A novel method for the determination of oxytocin using high-performance liquid chromatography (HPLC) with online ultraviolet (UV) irradiation and a chemiluminescence detection system was developed. This method is based on the discovery that tyrosine, a constituent amino acid of oxytocin, generates chemiluminescence when mixed with luminol and irradiated with UV light. The chemiluminescence reaction system was integrated into an HPLC system to facilitate the determination of oxytocin in samples containing complex matrices. The chromatographic separation was performed on a COSMOSIL PBr column using a mobile phase consisting of acetonitrile and 1.0 mM Tris-HNO₃ buffer (pH 7.4) in a ratio of 25/75 (v/v, %), respectively. The optimal conditions for UV irradiation and chemiluminescence reaction were investigated, and the method showed good linearity over the concentration range of 0.05-5.0 μM with a correlation coefficient of 0.994. The limit of detection (LOD, S/N = 3) was 14 nM, and the intra- and inter-day relative standard deviations (RSD) were less than 3.2 and 2.7 %, respectively. The proposed method was successfully applied to the determination of oxytocin in bovine milk samples, with good recoveries ranging from 85.5 % to 107 %. The developed method offers a highly sensitive and selective determination of oxytocin without the need for complicated pretreatment procedures, expensive reagents, or complex equipment, making it a promising tool for evaluating the biological effects of oxytocin in various samples.

A poly(butylene terephthalate) (PBT)-coated silica gel particle was introduced as a stationary phase in liquid chromatography, and the retention mechanism of the PBT stationary phase was investigated by measuring the retention of various aromatic and polar compounds including nucleosides under various types of mobile phase conditions. The retention of aromatic compounds with alkyl functional group(s) in the chemical structure was studied using methanol or acetonitrile as the mobile phase component. Even in the acetonitrile/water mobile phase, where π–π interactions were normally suppressed, the retention trend based on the typical hydrophobic behavior was found, confirming that π–π interactions could play a small role in the specific retention behavior. In the retention tendency for disubstituted benzenes using acetonitrile/water mobile phases with different water contents, the mobile phase composition could have a minimal effect on the selectivity for these isomers. However, for the separation of nitrotoluene, an excellent selectivity was observed when acetonitrile/water = 50/50 was used as the mobile phase. This is presumably because nitrotoluene, which has a high dipole moment, has a large contribution from dipole-dipole interactions on the PBT stationary phase. Comparing the retention of nucleoside on the PBT and typical octadecylsilica stationary phases, it was shown that a larger retention for these analytes on the PBT stationary phase. The retention mechanism of the PBT stationary phase could include a dipole-dipole interaction in addition to a hydrophobic interaction, resulting in a unique retention trend of various isomeric aromatic compounds on the PBT phase.

Scopoletin (Sco) and its analogues (Scos) are included in various functional food materials. Quantitation of Scos in foods is useful for quality evaluation purposes. In this study, an HPLC-fluorescence method was developed for analyzing Scos emitting native fluorescence in functional food materials. Sco, esculetin (Est), umbelliferone (Umb), isoscopoletin, isofraxidin, esculin, fraxetin, fraxin, and nodakenin could be separated within a retention time of 25 min by gradient elution of a Wakopak Handy ODS column (100 × 4.6 mm i.d., 6 μm) using 15 mmol/L phosphate buffer (pH 2.5) and acetonitrile. The limits of detection for Scos at a signal-to-noise ratio of 3 ranged from 7.10×10−3 to 1.39 ng/mL. Additionally, sufficient validation parameters for quantitating Scos in Noni products were obtained, with an accuracy of 67.9-117.3 %, intra-day precision of <11.7 %, inter-day precision of 9.0 %, and recovery of 67.7-114.0 %. Moreover, the proposed method was applicable to the quantitation of Scos in four Noni products. Sco, Esn, and Umb were in the ranges 9.17-34.5 μg/mL, 39.3-210.3 ng/mL, and 54.5-227.3 ng/mL, respectively, in all samples examined. The method is thus considered useful for quantifying levels of Scos in functional food materials.

