Richard W. Linton

Rapid Uptake of Aluminum into Cells of Intact Soybean Root Tips (A Microanalytical Study Using Secondary Ion Mass Spectrometry).

A wide range of physiological disorders has been reported within the first few hours of exposing intact plant roots to moderate levels of Al3+. Past microanalytic studies, largely limited to electron probe x-ray microanalysis, have been unable to detect intracellular Al in this time frame. This has led to the suggestion that Al exerts its effect solely from extracellular or remote tissue sites. Here, freeze-dried cryosections (10 [mu]m thick) collected from the soybean (Glycine max) primary root tip (0.3–0.8 mm from the apex) were analyzed using secondary ion mass spectrometry (SIMS). The high sensitivity of SIMS for Al permitted the first direct evidence of early entry of Al into root cells. Al was found in cells of the root tip after a 30-min exposure of intact roots to 38 [mu]M Al3+. The accumulation of Al was greatest in the first 30 [mu]m, i.e. two to three cell layers, but elevated Al levels extended at least 150 [mu]m inward from the root edge. Intracellular Al concentrations at the root periphery were estimated to be about 70 nmol g-1 fresh weight. After 18 h of exposure, Al was evident throughout the root cross-section, although the rate of accumulation had slowed considerably from that during the initial 30 min. These results are consistent with the hypothesis that early effects of Al toxicity at the root apex, such as those on cell division, cell extension, or nutrient transport, involve the direct intervention of Al on cell function.

Evaluation of matrix-assisted laser desorption ionization mass spectrometry for polymer characterization

A protocol for the preparation of polymeric samples for time-of-flight matrix-assisted laser desorption ionization mass spectrometry (TOF-MALDI-MS) analysis was developed. Dithranol was identified as a good matrix for polystyrene (PS), and the addition of silver for cationization of molecules was determined to be necessary. Based on this preparative method, low molecular weight samples of other polymers [polyisoprene, polybutadiene, poly(ethylene oxide), poly(methyl methacrylate), and polydimethylsiloxane] were analyzed with molecular weights up to 49 ku. The effects of laser intensity were determined to influence the molecular weight distribution of intact oligomers, most significantly for low molecular weight polymers. Linear and reflectron modes of analysis were evaluated; better signal intensity and resolution were obtained in the reflectron mode. The TOF-MALDI-MS measurements are compared with time-of-flight secondary ion mass spectrometry (TOF-SIMS) and gel permeation chromatography (GPC) for the same polymers. The Mn values calculated by TOF-MALDI-MS consistently are higher than values calculated by TOF-SIMS for all classes of polymers with molecular weights up to 8 ku. The molecular weights of the PS calculated from TOF-MALDI-MS are in good agreement with GPC (±10%). The composition of the terminal group on a polymer chain may affect the ion yields. The ion yields of intact oligomers were evaluated as a function of end group composition for both TOF-MALDI-MS and TOF-SIMS. The slight disparity of results between TOF-SIMS and TOF-MALDI-MS for the perfluoroalkyl-terminated PS suggests that the oligomers are desorbed preferentially from the surface in the TOF-SIMS analysis, rather than having an increased ionization probability.

The Early Entry of Al into Cells of Intact Soybean Roots (A Comparison of Three Developmental Root Regions Using Secondary Ion Mass Spectrometry Imaging)

Al localization was compared in three developmental regions of primary root of an Al-sensitive soybean (Glycine max) genotype using secondary ion mass spectrometry. In cryosections obtained after a 4-h exposure to 38 [mu]M [Al3+], Al had penetrated across the root and into the stele in all three regions. Although the greatest localized Al concentration was consistently at the root periphery, the majority of the Al in each region had accumulated in cortical cells. It was apparent that the secondary ion mass spectrometry 27Al+ mass signal was spread throughout the intracellular area and was not particularly intense in the cell wall. Inclusion of some cell wall in determinations of the Al levels across the root radius necessitated that these serve as minimal estimates for intracellular Al. Total accumulation of intracellular Al for each region was 60, 73, and 210 nmol g-1 fresh weight after 4 h, increasing with root development. Early metabolic responses to external Al, including those that have been reported deep inside the root and in mature regions, might result directly from intracellular Al. These responses might include ion transport events at the endodermis of mature roots or events associated with lateral root emergence, as well as events within the root tip.

