An inventory control system for spare parts at a refinery: An empirical comparison of different re-order point methods

Inventory control of spare parts is essential to many organizations, since excess inventory leads to high holding costs and stock outs can have a great impact on operations performance. This paper compares different re-order point methods for effective spare parts inventory control, motivated by a case study at a large oil refinery. Different demand modeling techniques and inventory policies are evaluated using real data.     

A simple coupled mode model for near band-edge phenomena in grated waveguides

The paper presents a simple coupled mode model to describe near band-edge phenomena in the spectral response of waveguides with a section containing a rectangular grating. It is shown that nearly all relevant parameters can be determined in reasonable approximation from only the dispersion curve and one adjustable, proportionality constant. The theory is compared with results from numerical calculations and experiments.     

Scheduling preventive railway maintenance activities

A railway system needs a substantial amount of maintenance. To prevent unexpected breakdowns as much as possible, preventive maintenance is required. In this paper we discuss the preventive maintenance scheduling problem (PMSP), where (short) routine activities and (long) unique projects have to be scheduled in a certain period. To reduce costs and inconvenience for the travellers and operators, these activities should be scheduled together as much as possible. We present two versions of the PMSP, one with fixed intervals between two consecutive executions of the same routine work, and one with only a maximum interval. Apart from giving a math programming formulation for the PMSP and for its extension we also present some heuristics. In addition, we compare the performance of these heuristics with the optimal solution using some randomly generated instances.     

Ultrafast and high-throughput mass spectrometric assay for therapeutic drug monitoring of antiretroviral drugs in pediatric HIV-1 infection applying dried blood spots

Kaletra® (Abott Laboratories) is a co-formulated medication used in the treatment of HIV-1-infected children, and it contains the two antiretroviral protease inhibitor drugs lopinavir and ritonavir. We validated two new ultrafast and high-throughput mass spectrometric assays to be used for therapeutic drug monitoring of lopinavir and ritonavir concentrations in whole blood and in plasma from HIV-1-infected children. Whole blood was blotted onto dried blood spot (DBS) collecting cards, and plasma was collected simultaneously. DBS collecting cards were extracted by an acetonitrile/water mixture while plasma samples were deproteinized with acetone. Drug concentrations were determined by matrix-assisted laser desorption/ionization-triple quadrupole tandem mass spectrometry (MALDI-QqQ-MS/MS). The application of DBS made it possible to measure lopinavir and ritonavir in whole blood in therapeutically relevant concentrations. The MALDI-QqQ-MS/MS plasma assay was successfully cross-validated with a commonly used high-performance liquid chromatography (HPLC)–ultraviolet (UV) assay for the therapeutic drug monitoring (TDM) of HIV-1-infected patients, and it showed comparable performance characteristics. Observed DBS concentrations showed as well, a good correlation between plasma concentrations obtained by MALDI-QqQ-MS/MS and those obtained by the HPLC-UV assay. Application of DBS for TDM proved to be a good alternative to the normally used plasma screening. Moreover, collection of DBS requires small amounts of whole blood which can be easily performed especially in (very) young children where collection of large whole blood amounts is often not possible. DBS is perfectly suited for TDM of HIV-1-infected children; but nevertheless, DBS can also easily be applied for TDM of patients in areas with limited or no laboratory facilities.     

Three-dimensional photoacoustic imaging of blood vessels in tissue

We applied photoacoustics as a tissue tomography technique for the detection of blood concentrations, e.g., angiogenesis around tumors. We imaged blood vessels in highly scattering samples, using 532-nm light, to depths of ~1 cm . The samples were real tissue (chicken breast) or 10% dilutions of Intralipid-10%. The blood flowed through nylon capillaries. Polyvinylidene difluoride (PVdF) piezoelectric detectors were used in a surface-scanning mode. We demonstrate the sensitivity of the technique by photoacoustic detection of single red blood cells upon a glass plate. Lateral resolution is limited by the detector diameter (200 microm). The depth resolution is ~10 microm.     

