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C D Tran

Publications and source records attributed to C D Tran.

16 recordsLinked to original sources
PubMed1996

Near-infrared detection of flow injection analysis by acoustooptic tunable filter-based spectrophotometry.

The instrumentation development of a near-infrared (near-IR) spectrophotometer based on an acoustooptic tunable filter (AOTF) and its application as a detector for flow injection analysis (FIA) are reported. In addition to being compact and all solid state, this AOTF-based instrument is very sensitive, has high resolution, and can be rapidly scanned. The latter advantage make it uniquely suited as a detector for FIA, in that it can rapidly record the whole near-IR absorption spectrum of a mixture passing through the FIA flow cell. Subsequent treatment of the recorded spectra with multivariate calibration methods makes it possible to use the FIA, for the first time, for such applications as the simultaneous determination of trace amounts of water and benzene in ethanol. Because all organic compounds absorb light in the near-infrared region, this AOTF-based near-IR detector can serve as a universal detector for FIA; as a consequence, applications of the FIA techniques can be expanded to other areas which are not possible otherwise.

Benzene↗
PubMed1994

Universal spectropolarimeter based on overtone circular dichroism measurements in the near-infrared region.

A novel and highly sensitive circular dichroism spectropolarimeter for the near-infrared region has been developed. In this instrument, a solid state titanium-sapphire laser that can be spectrally tuned from 670 to 1030 nm was used as the light source. The laser beam was converted into linearly polarized light by a polarizer and into left circularly polarized light and right circularly polarized light at 42 kHz by a photoelastic modulator (PEM). A limit of detection of 1.1 x 10(-6) AU was achieved by this instrument for (+)-Co(en)3(3+) at 765 nm. Further improvement including employing double modulation (at 42 kHz by the PEM and at 85 Hz by a chopper just before the laser beam was converted to CPL), demodulating and amplifying the signal with high-performance lock-in amplifiers, was made to enable the instrument to have the required sensitivity for the measurements of the circular dichroism of overtones and combination transitions of saturated chiral compounds, e.g., (R)- and (S)-camphor, (R)- and (S)-2-octanol, and (R)- and (S)-2-amino-1-octanol. Because the measured CD spectra originate from the overtones and combination transitions of the C-H and O-H groups, the spectropolarimeter can be used to detect virtually any compounds that have O-H and/or C-H groups.

Aluminum Oxide↗
PubMed1994

Chiral detection in high-performance liquid chromatography by vibrational circular dichroism.

A novel chiral detector for high-performance liquid chromatography has been developed. This detector is based on the measurement of circular dichroism of chiral effluents in the infrared region, i.e., vibrational circular dichroism (VCD). In this instrument, a solid-state spectral tunable (from 2.4 to 3.5 microns) F-center laser was used as the light source. The linearly polarized laser beam was converted into left circularly polarized light (LCPL) and right circularly polarized light (RCPL) at 42 kHz by means of a photoelastic modulator. The intensity of the LCPL and RCPL transmitted through the sample was measured by a liquid nitrogen cooled indium antimonide detector. Double modulation was employed to reduce the noise associated with the laser beam. Specifically, the linearly polarized laser beam, prior to being converted to CPL, was modulated at 85 Hz by a mechanical chopper. Demodulation and amplification were accomplished with the use of two lock-in amplifiers. In its present configuration, the instrument can be used to measure the VCD of O-H groups. Its sensitivity is so high that it was able, for the first time, to detect chirally (with limits of detection of micrograms) (R)- and (S)-2,2,2-trifluoro-1-(9- anthryl)ethanol and (R)- and (S)-benzoin when these compounds were chromatographically separated from the corresponding racemic mixtures by a Chiralcel-OD column. The main advantage of this chiral detector is, however, its universality; i.e., it can be used to virtually detect any chiral compounds which has O-H group (e.g, aliphatic alcohols such as 2-octanol).

Chromatography, High Pressure Liquid↗
PubMed1994

Thermal lens technique for sensitive and nonintrusive determination of isotopic purity.

A new technique in which absorption in the infrared region is sensitively monitored in the visible has been developed. This was accomplished using the visible laser to probe the thermal lens effect induced in a sample as a consequence of its absorption of radiation in the infrared. The sensitivity of the technique is much higher than that of conventional transmission measurements because, in addition to its inherent ultrasensitivity, infrared absorption is monitored in the visible region, which is relatively less noisy, and is detected by a phase-lock-detecting method. The technique is so sensitive that it can be used to determine the isotopic purity of methanol and its 13C and deuterated samples by measuring, not directly the absorption of the C-H group but rather indirectly the absorption of the O-H group. With use of the multivariate calibration method to analyze the data, this technique can be utilized for the nondestructive, noninvasive, and sensitive determination of isotopic impurity in methanol, 13CH3OH, CD3OH, CD2HOH, and CDH2OH at concentrations as low as 10(-3)% (w/w).

