Risk-based approach for the transfer of quantitative methods: bioanalytical applications

The transfer of analytical methods from a sending laboratory to a receiving one requires to guarantee that this last laboratory will obtain accurate results. Undeniably method transfer is the ultimate step before routine implementation of the method at the receiving site. The conventional statistical approaches generally used in this domain which analyze separately the trueness and precision characteristics of the receiver do not achieve this. Therefore, this paper aims first at demonstrating the applicability of two recent statistical approaches using total error-based criterion and taking into account the uncertainty of the true value estimate of the sending laboratory, to the transfer of bioanalytical methods. To achieve this, they were successfully applied to the transfer of two fully automated liquid chromatographic method coupled on-line to solid-phase extraction. The first one was dedicated to the determination of three catecholamines in human urine using electrochemical detection, and the second one to the quantitation of N-methyl-laudanosine in plasma using fluorescence detection. Secondly, a risk-based evaluation is made in order to understand why classical statistical approaches are not sufficient to provide the guarantees that the analytical method will give most of the time accurate results during its routine use. Finally, some recommendations for the transfer studies are proposed.     

Thermochemistry of N-N containing ions: a threshold photoelectron photoion coincidence spectroscopy study of ionized methyl- and tetramethylhydrazine

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2(+*)) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2(+) (m/z 45), loss of a methyl radical to form NHNH2(+) (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2(+*). Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)2NNCH3(+) (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2(+*). The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (delta(f)H0) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E0) obtained from the fitting procedure: delta(f)H0[(CH3)2NNCH3(+)] = 833 +/- 5 kJ mol(-1), delta(f)H0 [C3H8N2(+*)] = 1064 +/- 5 kJ mol(-1), delta(f)H0[CH2NHNH2(+)] = 862 +/- 5 kJ mol(-1), delta(f)H0[NHNH2(+)] = 959 +/- 5 kJ mol(-1), and delta(f)H0[N2H2(+*)] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to h nu approximately 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.     

Coupled diffusion and phase transition: Phase fields,constraints, and the Cahn–Hilliard equation

We develop a constrained theory for constituent migration in bodies with microstructure described by a scalar phase field. The distinguishing features of the theory stem from a systematic treatment and characterization of the reactions needed to maintain the internal constraint given by the coincidence of the mass fraction and the phase field. We also develop boundary conditions for situations in which the interface between the body and its environment is structureless and cannot support constituent transport. In addition to yielding a new derivation of the Cahn–Hilliard equation, the theory affords an interpretation of that equation as a limiting variant of an Allen–Cahn type diffusion system arising from the unconstrained theory obtained by considering the mass fraction and the phase field as independent quantities. We corroborate that interpretation with three-dimensional numerical simulations of a recently proposed benchmark problem.

     

Photocontrolled Synthesis of n-Type Conjugated Polymers

Current approaches to synthesize π-conjugated polymers (CPs) are dominated by thermally driven, transition-metal-mediated reactions. Herein we show that electron-deficient Grignard monomers readily polymerize under visible-light irradiation at room temperature in the absence of a catalyst. The product distribution can be tuned by the wavelength of irradiation based on the absorption of the polymer. Conversion studies are consistent with an uncontrolled chain-growth process; correspondingly, chain extension produces all-conjugated n-type block copolymers. Preliminary results demonstrate that the polymerization can be expanded to donor–acceptor alternating copolymers. We anticipate that this method can serve as a platform to access new architectures of n-type CPs without the need for transition-metal catalysis.     

Sharp-Interface Nematic-Isotropic Phase Transformations With Flow

We develop a sharp-interface theory for phase transformations between the isotropic and uniaxial nematic phases of a flowing liquid crystal. Aside from conventional evolution equations for the bulk phases and corresponding interface conditions, the theory includes a supplemental interface condition expressing the balance of configurational momentum. As an idealized illustrative application of the theory, we consider the problem of an evolving spherical droplet of the isotropic phase surrounded by the nematic phase in a radially-oriented state. For this problem, the bulk and interfacial equations collapse to a single nonlinear second-order ordinary differential equation for the radius of the droplet—an equation which, in essence, expresses the balance of configurational momentum on the interface. This droplet evolution equation, which closely resembles a previously derived and extensively studied equation for the expansion of contraction of a spherical gas bubble in an incompressible viscous liquid, includes terms accounting for the curvature elasticity and viscosity of the nematic phase, interfacial energy, interfacial viscosity, and the ordering kinetics of the phase transformation. We determine the equilibria of this equation and study their stability. Additionally, we find that motion of the interface generates a backflow, without director reorientation, in the nematic phase. Our analysis indicates that a backflow measurement has the potential to provide an independent means to determine the density difference between the isotropic and uniaxial nematic phases.     

