News free amplitude, and afixeddamping of the cantilever oscillation supplies the feedback signal to maintain a constant tipsample separation during imaging. The authors reported an improvement in image quality over corresponding contact images. (Rev. Set. Instrum. 1997, 68, 2082-90)
Schematic design of thermally driven ac-mode AFM. (Adapted with permission from Rev. Scii .nstrum. Copyright 1997 American Institute ef Physics.)
MS measurements of bacterial glycolipids
Attached to an electrode When microcrystals are mechanically attached to an electrode and then placed in a suitable solvent, the reactions defining the charge and masstransfer processes can eitiier be totally confined to the solid or solution state or can occur in both phases. However, there are few details about the mechanism of these processes and the phase (s) in which they occur. Alan M. Bond at Monash University (Australia) and Dirk A. Fiedler at La Trobe University (Australia) employed in situ rotating ringdisk electrode and simultaneous electrochemical electron spin resonance (ESR) techniques to study products created during and after electrochemical treatment of compounds attached to an electrode surface in die microcrys-
In situ generation of the solution ESR spectrum of [TCNQ]- in Li* buffer on Pt electrodes. (a) Initial surface attached TCNQ; (b) complete reduction to [TCNQ]~; (c) complete regeneration of TCNQ; (d) complete reduction to [TCNQ]~ after regeneration of TCNQ as in (c). (Adapted with permission from J. Electrochem. Soc. Copyright 1997 The Electrochemical Society.)
Pathogenic Gram-negative bacteria have talline state. Results for the reduction proteins, polysaccharides, and glycolipids of 7 7 8 8-tetracyanoquinodimetiiane in their outer membranes that help die bac(TCNQ) and oxidation of trans-Crteria successfully colonize human tissue. (CO)2 (1 2-bis(diphenylphosphino) ethThe glycolipids, or lipoollgosaccharides ane) showed that the two compounds (LOS), contain two parts—a hydrophobic, have similar mechanisms despite their anionic lipid A and a complex ollgosacchasubstantial chemical and physical difride linked to the lipid through an acidic sugar—that extend into die external environment and interact with host cell surfaces. The combination of lipophilic, polar, adducts, and extensive fragmentation and ionic moieties makes LOS insoluble in caused by p-elimination of phosphoric virtually every solvent, rendering me mole- acid. Witii delayed extraction, die mass cules difficult to analyze. resolution of the spectra and stability of the molecular ions were improved. High Bradford W. Gibson and co-workers at laser powers easily cleaved the LOS moleUniversity of California-San Francisco and cules between the oligosaccharide and at PerSeptive Biosystems have used delayed extraction MALDI-TOF in die llnear, lipid A moieties, fragmenting the molereflectron, and post-source decay modes to cules. Post-source decay yielded similar characterize LOS from Haemophilus influ- fragments that were more limited in number. Both types of fragmentation provided enzae and Haemophilus ducreyi. LOS Lrepastructural information tiiat was valuable in rations were treated with hydrazine to reassigning the phosphate and phosphoetiimove O-linked fatty acids from the lipid A moiety, making the LOS water soluble and anolamine substitution pattern. more amenable to co-crystallization witii In addition, they analyzed a crude exstandard MALDI matrices such as 2,5-dihy- tract of intact Salmonella LOS Sb solubilizdroxybenzoic acid and 1-hydroxyquinoline. ing it with EDTA and pulsed sonication. The hydrazine treatment also decreases the The mass spectrum contained many of heterogeneity of die molecules and rethe same LOS-glycoforms that were in the moves impurities. spectra of the O-deacylated preparation. Continuous extraction resulted in Despite the success of these analyses, mass spectra with broad peaks, many salt the authors commented that it is uncer452 A
Analytical Chemistry News & Features, August 1, 1997
ferences. Their data indicated that a solid-state nucleation/growth mechanism confined to the electrode surface dominated the electrochemical processes at potentials where voltammetric peaks are observed. (J. Electrochem. Soc. 1997,144,1566-74)
tain whether the technique can be further improved to accommodate the low-level analysis required for clinical sampling experiments. (J. Am. .oc. Mass Spectrom. 1997,8,645-58)
Positive-ion MALDI-TOF spectrum of S. typhimurium Ra strain LOS; inset shows expansion of delayed extraction spectrum. (Adapted with permission of Elsevier Science. Copyright 1997 American Society for Mass Spectrometry.)
