Untersuchung von Milch und milchhaltigen Waren

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Literatur

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The surface trans effect: influence of axial ligands on the surface chemical bonds of adsorbed metalloporphyrins

The chemical bond between an adsorbed, laterally coordinated metal ion and a metal surface is affected by an additional axial ligand on the metal ion. This surface analogon of the trans effect was studied in detail using monolayers of various M(II)-tetraphenylporphyrins (MTTPs, M = Fe, Co, Zn) and their nitrosyl complexes on a Ag(111) surface. X-ray photoelectron spectroscopy (XPS) shows that the oxidation state of the Fe and Co (but not Zn) ions in the MTPP monolayers is reduced because of the interaction with the substrate. This partial reduction is accompanied by the appearance of new valence states in the UV photoelectron and scanning tunneling spectra (UPS and STS), revealing the covalent character of the ion-substrate bond. Subsequent coordination of nitric oxide (NO) to the metal ions (Fe, Co) reverses these surface-induced effects, resulting in an increase of the oxidation states and the disappearance of the new valence states. Removal of the NO ligands by thermal desorption restores the original spectroscopic features, indicating that the described processes are fully reversible. The NO coordination also changes the spin state and thus the magnetic properties of the metal ions. Density-functional theory (DFT) calculations on model systems provide structural and energetic data on the adsorbed molecules and the surface chemical bond. The calculations reveal that competition effects, similar to the trans effect, play a central role and lead to a mutual interference of the two axial ligands, NO and Ag, and their bonds to the metal center. These findings have important implications for sensor technology and catalysis using supported planar metal complexes, in which the activity of the metal center is sensitively influenced by the substrate.     

Preliminary demonstration of an IonCCD as an alternative pixelated anode for direct MCP readout in a compact MS-based detector

We report on the preliminary testing of a new position-sensitive detector (PSD) by combining a microchannel plate (MCP) and a charge-sensitive pixilated anode with a direct readout based on charge-coupled detector (CCD) technology, which will be referred to as IonCCD (Hadjar et al. J Am Soc Mass Spectrom 22(4):612–623, 2011; Johnson et al. J Am Soc Mass Spectrom 22(8):1388–1394, 2011; Hadjar et al. J Am Soc Mass Spectrom 22(10):1872–1884, 2011). This work exploits the recently discovered electron detection capability of the IonCCD (Hadjar et al. J Am Soc Mass Spectrom 22(4):612–623, 2011), allowing it to be used directly behind an MC. This MCP-IonCCD configuration potentially obviates the need for electro-optical ion detector systems (EOIDs), which typically feature a relatively difficult-to-implement 5-kV power source as well as a phosphorus screen behind the MCP for conversion of electrons to photons prior to signal generation in a photosensitive CCD. Thus, the new system (MCP-IonCCD) has the potential to be smaller, simpler, more robust, and more cost efficient than EOID-based technologies in many applications. The use of the IonCCD as direct MCP readout anode, as opposed to its direct use as an ion detector, will benefit from the instant three-to-four-order-of-magnitude gain of the MCP with virtually no additional noise. The signal/noise gain can be used for either sensitivity or speed enhancement of the detector. The speed enhancement may motivate the development of faster IonCCD readout speeds (currently at 2.7 ms) to achieve the 2 kHz frame rate for which the IonCCD chip was designed, a must for transient signal applications. The presented detector exhibits clear potential not only as a trace analysis detector in scan-free mass spectrometry and electron spectroscopy but also as a compact detector for photon and particle imaging applications.     

Syntheses and Characterization of (C2F5)3BCO and (C3F7)3BCO

The new tris(perfluoroalkyl)borane carbonyls, (C₂F₅)₃BCO and (C₃F₇)₃BCO, were prepared by means of a novel synthetic route using commercially available precursors by reacting K[(C₂F₅)₃BCOOH] and K[(C₃F₇)₃BCOOH] with concentrated sulfuric acid in the last step. The carboxylic acids, K[(C₂F₅)₃BCOOH] and K[(C₃F₇)₃BCOOH], were prepared by oxidative cleavage of the C?C triple bonds in Cs[(C₂F₅)₃BC?CPh] and Cs[(C₃F₇)₃BC?CPh] in a two‐step process to yield K[(C₂F₅)₃BCO? COPh] and K[(C₃F₇)₃BCO? COPh] as isolable intermediates. Crystal structures were obtained of K[(C₂F₅)₃BCO? COPh], K[(C₂F₅)₃BCOOH] ? H₂O, (C₂F₅)₃BCO, K[(C₃F₇)₃BCOOH] ? 2 H₂O, and (C₃F₇)₃BCO. In the crystal structures of (C₂F₅)₃BCO and (C₃F₇)₃BCO the C?O bond lengths are 1.109(2) and 1.103(5) Å, respectively, which are among the shortest observed to date. Tris(pentafluoroethyl)borane carbonyl and (C₃F₇)₃BCO slowly decompose at room temperature to yield CO, difluoroperfluoroalkylboranes and perfluoroalkenes. The decomposition of (C₂F₅)₃BCO was found to follow a first‐order rate law with E_a=107 kJ mol^?1.