Carbon Isotopes↗
PubMed1993

Spectrofluorometer based on acousto-optic tunable filters for rapid scanning and multicomponent sample analyses.

Advantages of the acousto-optic tunable filter (AOTF), namely its ability for fast scanning and multiple-wavelength diffraction, were exploited to develop a novel, all solid-state, nonmoving parts spectrofluorometer. This instrument is based on the use of two AOTFs: one for excitation and the other for emission. The first AOTF was used to specifically diffract white incident light into a specific wavelength(s) for excitation. Depending on the needs, the second AOTF (i.e., the emission AOTF) can be used as either a very fast dispersive device or a polychromator. In the first configuration, the sample was excited by a single-excitation wavelength; the emitted light was analyzed by the emission AOTF, which was scanned very fast. A speed of 4.8 A was found to be the fastest speed which the AOTF can be scanned with a reasonable S/N and resolution. With this speed, a spectrum of 150 nm can be measured in 312 microseconds. Faster scanning is possible but, because of the limitation due to the speed of the acoustic wave, may undesiredly lead to degradation in the S/N and spectral resolution. In the second configuration, both AOTFs were used as a polychromator. Several different rf signals were simultaneously applied into the first AOTF to provide multiple-excitation wavelengths. The emission was simultaneously analyzed at several wavelengths by the emission AOTF. With this configuration, the fluorometer can be used for the analysis of multicomponent samples, and the maximum number of components it can analyze is, in principle, a x b, where a and b are the number of excitation and emission wavelengths, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Eosine Yellowish-(YS)↗
PubMed1992

Acousto-optic tunable filter as a polychromator and its application in multidimensional fluorescence spectrometry.

Acousto-optic tunable filter (AOTF) is an electronically driven dispersive device which operates on the principle of acousto-optic interaction in an anisotropic medium. Incident white light will be diffracted by the AOTF into a specific wavelength when a specific rf is applied to it. The diffracted light needs not be a monochromatic light. Multiwavelength light can be diffracted from the AOTF when several rf signals are simultaneously applied into the filter. Compared to conventional polychromators, advantages of this electronic AOTF polychromator include its ability to individually amplitude-modulate each wavelength of the diffracted multiwavelength light at different frequency. This is accomplished by individually and sinusoidally modulating each applied rf signal at the desired frequency. This feature makes it possible to develop a novel AOTF-based multidimensional fluorimeter in which the sample was simultaneously excited by two different wavelengths (514.5 and 488.0 nm) whose amplitudes were sinusoidally modulated at two different frequencies (100 and 66 Hz). Multicomponent samples, e.g., mixtures of rhodamine 6G and rhodamine B, were successfully analyzed using this novel fluorimeter and the developed data analysis.

Filtration↗
PubMed1992

Multiwavelength thermal lens spectrophotometer based on an acousto-optic tunable filter.

The instrumentation development of a novel, all solid-state, nonmoving parts, fast-scanning and wide-tuning range multiwavelength thermal lens spectrophotometer based on the acousto-optic tunable filter (AOTF) is described. Initially, the essential electronic driver was developed to facilitate the systematic characterization of the paratellurite (TeO2) AOTF and to demonstrate that this filter can be successfully and uniquely used as an all solid-state, nonmoving parts dispersive device to rapidly diffract white incident light into a selected color beam, to amplitude modulate the diffracted monochromatic light, and to keep its intensity constant. The multiwavelength thermal lens instrument was subsequently constructed using this AOTF, and preliminary results on advantages of this spectrophotometer such as its ability to characterize trace chemicals and to analyze multicomponent samples are delineated.

Acoustics↗
PubMed1991

High-performance liquid chromatographic separation of racemic and diastereomeric mixtures of 2,4-pentadienoate-iron tricarbonyl derivatives.

beta-Cyclodextrin chiral stationary phase facilitates the chiral separation of the (+/-)-methyl-5-formyl-2,4-pentadienoate-iron tricarbonyl (1) racemic mixture. The separation of oxazolidine derivatives 2 and 3 diastereomers were achieved with a C18 column but the compounds underwent in-column hydrolysis to give (-)- and (+)-1, respectively. This hydrolysis was exploited for the determination of 2 and 3 by the beta-cyclodextrin column, namely 2 and 3 were initially and completely hydrolyzed in the column to give (-)- and (+)-1 and this racemic mixture was then separated by this chiral column.

