Trace element analysis by laser plasma mass spectrometry

Summary A double focusing mass spectrometer was equipped with a laser plasma ion source. This instrument has been used for multi-element analysis of solid materials. Laser plasma mass spectrometry (LMS) permits direct analysis of both conducting and non-conducting samples. The abundances of about 30 trace elements can be determined simultaneously in rock samples. A great advantage of LMS is that most elements are ionized with about the same efficiency, this means that only small corrections are necessary for quantitative analysis. Solutions which have been dried on a quartz disk can also be analyzed. This technique yields mass spectra which are free of interferences of molecules or cluster-ions.

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Spurenelementanalyse durch Laser-Plasma-Massenspektrometrie

     

Multi-ion counting-spark source mass spectrometry (MIC-SSMS): A new multielement technique in geo- and cosmochemistry

The photoplate detection system of a spark source mass spectrometer has been recently replaced by a detector array consisting of 20 separate small channeltrons for simultaneous ion counting of up to 20 trace elements. The new multi-ion counting – spark source mass spectrometry (MIC-SSMS) technique combines the advantages of conventional SSMS with modern on-line detection of elements. It has important analytical features, such as simple and fast solid-state sample preparation, high precision (about 1–2%) and accuracy (4%) using multielement isotope dilution, high sensitivity which leads to short measuring times (10–50 min) and low detection limits (about 0.001–0.01 μg/g).     

DFB laser diodes in the wavelength range from 760 nm to 2.5 microm

We present a novel device technology to produce DFB laser diodes which are suitable for tunable diode laser spectroscopy. The new technological approach employs lateral metal distributed feedback (DFB) gratings in close proximity to the laser ridge which results in single mode emission with high spectral purity and output powers as required for most spectroscopic applications. Over the entire wavelength range from the visible (760 nm) up to the near-infrared (2.5 microm) single mode emission can be obtained for devices based on different semiconductor systems such as GaAs, InP and GaSb. Typical side mode suppression ratios are better than 35 dB for cw-room temperature operation and narrow linewidths ensure high spectroscopic resolution.     

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.     

Analysis of the cryptophycin P450 epoxidase reveals substrate tolerance and cooperativity

Cryptophycins are potent anticancer agents isolated from Nostoc sp. ATCC 53789 and Nostoc sp. GSV 224. The most potent natural cryptophycin analogues retain a beta-epoxide at the C2'-C3' position of the molecule. A P450 epoxidase encoded by c rpE recently identified from the cryptophycin gene cluster was shown to install this key functional group into cryptophycin-4 (Cr-4) to produce cryptophycin-2 (Cr-2) in a regio- and stereospecific manner. Here we report a detailed characterization of the CrpE epoxidase using an engineered maltose binding protein (MBP)-CrpE fusion. The substrate tolerance of the CrpE polypeptide was investigated with a series of structurally related cryptophycin analogues generated by chemoenzymatic synthesis. The enzyme specifically installed a beta-epoxide between C2' and C3' of cyclic cryptophycin analogues. The kcat/Km values of the enzyme were determined to provide further insights into the P450 epoxidase catalytic efficiency affected by substrate structural variation. Finally, binding analysis revealed cooperativity of MBP-CrpE toward natural and unnatural desepoxy cryptophycin substrates.