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
