On the simulation of the pivot bearing inside HDD

The pivot bearing is an important mechanical component in HDD. Several quasi-rigid (QR) body motion modes of the HDD are related to the stiffness of the pivot bearing such as the axial translation mode, the lateral translation mode and the rocking mode. In the shock simulation of the HDD, the pivot bearing is often simplified to identical theoretical models wherein the bearing is simulated with springs or beams. The overall axial stiffness and the radial stiffness of these models are often carefully checked and match well with the experimental values. However, how well these theoretical models represent the rotational stiffness of the bearing is still not fully understood. This is partly due to the difficulties in obtaining the experimental rotational stiffness of the pivot bearing. In this paper, three different theoretical models are examined for the simulation of the bearing. The rotational stiffness of these three theoretical models is derived analytically. The theoretical formulations are verified with the finite element analysis results.     

Mechanical properties of 6061 Al-Mg-Si alloy after very rapid heating

The effects of time, temperature and strain rate on the yield strength determined at elevated temperature have been investigated for 6061-T6 Al-Mg-Si alloy. To achieve very short times-at-temperature, nanosecond pulse heating produced by electron beam energy deposition was used along with one-dimensional stress-wave loading. When a relatively thick sample is heated in this way it cannot expand on the same time scale as the temperature increase. As a result, stress relief waves propagate in the material after energy deposition, and this deformation occurring at high strain-rates and elevated temperatures produces microstructural changes that reduce the strength of the alloy on the time scale of a few μs. This strength reduction occurs in addition to that due to the lowered shear modulus at-temperature, and is an essentially permanent change reflected in the greatly reduced room temperature strength of the material following nanosecond pulse heating.If the material is heated slowly enough so that the sample can expand as the temperature increases, and if the soaking time at temperature is less than required for microstructural changes in the alloy (e.g. approximately 3 s at 260°C), the yield strength measured at-temperature under wave propagation conditions drops in proportion to the shear modulus. In addition, the yield strength measurement is sensitive to the rate of deformation at elevated temperatures even for short times-at-temperature. The nature of this sensitivity is discussed in terms of thermally-activated dislocation motion.     

High-performance liquid chromatographic method for the determination of salicylic acid and its metabolites in urine by direct injection

A direct injection method has been developed for the determination of salicylic acid and its metabolites in urine. Urine samples are treated with hydroxylamine to convert salicyl acyl glucuronide to salicylhydroxamic acid, which can be accurately quantitated by direct injection into a high-performance liquid chromatographic system along with salicylic acid, gentisic acid and salicyluric acid. Salicyl phenolic glucuronide is quantitated by difference after hydrochloric acid hydrolysis at 65 degrees C with no loss of salicylic acid by sublimation or hydrolytic loss of salicyluric acid. This method has been applied to urine samples from human subjects and the results are discussed.     

A molecular model to explain paclitaxel and docetaxel sensitivity changes through adduct formation with primary amines in electrospray ionization mass spectrometry

The objective of this study was to adopt a molecular modeling approach to understand changes in signal intensity due to adduct formation with short-chain alkylamines for two anticancer agents, paclitaxel and docetaxel, during electrospray mass spectrometric analysis. We describe a simple and intuitive modeling procedure using a comparison of hydropathic interaction (HINT) scores to explain differences in responses of amine adducts formed with the two analytes. The responses of paclitaxel and docetaxel were generally enhanced considerably (up to approximately 500% in some instances) on adding the amines. However, for the docetaxel adduct formed with added decylamine in the mobile phase, the response dropped by 32%. A mechanistic understanding for this behavior is proposed, and binding scores calculated from corresponding molecular models were found to be consistent with the trend obtained from mass spectrometric data.     

Combining immunoassays with chromatographic and electrophoretic separation techniques — a review

The combination of immunoassays with separation techniques such as chromatography and electrophoresis can provide both selectivity and sensitivity that is competitive with any method currently available for molecular analysis. Immunoassays can be carried out on-line and off-line, pre and post separation. The on-line post separation mode is the most promising for routine analysis because of the high throughput that can be achieved but also provides the greatest challenge with regard to compatibility of the interfaced systems. This paper reviews the various approaches that have been researched from a practical immunochemical point of view with emphasis on the special problems incurred with matrix compatibility for on-line post separation systems.     

A LC-electrospray tandem MS method for the analysis of naltrexone in canine plasma employing a molecular model to demonstrate the absence of internal standard deuterium isotope effects

A simple and sensitive method is described for the determination of naltrexone (NAL), an opioid antagonist, in dog plasma. Sample processing involved a single step liquid-liquid extraction, followed by evaporation of the supernatant, and reconstitution of the residue prior to injection into the liquid chromatograph. The peak height ratio of NAL to [15,15,16-(2)H] naltrexone (NAL-d(3)) was used for quantitation. Observation of the chromatograms for NAL and NAL-d(3) revealed that the mean retention times of the compounds were 1.32 and 1.31 min, respectively. The almost identical retention times possibly accounted for the absence of matrix effects influencing quantitation. Molecular mechanics calculations using SYBYL software were carried out to qualitatively and quantitatively assess analyte and isotopic internal standard stationary phase interactions. Binding energy values of -10.22 and -10.26 kcal/mole were obtained for NAL and NAL-d(3), respectively. These data predict, semi-quantitatively, the absence of deuterium isotope effects that may influence quantitation. Calibration curves were linear from 10 pg/mL to 5014 pg/mL with a weighting factor of 1/x. Precision and accuracy and reverse predicted concentration residuals were within 15%. The method has been used successfully for the analysis of plasma samples from a pilot subcutaneous implantation study in dog.     

