High-performance liquid chromatography with post-column derivatisation and fluorescence detection for sensitive determination of aflatoxin M1 in milk and cheese

A new HPLC method with fluorescence detection using pyridinium hydrobromide perbromide as a post-column derivatising agent has been developed to determine aflatoxin M1 in milk and cheese. The detection limits were 1 ng/kg for milk and 5 ng/kg for cheese. The calibration curve was linear from 0.001 to 0.1 ng injected. The method includes a preliminary C18-SPE clean-up and the average recoveries of Aflatoxin M1 from milk and cheese, spiked at levels of 25-75 ng/kg and 100-300 ng/kg, respectively, were 90 and 76%; the precision (RSDr) ranged from 1.7 to 2.6% for milk and from 3.5 to 6.5% for cheese. The method is rapid, easily automatable and therefore useful for accurate and precise screening of aflatoxin M1 in milk and cheese.     

Enantioselective organocatalytic conjugate addition of aldehydes to nitrodienes

The asymmetric organocatalyzed Michael addition of aldehydes to alpha,beta-gamma,delta-unsaturated nitro compounds has been accomplished using only 5 mol % of (S)-diphenylprolinol silyl ether and 2 equiv of aldehyde in a mixture of ethanol and water (5% v/v). The Michael adducts were obtained in good yields, diastereoselectivities up to 94/6, and ee's up to 99%. This process provides synthetically useful compounds which can easily lead to more complex molecules, as exemplified with substituted tetrahydropyran or cyclohexene.     

Low density spectrum of transverse NMR in molecular gases. Application to hydrogen

The low density behaviour of the spectral lineshape function associated with the relaxation of the transverse components of the nuclear magnetization of a homonuclear diatomic gas of spin $\text{1}\text{2}$ nuclei is studied from the point of view of a recent kinetic theory approach to NMR in molecular gases. It is shown that as a critical density is approached (from above), the relaxation (in time-domain) passes over from exponential to non-exponential behaviour. For the special case in which the spin—rotation relaxation mechanism predominates, an analytic solution to the problem has been given, while for molecular hydrogen which has, in addition to the aforementioned mechanism, dipolar relaxation, the behaviour of the lineshape function has been obtained numerically. The critical density at which the relaxation passes from exponential to nonexponential in H₂ is found to be of the order of 3 × 10^?3 amagats which lies, at present, still outside experimental accessibility. One important consequence of this result is that the traditional Abragam formula for the transverse relaxation time T₂ is clearly seen to be invalid below ≈ 5 × 10^?3 amagats.     

Renormalized theory of the time-dependent pair distribution function. I. General formulation

A classical renormalized theory of a time-dependent pair-distribution function (TDPDF), previously introduced by Oppenheim and Bloom, is presented. An equation of motion for the TDPDF is derived in which the memory function of the system appears. This is then split into a part which contains only static correlation functions and a part which describes the dynamics. The mean field approximation is discussed in some detail and contact is made witn the theory of Oppenheim and Bloom.Work supported in part by a National Research Council of Canada operating grant.     

Low density relaxation of the transverse nuclear magnetization in molecular gases

A time-domain analysis of the relaxation of the transverse nuclear magnetization in low density gases is presented. Attention is focussed upon the transition region where the phenomenological Bloch equation breaks down and non-exponential behaviour sets in. The analysis is based upon a perturbation parameter which makes it possible to obtain analytical results in an extremely, simple way and to a good degree of approximation. It corroborates the results recently obtained in the frequency-domain analysis of the same problem.     

Towards universal enrichment nanocoating for IR-ATR waveguides

Polymer multilayered nanocoating capable of concentrating various chemical substances at IR-ATR waveguide surfaces is described. The coating affinity to an analyte played a pivotal role in sensitivity enhancement of the IR-ATR measurements, since the unmodified waveguide did not show any analyte detection.     