Characterization of silanol reactivity and acidity on octadecyl-bonded chromatographic supports by29Si solidstate nuclear magnetic resonance spectroscopy and surface titration

Abstract Surface analysis and chromatographic data are used to examine the structure of surface groups in a reversed-phase chromatographic environment involving an octadecyl-derivatized silica substrate. The support is characterized by 29 Si solid-state nuclear magnetic resonance spectroscopy and surface titration, and results are compared to other spectroscopic methods for the quantitation of surface species. Quantitative nuclear magnetic resonance data permits examination of the number and reactivity of single versus geminal hydroxyl sites as a function of octadecyldimethylsilyl coverage. It is postulated that an acidic, reactive subset of surface silanols exists that includes a large proportion of the geminal silanol sites, and a subset of the single silanol sites including hydrogen-bonded species such as the vicinal silanols. Knowledge of surface structure helps to explain various types of chromatographic behavior including the inaccessibility of some surface region silanols at moderate silane coverages, the partition-like behavior of octadecyl substrates, and the pH-dependent retention of non-polar solutes in reversed-phase chromatography.

A microcomputer based digital imaging system for ion microanalysis

A microcomputer based digital imaging system was developed for a Cameca IMS‐3f ion microscope permitting real‐time digital acquisition of secondary ion images. Image signal‐to‐noise enhancement results from random noise reduction by real‐time ensemble averaging and from a reduction of pattern noise in the charge injection device (CID) array by subtraction of blank frames. Acquired images comprise 244times248 pixel arrays with 8‐bit intensity resolution. Images are displayed on a colour monitor in a grey scale or pseudo‐colour using one of four programmable lookup tables. Image processing software permits off‐line ion image enhancement and manipulation as well as multitechnique digital image correlations. System capability is illustrated by a biological example involving digital imaging studies of Al distribution in osteomalacic bone tissue, including correlative light microscopy and ion microanalysis.

Removing sulfur from gold using ultraviolet/ozone cleaning

Irradiating surfaces with ultraviolet ~UV! light has been demonstrated to be an effective means of eliminating surface contamination from a variety of substrates. The mechanism of this UV cleaning procedure involves UV excitation of surface species and conversion of molecular oxygen to ozone and atomic oxygen. These strong oxidizers then decompose UV excited organic surface contaminants to volatile groups such as CO, CO2, N2, etc. that can desorb, and inorganics are converted to highly oxidized states that can be readily removed by rinsing with ultrapure water. While solvent cleaning protocols are effective in some applications, contaminants are often only partially removed, and impurities present in the cleaning solvents can be introduced. The UV/ozone method is primarily a dry process that cleans substrates more rigorously than solvent methods and only requires ultrapure water for removing certain inorganics. Thus, the use of excess and often caustic solvents is avoided, and the possibility of contamination from impurities introduced by these solvents is eliminated as well. UV/ozone cleaning also contains advantages over the conventional dry method of ion plasma cleaning. Sample ion bombardment in plasma cleaning can induce surface roughening at the atomic scale. However, the UV/ozone process only affects adsorbed surface species and leaves an inert substrate such as gold intact. This is advantageous for scanning microscopy studies where atomically flat surfaces are often needed. Although numerous studies have examined the effectiveness of the UV/ozone method in cleaning semiconductors such as silicon, relatively few have investigated UV/ozone treating metal surfaces, and these studies only concern the elimination of hydrocarbons. In recent years the selfassembly of sulfur-containing molecules via strong sulfur– gold bonding such as alkanethiols on gold has become a topic of considerable interest, and characterizing the sulfur from such thiol monolayers is often beneficial in understanding monolayer properties. Because sulfur readily adheres to gold, however, freshly deposited gold samples exposed to ambient conditions are easily contaminated with any sulfur compounds present in the laboratory air. Attempts by the authors to remove such sulfur contaminants with solvents including nitric acid and piranha solution were unsuccessful. Thus, a noninvasive means of providing gold substrates free of sulfur contaminants prior to thiol self-assembly is needed. Gold bound alkylthiolates irradiated with UV light have been shown to convert to alkylsulfonates. Recently, this idea has been utilized to photopattern self-assembled ~SA! alkanethiol monolayers on gold in which spatially patterned sulfonates formed from thiolate UV exposure are rinsed from

