Aspects of matrix effects in applications of liquid chromatography–mass spectrometry to forensic and clinical toxicology—a review

In the last decade, liquid chromatography coupled to (tandem) mass spectrometry (LC-MS(-MS)) has become a versatile technique with many routine applications in clinical and forensic toxicology. However, it is well-known that ionization in LC-MS(-MS) is prone to so-called matrix effects, i.e., alteration in response due to the presence of co-eluting compounds that may increase (ion enhancement) or reduce (ion suppression) ionization of the analyte. Since the first reports on such matrix effects, numerous papers have been published on this matter and the subject has been reviewed several times. However, none of the existing reviews has specifically addressed aspects of matrix effects of particular interest and relevance to clinical and forensic toxicology, for example matrix effects in methods for multi-analyte or systematic toxicological analysis or matrix effects in (alternative) matrices almost exclusively analyzed in clinical and forensic toxicology, for example meconium, hair, oral fluid, or decomposed samples in postmortem toxicology. This review article will therefore focus on these issues, critically discussing experiments and results of matrix effects in LC-MS(-MS) applications in clinical and forensic toxicology. Moreover, it provides guidance on performance of studies on matrix effects in LC-MS(-MS) procedures in systematic toxicological analysis and postmortem toxicology.     

Catalytic and stoichiometric reactivity of β-silylamido agostic complex of Mo: intermediacy of a silanimine complex and applications to multicomponent coupling

The reaction of complex (ArN═)(2)Mo(PMe(3))(3) (Ar = 2,6-diisopropylphenyl) with PhSiH(3) gives the β-agostic NSi-H···M silyamido complex (ArN═)Mo(SiH(2)Ph)(PMe(3))(η(3)-ArN-SiHPh-H) (3) as the first product. 3 decomposes in the mother liquor to a mixture of hydride compounds, including complex {η(3)-SiH(Ph)-N(Ar)-SiHPh-H···}MoH(3)(PMe(3))(3) characterized by NMR. Compound 3 was obtained on preparative scale by reacting (ArN═)(2)Mo(PMe(3))(3) with 2 equiv of PhSiH(3) under N(2) purging and characterized by multinuclear NMR, IR, and X-ray diffraction. Analogous reaction of (Ar'N═)(2)Mo(PMe(3))(3) (Ar' = 2,6-dimethylphenyl) with PhSiH(3) affords the nonagostic silylamido derivative (Ar'N═)Mo(SiH(2)Ph)(PMe(3))(2)(NAr'{SiH(2)Ph}) (5) as the first product. 5 decomposes in the mother liquor to a mixture of {η(3)-PhHSi-N(Ar')-SiHPh-H···}MoH(3)(PMe(3))(3), (Ar'N═)Mo(H)(2)(PMe(3))(2)(η(2)-Ar'N═SiHPh), and other hydride species. Catalytic and stoichiometric reactivity of 3 was studied. Complex 3 undergoes exchange with its minor diastereomer 3' by an agostic bond-opening/closing mechanism. It also exchanges the classical silyl group with free silane by an associative mechanism which most likely includes dissociation of the Si-H agostic bond followed by the rate-determining silane σ-bond metathesis. However, labeling experiments suggest the possibility of an alternative (minor) pathway in this exchange including a silanimine intermediate. 3 was found to catalyze dehydrogenative coupling of silane, hydrosilylation of carbonyls and nitriles, and dehydrogenative silylation of alcohols and amines. Stoichiometric reactions of 3 with nitriles proceed via intermediate formation of η(2)-adducts (ArN═)Mo(PMe(3))(η(2)-ArN═SiHPh)(η(2)-N≡CR), followed by an unusual Si-N coupling to give (ArN═)Mo(PMe(3))(κ(2)-NAr-SiHPh-C(R)═N-). Reactions of 3 with carbonyls lead to η(2)-carbonyl adducts (ArN═)(2)Mo(O═CRR')(PMe(3)) which were independently prepared by reactions of (ArN═)(2)Mo(PMe(3))(3) with the corresponding carbonyl O═CRR'. In the case of reaction with benzaldehyde, the silanimine adduct (ArN═)Mo(PMe(3))(η(2)-ArN═SiHPh)(η(2)-O═CHPh) was observed by NMR. Reactions of complex 3 with olefins lead to products of Si(ag)-C coupling, (ArN═)Mo(Et)(PMe(3))(η(3)-NAr-SiHPh-CH═CH(2)) (17) and (ArN═)Mo(H)(PMe(3))(η(3)-NAr-SiHPh-CH═CHPh), for ethylene and styrene, respectively. The hydride complex (ArN═)Mo(H)(PMe(3))(η(3)-NAr-SiHPh-CH═CH(2)) was obtained from 17 by hydrogenation and reaction with PhSiH(3). Mechanistic studies of the latter process revealed an unusual dependence of the rate constant on phosphine concentration, which was explained by competition of two reaction pathways. Reaction of 17 with PhSiH(3) in the presence of BPh(3) leads to agostic complex (ArN═)Mo(SiH(2)Ph)(η(3)-NAr-Si(Et)Ph-H)(η(2)-CH(2)═CH(2)) (24) having the Et substituent at the agostic silicon. Mechanistic studies show that the Et group stems from hydrogenation of the vinyl substituent by silane. Reaction of 24 with PMe(3) gives the agostic complex (ArN═)Mo(SiH(2)Ph)(PMe(3))(η(3)-NAr-Si(Et)Ph-H), which slowly reacts with PhSiH(3) to furnish silylamide 3 and the hydrosilylation product PhEtSiH(2). A mechanism involving silane attack on the imido ligand was proposed to explain this transformation.     