Assessment of matrix effects and determination of niacin in human plasma using liquid-liquid extraction and liquid chromatography-tandem mass spectrometry

A simple, sensitive and rapid liquid-liquid extraction method for the analysis of nicotinic acid (niacin) and its labeled internal standard nicotinic acid-d4 (niacin-d4) in human plasma was developed and validated. The analyte and its internal standard were isolated from acidified plasma using a single liquid-liquid extraction procedure with methyl-t-butyl ether. The extracted samples were analyzed by liquid chromatography-tandem mass spectrometry in positive electrospray ionization mode with multiple reaction monitoring. The calibration curves were linear in the measured range between 5 and 1000 ng/mL and the limit of detection was calculated as 122 pg/mL. The method required 250 microL of human plasma and the total run time between injections was 3.5 min. Matrix effects were assessed by post-column infusion experiments, phospholipids monitoring and post-extraction addition experiments. The extraction of phospholipids and niacin from plasma was studied under acidic, neutral and basic conditions. Acidic conditions were optimal for both the recovery of niacin and the removal of phospholipids; the degree of matrix effects for niacin was determined to be 2.5%. It was concluded that effective removal of matrix components can overcome low recovery issues associated with liquid-liquid extractions of polar analytes.     

An extended stable isotope‐labeled signature peptide internal standard for tracking immunocapture of human plasma osteopontin for LC‐MS/MS quantification

A stable isotope‐labeled signature peptide, whose sequence corresponds to the human osteopontin (hOPN) specific antibody epitope, was evaluated as an internal standard to compensate for immunocapture variability during quantification of hOPN by immunoaffinity‐coupled LC‐MS/MS. Immunocapture variability was induced by varying the antibody amount per well from 150 to 4500 ng and analysis was carried out with internal standards added before and after the immunocapture step. The immunocapture variability ranged from ?80.9 to 77.0% when the IS was added after immunocapture and from ?37.5 to 20.3% when the internal standard was added before immunocapture. The lower variability demonstrates the ability of stable labeled isotope internal standard peptide to compensate for variation during immunocapture. Copyright © 2015 John Wiley & Sons, Ltd.     

Two-dimensional analysis of the split hopkinson pressure bar system

The split Hopkinson pressure bar is widely used to measure the dynamic properties of solid materials. This paper presents the results of the first comprehensive two-dimensional numerical analysis of the technique, and quantitatively describes the effects of realistic friction and of variations in both the specimen geometry and the imposed strain-rate on the validity of the assumptions used in analyzing experimental data. A two-dimensional axisymmetric numerical analysis is used to compute all components of the stress, strain and strain-rate tensors at each mesh point within the specimen and the elastic bars. The calculated response of the pressure bars is used to reconstruct the stress-strain behavior of the specimen and this is compared to both the input stress-strain curve and the actual calculated stress-strain states in the specimen. Thus, the validity of the assumptions and the corrections used in the analysis of the data is determined.Inertia and friction between the specimen and the elastic bars affect the response of the specimen differently for different length-to-diameter ratios. Inertia effects produce stress waves propagating radially and axially in the specimen and may result in an oscillating reconstructed stress-strain curve. If the ends of the specimen are well lubricated and care is taken to minimize the effects of inertia, the reconstructed stress-strain curve agrees with the input. However, serious stress and strain nonuniformity exists when the ends are not lubricated and this results in a reconstructed stress-strain curve where, for any given strain, the stress magnitude is larger than the correct value. A comparison of the calculations with experiment shows excellent agreement for various interface conditions. Finally, the inertia correction of E.D.H. Davies and S.C. Hunter (1963) is found to be reasonable.     

Determination of amphetamine in dog plasma by gas chromatography with mass selective detection

This paper describes the validation of an analytical method for the determination of amphetamine in beagle dog plasma by gas chromatography coupled to mass spectrometry (GC-MS). d-Amphetamine-d(6) was used as the internal standard. The method consisted of a rapid single-step liquid-liquid extraction and derivatization of amphetamine with 2,2,2-trichloroethyl chloroformate, followed by sensitive GC-MS detection. This is the first report utilizing the combination of trichloroethyl chloroformate as a derivatization reagent and a deuterated amphetamine analog as an IS for the quantification of amphetamine in plasma. The method was validated in terms of specificity, curve fit, precision, accuracy, recovery and stability, and was acceptable according to FDA draft guidelines for validation of bioanalytical methods. The limit of detection was 0.65 ng/mL. The calibration range was 5-150 ng/mL. The validated method was successfully employed for the quantitation of amphetamine in dog plasma samples for pharmacokinetic profiling.