A Direct Deposition Method for Coupling Matrix‐assisted Laser Desorption/Ionization Mass Spectrometry with Gel Permeation Chromatography for Polymer Characterization

Matrix-assisted laser desorption/ionization (MALDI) measurements of polymeric materials of narrow polydispersity provide molecular weight measurements that agree closely with those from conventional tools such as gel-permeation chromatography (GPC). However, materials that have a broader molecular weight distribution (polydispersity) > 1.1) have been shown to diminish the accuracy of the MALDI results. In a recent effort to overcome this limitation, some researchers have utilized GPC prior to MALDI analysis to collect fractions with narrower molecular weight distributions. In our research, the GPC eluent was spray deposited onto a rotating matrix-coated substrate and the resulting polymer ‘trail’ was characterized directly by MALDI. Specifically, molecular weight information was determined using various points along poly(methyl methacrylate) sample ‘trails’ deposited on a matrix of trans-3-indoleacrylic acid. The overall number-average molecular weight (M n) and molecular weight distribution (MWD) for each of the samples were calculated. The Mn values are in fairly good agreement with manufacturers values, though the MWD values determined by MALDI are narrower.

The role of secondary ion mass spectrometry (SIMS) in biological microanalysis: technique comparisons and prospects

The virtues and limitations of SIMS ion microscopy are compared with other spectroscopic techniques applicable to biological microanalysis, with a special emphasis on techniques for elemental localization in biological tissue (electron, X‐ray, laser, nuclear, ion microprobes). Principal advantages of SIMS include high detection sensitivity, high depth resolution, isotope specificity, and possibilities for three‐dimensional imaging. Current limitations, especially in comparison to X‐ray microanalysis, center on lateral spatial resolution and quantification. Recent SIMS instrumentation advances involving field emission liquid metal ion sources and laser post‐ionization will help to minimize these limitations in the future. The molecular surface analysis capabilities of static SIMS, especially with the new developments in commercial time‐of‐flight spectrometers, are promising for application to biomimetic, biomaterials, and biological tissue or cell surfaces. However, the direct microchemical imaging of biomolecules in tissue samples using SIMS will be hindered by limited concentrations, small analytical volumes, and the inefficiencies of converting surface molecules to structurally significant gas phase ions. Indirect detection using elemental or isotopically tagged molecules, however, shows considerable promise for molecular imaging studies using SIMS ion microscopy.

Transmission near-infrared technique for evaluation and relative quantitation of surface groups on silica

Abstract A qualitative and quantitative infrared method for surface group determination on silica gel is presented. Carbon tetrachloride is used to reduce scattering so that the sample transmits light throughout the region of interest (near-infrared: 8000-4000 cm−1. The technique is shown to be useful in studying water adsorption, 2H2O exchange and trimethylchlorosilane reaction with silanol groups on the silica surface.

Determination of chromium speciation in environmental particles. Multitechnique study of ferrochrome smelter dust

The chemical speciation of particulate Cr has a significant influence on its environmental impact, with Cr/sup 6 +/ compounds of greatest current concern. A novel analytical regimen was evaluated for the determination of the concentration, speciation, and potential bioavailability of environmental Cr by using the example of ferrochrome smelter dust. Various wet chemical extractions were compared for possible use in the routine environmental monitoring of bioavailable Cr and Cr/sup 6 +/. Roughly half of the total Cr in the primary smelter dust was extractable by routine acid/base leaching (bioavailable), of which about 40% was Cr/sup 6 +/. The Cr/sup 6 +/ exists as Cr/sub 2/O/sub 7//sup 2 -/ or CrO/sub 4//sup 2 -/ and predominates in submicron particles probably formed during smelting. The remainder of the Cr is primarily insoluble Cr/sub 2/O/sub 3/ which is located in large particles similar to the original chromite ore.