Recent applications of liquid chromatography–mass spectrometry to residue analysis of antimicrobials in food of animal origin

Residual antimicrobials in food constitute a risk to human health. Although epidemiological data on the real magnitude of their adverse effects are very scarce, they indicate that food could be an important vehicle for evolution and dissemination of antimicrobial-resistant bacteria. Public health agencies in many countries rely on detection by mass spectrometry (MS) for unambiguous identification of residues of antimicrobial agents in animal food products for human consumption. The introduction of relatively inexpensive and robust liquid chromatography (LC)–MS systems has given a strong impulse to the development of confirmatory methods for the above medicines in foodstuffs. The initial part of this review, after a brief introduction into the field of antimicrobials, is dedicated to the most important EU regulations and directives for control of residues of these substances in animal products. The main attention in this review is on the sample-treatment and MS detection systems in use today for analysing the most important classes of antimicrobials in various biological matrices (milk, animal tissues, eggs, and honey). As evidenced by this review, reversed-phase LC combined with tandem MS, usually triple-quadrupole MS (QqQMS), is currently the preferred technique in most residue analysis of a single-class of antimicrobials. A recently emerging analytical strategy is that of developing methods for detecting a large variety of veterinary drugs belonging to different classes, including pesticides (multi-class residue analysis). To do this, simple and generic extraction and separation techniques applicable to a broad range of compounds differing in physical and chemical properties have been adopted. Such methods are still based mainly on LC–QqQMS. Emerging alternative MS detection systems are time-of-flight MS, which provides accurate mass of the analyte(s), or Q–linear ion trap (IT) MS that eliminates some limitations of ITMS ⁿ .     

Positive and negative fast-atom bombardment—collision-activated dissociation—linked-field scanned mass spectra of leucine enkephalin

Negative- and positive-ion fast-atom bombardment—collision-activated dissociation—linked-field (B/E) scan mass spectral data are compared for the opioid pentapeptide leucine enkephalin. An analysis of the fragmentation pathways which are specific to each ionization mode is presented. Data from the two ionization modes are significant from several analytical viewpoints—complementarity, facilitated amino acid sequence determination of nanomol amounts of pepfide, and a provision for higher level specificity for the measurement of endogenous peptides.     

Determination of nicotine in tobacco with second-order spectra data of charge-transfer complex in ethanol–water binary solvents processed by parallel factor analysis

A new spectrophotometric method for the determination of nicotine in mixtures without pre-separation has been proposed. Nicotine could react with 2,4-dinitrophenol through a charge-transfer reaction to form a colored complex. The second-order data from the visible absorption spectra of the complex in a series of ethanol–water binary solvents with various water volume fractions could be expressed as the combination of two bilinear data matrices. With the bilinear model, the second-order spectra data of mixtures containing nicotine and other interferents could be analysed by using second-order calibration algorithms, and the determination of nicotine in the mixtures could be achieved. The algorithm used here was parallel factor analysis. The method has been successfully used to determine nicotine in tobacco samples with satisfactory results.     

Novel Frechet-type dendritic BINOL ligands and their applications in Ti（IV） complex catalyzed asymmetric addition of diethylzinc to aldehydes

New Frechet-type dendritic BINOL ligands bearing several BINOL units at the periphery have been successfully synthesized. The （dendritic BINOL） Ti（IV） complexes were proved to be efficient catalysts for the enantioselective addition of diethylzinc to aromatic aldehydes. Moreover, these ligands could be quantitatively recovered and reused at least five times without a loss of the yield and enantioselectivity.     

