Distribution characteristics of methamphetamine and amphetamine in urine and hair specimens collected from alleged methamphetamine users in northern Taiwan

This study was conducted to better understand the distribution characteristics of methamphetamine and amphetamine in urine and hair specimens collected from alleged methamphetamine users in the local population. It is anticipated that the data hereby obtained will be helpful to the interpretation of the time and pattern of drug use. Eight alleged methamphetamine-using arrestees from Keelung Police Department (north of Taipei, Taiwan) consented to contribute both urine and hair specimens. Each arrestee contributed seven urine specimens collected at 0, 12, 24, 48, 72, 96, and 120 h, respectively, after the arrest. Hair specimens were cut into 2-cm sections. The limits of detection and quantitation of the urine protocol were 40 and 50 ng/mL, respectively, for both amphetamine and methamphetamine, while the corresponding limits of detection and quantitation for the hair protocol were 0.8 and 1.0 ng/mg, respectively. The concentration variations of methamphetamine and amphetamine in the urine specimens exhibited three distinct patterns: (a) continuous decrease in the analytes’ concentrations for specimens collected at hours 0-120; (b) increase in the analytes’ concentrations in specimens collected at hours 0-12, followed by decrease; (c) increase in analytes’ concentrations in specimens collected at later times. Together with the amphetamine/methamphetamine concentration ratios found in these urine specimens, the observed trends in the changes of the analytes’ concentrations are helpful for the interpretation on the time of drug use. Unlike urine specimens, amphetamine/methamphetamine concentration ratios in various hair specimens and hair sections remain relatively constant.     

Delay effect in models of population growth

First, we systematize earlier results on the global stability of the model of population growth. Second, we investigate the effect of delay on the asymptotic behavior when the nonlinearity f is a unimodal function. Our results can be applied to several population models [Elements of Mathematical Ecology, 2001 [7]; Appl. Anal. 43 (1992) 109-124; Math. Comput. Modelling, in press; Funkt. Biol. Med. 256 (1982) 156-164; Math. Comput. Modelling 35 (2002) 719-731; Mat. Stos. 6 (1976) 25-40] because the function f does not need to be monotone or differentiable. Specifically, our results generalize earlier result of [Delay Differential Equations with Applications in Population Dynamics, 1993], since our function f may not be differentiable.     

A damage model with evolving nonlocal interactions

We present a damage model for softening materials with evolving nonlocal interactions. The thermodynamic implications and the material stability issue are addressed. The proposed nonlocal averaging scheme provides the obtained constitutive models with an evolving nonlocal interaction which is activated only when damage occurs. In the analysis of structures made of quasi-brittle materials, this feature helps not only to overcome some issues with the incorrect initiation of damage but also to better control the evolving size of the active fracture process zone. This is an essential feature that is usually not considered in depth in many existing nonlocal approaches to the continuum modelling of quasi-brittle fracture. Numerical examples are given to demonstrate features of the proposed modelling approach.