The M/M/1 queue with inventory, lost sale, and general lead times

We consider an M/M/1 queueing system with inventory under the $(r,Q)$ policy and with lost sales, in which demands occur according to a Poisson process and service times are exponentially distributed. All arriving customers during stockout are lost. We derive the stationary distributions of the joint queue length (number of customers in the system) and on-hand inventory when lead times are random variables and can take various distributions. The derived stationary distributions are used to formulate long-run average performance measures and cost functions in some numerical examples.     

Bioassay-guided isolation of antioxidants from Astragalus altaicus by combination of chromatographic techniques

An efficient method for bioassay-guided preparative isolation of antioxidants from the n-butanol extract of Astragalus altaicus Bunge was ingeniously developed by combination of silica gel column chromatography and high-speed counter-current chromatography. Under the bioassay-guidance of antioxidant activities, the antioxidants were gradually separated from the crude sample of Astragalus altaicus Bunge by silica gel column chromatography and high-speed counter-current chromatography. Silica gel column chromatography separation was performed with chloroform, chloroform-methanol (100:1~5:1, v/v) and chloroform-methanol-water (5:1:0.1~2:1:0.1, v/v/v). High-speed counter-current chromatography separation was performed with a two-phase solvent system composed of ethyl acetate-n-butanol-water (2:1:6, v/v/v), which was successfully selected by thin layer chromatography analysis, at a flow rate of 1.5 mL/min. As a result, isorhamnetin-3-gentiobioside (20.8 mg), rutin (82.0 mg), and narcissin (12.8 mg) were obtained for the first time from 200 mg of the crude sample, ABS-5 of Astragalus altaicus Bunge. The purities were all at over 95% by high-performance liquid chromatography analysis, and their structures were unambiguously identified by mass spectroscopy, (1) H, and (13) C nuclear magnetic resonance spectroscopy. Antioxidant activities of the three compounds were also assayed by in vitro ABTS [2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulphonic acid) diamonium salt] radical cation scavenging activity. Among them, rutin possessed the highest antioxidant capacity with SC(50) value of 22.15 μg/mL.     

Förster-type energy transfer mechanism in PF2/6 to MEH-PPV conjugated polymers

Energy transfer mechanism between Poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF_2/6) as a donor and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as an acceptor in different solvents has been studied using steady-state emission measurements. Four different solvents namely, tetrahydrofuran (THF), toluene, chlorobenzene (C.B) and benzene have been used in this study. The absorption and luminescence behaviors of the samples are measured at a fixed concentration of donor (0.1 μM) while the concentrations for acceptor are kept in the range of 0.1–1.0 μM. Based on these measurements, the energy transfer properties namely quenching rate constant (k_SV), energy transfer rate constant (k_ET), energy transfer probability (P_DA), transfer efficiency (η) and critical distance of energy transfer (R_o) are calculated. The use of THF resulted in the highest energy transfer. Long range dipole–dipole interaction between the excited donor and ground state acceptor molecules is the dominant mechanism responsible for the energy transfer as proven by the large values of R_o.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

490 fs pulse generation from a passive C-band AlGaInAs/InP quantum well mode-locked laser

We report femtosecond pulses from a passive C-band two-section AlGaInAs/InP mode-locked laser with a monolithically integrated passive waveguide made by quantum well intermixing. Without any external pulse compression, Lorentzian pulses are generated at a repetition frequency of ~38 GHz with 490 fs pulse duration, which is, to the best of our knowledge, the shortest pulse from any directly electrically pumped quantum well semiconductor mode-locked laser. The mode-locking range is relatively large and the ultranarrow pulse width is very stable over a broad range of driving conditions.     

Studies on the effect of column angle in centrifugal helix counter-current chromatography

The performance of the coiled column of centrifugal counter-current chromatography was investigated by changing the angle between column axis and centrifugal force in the separation of dipeptides or DNP-amino acids each with suitable two-phase solvent systems. In general, retention of the stationary phase (Sf) decreased, and peak resolution (Rs) increased as the column angle was increased. The first series of experiments was performed using a polar two-phase solvent system composed of 1-butanol–acetic acid–water (4:1:5, v/v/v) to separate two dipeptide samples, Trp-Tyr and Val-Tyr, at a flow rate of 1 ml/min at 1000 rpm. When the column angle was changed from 0° to 90°, Rs increased from 1.05 (Sf = 60.1%) to 1.17 (Sf = 38.7%) with the lower phase mobile and from 1.02 (Sf = 67.8%) to 1.14 (Sf = 47.4%) with the upper phase mobile, respectively. The second series of experiments was similarly performed with a more hydrophobic two-phase solvent system composed of hexane–ethyl acetate–methanol–0.1 M hydrochloric acid (1:1:1:1, v/v/v/v) to separate three DNP-amino acids, DNP-glu, DNP-β-ala and DNP-ala, at a flow rate of 1 ml/min at 1000 rpm. When the column angle was changed from 0° to 90°, Rs increased from 1.38 (1st peak/2nd peak) and 1.20 (2nd peak/3rd peak) (Sf = 61.1%) to 1.66 and 1.45 (Sf = 34.4%) with the lower phase mobile and from 1.14 and 0.63 (Sf = 72.2%) to 1.53 and 0.87 (Sf = 51.1%) with the upper phase mobile, respectively. The overall results of our studies indicate that increasing the column angle against the radially acting centrifugal force enhances the mixing of two phases in the column to improve the peak while decreasing the stationary phase retention by interrupting the laminar flow of the mobile phase.     