Molecular structure of imidazolate bridged binuclear zinc complex and its single crystal ESR spectra doped with bridged Cu—Zn complex

The X-ray crystal structure of [(dtma)ZnImZn(dtma)]ClO_4·2.5H_2O (Hdtma=4-Diethyl-enetriamineacetic acid) was determined.The crystal is of orthorhombic,space group Pbcn with a-14.104(5),b=14.897(5),c=25.384(9),and Z=8.The least-square refinement of the structureleads to conventional R factor of 0.066.The magnetic properties of [(dtma)CulmZn(dtma)]CIO_4·2.5H_2Owere investigated.From the single crystal ESR spectra of Zn—Im—Zn dimer doped withCu—Im—Zn complex,the anisotropic g and A tensors and electronic spin-density of the Cu—Zncomplex are obtained and the bonding nature of Cu is discussed.     

Do electrospray mass spectra of surfactants mirror their aggregation state in solution?

One important feature in the gas phase chemistry of surfactants is to ascertain whether their aggregates produced by electrospray ionization reflect those formed in the starting solution. With this aim, we have performed ESI-MS, ESI-MS/MS and ER-MS spectra of bis(2-ethylhexyl)sulfosuccinate (AOTNa) solutions in different solvents, i.e. water, water/methanol, methanol and n-hexane. The results clearly indicate that, notwithstanding the strongly different aggregation state in solution (direct micelles in water and in water/methanol, molecular dispersion in methanol and reverse micelles in n-hexane) and marked effects of the solvent polarity on the total ionic current, the surfactant aggregates in gas phase show identical structural features. Analogous conclusions can be drawn analyzing the infrared multiple photon dissociation (IRMPD) spectra of AOTNa solutions in water/methanol and n-hexane. Moreover, according to the idea that gas phase can be considered an apolar environment par excellence, data consistently suggest a reverse micelle-like aggregation. Some peculiarities of the mechanisms leading to aggregate formation through electrospray ionization of surfactant solutions in solvent media with different polarity have been also discussed.     

Response of peptide intensity to concentration in ESI–MS-based proteome

High-throughput quantification with label-free methods has received considerable attention in electrospray ionization(ESI)-mass spectrometry(MS),but the manner by which MS signals respond to peptide concentration remains unclear in proteomics.We developed a new mathematical formula to describe the intrinsic log-log relationship between the MS intensity response and peptide concentration in an analytical ESI process.Experimental results showed that the calibration curve is fairly fit to the log-log formula with a linear dynamic range of approximate four to five orders of magnitude.However,we found that the ionization of analytical peptides can be severely suppressed by coexisting matrix peptides,such that the calibration curve can be poorly leveled off on both ends.Our study suggests that the interferences from coexisting matrix peptides should be reduced in the ESI process to use the log-log calibration curve successfully for the high-throughput quantification.     

Triplet—triplet absorption and polarization spectra of phenazine

Employing laser photoselection spectroscopy, the triplet—triplet (T—T) polarization spectrum over the visible portion of the spectrum of phenazine was recorded. The strong T—T absorption band located in the violet portion of the spectrum is differently polarized from the flat band stretching from the blue to the red portion of the spectrum. This indicates the presence of two electronic transitions. The portion located in the violet spectral region is long-axis-polarized and presents the ³B⁻_3g ← ³B⁺_2u transition and the long-wavelength portion to the short-axis-polarized ³A⁻_1g ← ³B⁺_2u transition.     

Self-assembly of artificial architectures in living cells-design and applications

Self-assembly exists widely in natural living system and artificial synthetic material system.Administration of self-assemblies of artificial architectures in living cells can be used to explore the molecular physicochemical fundamentals and operating mechanisms of living system,and consequently promote the development of biomedicine.In order to mimic naturally occurring self-assemblies and realize controllable functions,great efforts have been devoted to constructing dynamic assembly of artificial architectures in living cells by responding to intracellular specific stimuli,which can be used to regulate morphology,behaviors and fate of living cells.This review highlights the recent progress on artificial self-assembly in living cells.The molecular fundamentals and characteristics of intracellular environment that can induce the self-assembly of artificial architectures are introduced,and the representative work on dynamic artificial self-assembly in living cells is sketched chronologically.Moreover,intracellular stimuli-mediated pathways of artificial assembly in living cells are categorized,biological effects caused by intracellular self-assembly are summarized,and biomedical applications focusing on therapy and imaging are described.In the end,the perspective and challenges of artificial self-assembly in living cells are fully discussed.It is believed that the grand advances on artificial self-assembly in living cells will contribute to elaborating the molecular mechanisms in cells,and further promoting the biologically and medically-related applications in the future.     