Chromatography, High Pressure Liquid↗
PubMed1990

Thermal lens-circular dichroism detector for high-performance liquid chromatography.

A novel and ultrasensitive chiral detector for high-performance liquid chromatography has been developed. This detector is based on the measurement of circular dichroism of chiral effluents by the thermal lens effect. In this instrument, the chromatographic effluent was sequentially excited by left circularly polarized laser light (LCPL) and right circularly polarized laser light (RCPL); both of these excitation beams were derived from the same argon ion laser whose linearly polarized output was transformed into circularly polarized light by means of a Pockels cell. The heat generated as a consequence of the sample absorption of the LCPL and RCPL was measured by the probe laser beam collinearly overlapping with the two excitation beams. A lock-in amplifier was used to measure the thermal lens-circular dichroism (TL-CD) signal which corresponds to the difference in the thermal lens signals produced by the LCPL and RCPL excitation beams. In addition to its high sensitivity, the advantages of this TL-CD chiral detector include its ability to provide, directly and in real time, information on the chirality (i.e., circular dichroism) and optical purity of chiral samples. A detection limit of 7.2 ng was achieved for (-)-tris(ethylenediamine)cobalt(III) (k' = 0.45) as well as for the (+)-tris(ethylenediamine)cobalt(III) (k' = 1.40) when these two enantiomers were chromatographically separated from the corresponding racemic mixture through the use of bis(mu-d-tartrato)diantiomonate(III) ion pair reversed-phase chromatography. This limit of detection was found by using a 10-microL flow cell and having 5-mm path length and 6-mW excitation laser beam (lambda = 514.5 nm) modulated at 2 Hz.

Chromatography, High Pressure Liquid↗
PubMed1986

A review of techniques of crown fabrication for existing removable partial dentures.

Fourteen methods for making a crown to fit an RPD have been described. The techniques have been briefly reviewed and classified to indicate whether crown pattern construction is direct, direct-indirect, or indirect, and whether the RPD is needed in the laboratory phase. The choice of a technique appears to be a matter of personal preference for the dentist.

Acrylic Resins↗
PubMed1985

Studies of the fluorescence from tryptophan in melittin.

The fluorescence lifetime and rotational correlation time of the tryptophan residue in melittin, as both a monomer and tetramer, have been measured between pH 6 and 11. The fluorescence decays are non-exponential and give lifetimes of 0.7 +/- 0.1 ns and 3.1 +/- 0.1 ns. This emission is consistent with a model in which the tryptophan residue is in slightly different environments in the protein. In a dilute solution of monomer the mean fluorescence lifetime is 2.3 +/- 0.1 ns, below pH 10, but falls to 1.7 ns at higher pH. In contrast, the melittin tetramer has a mean fluorescence lifetime of only 2.2 ns at pH 6, which falls to 1.9 ns by pH 8, and falls again above pH 10 to the same value as in monomeric melittin. The behaviour between pH 6 and 8 is explained as the quenching of the Trp residue by lysine groups, which are near to the Trp in the tetramer but in the monomer, are too distant to quench. Fluorescence anisotropy decays show that the Trp residue has considerable freedom of motion and the range of "wobbling" motion is 35 +/- 10 degrees in the tetramer.

Animals↗
PubMed1982

Secondary structure and dynamics of glucagon in solution.

A correlation between the secondary structure of glucagon determined by circular dichroism and its dynamic behaviour as obtained from picosecond fluorescence anisotropy is demonstrated. The CD data show that the percentage of alpha-helix decreases with increasing temperature, but the rotational relaxation time of the glucagon increases with temperature. These observations suggest that the protein's shape changes with temperature in such a way that its volume is larger at 38 degrees C than at 5.5 degrees C. The fluorescence anisotropy of glucagon decays biexponentially at each temperature studied and at 26 degrees C the rotation lifetimes are 1670 and 307 ps at pH 10.2 and 2147 and 517 ps at pH 2.2. It is proposed that the shorter decays are due to the restricted motion of the single tryptophan residue while rotation of the whole protein is responsible for the longer decays. The calculated rotational diffusion coefficient, Dw, of the tryptophan residue is much smaller, (ie. has a larger apparent volume) than that of a free tryptophan in solution. The hydrophobic interactions between residues Phe-22 to Leu-26 are probably responsible for the larger apparent volume in the protein compared to solution and will stabilize this part of the protein. The rotational diffusion of aggregated glucagon is also discussed.

Circular Dichroism↗