• Bulletin of the Korean Chemical Society
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• v.28 no.6
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• pp.1070-1072
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• 2007

• Journal of the Korean Chemical Society
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• v.38 no.3
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• pp.221-223
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• 1994

We tried to apply the linear solvation energy comparison method to solute retention in normal phase liquid chromatography. Correlation coefficients of regression of lnk' collected in a fixed eluent against solute polarity indices have proven to be lower than those obtained from reversed phase liquid chromatography data. This event can be attributed to complexity of solute retention process in normal phase liquid chromatography. We concluded from the regression results that each specific polarity of the stationary phase is greater than that of the mobile phase and that the difference in each polarity between the stationary phase and the mobile phase decreases as the volume fraction(${\phi}$) of the more polar solvent in the mobile phase increases. Correlations of lnk' of a single solute against solvent polarity indices have proven to be meaningless owing to covariance among the solvent polarity indices. Instead, a good linear relationship between lnk' and solvent ${\pi}^*$ was observed, and its linearity is better than that between lnk' and ${\phi}$.

• Archives of Pharmacal Research
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• v.17 no.4
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• pp.244-248
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• 1994

A stereoselective and sensitive high performance liquid chromatography using fluoresecence deterctor was examined for the determination of R(-) and S(+)-salbutamol in human plasma. Solid phase extraction method using silica as sorbent was used to extract salbutamol racemates from the plasma matrices. After fractionation and freeze-drying of the eluates containing salbutamol racemates, they were separated and quantified on a chirla stationary column. The detection limit of each enantiomer was 2 ng/ml in human plasma (S/N=3).

• Structural Engineering and Mechanics
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• v.70 no.5
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• pp.623-637
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• 2019

This paper analyzes the non-stationary vibration and super-harmonic resonances in nonlinear dynamic motion of viscoelastic nano-resonators. For this purpose, a new coupled size-dependent model is developed for a plate-shape nano-resonator made of nonlinear viscoelastic material based on modified coupled stress theory. The virtual work induced by viscous forces obtained in the framework of the Leaderman integral for the size-independent and size-dependent stress tensors. With incorporating the size-dependent potential energy, kinetic energy, and an external excitation force work based on Hamilton's principle, the viscous work equation is balanced. The resulting size-dependent viscoelastically coupled equations are solved using the expansion theory, Galerkin method and the fourth-order Runge-Kutta technique. The Hilbert-Huang transform is performed to examine the effects of the viscoelastic parameter and initial excitation values on the nanosystem free vibration. Furthermore, the secondary resonance due to the super-harmonic motions are examined in the form of frequency response, force response, Poincare map, phase portrait and fast Fourier transforms. The results show that the vibration of viscoelastic nanosystem is non-stationary at higher excitation values unlike the elastic ones. In addition, ignoring the small-size effects shifts the secondary resonance, significantly.

• Journal of Power Electronics
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• v.16 no.1
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• pp.297-309
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• 2016

Grid-connected inverters (GCIs) with an LCL output filter have the ability of attenuating high-frequency (HF) switching ripples. However, by using only grid-current control, the system is prone to resonances if it is not properly damped, and the current distortion is amplified significantly under highly distorted grid conditions. This paper proposes a synchronous reference frame equivalent proportional-integral (SRF-EPI) controller in the αβ stationary frame using the parallel virtual resistance-based active damping (PVR-AD) strategy for grid-interfaced distributed generation (DG) systems to suppress LCL resonance. Although both a proportional-resonant (PR) controller in the αβ stationary frame and a PI controller in the dq synchronous frame achieve zero steady-state error, the amplitude- and phase-frequency characteristics differ greatly from each other except for the reference tracking at the fundamental frequency. Therefore, an accurate SRF-EPI controller in the αβ stationary frame is established to achieve precise tracking accuracy. Moreover, the robustness, the harmonic rejection capability, and the influence of the control delay are investigated by the Nyquist stability criterion when the PVR-based AD method is adopted. Furthermore, grid voltage feed-forward and multiple PR controllers are integrated into the current loop to mitigate the current distortion introduced by the grid background distortion. In addition, the parameters design guidelines are presented to show the effectiveness of the proposed strategy. Finally, simulation and experimental results are provided to validate the feasibility of the proposed control approach.

• The Transactions of the Korean Institute of Power Electronics
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• v.20 no.6
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• pp.517-525
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• 2015

A grid connected inverter must be operated as the main electricity source under an isolated condition caused by the grid problem. Conventionally, the dual loop controller is used for the grid inverter, and the controller is used for control under the stand-alone mode. Generally, the PI(Proportional - Integral) controller is highly efficient under a synchronous reference frame, and stable control can be available. However, in this synchronous frame-based control, high-quality DSP is required because many sinusoidal calculations are necessary. When the PI control is conducted under a stationary frame, the controller constructions are made simple so that they work even with a low-price micro controller. However, given the characteristics of the PI controller, it should be designed with the phase of reference voltage considered. Otherwise, the phase delay of the output voltage can occur. Although the current controller also has a higher bandwidth than the voltage controller, distortion of the voltage is difficult to avoid only by the rapid response of the PI controller, as a sudden load change can occur in the nonlinear load. In this study, a new control method that solves the voltage controller bandwidth problem and rapidly copes with it even in the nonlinear load situation is proposed. The validity of the proposed method is proved by simulation and experimental results.