PHOTOREACTIVATING ENZYME FROM Streptomyces griseus—VI. ACTION SPECTRUM AND KINETICS OF PHOTOREACTIVATION

Abstract— A high resolution action spectrum for photoreactivation was determined using purified photoreactivating enzyme from Streptomyces griseus. Conversion of pyrimidine dimers in UV-irradiated DNA, the substrate for photoreactivating enzyme, was measured with a Haemophilus influenzae transformation assay. A high similarity was found between action spectrum (max. at 445 nm) and the long wavelength absorption band (max. at 443 nm)of photoreactivating enzyme. In addition to the400–470 nm region considerable photoreactivation was found with wavelengths between 280 and 320 nm. No evidence was obtained for the presence of nonenzymatic photoreactivation. Comparison of in vitro and in vivo action spectra revealed that the sharp peak at 313 nm found in vivo is probably the result of counteracting photoreactivation and inactivation effects. Comparison of the action spectrum with the absorption spectrum of 8-hydroxy-10-methyl-5-deazaisoalloxazine in an aprotic dipolar solvent (which serves as a model for the 8-hydroxy-5-deazaflavin chromophore in photoreactivating enzyme) indicates the possible presence of other chromophore(s) involved in the photorepair process. From kinetic measurements and flash experiments values were obtained for the rate constants of the photoreactivation reaction. The quantum yield of photoreactivation was estimated to be approximately 1.     

Binding of a Diphosphorylated-ITAM peptide to spleen tyrosine kinase (Syk) induces distal conformational changes: A hydrogen exchange mass spectrometry study

Structural flexibility plays a crucial role in protein function. To assess whether specific structural changes are associated with the binding of an immunoreceptor tyrosine-based activation motif (ITAM) to the tandem Src homology-2 domains (tSH2) of the spleen tyrosine kinase [EC 2.7.7.112] (Syk), we used an approach based on protein hydrogen/deuterium exchange in the presence and absence of the diphosphorylated ITAM peptide. The protein deuterium uptake by the intact Syk protein was monitored in time by electrospray mass spectrometry, which revealed a dramatic relative decrease in deuterium uptake when the protein was bound to the ITAM peptide, suggesting an overall change in protein dynamics. Subsequently, the deuterium incorporation of individual segments of the protein was investigated using proteolysis and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) peptide mass-analysis, which revealed that several regions of Syk tSH2 are significantly more protected from exchange in the presence of the ITAM peptide. Four protected regions encompass the phosphotyrosine and hydrophobic binding sites on the SH2 domains, whereas two other protected regions are located in the inter-SH2 linker motif and do not make any direct contacts with the peptide. Interestingly, our data suggest that binding of the ITAM peptide to Syk tSH2 induces distal structural effects on the protein that stabilize the inter-SH2 linker region, possibly by raising the degree of helical structure upon binding.     

Partitioned-GMRES in Domain Decomposition with Approximate Subdomain Solution

Solution of large linear systems encountered in computational fluid dynamics often leads to some form of domain decomposition, especially when it is desired to use parallel machines. In this paper P-GMRES, a partitioned modification of GMRES, is applied to such problems. It is shown that P-GMRES converges faster than GMRES if the subdomains are solved exactly, and that P-GMRES requires less communication in the computation of the inner products. Also, approximate solutions for the subdomains by an inner preconditioned GMRES iteration are considered, in combination with a restarted version of P-GMRES. It turns out that rather crude tolerances are allowed, and that a good strategy is to vary the tolerance for the subdomains in the course of the outer iteration.This revised version was published online in October 2005 with corrections to the Cover Date.     

Describing the soil physical characteristics of soil samples with cubical splines

The Mualem-Van Genuchten equations have become very popular in recent decades. Problems were encountered fitting the equations’ parameters through sets of data measured in the laboratory: parameters were found which yielded results that were not monotonic increasing or decreasing. Due to the interaction between the soil moisture retention and the hydraulic conductivity relationship, some data sets yield a fit that seems not to be optimal. So the search for alternatives started. We ended with the cubical spline approximation of the soil physical characteristics. Software was developed to fit the spline-based curves to sets of measured data. Five different objective functions are tested and their results are compared for four different data sets. It is shown that the well-known least-square approximation does not always perform best. The distance between the measured points and the fitted curve, as can be evaluated numerically in a simple way, appears to yield good fits when applied as a criterion in the optimization procedure. Despite an increase in computational effort, this method is recommended over the least square method.