Homonuclear mixing sequences for perdeuterated proteins

We tested the performance of several (13)C homonuclear mixing sequences on perdeuterated microcrystalline ubiquitin. All sequences were applied without (1)H decoupling and at relatively low MAS frequencies. We found that RFDR gave the highest overall transfer efficiency and that DREAM performs surprisingly well under these conditions being twice as efficient in the aliphatic region of the spectrum than the other mixing sequences tested.     

XUV free-electron laser desorption of NO from graphite (0001)

We report results of femtosecond laser induced desorption of NO from highly oriented pyrolytic graphite using XUV photon energies of hν = 38 eV and hν = 57 eV. Femtosecond pulses with a pulse energy of up to 40 μJ and about 30 fs duration generated at FLASH are applied. The desorbed molecules are detected with rovibrational state selectivity by (1 + 1) REMPI in the A(2)Σ(+) ← X(2)Π γ-bands around λ = 226 nm. A nonlinear desorption yield of neutral NO is observed with an exponent of m = 1.4 ± 0.2. At a fluence of about 4 mJ/cm(2) a desorption cross section of σ(1) = (1.1 ± 0.4) × 10(-17) cm(2) is observed, accompanied with a lower one of σ(2) = (2.6 ± 0.3) × 10(-19) cm(2) observable at higher total fluence. A nonthermal rovibrational population distribution is observed with an average rotational energy of = 38.6 meV (311 cm(-1)), a vibrational energy of = 136 meV (1097 cm(-1)) and an electronic energy of = 3.9 meV (31 cm(-1)).     

Accurate background monitoring circuit for BC6-equipped varian-techtron AA5 and AA6 atomic absorption spectrometers

A low-cost circuit permits the accurate simultaneous measurement of both background-corrected atomic absorption spectrometric signals and background-only signals.     

Observation of highly flexible residues in amyloid fibrils of the HET-s prion

We report the observation of undetected (until now) residues of the prion protein fragment HET-s(218-289) which give rise to well-resolved (13)C, (15)N, and (1)H NMR resonances under high-resolution magic-angle spinning (HRMAS) conditions. The observed signals belong to large polymeric units as shown by measuring the lateral diffusion constants. The amino acids identified in the spectra are compatible with their localization in the segments of the protein which could not be detected in earlier solid-state NMR experiments. The observed chemical shifts indicate that these residues are in a random-coil conformation. Complementary experiments which detect only dynamic or static residues, respectively, strongly suggest that they belong to different parts of the same molecule.     

Electrothermal atomic absorption spectroscopy - preseht understanding and future needs

A comparison is made between Massmann-type furnaces (with and without the L'vov platform) and constant temperature atomizers. It is shown that there is no major difference between these types of furnaces with regard to peak height sensitivities. On the other hand, the Massmann-type furnaces shoved to a greater extent susceptibilities towards matrix interference effects. The effect of the sample residence time on gas phase interference effects has been investigated at various constant temperatures for lead in large excesses of iron chloride and sodium sulphate, respectively. These experimental results are discussed and they are correlated to data obtained by high temperature equilibrium calculations. As a conclusion we found that there is a need for a better control of the gas phase inside graphite tubes. Advantages of separating the volatilization and atomization processes are discussed. The potentialities of constant temperature atomizers for atomic emission spectroscopy are lined out.Since its inception, conventional GFAAS has been developed considerably with regard to methodology and instrumentation. The technique has been essentially improved by the introduction of e.g., automatic sample devices, the L'vov platform technique, matrix modifications, pyrolytically coated graphite, automatic background correctors, adequate signal evaluation and rapid controlled heating of the atomizers. In spite of this progress there still remain problems in connection with the vaporization/atomization of samples. In conventional Massmann-type furnaces, the temperature at which an element is vaporized depends on its volatility and usually effective atomization temperatures are often too low for complete atomization. An additional disadvantage comprises difficulties in relating absorbance signals, which may originate from different atomization intervals, to true amounts of an element. Many of these problems inherent in Massmann-type furnaces can be eliminated by vaporizing samples into atomizers which are kept at a constant temperature. This concept was employed in the first graphite furnace ever built for analytical AAS [l], but due to the technical complexity of the isothermal approach, it has only been realized on a minor scale and therefore little is known about its limitations.By vaporizing samples from a platform [2,3] inserted into Massmann-type furnaces, the problems arising from non-isothermal atomization can often be minimized in a relatively simple way. In particular for volatile elements it is possible to approach conditions of constant temperature atomizers by the combined use of the platform technique with an element stabilizing modifier solution [4,5].The aim of this paper is to characterize isothermal as well as Massmann-type atomizers (equipped with and without platforms) with respect to sensitivity and susceptibility to interference effects as well as identifying future needs in order to develop the graphite furnace technique further.     

The determination of mercury in air samples by flameless atomic absorption

A method is described for the determination of mercury in air by furnace atomic absorption. The porous graphite cups used ordinarily fail to adsorb mercury vapors from the air. When these same porous graphite cups are plated on the inner surface with a thin layer of gold the retention of mercury from an air sample passed through the cup is shown to be quantitative. The sensitivity of the analytical system is 3 × 10^?10 g which is equivalent to 0.6 $\text{μg}\text{m}{}^3$ for a 500 cm³ air sample. Relative S.D. is 11.2%. The method described should also prove useful for other types of flameless devices.     

Improved resolution in (13)C solid-state spectra through spin-state-selection

The application of a spin-state-selective coherence transfer experiment (INADEQUATE-SSS) to solid-state NMR spectroscopy is described. Two-dimensional (13)C double-quantum/single-quantum spectra without J splittings in both dimensions lead to enhanced spectral resolution. The method is demonstrated to significantly improve the spectral resolution of the crowded C'-C(alpha) region of two proteins.