General theory of wavemaker,I. theoretical results

A general theory of the wavemaker is presented based on a recent formulation of the water wave equations by Hui and Tenti. It exploits the fact that the free surface is a surface of constant pressure in order to make the surface boundary conditions linear and to be evaluated at a fixed boundary. The main features of the present theory are as follows: First, it applies to any weakly nonlinear wavemaker. Second, the full initial-boundary value problem is solved, thus including the transient effects in contrast to the classical approaches. Third, the finite amplitude (weakly nonlinear) effects are explicitly calculated. Finally, it is notable from a mathematical standpoint that the complicated second-order problem can be transformed to the form of the linear problem, and can therefore be solved by identical techniques.

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Résumé Nous présentons une étude théorique générale des batteurs à houle fondée sur une formulation récente des équations d'un fluide parfait, pesant et incompressible, par Hui and Tenti, dans laquelle la pression est une variable indépendante et la condition aux limites à la surface libre devient une condition linéaire portant sur une frontière fixe. Cette théorie est applicable à un batteur quelconque, et nous résolvons le problème de Cauchy en incorporant les effets transitoires, contrairement aux travaux classiques. En particulier nous calculons explicitement les corrections nonlinéaires du second ordre, ce qui est notable du point de vue mathématique parce que nous pouvons ramener la forme du problème du second ordre à celle du premier ordre.

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This work was supported in part by the Natural Sciences and Engineering Research Council of Canada through grants to G. Tenti and W. H. Hui.     

A Quasi-linear Viscoelastic Constitutive Equation for the Brain: Application to Hydrocephalus

Hydrocephalus is a condition which occurs when an excessive accumulation of cerebrospinal fluid in the brain causes enlargement of the ventricular cavities. Modern treatments of shunt implantation are effective, but have an unacceptably high rate of failure in most reported series. One of the common factors causing shunt failure is the misplacement of the proximal catheter's tip, which can be remedied if the healed configuration of the ventricular space can be predicted. In a recent study we have shown that this is accomplished by a mathematical model which requires as input the knowledge of the speed at which the ventricular walls move inwardly. In this paper we report on a theoretical method of calculating this speed and show that it will become of great practical usefulness as soon as more experimental results become available.     

Non-resonant absorption by a dilute gas of symmetric top molecules

A simple kinetic theory of non-resonant absorption by a dilute gas of symmetric top molecules is presented. Attention is focussed on the case where absorption occurs in the presence of static magnetic and electric fields. The macroscopic polarization of interest is found to behave differently in the two cases, but the lineshape has the same simple lorentzian form to a very good approximation. The predictions of the present theory agree with recent experimental observations, and the discrepancies of a previous kinetic theory are analysed and shown to be removable.     

A new approach to steady flows with free surfaces

The free boundary problem of a two-dimensional progressive wave (the Stokes wave) is reduced to a fixed boundary problem by regarding the ordinatey=y(x,)–x being the abscissa and the stream function — as the dependent variable. The Stokes wave is thus calculated analytically and explicitly to very high order via a recursion formula. For convenience, this is carried out with the help of the symbolic computation program MACSYMA. This approach is shown to be equally applicable to rotational flow, and in particular the class of exact flows for which the pressure is a function of is obtained. The extension of the method to three-dimensional Stokes waves is discussed.

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Sommaire Le probléme des houles (ondes de Stokes) est habituellement formulé comme un probléme aux limites en deux dimensions avec une surface libre. Dans ce travail nous considérons l'axe vertical comme une fonction de l'abscissex et de la fonction de courant,y=y(x,), grâce à quoi la condition aux limites porte sur une ligne libre d'équation connue (=0). Nous utilisons le programme de calcul symbolique MACSYMA pour calculer l'onde de Stokes analytiquement et explicitement. Nous démontrons aussi que cette méthode peut être utilisé dans le cas d'un mouvement rotationnel tel que, par exemple, la classe des solutions exactes où la pression n'est qu'une fonction de. Enfin nous discutons la possibilité d'utiliser cette méthode pour l'étude des ondes de Stokes en trois dimensions.