For the sensitive fluorescence detection of tryptophan (Trp) enantiomers derivatized with 4-fluoro-7-nitro-2,1,3- benzoxadiazole (NBD-F), a photo-reactor was designed and integrated into a chiral high-performance liquid chromatographic (HPLC) system with tandemly-connected ultraviolet/visible (UV/VIS) and fluorescence detectors. The photo-reactor was inserted between the two detectors. After monitoring the separation of the NBD-Trp enantiomers by the absorbance (470 nm), light-irradiation (200 nm) was performed and the peaks derived from both enantiomers were fluorometrically detected (ex. 470 nm, em. 530 nm). To improve the fluorescence detection sensitivity, the effects of the compositions of the organic solvents in the mobile phase and the flow rate were investigated. As a result, using an acetonitrile solution containing 0.3 % formic acid at the flow rate of 100 µL/min, the highest fluorescence intensity (the ratio of the intensity obtained by a fluorescence detector and that obtained by a UV/VIS detector) derived from NBD-d-Trp was acquired. The obtained ratio was approximately 1600 times higher than that obtained when a mixed solution of acetonitrile/methanol (50/50, v/v) containing 0.1 % formic acid at the flow rate of 200 µL/min was adopted as the commonly-used mobile phase conditions for separating NBD-Trp enantiomers using a Pirkle-type column. These results indicate that the developed photo-reactor is effective for the sensitive detection of Trp enantiomers (lower limit of detection: 5 fmol/injection) and promotes further clarifications, such as tissue distribution and amounts under various physiological conditions in mammals.

For highly sensitive analyses of cationic analytes in microchip electrophoresis (MCE), large-volume dual preconcentration by isotachophoresis and stacking (LDIS) was combined with field-amplified sample injection (FASI) on Y-shaped channel microchips. To enrich cationic compounds by LDIS-FASI, the microchannel was coated with neutral poly(vinyl alcohol) and cationic poly(allylamine) mixture to obtain weakly and positively-charged surface, generating a reversal electroosmotic flow (EOF). In the conventional LDIS analysis of histamine, the sensitive enhancement factor (SEF) was evaluated to be 142, whereas in LDIS-FASI that was increased to 1572 at the FASI injection time of 90 s.

This study proposes a novel analytical method for the sensitive and rapid detection of residual solvents in pharmaceutical formulations. In this method, a powdered tablet is dissolved and treated via purge and trap extraction using a functional polytetrafluoroethylene (PTFE) membrane and a needle-type extraction device. Specifically, the powdered sample and sodium chloride were added to 5 mL of pure water in a glass solid-phase extraction cartridge, followed by stirring until the dissolution of the solids. Subsequently, 100 mL of headspace gas was collected using a needle-type extraction device packed with Carbopack X and carbon molecular sieve as adsorbents. During the active sampling, a purging gas was introduced into the solution from the bottom of the cartridge through a functional PTFE membrane, which retained the solution while allowing the purging gas to pass. The typical sample collection time was 10 min. Following the sample collection, the extraction needle was directly inserted into a heated gas chromatographic injection port, and the extracted analytes were thermally desorbed and analyzed using a flame ionization detector. The limits of detections for benzene, carbon tetrachloride, 1,2-dichloroethane, 1,1-dichloroethylene, and 1,1,1-trichloroethane (Class 1 compounds) were 0.005, 0.5, 0.005, 0.01, and 0.01 w/w ppm, respectively. The recovery of the target compounds did not depend on the formulation, and the proposed method demonstrated sufficient recovery and sensitivity for hydrophobic analytes, whereby allowing simple and rapid sample preparation.

A highly sensitive, accurate, and selective LC–MS/MS method for the simultaneous quantification of polycarboxylic acids in fruits was developed by combining the derivatization of polycarboxylic acids, oxygen–heavy oxygen exchange reactions, and weak cation exchange solid-phase extraction. The derivatization reagent used in this method, namely TPP-AEP, effectively derivatizes polycarboxylic acids and enables highly sensitive LC–MS/MS analysis. Combined with the synthesis of internal standards by oxygen–heavy oxygen exchange reaction and selective SPE using a weak cation exchange column, 19 polycarboxylic acids could be accurately and selectively quantified. This method was applied to the quantitative analysis of polycarboxylic acids in fruits and 100% fruit juices from concentrated fruits, and the differences in their compositions were elucidated. The developed method exhibited good detection sensitivity, accuracy, and selectivity. These results are useful for preventing the counterfeiting of food products and improving the quality of fruit juice from the concentrate.