News
Simplified ESI spectra Electrospray ionization (ESI) techniques are notorious for yielding complex mass spectra, because the resulting ions are produced in various charge states. Unlike MALDI, which yields predominantly singly-charged ions and therefore relatively simple mass spectra, ESI is generally thought to be unsuitable for the analysis of mixtures. But Lloyd M. Smith and co-workers at the University of WisconsinMadison have found a way to reduce the charge state of ions produced by ESI, thereby yielding predominantly singlycharged species. Charge reduction is accomplished in a controlled manner by exposing the electrospray plume to a bipolar ionizing gas. Multiply-charged electrospray ions are neutralized when they collide with the bipolar ions. The rate of the collisional process is dependent on the concentration of bipolar
ions and provides a way to control the charge state of ESI-generated ions. ESI-generated ions were analyzed for a simple protein mixture on an orthogonal time-of-flight mass spectrometer. In the absence of charge reduction, a complex mass spectrum with about 50 peaks was obtained (18 corresponding to various charge states). However, with the use of charge reduction, a mass spectrum with only eight major peaks was obtained. The simplified spectra were obtained only at the expense of signal intensity. Because the charge-reduction process neutralizes many ions, which go undetected, signal intensities in the charge-reduced spectra were significantly lower than those in the spectra obtained without charge reduction However the authors claim that a reduction in chemical noise and simplification of spectra tend to increase detection sensitivity (Science 1999 283 194-97)
The origin of limestone fluorescence
Mullins and colleagues took another approach, looking at six limestone samples that were similar in composition but differFluorescence can be used to identify the ent in how they were formed. Fluorescence provenance of the stone in monuments or to intensities, lifetimes, and emission spectra search for underground petroleum. One of were recorded and compared with those of the best known fluorescent rocks is limecrude oil and two stone, although the limestone compoorigin of its fluoresnents, bitumen and cence has not been kerogen. Plots of pinpointed. Howfluorescence red ever, a new paper by shifts versus excitaOliver C. Mullins tion wavelength and colleagues at were also made. Schlumberger-Doll Research shows The researchers that the fluoresfound that the fluocence properties of rescence properties limestone are simiof the limestone lar to those of crude samples followed oils which derive the same trends as their fluorescence crude oil. Because Fluorescence red shifts plotted against from organic crude oil derives its excitation wavelength. The limestone samples sources properties from orobey the same trends as crude oil and a bitumen. (Adapted with permission. Copyright ganic materials, this Because the 1998 Society for Applied Spectroscopy.) result strongly supmain ingredient in ports the idea that limestone is calcium limestone fluorescence has an organic oricarbonate fossils, many researchers have gin, the researchers said. In addition the assumed that the fluorescence is organic in properties of limestone fluorescence were nature. Some studies have found similarities related to the maturation of the rocks Bebetween the fluorescence of organic compocause the same trend is seen with crude nents extracted from limestone and the fluooils Mullins and colleagues suggest that rescence of whole rocks. But limestones these disparate materials mav be unified by vary substantially in composition, making it a sinele framework (Abbl Sbectross difficult to generalize, and iron and other 1 9 9 8 52 1606-13) trace metals have not been ruled out. 170 A
Analytical Chemistry News & Features, March 1, 1999
Redox switching Electroactive polymer films are commonly applied to electrode surfaces to modify their electrochemical responses. In order to design such films with specified properties, it is crucial to understand the dynamics of all the processes that accompany redox switching. It is often necessary to couple electrochemical methods with nonelectrochemical (or 'hyphenated") techniques to obtain information regarding electrode/electrolyte interfacial processes. Various nonelectro:hemical probe techniques (e.g., UV, risible and IR ttansmission/reflection spectroscopies; fluorescence; and the slectrochemical quartz-crystal microbalance) have been used to investigate changes in film surface populations caused by redox transformations Stanley Bruckenstein of the State University of New York at Buffalo and A Robert Hillman of the Universifv of I Bicester (XI K) describe a mathematical theory that relates all nrobe responses (based fare pormlation chancres) to electrochemdcal control functions involved in film reIn their analysis of electroactive film "edox switching under permselective conditions, the authors describe the film state in terms of three parameters—po;ential, charge level, and species. The theory is general in nature; however, it is specifically discussed with regard to relox switching of electroactive films that incorporate three elementary steps— :oupled electron/ion transfer, solvent transfer, and film structural change. Data from any probe technique that yields information regarding the population of nobile species in the film can be interpreted by the theory. Coulostatic and potentiostatic control functions provide the most straightforward qualitative separation of die elementary steps involved in film redox switching. In addition, they provide a simple ray to quantitatively determine rate constants and other relevant kinetic parameters. A 3-D vector/scheme of cubes ap)roach is used to visualize possible nechanisms for film redox switching. fhe three axes are coupled electron/ion transfer, solvent transfer, and polymer •econfiguration. (J. Phys. Chem. B1998, 102,10826-35)