• Journal of Positioning, Navigation, and Timing
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• v.8 no.3
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• pp.95-105
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• 2019

A method that applies space-time adaptive signal processing (STAP) algorithm based on an array antenna consisting of multiple antenna elements has been known to be effective to remove wide-band jamming signals in GPS receivers. However, the occurrence of phase distortion in navigation signals has been a problem when navigation signals, from which jamming signals are removed using STAP, are supplied to global positioning system (GPS) receivers. This paper verified the navigation performance degradation as a result of phase distortion. To mitigate this phenomenon, this paper proposes a mode switching scheme, in which a bypass mode is adopted to make the best use of the tracking performance of receivers without performing signal processing when jamming signals are not present or weak, and a STAP mode is employed when jamming signals exceed the threshold value. In this paper, the mode switching scheme is proposed for two environments: when receivers are stationary, and when receivers are moving. This paper confirmed that the performance of position error improved because phase distortion could be excluded due to STAP if the bypass mode was adopted under a condition where the jamming signal power level was below the threshold value in an environment where receivers were stationary. However, this paper also observed that the navigation failed due to the instability of tracking performance of receivers due to phase distortion that occurred at the switching time, although the number of switching could be reduced dramatically by proposing a dual threshold scheme of on- and off-thresholds that switched a mode due to the array antenna characteristics of varying gains according to the jamming signal incident direction in an environment where receivers were moving. The analysis results verified that running the STAP algorithm at all times is more efficient than the mode switching, in terms of maintaining stable navigation and ensuring position error performance, to remove jamming signals in an environment where receivers were moving.

• Analytical Science and Technology
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• v.32 no.5
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• pp.163-172
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• 2019

Methamphetamine (MA) is currently the most abused illicit drug in Korea and its major metabolite is amphetamine (AP). As MA exist as two enantiomers with the different pharmacological properties, it is necessary to determine their respective amounts in a sample. Thus a chiral stationary phase liquid chromatography-tandem mass spectrometric (LC-MS/MS) method was developed for identification and quantification of d-MA, l-MA, d-AP, and l-AP in human urine. Urine sample ($200{\mu}L$) was diluted with pure water and purified using solid-phase extraction (SPE) cartridge. A $5-{\mu}L$ aliquot of SPE treated sample solution was injected into LC-MS/MS system. Chiral separation was carried out on the Astec Chirobiotic V2 column with an isocratic elution for each enantiomer. Identification and quantification of enantiomeric MA and AP was performed using multiple reaction monitoring (MRM) detection mode. Linear regression with a $1/x^2$ as the weighting factor was applied to generate a calibration curve. The linear ranges were 25-1000 ng/mL for all compounds. The intra- and inter-day precisions were within 3.6 %, while the intra- and inter-day accuracies ranged from -5.4 % to 11.8 %. The limits of detection were 2.5 ng/mL (d-MA), 3.5 ng/mL (l-MA), 7.5 ng/mL (d-AP), and 7.5 ng/mL (l-AP). Method validation parameters such as selectivity, matrix effect, and stability were evaluated and met acceptance criteria. The applicability of the method was tested by the analysis of genuine forensic urine samples from drug abusers. d-MA is the most common compound found in urine and mainly used by abusers.

• Proceedings of the KIEE Conference
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• 2000.07b
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• pp.1108-1110
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• 2000

The BLDCM(Brushless DC motor) has been the Trapezoidal Back Electromotive Force(EMF) due to a surface magnet rotor with nonlinear distribution and full-pitch windings. Theoretically, it should be fed with rectangular phase current in order to minimize torque ripple. But, because voltage source inverter drives BLDCM, perfectly rectangular phase currents are not available. Now in this paper, using fourier series coefficients, calculating the coefficients of harmonic current within available orders and each harmonic component are controlled on stationary frame. Only using four switches, low cost and small size drive can be made and proposed method will be more useful in industrial. Simulation and experimental results prove the validity of the proposed method.

• YAKHAK HOEJI
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• v.25 no.1
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• pp.1-7
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• 1981

Glycyrrhizin (GA) content in licorice was determined by a couple of methods using HPLC, respectively. In Method(I), GA content itself was determined from the licorice aqueous extract, while in Method (II) glycyrrhetinic acid (GHeA ; the aglicone of GA) content corresponding to the quantity of GA was measured from the chloroform extract of the hydrolyzed product of licorice aqueous extract. A reverse phase column Hibar Lichrosorb RP-18 (E. Merck) was used as the stationary phase. As the mobile phase MeOH: $H_{2}O$(0.05M-$NaH_{2}PO_{4}$)=58 : 42 solution in Method (I), and MeOH: $H_{2}O $: AcOH=78; 19: 3 solution in Method (II) were suitable, respectively. The value obtained by Method (II) appeared slightly higher than that by Method (I). The effect of some other herbal drugs on the assay of GA quantity in mixed sample was also observed in both above two methods. By Method (I) Cassiae Cortex, Rehmaniae Rhizoma, Paeoniae Radix, and Angelicae Radix gave the subtractive effect on the amount of GA compared with the value from licorice alone. In the case of Method (II) Cassiae Cortex and Rehmaniae Rhizoma appeared to have subtractive effect but Paeoniae Radix and Angelicae Radix scarcely showed any influence. Pachymae Fungus did not affect the GA content at all. It seems that glycyrrhizin in licorice interacts with certain components of other herbal drugs.