7 M under He-Cd laser excitation or 3.4 × 10−7 M under semiconductor laser excitation.

Effect of Laser Pulse Width on Ionization Efficiency in Supersonic Beam-Multiphoton Ionization-Mass Spectrometry

The efficiency in multiphoton ionization is calculated for various molecules with different lifetimes, based on a kinetic model using rate equations. The results are strongly dependent on the types of resonance ionization, i.e., ionization through congested intermediate levels or ionization through a discrete intermediate level. In the former case, a laser pulse shorter than the lifetime is always preferred for efficient ionization. In the latter case, a 10-ps laser pulse provides the maximum ionization efficiency for a molecule with a lifetime in excess of 10 ps. For a molecule having a lifetime shorter than 10 ps, the use of a laser whose pulse width is adjusted to the same value as the lifetime is suggested. The results obtained by a computer simulation are in reasonably good agreement with the experimental data obtained for monohalogenated benzenes.

Resonant and nonresonant multiphoton ionization processes in the mass spectrometry of explosives.

Multiphoton ionization processes were studied for three types of explosives using a line-tunable ultraviolet femtosecond laser. When peroxides such as triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD) were ionized through a nonresonant two-photon process, a molecular ion was dominantly observed by reducing the excess energy remaining in the ion. However, an aromatic nitro compound such as 2,4,6-trinitrotoluene (TNT) produced large signals arising from molecular and fragment ions by resonant two-photon ionization. In addition, only fragment ions were produced from a nonaromatic nitro compound such as 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), even when a resonant two-photon ionization process was employed, suggesting that a further reduction in excess energy would be necessary if a molecular ion were to be observed.

Immunoassay for human serum albumin using capillary electrophoresis–semiconductor laser-induced fluorometry

Capillary electrophoresis combined with semiconductor laser-induced fluorometry was applied to an immunoassay of human serum albumin. Human serum albumin was labeled with a fluorescent molecule (Cy5), which has an absorption maximum at 649 nm. The labeled albumin was purified by ultrafiltration in order to reduce signals, which are unreacted labeling reagent, product, and fragment products derived there from. After the purification, no signal for unreacted labeling reagent and fragment products was detectable in the electropherogram of the labeled albumin. The labeled albumin was then reacted with anti-albumin to form an immunocomplex, which was separated from the excess free albumin. The competitive immunoassay was used in the determination of human serum albumin in a controlled serum sample, using the labeled albumin. The obtained value was found to be 0.21 +/- 0.02 mg/ml, which is in good agreement with other known values.

Diode lasers in analytical chemistry

In this review article, I report the structure and characteristics of a diode laser and the figure of merits in the application to analytical spectroscopy. This diode laser is currently used in absorption-based and fluorescence-based spectrometries. Due to their ultra-high detectability, single atoms and molecules can be detected in the extreme case. Moreover, selectivity is improved by time-resolved spectrometry and also by a combination with separation techniques such as chromatography. Second harmonic generation is employed to extend the laser wavelength, in order to expand the analytical application. Finally, limitations of the diode laser are pointed out, and the problem, which should be solved in the near future, is also described.