DFT study of chemical mechanism of pre-SERS spectra in Pyrazine–metal complex and metal–Pyrazine–metal junction

The chemical mechanism of Normal Raman Scattering (NRS) and pre-surface enhanced Raman scattering (pre-SERS) spectra for Pyrazine–Ag₂ complex, Ag₂–Pyrazine–Ag₂ junction and Ag₂–Pyrazine–Au₂ junction were investigated with density functional theory (DFT) and charge difference densities (CDDs) for the first time. The NRS intensities of the above three structures enhanced obviously relative to isolated Pyrazine and the enhancement mechanism was confirmed to be static chemical enhancement. The pre-SERS intensities of the above three structures enhanced evidently compared to corresponding NRS intensities, and the enhancement mechanism was confirmed to charge transfer (CT) resonance Raman enhancement. The largest enhanced orders of NRS and pre-SERS intensities among the three structures were up to 10³ and 10⁵, respectively. Compared the intensity of pre-SERS with corresponding intensity of NRS spectra, the enhancement effect of Pyrazine–Ag₂ complex was larger than the others. Intramolecular and intermolecular CT on resonant electronic transition were described by CDDs.     

Classification of time-of-flight secondary ion mass spectrometry spectra from complex Cu–Fe sulphides by principal component analysis and artificial neural networks

Artificial neural network (ANN) and a hybrid principal component analysis-artificial neural network (PCA-ANN) classifiers have been successfully implemented for classification of static time-of-flight secondary ion mass spectrometry (ToF-SIMS) mass spectra collected from complex Cu–Fe sulphides (chalcopyrite, bornite, chalcocite and pyrite) at different flotation conditions. ANNs are very good pattern classifiers because of: their ability to learn and generalise patterns that are not linearly separable; their fault and noise tolerance capability; and high parallelism. In the first approach, fragments from the whole ToF-SIMS spectrum were used as input to the ANN, the model yielded high overall correct classification rates of 100% for feed samples, 88% for conditioned feed samples and 91% for Eh modified samples. In the second approach, the hybrid pattern classifier PCA-ANN was integrated. PCA is a very effective multivariate data analysis tool applied to enhance species features and reduce data dimensionality. Principal component (PC) scores which accounted for 95% of the raw spectral data variance, were used as input to the ANN, the model yielded high overall correct classification rates of 88% for conditioned feed samples and 95% for Eh modified samples.     

Novel Fréchet-type dendritic BINOL ligands and their applications in Ti(IV) complex catalyzed asymmetric addition of diethylzinc to aldehydes

New Fréchet-type dendritic BINOL ligands bearing several BINOL units at the periphery have been successfully synthesized. The (dendritic BINOL) Ti(Ⅳ) complexes were proved to be efficient catalysts for the enantioselective addition of diethylzinc to aromatic aldehydes.Moreover,these ligands could be quantitatively recovered and reused at least five times without a loss of the yield and enantioselectivity.     

Blue shifts and unusual intensity changes in the infrared spectra of the enflurane···acetone complexes: spectroscopic and theoretical studies

Blue-shifting C-H···O hydrogen-bonded complexes between enflurane (CHFCl-CF(2)-O-CHF(2)) and deuterated acetone have been identified in CCl(4) solution by FT-IR spectroscopy. For the two ν(C-H) stretching vibrations of enflurane the observed blue shifts are +17 and +11 cm(-1). The corresponding two infrared ν(C-H) bands show the opposite changes of their intensity, one is decreasing, and the other is significantly increasing, upon formation of the hydrogen bonding. The structures, binding energies, and theoretical infrared spectra of the enflurane-acetone complexes were calculated by MP2 and B3LYP methods using the 6-311++G(d,p) basis set. The interaction energies were evaluated by the complete basis set limit (CBS) calculations at the HF, MP2, and CCSD(T) levels of theory. Although the MP2 method slightly overestimates the blue shifts, the MP2 predicted frequency difference and the relative IR intensities of two ν(C-H) stretching bands for the enflurane-acetone complexes show good agreement with experiment. Unfortunately, the B3LYP method predicts incorrect IR intensities of these hydrogen-bonded systems. The NBO analysis was performed to unravel the origin of the unusual intensity changes of two ν(C-H) stretching bands, in enflurane complexes.     

Assignment of the UV—photoelectron spectra of cyclobutene and its syn- and anti-dimers

Ab initio CI studies of the UV-PES of the title compounds are described. The assignments are discussed in relation to through-bond and through-space effects.     

The vibrational spectra of germane and silane derivatives—XI. Vibrational spectra and normal co-ordinate analyses of some thiogermanes

The i.r. and Raman spectra of the thiogermanes, (CH₃)_nH_3-n, GeSCH₃(n = 0–3) are reported and the fundamentals assigned. A normal co-ordinate analysis based on a modified valence force field supports the a priori assignments for all of the fundamental frequencies except the torsional modes.