N ews
You light up my micelle The double-chain surfactant Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate) has been extensively investigated for its ability to form reverse micellar aggregates in nonpolar solvents and to solubilize relatively large amounts of water. In order to characterize such reverse micelle struc-
Resolution of the Prodan emission spectrum. AOT concentration is 0.200 M, the water-to-AOT ratio is 2, and excitation is at 350 nm. Four overlapping Gaussian curves contribute to the overall spectrum.
tures, fluorescence techniques that use molecular probes to reflect their microenvironment and exhibit a particular affinity for one or more of the micelle regions (the water pool, the AOT interface, or the surrounding hydrocarbon solvent phase) have been employed. However, an ideal fluorescence probe for the water/AOT/ alkane system would distribute itself over all the regions of the reverse micelle and simultaneously display fluorescence properties indicative of the local environment. Concurrent variations in the properties of the individual micelle domains could then be distinguished with a probe capable of partitioning into each region. Kerry K Karukstis and colleagues at Harvey Mudd College used the neutral hydrophobic probe Prodan (6-propanol-2(dimethyl amino) naphthalene) to investigate the ternary system AOT/heptane/ water because of the probe's solubility in various media, its lack of electrostatic interaction with the anionic head groups of AOT, and its ability to display appreciable shifts in the wavelength of maximum
Kalman filters organics in the Chesapeake Bay
In the shadows Chemical modifiers are often added to the matrix of graphite furnace AAS samples to remove interferences either by thermally stabilizing the analyte or by increasing the volatility of matrix components. Unfortunately, this procedure can result in undesirable background levels or depression of the analyte signal, possibly because of condensation of the matrix vapors upon reaching a point of supersaturation. The condensation can cause nonspecific background absorption by light-scattering microparticles, and if the particles are not temporally and spatially uniform, deuterium arc lamp background correction C3T1 generate erroneous results. Chuni L. Chakrabarti and co-workers at Carleton University (Canada), the National Research Council of Canada, and the Geological Survey of Canada have used shadow spectral digital imaging (SSDI) to study vapor condensation in GFAAS. SSDI uses a charge-coupled device to record images of the atomization process. This method marks an improvement over previous techniques in that it yields quantitative data, makes data acquisition more convenient and facilitates computer data manipulation. They used SSDI to investigate the condensation of Au and various chemical modifiers, including MgCl2, NaCl, 524 A
emission with a variation in solvent. This probe also exhibits measurable fluorescence intensities in polar and nonpolar media. They performed a nonlinear leastsquares analysis on a single fluorescence emission spectrum to a sum of overlapping Gaussian curves by using an iterative Marquardt-Levenberg fitting algorithm and observed four principal microenvironments for Prodan, including an inner freewater pool, a bound-water region, the AOT interface, and the surrounding hydrocarbon solvent phase. As the surfactant concentration and molar ratio of water to surfactant were varied, it was possible to observe the changes in the emission characteristics of Prodan as it responded to specific micellar structural features such as the heterogeneity of the water pool variable polarities of the bound and free water regions the hydrophobicity and permeability of the surfactant interface and the hydration of Na+ counterions in the bound water region (/ Phvs Chem 1996 100 11133-38)
Absorbance contour maps of the vaporization of 20 ug Au at 1800 °C under gas-stop mode; (ii) 750 ms, (iv) 850 ms, (vii) 1000 ms, and (viii) 2000 ms. (Adapted with permission of the Society for Applied Spectroscopy.)
