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61.
Some important analytes in ICP-MS are interfered by plasma argon or argon species, such as ArO+ and 56Fe, 40Ar and K or Ca. One approach to overcome this interference is the use of reduced forward power and a metal shield inserted between torch and load coil. These so called cold plasma conditions reduce the background caused by argon species and the formerly interfered analytes can be easily detected in the ng/g-range. Other elements in the lower mass region also profit from these conditions even when they are not interfered in normal plasma mode. The limits of detection are improved due to reduced background noise level and enhanced ion transmission. On the other hand, the reduced power fed to the plasma lowers the analytical performance and makes it susceptible to matrix effects. Elements of higher mass generally show higher detection limits compared to normal plasma mode.  相似文献   
62.
A Simple and Flexible Synthesis of Pyrroles from α , β -Unsaturated Sulfones: The addition of alkyl isocyanoacetates and isocyanoacetonitrile to α,β-unsaturated sulfones affords a convenient and broad access to pyrroles with unusual substitution patterns (see Scheme 2). The α,β-unsaturated sulfones required as starting materials are easily obtained from different olefines.  相似文献   
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Well known sources of contamination are the laboratory environment, vessels, reagents and handling of the sample. Attention is drawn to the importance of cationic impurities included in the ion-exchangers used for trace-matrix separations (TMS) at the sub-ng/g-level. The usual rinsing and conditioning of the resins does not remove all adhering metals and thus leads to contaminated eluates in analytical work. High and badly reproducible blanks are observed, in particular for Na, Mg, Al, K, Cr, Fe, Zn and Ba. They are introduced during the polymerization process and are not ionically bound but obviously included in the polymer matrix. Any time when another “nest” of inclusion is opened in the resin column by mechanical stress, the next irreproducible event of contamination occurs.  相似文献   
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Summary. This paper develops renewal theory for a rather general class of random walks SN including linear submartingales with positive drift. The basic assumption on SN is that their conditional increment distribution functions with respect to some filtration ?? are bounded from above and below by integrable distribution functions. Under a further mean stability condition these random walks turn out to be natural candidates for satisfying Blackwell-type renewal theorems. In a companion paper [2], certain uniform lower and upper drift bounds for SN, describing its average growth on finite remote time intervals, have been introduced and shown to be equal in case the afore-mentioned mean stability condition holds true. With the help of these bounds we give lower and upper estimates for H * U(B), where U denotes the renewal measure of SN, H a suitable delay distribution and B a Borel subset of IR. This is then further utilized in combination with a coupling argument to prove the principal result, namely an extension of Blackwell's renewal theorem to random walks of the previous type whose conditional increment distribution additionally contain a subsequence with a common component in a certain sense. A number of examples are also presented.  相似文献   
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We analyze the time-dependent spectrum of eigenvalues of the correlation matrix for multivariate EEG data at the transition to epileptic seizures. By a mechanism of level repulsion between states at both edges of the spectrum of the correlation matrix, relevant information about quantitative correlation changes is reflected in the largest and smallest eigenvalues and corresponding eigenvectors. By the application of measures from random matrix theory we provide evidence that statistically relevant information can be obtained both at the upper and the lower end of the spectrum. In addition, information about spatial characteristics of correlation changes can be extracted.  相似文献   
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Bicycloalkenones can be generated by intramolecular Wittig-reaction, even if the olefinic bond formed has a relative high strain energy.  相似文献   
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