N-Alkylnicotinium halides: A class of cationic matrix additives for enhancing the sensitivity in negative ion Fast-Atom bombardment mass spectrometry of polyanionic analytes

The addition of some surfactants to the fast-atom bombardment (FAB) matrix previously has been demonstrated to enhance analyte signals in fast-atom bombardment mass spectrometry. In particular, cationic surfactants appear to enhance the negative ion FAB detectability of analytes that exist as anionic species in the matrix solution. It has been proposed that the charged surfactant concentrates the oppositely charged analyte near the surface, which results in larger signals for the analyte. Cationic surfactants that contain a fixed positive charge and an additional basic site were prepared with different hydrophobic moieties and were evaluated for their effectiveness as FAB matrix additives. The compound N-octylnico-tinium bromide (ONBr) is shown to improve greatly the analyte-related signals in negative ion fast-atom bombardment mass spectrometry for a variety of polyanionic analytes, relative to other surfactants (e.g., cetylpyridinium salts). This surfactant not only enhances detectability, but also simplifies the pseudomolecular ion region of the resulting spectra by reducing or eliminating metal cation adduct peaks. The simple mechanism of enhancement via surface activity is evaluated, and alternative mechanisms are considered. It is clearly shown that ONBr, as a FAB matrix additive, will allow mass spectrometry to be used for the analysis of anionic compounds that normally exhibit very low responses.     

Neue Synthesewege für Thiohalogeno- und Cyclothioarsenate(III). Kristallstrukturen von PPh4[As2SBr6] · CH3CN und PPh4[SAsS5]

Novel Routes to the Synthesis of Thiohalogeno- and Cyclothioarsenates(III). Crystal Structures of PPh₄[As₂SBr₆] · CH₃CN and PPh₄[SAsS₅] By reactions of (PPh₄)₂[As₂Cl₈] and (PPh₄)₂[As₂Br₈] with Na₂S₄ in acetonitrile (PPh₄)₂[As₂SCl₆] · CH₃CN and (PPh₄)₂[As₂SBr₆] · CH₃CN were obtained, respectively. Using K₂S₅, PPh₄[As₂SCl₅] and PPh₄[SAsS₅] were the products. The latter can also be obtained from PPh₄[As₂SCl₅] and Na₂S₄, while PPh₄[As₃S₃Br₄] is formed from PPh₄[As₂SBr₅] with K₂S₅. Two X-ray crystal structure determinations were performed. PPh₄[As₂SBr₆] · CH₃CN: triclinic, P1 , Z = 2, a = 1200.4(7), b = 1507.3(6), c = 1594.4(8) pm, α = 81.59(2), β = 78.22(3), γ = 80.58(2)°, R = 0.096 for 2298 observed reflexions. The structure contains [As₂SBr₆]^2? -ions in which the two Sb atoms are joined via one S and two Br atoms. PPh₄[SAsS₅]: triclinic, P1 , Z = 2, a = 1133.9(4), b = 1142.5(4), c = 1186.9(5) pm, α = 102.77(4), β = 107.74(3), γ = 106.65(3)°, R = 0.043 für 2677 reflexions. In the [SAsS₅]^? -ion an AsS₅ ring in the chair conformation is present.     

Optimal use of hub facilities: A two-hub model with fixed arc costs

Summary We introduce a model of a communication network design problem involving the utilization of hub facilities. That is, for a problem with two sets of customers and no intraset demand we seek to determine how the hub node associated with each set should be utilized. We assume that the only costs are the fixed costs associated with creating each of the three types of connecting arcs. A key parameter is the “group” size which is the number of communication circuits which can be bundled together in an arc. The optimal design depends strongly on how closely the arcs can be filled to capacity. The general demand problem is shown to be NP-Hard. However, for unit demand, we derive an almost “all or nothing” result which specifies that all flow should be direct node-to-node or, on the other hand, all or almost all flow should go via the hubs. Research supported in part by Grant SAB-94-0115 from the Spanish Interministerial Commission of Science and Technology while this author was on sabbatical leave at the Polytechnic University of Catalonia in Barcelona.     

Effects of head extension and tongue protrusion on voice perturbation measures

Head extension with protruded tongue is the position for video-laryngoscopy and simultaneous glottographic recordings including photoglottographic signals. This study investigated the effect of head extension and tongue protrusion on the measures of fundamental frequency, frequency perturbation (jitter), and amplitude perturbation (shimmer). Acoustic signals recorded during sustained vowels were obtained from 49 women and 66 men with no speech or voice disorders in different head-tongue positions. Head extension was associated with increased fundamental frequency and decreased shimmer. In men, head extension did not appear to affect jitter. When the tongue was protruded, head extension tended to lower jitter. For both genders, tongue protrusion was associated with decreased fundamental frequency with head extension. In the men, tongue protrusion tended to increase shimmer when the head was in the neutral position. In the women, tongue protrusion was associated with increased jitter and increased shimmer and was most evident in the head-neutral position. These findings supported a physical linkage hypothesis of the relationship between vocal tract configuration and vocal fold vibration, suggesting that head-tongue position must be taken into account when comparing voice measures.     

An Insert/Delete Heuristic for the Travelling Salesman Subset-Tour Problem with One Additional Constraint

The Travelling Salesman Subset-tour Problem (TSSP) differs from the well-known Travelling Salesman Problem (TSP) in that the salesman is not required to visit every city. Many problems, such as the prize collecting TSP, the orienteering problem, and the time constrained TSP, can be viewed as TSSPs with one additional constraint (TSSP + 1). In this paper, we present a heuristic approach for the TSSP + I class of problems. The general philosophy of our approach is to maintain tour feasibility with respect to the TSSP subproblem. This allows us to begin our approach with any TSSP tour. In each step, the insertion or deletion of a city is performed either to reduce infeasibility in the additional constraint or to improve upon the minimization objective. We present computational results that show our approach is superior to approaches using just insertion, and thus demonstrate the importance of considering the possible deletion of cities.     

Coercivity of the Single Layer Heat Potential

The single layer heat potential operator, K, arises in the solution of initial-boundary value problems for the heat equation using boundary integral methods. In this note we show that K maps a certain anisotropic Sobolev space isomorphically onto its dual, and, moreover, satisfies the coercivity inequality $ < K_{q,q} >\geq c\|q\|^2$. We thereby establish the well-posedness of the operator equation $K_q=f$ and provide a basis for the analysis of the discretizations.