Evaluation on the performance of four different column models mounted on the compact type-I coil planet centrifuge

Optimal positions of coiled separation columns on the type-I centrifuge were determined for four typical two-phase solvent systems to obtain the best separation efficiency (resolution and retention of stationary phase) for each with a suitable set of test samples. A set of short coiled columns is connected in series and mounted around the holder hub in four different ways: (model A) the tail of one unit with left-handedness was connected to the head of the next unit with right-handedness (TL-HR); (model B) the tail of one unit with left-handedness was connected to the tail of the next unit with right-handedness (TL-TR); (model C) the tail of one unit with left-handedness was connected to the tail of the next unit with left-handedness (TL-TL); (model D) the tail of one unit with left-handedness was connected to the head of the next unit with left-handedness (TL-HL). The results indicated that the performance of model D was the best among the four models. High revolution speed (800 rpm) is favorable to separation using the moderately hydrophobic solvent system of hexane-ethyl acetate-methanol-0.1M HCl (1:1:1:1, v/v) (HEMW), while lower revolution speed (600 rpm) is beneficial to the separation with polar solvent system of 1-butanol-acetic acid-water (19:1:20, v/v) (BAW).     

Evaluation of the effect of column orientation in type-I high-speed counter-current chromatography

The performance of type-I high-speed counter-current chromatography was evaluated by changing the column inclination against the rotating centrifugal force field. The separations were performed with two different solvent systems composed of 1-butanol–acetic acid–water (4.75:0.25:5, v/v) (BAW) and hexane–ethyl acetate–methanol–0.1 M HCl (1:1:1:1, v/v) (HEMW) using dipeptides and DNP-amino acid as test samples, respectively. A set of short coiled columns connected in series is mounted around the holder hub in two different ways: in the parallel orientation, all column units are arranged in parallel to each other and mounted on the holder at various angles against the horizontal plane. In the zigzag configuration, the neighboring units of the same column are mounted symmetrically forming various angles apart. In the parallel configuration, for both the BAW and HEMW systems, Sf (the retention of stationary phase) first increased as the column angle decreased from 90° to 60° and then decreased, as the column angle further decreased from 60° to 0°, while Rs (peak resolution) continually declined over the entire column angle range from 90° to 0°. But, for both solvent systems, with the zigzag configuration, retention of stationary phase and resolution both decreased as the column angle decreased from 90° to 0°. In general, Sf and Rs for separation of dipeptides in the BAW system, from 90° to 15°, is better for the parallel orientation than for the zigzag configuration. However, at 0°, Sf and Rs are better for the zigzag orientation. In the DNP-amino acid separation with the HEMW system, retention of the stationary phase and Rs for the parallel orientation is better than that for the zigzag orientation from 90° to 30°, whereas from 30° to 0° the results are opposite. Over all results of our studies revealed that the formally used column orientation [5] at 90° inclination yields the highest peak resolution in both solvent systems.     

Co-microprecipitation/flotation of trace amounts of cadmium from environmental samples through its complexation with iodide and neutralization with cetyltrimethylammonium bromide in the presence of perchlorate ions

ABSTRACT

A fast, cost-effective and reliable method is presented for separation and preconcentration of trace amounts of cadmium based on co-precipitation and flotation prior to its flame atomic absorption (AAS) spectrometric determination. Cadmium (II) was complexed with iodide and neutralized with cetyltrimethylammonium bromide (CTAB) in the presence of perchlorate ions. This resulted in the formation of a bulky precipitate containing the ternary complex of CdI₄(CTA)₂, floating on the top of the solution. The aqueous layer was then simply drained out, the floated layer was dissolved in 1.0 mL of acetonitrile, and its Cd content was determined by flame atomic absorption spectrometry. The parameters affecting the designed separation method such as KI, CTAB, and ClO₄^? concentration, pH, ionic strength, volumes of sample and dissolving solvent, and extraction time were studied and optimized. For preconcentration of 200 mL of the sample, the calibration graph was linear in the range of 1–30 μg L^?1 of cadmium with a correlation coefficient of 0.9997, enhancement factor of 194 and a limit of detection of 0.18 μg L^?1. The relative standard deviation for seven replicate determinations at 20 μg L^?1 levels of cadmium was found to be 2.1%. The effect of the presence of different common cations and anions on the separation and determination of Cd(II) by the developed method was studied. The method was successfully applied to the determination of trace amounts of Cd(II) ions in different types of real samples including tap water, polluted industrial wastewater, dust, and soil with the recoveries in the range of 95.3 to 103.4.