(NH4)2HP04, La(N03)3, and a mixture of Pd and MgtNOj^, in the graphite tube. Temporal and spatial nonuniformity, attributed to the gasflowpatterns that developed during heating, was observed in the light scattering. Condensation was also influenced by the sample injection hole and the location of sample deposition. The use of a graphite platform or a low argon purge gas flow reduced the light scattering. (Appl. Spectrosc. 1996, 50,715-31)
Analytical Chemistry News & Features, September 1, 1996
The Chesapeake Bay Fall Line Toxics Monitoring Program (CBFLP) conducts studies to determine the annual loads and transport fluxes of trace contaminants in the bay's tributaries. In one study, ultratrace analytical methods were used to determine the concentrations and transport fluxes of selected organic contaminants above the river fall lines (regions where upland streams meet the lowland river) of the Susquehanna, Potomac, and James rivers, which together make up 75% of the total basin area that supports the bay watershed. Twenty-six samples, taken almost monthly from March 1992 until February 1993, were analyzed for atrazine, metolachlor, fluoranthene, and total PCBs. Because the concentrations of organic contaminants in surface water can be highly variable, researchers would like to be able to estimate concentrations for those months in which samples were not taken. J. Terry Godfrey and Gregory D. Foster of George Mason University used the data from the 26 samples to develop a Kalman filter for predicting concentrations of organic contaminants in fluvial transport in Chesapeake Bay tributaries. Their
model assumes a knowntimeseries of concentration and environmental parameter data and also assumes stationary Gaussian statistics describing the performance of these quantities. They made 92 comparisons across all tributaries and compounds by using substitution, and for known concentrations they obtained afilteredor smoothed value that reflects the correction attributable to the estimated noise in the measurements and the underlying environmental system. Their values were within a factor of 4-5 of the measured concentration in most Plot of the logarithm of predicted versus cases, which is within the accuracy goals for this watershed. (Environ. Sci. Technol. measured concentrations for the three rivers: James (m), Susquehanna (o)) and 1996 30 2312-17) Potomac (Q).
What goes on at the tip When an STM tip approaches a surface, it often induces removal of material. It has been speculated that an electrochemical mechanism could be responsible for STMinduced surface transformations in air, because most surfaces have a thin layer of water that could act as an electrolyte solution to support low-current electrochemical processes. For example, in studies of STM-induced modification of nominally
STM images ofn-octadecyl mercaptan SAMs before and after patterning (white square), (a) prepattern; (b) and (f) in air at high humidity; (c) in air at low humidity; and (d) and (e) in nitrogen at high and low humidities, respectively.
naked conducting surfaces such as graphite andtitanium,it has been proposed that water was reduced at the tip (cathode) and that patterning resulted from oxidation of the substrate (anode). As part of their ongoing work to control patterning and better understand the nature of the processes involved in tip-induced surface modification, Richard M. Crooks and colleagues at Texas A&M University investigated scanning probe-induced electrochemical patterning of nominally naked Au(lll) and M-alkanethiol-coated Au(lll) surfaces in controlled > ~ 70% relative humidity. They found that patterning proceeded at biases above ~ +2.3 V because a thin layer of water adsorbed to the tip and surface had acted as an ultrathin-layer electrochemical cell. This low-energy selfassembled monolayer (SAM) restricts the dimensions of the highly resistive solution in the tip-sample gap, confines the patterning to the immediate vicinity of the tip, passivates unetched regions of the Au(lll) substrate, and retards the surface mobility of gold atoms, thereby stabilizing the patterns. Without SAMs patterns were not reproducible and the Au(lll) rapidly annealed to its pre-etch form At < ~ 25% relative humidity ,he amount of water on the SAM surface was insufficient to support electrochemistry and an insignificant amount of patterning was observed at sample biases up to +5 0 V The authors note that these observations nrovide a convenient method for studyine- electrochemical phenomena on a 1 to 100 nm scale and electrochemi n
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Simplifying interpretation of CID spectra One of the most popular ways to sequence proteins is by tryptic digestion followed by LC/ESIMS of the resulting peptides. Most of thetime,scission of the peptide bonds dominates the observed fragmentation pathways, and the resulting spectrum consists mostly of easily identifiable series of ions. However, when unexpected fragmentation at other positions of the peptide chain, multiple cleavages, or loss of water or ammonia occur, interpretation of the spectrum becomes more complicated. In an effort to determine the conditions under which the most easily interpreted collision-induced dissociation (CID) spectra of proteins can be obtained, Brian T. Chait and colleagues at Rockefeller University have investigated the effect of the collision energy and collision cell gas pressure on the fragmentation of a series of doubly protonated tryptic peptides. The most informative fragmentation spectrum is obtained when the primary fragment ions approach their maximum abundances. The researchers found that this occurs when the sum of the intensities of the three strongest fragment ions reaches about 1.5 times the remaining parent ion intensity. To compare peptides,
CID spectra of the octapeptide ASHLGLAR at three acceleration voltages. Note the increase in the relative abundances of smaller fragments in the 30-V spectrum. (Adapted with permission of the American Society for Mass Spectrometry.)
Analytical Chemistry News & Features, September 1, 1996 5 2 5 A