Quantification of 2-acetyl-1-pyrroline in rice by stable isotope dilution assay through headspace solid-phase microextraction coupled to gas chromatography-tandem mass spectrometry

A new and convenient synthesis of 2-acetyl-1-pyrroline (2AP), a potent flavor compound in rice, and its ring-deuterated analog, 2-acetyl-1-d₂-pyrroline (2AP-d₂), was reported. A stable isotope dilution assay (SIDA), involving headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-positive chemical ionization-ion trap-tandem mass spectrometry (GC-PCI-IT-MS-MS), was developed for 2AP quantification. A divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber was used for HS-SPME procedure and parameters affecting analytes recovery, such as extraction time and temperature, pH and salt, were studied. The repeatability of the method (n = 10) expressed as relative standard deviation (RSD) was 11.6%. A good linearity was observed from 5.9 to 779 ng of 2AP (r² = 0.9989). Limits of detection (LOD) and quantification (LOQ) for 2AP were 0.1 and 0.4 ng g⁻¹ of rice, respectively. The recovery of spiked 2AP from rice matrix was almost complete. The developed method was applied to the quantification of 2AP in aerial parts and grains of scented and non-scented rice cultivars.     

CuO Nanospheres Based Nonenzymatic Glucose Sensor

CuO nanospheres, synthesized by a simple one‐step hydrothermal method, have been applied to modify the glassy carbon (GC) electrode for sensitive nonenzymatic glucose detection. The CuO nanospheres modified electrode, compared to the Nafion modified GC electrode, exhibits an enhanced electrocatalytic property for direct glucose oxidation and shows a fast response and a high sensitivity for the amperometric detection of glucose. It has been determined that the dissolved oxygen is not involved in glucose oxidation and the high concentration of NaCl does not poison the electrode. These results also indicate that CuO nanospheres have great potential application in electrochemical detection.     

Threshold-photoelectron spectroscopic study of methyl-substituted hydrazine compounds

The valence shell electronic structures of methylhydrazine (CH(3)NHNH(2)), 1,1-dimethylhydrazine ((CH(3))(2)NNH(2)) and tetramethylhydrazine ((CH(3))(4)N(2)) have been studied by recording threshold and conventional (kinetic energy resolved) photoelectron spectra. Ab initio calculations have been performed on ammonia and the three methyl substituted hydrazines, with the structures being optimized at the B3-LYP/6-31+G(d) level of theory. The ionization energies of the valence molecular orbitals were calculated using the Green's function method, allowing the photoelectron bands to be assigned to specific molecular orbitals. The ground-state adiabatic and vertical ionization energies, as determined from the threshold photoelectron spectra, were IE(a) = 8.02 +/- 0.16 eV and IE(v) = 9.36 +/- 0.02 eV for methylhydrazine, IE(a) = 7.78 +/- 0.16 eV and IE(v) = 8.86 +/- 0.01 eV for 1,1-dimethylhydrazine and IE(a) = 7.26 +/- 0.16 eV and IE(v) = 8.38 +/- 0.01 eV for tetramethylhydrazine. Due to the large geometry change that occurs upon ionization, these IE(a) values are all higher than the true thresholds. New features have been observed in the inner valence region and these have been compared with similar structure in the spectrum of hydrazine. The effect of resonant autoionization on the threshold photoelectron yield is discussed. New heats of formation (Delta(f)H) are proposed for the three hydrazines on the basis of G3 calculations: 107, 94, and 95 kJ/mol for methylhydrazine, 1,1-dimethyhydrazine and tetramethylhydrazine, respectively. The previously reported Delta(f)H for tetramethylhydrazine is shown to be erroneous.