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Determination of Pesticides in Soil
Published in José L. Tadeo, Analysis of Pesticides in Food and Environmental Samples, 2019
Beatriz Albero, Rosa Ana Pérez, José L. Tadeo
The ionization technique most commonly used in GC–MS analysis is electron impact (EI), which produces characteristic ion fragments of compounds that are collected in spectral libraries. Full scan and selected ion monitoring (SIM) are the two working modes for EI–MS; SIM mode is more sensitive and selective than full scan. Most of the multiresidue methods developed in the last few years use MS as detection system as it offers the possibility of the simultaneous determination and identity confirmation of a large number of pesticides from different chemical classes in a single injection. Chemical ionization (CI) is a soft ionization technique that can work with two different polarities, positive (PCI) and negative (NCI), with minimal fragmentation which is useful to obtain molecular ions that are not observed in EI mass spectra. Atmospheric pressure ionization, commonly used in liquid chromatography (LC), is also a soft ionization in comparison to EI, resulting in less fragmentation, that is starting to be used coupled to GC. The generation of higher m/z ions as abundant ions makes the selection of precursor ions for tandem mass spectrometry (MS/MS) analysis easier and compound specific. GC–MS/MS allows the determination of pesticides in soil with good selectivity and high sensibility, reducing the sample treatment steps. Time of flight mass spectrometry (TOF-MS) is the result of the significant advances undergone by the analytical instrumentation that is being applied in the determination of pesticides since full mass-range spectrum and exact mass determination can be obtained for each pesticide without compromising sensitivity. Quadrupole time of flight (QTOF) is a hybrid analyzer that combines the benefits of obtaining accurate mass determination with performing a full spectral acquisition of the entire product ion-profile.
A comprehensive benchmark of the XMS-CASPT2 method for the photochemistry of a retinal chromophore model
Published in Molecular Physics, 2018
A CI is a point where two Born–Oppenheimer potential energy surfaces of same spin and symmetry becomes isoenergetic and exchange their electronic character [24]. The strong vibronic coupling nearby the CI leads to a breakdown of the Born–Oppenheimer approximation. The degeneracy at the CI is lifted by displacing the molecular geometry along two specific nuclear degrees of freedom (or branching vectors), namely the gradient difference vector (GDV; g) and nonadiabatic coupling vector (NACV, h). The GDV mostly parallel to the bond length alternation (BLA) coordinate while the NAC vectors correspond to the torsional motion with a strong nonadiabatic interaction between two adiabatic electronic states [18]. Displacement of CI along the remaining 3N-8 degrees of freedom retains the electronic degeneracy and this 3N-8 dimensional subspace (also called the intersection space) forms a set of infinite CI points with different geometry and energy. In previous studies, various methodologies have already been tested to characterise the CI seam. At the CI, the electronic states possess very different electron distribution, therefore, an appropriate electronic structure method with balanced static and dynamic electron correlation should provide an accurate geometry of the CI. Characterising the CIs has great significance for nonadiabatic processes as well as nonradiative photochemical reactions. The computational investigation of photochemical processes entails electronic structure methods to be able to correctly describe the potential energy surfaces of two intersecting states in the vicinity of a CI driving the photoisomerisation.
Photosensitivity, substituent and solvent-induced shifts in UV-visible absorption bands of naphthyl-ester liquid crystals: a comparative theoretical approach
Published in Liquid Crystals, 2014
P. Lakshmi Praveen, Durga P. Ojha
LCs are model materials for such applications as they are especially sensitive to electrical fields, exhibit short response times on the submicron scale and feature excellent transparency over a broad spectral range, from ultraviolet to visible to infrared. The addition of substituent atoms is the widely used technique in order to change the physical properties of LC molecules. In the calculation of electronic spectra, the CI method is widely employed. Using a CI method in combination with a semiempirical model Hamiltonian, an evaluation of absorption spectra of large organic molecules and LCs becomes possible.[38] The analysis of UV-Vis absorption spectra of the NAPHE1 and NAPHE2 molecules based on DFT data is given next.