A technique for the examination of polar ice using scanning electron microscopy

British Antarctic Survey, Cambridge, England, United Kingdom
Journal of Microscopy (Impact Factor: 2.33). 03/2002; 205(Pt 2):118-24. DOI: 10.1046/j.0022-2720.2001.00981.x
Source: PubMed


The microstructure and location of impurities in polar ice are of great relevance to ice core studies. We describe a reliable method to examine ice in the scanning electron microscope (SEM). Specimens were cut in a cold room and could have their surfaces altered by sublimation either before (pre-etching) or after (etching) introduction to the cryo-chamber of the SEM. Pre-etching was used to smooth surfaces, whilst etching stripped away layers from the specimen surface, aiding the location of particles in situ, and allowing embedded structures to be revealed. X-ray analysis was used to determine the composition of localized impurities, which in some cases had been concentrated on the surface by etching. Examining uncoated surfaces was found to be advantageous and did not detract from qualitative X-ray analysis. Imaging uncoated was performed at low accelerating voltages and probe currents to avoid problems of surface charging.

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Available from: Piers R. F. Barnes, Jun 09, 2015
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    • "These pressure– temperature conditions are in the water vapour field and ice is unstable and sublimes. The rate of sublimation is estimated to be about 0.5 µm min −1 at −100 • C and >1 µm min −1 at >–90 • C (Davy & Branton, 1970; Barnes et al., 2002; Waller et al., 2005). This sublimation rate would cause a loss of about 50 to 500 µm, respectively, from the sample surface over an 8–9 h period, which is typically the timescale required to characterize subgrain boundaries in a 1 cm 2 ice sample using EBSD. "
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    ABSTRACT: Naturally deformed ice contains subgrains with characteristic geometries that have recently been identified in etched surfaces using high-resolution light microscopy (LM). The probable slip systems responsible for these subgrain boundary types can be determined using electron backscattered diffraction (EBSD), providing the etch features imaged with reflected LM can be retained during EBSD data acquisition in a scanning electron microscope (SEM). Retention of the etch features requires that the ice surface is stable. Depending on the pressure and temperature, sublimation of ice can occur. The equilibrium temperature for a low pressure SEM operating at 1 × 10(-6) hPa is about -112°C and operating at higher temperatures causes sublimation. Although charging of uncoated ice samples is reduced by sublimation, important information contained in the etch features are removed as the surface sublimes. We developed a method for collecting EBSD data on stable ice surfaces in a low pressure SEM. We found that operating at temperatures of <-112°C reduced sublimation so that the original etch surface features were retained. Charging, which occurred at low pressures (<1.5 × 10(-6) to 2.8 × 10(-5) hPa) was reduced by defocusing the beam. At very low pressures (<1.5 × 10(-6) hPa) the spatial resolution with a defocused beam at 10 kV was about 3 μm in the x-direction at -150°C and 0.5 μm at -120°C, because at higher temperature charging was less and only a small defocus was needed to compensate it. Angular resolution was better than 0.7° after orientation averaging. Excellent agreement was obtained between LM etch features and EBSD mapped microstructures. First results are shown, which indicate subgrain boundary types comprised of basal (tilt and twist) and nonbasal dislocations (tilt boundaries).
    Journal of Microscopy 12/2010; 242(3):295-310. DOI:10.1111/j.1365-2818.2010.03471.x · 2.33 Impact Factor
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    • "The gold coating also resulted in suppression of Ca and Mg peaks compared with the background. Similar effects have been reported for characterization of dust particles [40], but were nevertheless overlooked in all previous reports on ferrography as well as in many papers dealing with bone and cartilage particles in general. Copper (Cu) coating was found to be a successful alternative, revealing S and P peaks that were otherwise masked (Fig. 1c). "
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    • "Wolff and Paren [12] suggested that the DC conductivity of polar ice could be due to the presence of liquid layers at the grain boundaries. In agreement with this idea, SEM-EDS studies have surmised the presence of sulfuric acid at the grain boundaries in Antarctic ice [13] [14] [15], and Fukazawa et al. [16] have found evidence for sulfuric acid at the triple junction of South Yamato and Nansen ice cores, Antarctica by micro-Raman spectroscopy . Rempel et al. [17] [18] suggested that the ions exist as acid solutions (sulfuric and nitric acid) displaced by premelting and anomalous diffusion through the vein network. "
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    ABSTRACT: We describe in detail our method of measuring the chemical forms of microparticles in polar ice samples through micro-Raman spectroscopy. The method is intended for solid ice samples, an important point because melting the ice can result in dissociation, contamination, and chemical reactions prior to or during a measurement. We demonstrate the technique of measuring the chemical forms of these microparticles and show that the reference spectra of those salts expected to be common in polar ice are unambiguously detected. From our measurements, Raman intensity of sulfate salts is relatively higher than insoluble dust due to the specific Raman scattering cross-section of chemical forms of microparticles in ice.
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