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M K Lamvik

Publications and source records attributed to M K Lamvik.

12 recordsLinked to original sources
PubMed1991

Temperature directly affects the rate of irradiation-induced mass loss from phosphatidylcholine multilayers.

We monitored the mass thickness of egg yolk phosphatidylcholine multilayers at several temperatures during electron irradiation. The rate of irradiation-induced mass loss was reduced substantially when this specimen was cooled to liquid nitrogen temperature from room temperature. Additional cooling to liquid helium temperature caused an additional reduction of mass-loss rate. The characteristic doses D(1/e), which are the slopes of the logarithm of the differential mass thickness against dose, were approximately 7 x 10(3) e/nm2 at 290 K, 8 x 10(4) e/nm2 at 130 K, and 1.4 x 10(5) e/nm2 at less than 10 K. The fractions of the original mass thickness that remained after arbitrarily high doses were about 69% at 290 K, 72% at 130 K, and 77% at less than 10 K.

Animals↗
PubMed1989

Calibration methods for quantitative image processing in electron microscopy.

An image can be represented digitally as a matrix of numbers. When those numbers are linearly related to a property of the object, such as mass per unit area, a simple integration of an image area leads to a total of that property, such as the mass of a particle that is represented in a selected area. Following techniques pioneered by Bahr and Zeitler, we illustrate the use of photographic densitometry of films exposed in an electron microscope to measure electron scattering. The transmission of an electron micrograph will be linear with respect to mass thickness for a particular value of background brightfield density, hence allowing determination of the mass of microscopic particles. We show here a digital computer method for conveniently establishing the linear condition by quantitative image processing using micrographs of polystyrene spheres. The method also serves to produce calibration curves for cases where the transfer from transmission to mass thickness is not linear. We also illustrate how an inexpensive computer is used to display and integrate regions of micrographs to determine particle mass.

Calibration↗
PubMed1989

Substrate properties affect the mass loss rate in collodion at liquid helium temperature.

A previous measurement showed that mass loss from collodion supported by thin carbon films was linear with electron exposure at liquid helium temperature. No other organic solid had shown a linear loss of mass at any temperature. When measurements of collodion were done using titanium supports, the loss of mass proceeded exponentially with exposure at liquid helium temperature. This result suggested that the differing electrical conductivities of these substrates might be the cause of the different mass loss effects. Carbon films, which are typically used at ambient temperatures, have much lower electrical conductivity at very low temperature than titanium films. This suggested that specimen preparation materials and techniques used routinely for room temperature studies may need to be modified when microscopy is done using superconducting objective lenses. For both substrates, the rate of mass loss is slowest at liquid helium temperature.

Carbon↗
PubMed1989

Correction for specimen movement after acquisition of element-specific electron microprobe images.

Because a long time is generally required to generate X-ray maps of specific elements by electron beam methods, images are subject to a loss of resolution due to stage movement. Methods have been previously described for correcting stage drift during exposure by sensing the drift and deflecting the beam to follow the stage; but these methods require modifications of the equipment. When the drift is not excessive, it is possible to correct a series of images after the exposure series is finished. Here we demonstrate two methods for correcting the drift, one based on manual assignment of specimen position and one on the use of cross-correlation functions to determine objectively the misalignment of images in the series. The success of the methods is illustrated in calcium-specific images of a bone section that show the collagen periodicity after drift correction.

Bone and Bones↗
PubMed1988

Dynamic hydration effects in an electron microscope cold stage.

Water can be a substantial proportion of the residual gas in modern electron microscopes even when frozen hydrated specimens are not used. During measurements of the mass thickness of thin collodion film specimens at low temperatures, it was found that a volatile surface layer (condensed water) modified the apparent rate of mass loss induced by radiation exposure. Mass loss can be enhanced by the presence of water (specimen "etching"), or mass loss can be masked by the dynamic adsorption of water to the specimen surface. The microscope or the grid can be a secondary source of the water; even with cold anticontaminator plates in the vicinity of the specimen, water can be desorbed by x-rays or backscattered electrons. In one typical situation, the mass loss rate appears reduced (due to water adsorption), but the ultimate damage is greater (due to etching). These results illustrate that care must be taken in interpreting mass thickness measurements made in the presence of water and that the lowest stage temperature does not necessarily produce the best observation conditions for all specimens.

Freezing↗
PubMed1987

Mass loss rate in collodion is greatly reduced at liquid helium temperature.

The mass thickness of collodion films has been monitored at several temperatures, under conditions typical of electron microscopy, through the use of an electron energy loss spectrometer. Compared to room temperature, only a five-fold reduction in the rate of mass loss was observed through the use of a commercial liquid nitrogen cooled stage; in contrast, the rate of mass loss was reduced more than one hundred fold when these films were held at liquid helium temperature.

Collodion↗
PubMed1976

Electronmicroscopic mass determination using photographic isodensity techniques.

Using Agfacontour, Kodalith, or similar films, it is possible to convert an electron micrograph into a map of regions of equal photographic density (isodensitites) by simple copying procedures. If images of polystyrene spheres and a calibrated density wedge are included in the copies, it is possible to quantitatively calibrate the isodensities in terms of mass per unit area of the specimen. In this way, electronmicroscopic mass determination may be accomplished without the need of densitometers or other photometric equipment.

Animals↗
PubMed1976

Minimization of dose as a criterion for the selection of imaging modes in electron microscopy of amorphous specimens.

A fundamental limitation in electron microscopy of organic specimens is radiation damage by the electron beam. To minimize damage it is necessary to have maximum information collection for a given dose. Various modes of operation of conventional and scanning transmission microscopes are compared with respect to their ability to detect small changes in specimen thickness or density with a given signal to noise ratio. Incoherent imaging is assumed, and this is expected to apply to amorphous specimens under a variety of microscope conditions. For either very thin or very thick specimens, the scanning transmission microscope is found to require nearly 10 times less dose than a conventional microscope for the same signal to noise ratio in the image. For specimens of intermediate thickness, scanning and conventional transmission electron microscopes are roughly equivalent.

Electrons↗
PubMed1975

High resolution scanning electron microscopy at the subcellular level.

Recently developed scanning electron microscopes provide sufficient resolution to allow useful observation of subcellular biological objects. Preparation methods for such objects need not be limited to the traditional coating and mounting procedures. Many methods developed for transmission electron microscopy are immediately adaptable to scanning electron microscopy. We show that a number of techniques are available to the microscopist which yield adequate contrast and high resolution. As examples we show skeletal muscle myofibrils dispersed to reveal thick filaments, uncoated on a thin carbon film; a tropomyosin tactoid, negatively stained with uranyl acetate; oncornavirus, conventionally coated; and T4 bacteriophage on an aluminium substrate.

Animals↗