摘要:

The role of RF quadrupoles in modern mass spectrometer and tandem mass spectrometer systems is described. A good deal of the success in the application of tandem quadrupole mass spectrometers and atmospheric pressure ionization mass spectrometers is due to new applications of the RF quadrupole ion guide. Developments that led from the use of very open, "transparent" electrodes in a cryopumped system to the opposite extreme of well-confined RF quadrupoles that operate advantageously at pressures of several mTorr are described. The ion containment properties of RF quadrupoles are remarkably insensitive to the accuracy of the electrode positions. This has allowed the use of novel quadrupole designs in order to provide unique solutions to instrumentational and physical-chemical problems.Key words: quadrupole, focusing, mass spectrometer.

ISSN:0008-4042    

DOI:10.1139/v98-073

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

This paper describes our efforts focused at defining the formation of diphenylmethane moieties in lignin during conventional kraft and soda pulping conditions. This objective was realized by confirming, initially, the assignment of a31P NMR signal as being due exclusively to the presence of phosphitylated diphenylmethane (DPM) phenolic hydroxyl groups. More specifically, softwood milled wood lignin (Picea mariana) was subjected to kraft pulping conditions in the presence and absence of varying amounts of formaldehyde. After quantitative recovery of the lignin, the31P NMR spectra were recorded and the spectra revealed selective signal growth in the region confined between 142.8 and 144.3 ppm, in accordance with previous model compound work and detailed calculations based on the Hammett principles. To further substantiate our conclusions we also carried out two series of isothermal (120°C) kraft and soda pulping experiments followed by quantitative determinations of the DPM moieties, and correlated our findings with the differences in chemistry known to occur between the two processes. Finally, diphenylmethane phenolic moieties were determined in isolated residual lignins from two western hemlock kraft pulp samples produced via an EMCC®mill protocol and a laboratory batch digester, respectively. These structures prevailed amongst the condensed phenolic units of the conventional pulp, providing additional evidence to support the fact that modern modified pulping technologies beneficially alter the structure of residual kraft lignin.Key words: phenyl groups, kraft pulping, modified kraft process, nuclear magnetic resonance (NMR), phosphorus spectroscopy.

ISSN:0008-4042    

DOI:10.1139/v98-060

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Using the published method of Wiedenbruch, bis(pentafluorophenyl)zinc,1, was prepared from anhydrous ZnCl2and 2 equiv. of LiC6F5in diethyl ether. Base-free1was obtained in 60-65% yield by repeated distillation of the initially formed bis(diethyl) ether adduct of1. The X-ray quality crystals of1were obtained from benzene solution. The molecular structure of1revealed a near linear geometry for the two-coordinate zinc center (C(1)-Zn-C(7) = 172.6(2)°), typical of monomeric ZnR2derivatives. In the crystal structure, stacking interactions between C6F5rings on adjacent molecules is a dominant motif, with ring centroid to ring centroid distances of 3.503 and 3.563 Å observed. A weak intermolecular C-F...Zn interaction between F(2) and an adjacent zinc center, as judged by the close contact of 2.849(2) Å, also appears to be an important aspect of the crystal structure. Compound1is an effective but nonselective C6F5transfer agent to BCl3;1: monoclinic, space groupP21/n,a= 11.902(2) Å,b= 7.732(2) Å,c= 13.735(2) Å, &bgr; = 110.58(1)°,V= 1183.4(4) Å3,Z= 4,R= 0.048,Rw= 0.069.Key words: organozinc complex, pentafluorophenyl transfer agent.

ISSN:0008-4042    

DOI:10.1139/v98-064

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

In organoammonium cations containing two or more N atoms, the H(N) atom introduced by the protonation may engage in an N→H(N)...Y bond to an external acceptor (anion or solvent molecule); in anintra-cation (intra-annular) N→H(N)...N' bond; or in a branched N→H(N)...N',Y bond(s), simultaneously intra-cation and to one or more external acceptors. The outcome of the competition for these bond types is determined by various factors, e.g., the size of the ring to be closed by insertion of the H(N) atom, steric accessibility of H(N), and the H-bond-forming strength of Y. Some of these are discussed in considerable detail and are exemplified by the hitherto unreported crystal structures of the tetraphenylborates (solvated or unsolvated) of the 2,2':6',2"-terpyridinium (1), 2,3-bis(2-pyridyl)pyrazinium (2), 2,2'-dipyridylammonium (3), 6,7-dihydro[b,j][1,10]phenanthrolinium (9), 2,4,7-triamino-6-pteridinium (triamterenium,10), proton-sponge (1-Me2N-8-Me2NH-naphthalene,11,12), and 9-amino-1,2,3,4-tetrahydroacridinium (tacrinium,13) cations. The crystal structures of the comparison compounds 2,3-bis(2-pyridyl)pyrazine (4) and its mono- (2A) and diprotonated (3) chlorides, and of 2-phenylpyridinium (6) and 7,8-benzoquinolinium (7,8) tetraphenylborates, have also been determined. The many interesting features of these crystal structures, such as the X→H(X)...phenyl bonding (X = N, O, C), are commented upon and crystallographic comparisons with numerous literature compounds are offered. Included in the discussion are the monoprotonated intrabridgehead cations of the bicyclic diamines of Alder et al., the N-H(N)-N' angle of which has been shown, analytically, to be dominated by the size of the smallest of the rings in these tricyclic systems.Key words: crystal structures, hydrogen bonding, intrabridgehead cations, proton sponges, tetraphenylborates.

ISSN:0008-4042    

DOI:10.1139/v98-077

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Escherichia coli&bgr;-galactosidase molecules were incubated with fluorogenic substrate in a capillary. Upon flushing the reactor contents past the detector, individual randomly distributed peaks of product were observed, each representing the activity of an individual enzyme molecule. Individual molecules were found to differ with respect to their activities. Molecules showed a 23-fold distribution of activities with the majority of molecules within a 4-fold distribution.Key words: single-molecule chemistry, &bgr;-galactosidase, capillary electrophoresis, laser-induced fluorescence, enzyme assay.

ISSN:0008-4042    

DOI:10.1139/v98-055

出版商:NRC Research Press    

年代:1998

数据来源: NRC