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 the 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 T h e authors note that these observations nrnvide a convenient method fnr «tiiHvine- p W t r o c h p m i r a l nhpnompna , tn 100 nm scalp anH pWrrochpmi callv altering volumes smaller than
100 nm3 (/ Phvs Chem 1996 100 i i n o e n1x llUob—vl)
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
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they defined the conversion ratio as the sum of abundances of the three strongest fragment ions divided by the remaining parent ion abundance and the optimal collision energy as the collision cell voltage at which the conversion ratio is 1.5. The authors found a narrow energy range (at a given pressure) in which the degree of fragmentation was acceptable for parent ions of a given mass, and that the optimum collision energy exhibits a strong linear correlation with parent ion mass. The use of a common collision energy and pressure, therefore, will result in suboptimal fragmentation of pep-
Identifying separated neurotransmitters Although neurons have no problem detecting neurotransmitters, researchers trying to measure these compounds at low concentrations in biological samples face a difficult task. These neuroactive molecules lack strong absorption features at wavelengths longer than the water cutoff, are often electrochemically inactive at potentials useful for biological samples, and are not easily labeled with fluorescent tags at low concentrations. Biosensors, and in this case, patch-clamp-based biosensors are one way to detect these biologically active analytes with a high degree of sensitivity Richard N. Zare and co-workers at Stanford University and GSteborg University (Sweden) combined a
tides that have a wide range of masses, such as tryptic digests of proteins. They suggest a three-step strategy for each eluting peak to obtain optimal sequence information: Use one or two scans in the single MS mode to determine the massto-charge ratio of the major component at the onset of elution of an LC peak, use real-time computer control to set the collision offset voltage to an optimal one interpolated for the mass of interest, and average the CID spectra scanned under optimal conditions for the rest of the eluting peak. (/. Am. Soc. Mass Spectrom 1996 7 677-81)
patch-clamp detector with capillary electrophoresis (CE) to probe the kinetics, conductance states, and open and close times for an active neuronal receptor-ion channel. By coupling this information with the electrophoretic mobility, the researchers obtained a multidimensional format for neurotransmitter identification. The patch-clamp detector used neurons freshly dissociated from the rat olfactory bulb as biosensors. These cells contain receptors to the wellknown neurotransmitter GABA (y-aminobutyrate acid) as well as to two types of glutamate transmitters. The outlet of the CE capillary was positioned within 5-25 um of the patch-clamp biosensor. Because CE is run at high voltages, the capillary wasfracturedand grounded at about 7 cm above the outlet to create afield-freeregion near the patch-clamp. The CE chemically separated the agonists before detection allowing the scientists to resolve singlechannel openings for each separated ligand Low conductance states were generally ill defined but events with lartre amplihides rnuld he rnnntitated This tvne nf tprhnrilniTv rnnlH have lVatinn • tn srrpp Vncr
Schematic showing the approximate geometry of the CE column outlet relative to the patch-clamp pipette. (Adapted with permission of the American Association for the Advancement of Science.)
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orphan receptor ligands and excitotoxms, as well as neurotransmitters in the mammalian brain, (Sc.ence 1996,272, 177v—82)
Analytical Chemistry News & &eatures, September r, 1,96
Distinguishing PAH isomers Because PAHs are mutagenic and ubiquitous in the environment, they have long been the subject of analytical research. Mass spectrometric methods are particularly suited for analysis of PAHs, because these compounds are semivolatile and occur as complex mixtures; electron ionization (EI) and chemical ionization (CI) with quadrupole or magnetic sector mass spectrometers have been effectively used to determine PAHs. Although distinguishing between the isomeric forms of PAHs using EI is difficult because the isomers tend to produce common intermediates that give identical losses upon high-energy ionization or collisional activation CI-MS allows isomer differentiation based on adduct formation Michael L Gross of Washington University along with Andy B Whitehill and M George of the University of Nebraska-Lincoln have determined the jras-phase reactivity of a series of PAHs with nucleophiles in an effort to achieve isomer differentiation through ion-molpcnle reactions and collisinnallv activated dprnmnnsition (CAD) snprtra PAH radical cations were reacted with model DNA bases (neutral pyridine or iV-methylimidazole) to produce adducts, and isomer differentiation was achieved by comparing CAD and metastable-ion spectra of the gas-phase adducts of isomeric PAHs. The fragmentation patterns of these gas-phase PAH/ nucleophile adducts are similar to those
CAD spectra of product ions from aa ionized mixture oo PAH and fi-methylimidazole. Benzo[e]pyrene ((op)) benzo[a]pyrene (middle), and perylene (bottom)) (Adapted with permission of the American Society for Mass Spectrometry..
