Phase Contrast Optical Microscopy Research Articles

An environmental survey in chrysotile asbestos milling processes in India.

Environmental monitoring to determine airborne asbestos fiber levels has been carried out in four different mills processing chrysotile asbestos in the Cuddapah District (Andhra Pradesh) of India. The "membrane filter method" comprising standard asbestos sampling techniques, acetone-triacetin method for sample preparation, fiber counting, and sizing using the phase contrast optical microscope were adopted in the study. Fiber concentrations both with respect to personal exposures and processing areas were found in most of the cases to be much higher than the prescribed Threshold Limit Value (TLVs) of the developed and developing countries for chrysotile asbestos. By optical microscopy, fiber length distribution showed 70% of fibers in the milling processes were in size range > 5-10 microns, whereas in > 10-20 and > 20 microns, 20% and 8%, respectively. Fiber identification for major elemental content, also done by using scanning electron microscope equipped with an energy dispersive X-ray analyzer, indicated the presence of tremolite along with chrysotile. The study stresses the urgent need to adopt suitable engineering controls at the dust generating sources to reduce the fiber level in the mill environment below the threshold limit.

Construction of a Portable Fiber Monitor Measuring the Differential Light Scattering from Aligned Fibers

This report describes experimental work in which three separate physical techniques have been combined to produce a portable monitor for airborne fibers. Air containing the fibers to be monitored is drawn through a rectangular, slotted inlet into the instrument at 4 L/min using a battery pump. The flow acceleration at this inlet aligns the fibers. Corona charging then enables the fibers to be precipitated onto a standard microscope slide while retaining their alignment. The difference in light scattering along and at right angles to this deposit is used to determine the concentration of fibers. Calibration of the instrument is achieved by counting the fibers on the slide by phase-contrast optical microscopy for a range of concentrations. A backup filter traps any fibers that escape the corona charging and precipitation region, and this filter can be analyzed by electron microscopy so that the fibers can be identified. Performance data are presented for fibers and compact particles sampled from a laborator...

Reaction kinetics in continuous silicon carbide reinforced titanium 15V-3Cr-3Al-3Sn

The reaction kinetics of a silicon carbide (SCS-6) reinforced titanium 15V-3Cr-3Al—3Sn composite have been characterized using a combination of phase contrast optical microscopy and scanning electron microscopy. The matrix is an all beta alloy that is a potential substitute for the more commonly used Ti-6A1-4V because it can be cold rolled into thin foils without expensive intermediate anneals. The composite panel was fabricated via diffusion bonding (Textron Specialty Materials) and was composed of eight plies [0°]8.. Samples were annealed under vacuum at temperatures ranging from 600°C to 950°C for periods of 4 to 96 hr. For a given temperature, the zone thickness was observed to increase linearly as a function of the square root of time. The data fit an Arrhenius type relationship where the slope of the Ink versus 1/T curve represents the activation energy for the process. Values for the activation energy,Q, are presented and compared for the 15-3 alloy, 6A1-4V, and other titanium matrix alloys.

STUDY OF THE COUNT-TO-MASS CONVERSION FACTOR FOR ASBESTOS FIBRES IN SAMPLES COLLECTED AT THE EMISSIONS OF THREE INDUSTRIAL PLANTS

Count-to-mass conversion factors for asbestos fibres in the stack emissions of three industrial plants were calculated. Dust samples deriving from different working processes were collected downstream from the fabric-based filtration systems. Gravimetric measurements were carried out utilizing an X-ray diffraction procedure based on the use of a silver filter that permits the correction for X-ray absorption by the sample. The number of fibres covered by regulations ('Regulated fibres') (F) was counted by phase-contrast optical microscopy (PCOM) on a section of the same filter as had been used for gravimetric measurements. The values of the conversion factors vary widely, ranging from 0.4 x 10(3) to 4.9 x 10(3) fibers per microgram (F microgram -1) according to the working process involved. They are always less than 20 x 10(3) F microgram -1, the value specified in the EEC Directive 217/87 concerning the prevention of asbestos pollution in the environment.

Controlling crosslink density of coreactive polymers in an extruder reactor

AbstractA common technique for the manufacture of polymer alloys is melt blending of polymers with specific chemical interactions. For instance, an amido ester crosslink forms when oxazoline‐grafted PS is mixed with acid‐grafted PE. The crosslink builds melt viscosity and stabilizes product morphology. Reaction control is critical since overreacted alloys from gels and underreacted alloys have gross composition gradients and/or tend to phase coalesce so that product properties are variable. Polymer reactions in plasticating compounders are particularly difficult control problems since several unit operations involving plastication, heat exchange, mixing, and separating of by‐products must be sequenced properly to control a reaction. Reactive polystyrene (OPS) and reactive polyethylene (APE) with oxazoline and carboxylic acid functionality, respectively, were melt blended in a ZSK compounding extruder under a variety of conditions. The extent of crosslink was monitored by the strong shear viscosity increase of the reacted product, the infrared absorption by the crosslink, and reactant depletion in the melt. The phase morphology was characterized by optical phase contrast microscopy. A response surface of product shear viscosity vs. the primary operational parameters of extrusion rate and extruder RPM is interpreted in terms of characteristic reaction time and development of reactive interfaces during the mixing process. Response surfaces result from storing coordinates and responses while exercising a new feedback loop control system.

Miscibility behaviour of the system [formula omitted

The miscibility behaviour of blends of polystyrene and poly( p-methylstyrene) was studied by differential scanning calorimetry and phase-contrast optical microscopy. Phase diagrams have been constructed for blends of different molar masses of the constituents. These blends exhibit upper critical solution temperature type of demixing, which is a rare phenomenon for high-molar-mass polymer blends. The Holes-Huggins theory, closely related to the Simha-Somcynsky hole theory, predicts this type of phase behaviour for this particular system in a semi-quantitative way. Theoretical predictions are made for the influence of pressure on the miscibility behaviour and for the excess volume and enthalpy for this blend.

Influence of thermal annealing in air on the electro-optic characteristics of chemical bath deposited non-stoichiometric cadmium zinc selenide thin films

Non-stoichiometric thin films of cadmium zinc selenide were grown onto glass substrates by the chemical bath deposition technique from elemental selenium, sodium sulphite, cadmium chloride, zinc chloride, deionised water and ammonia. As-deposited films were annealed at different temperatures up to 600 K. From their deposition temperature in air, their electrical conductivity, optical properties and microstructure were evaluated. The electrical conductivity increases rapidly with increasing annealing temperature and activation energy EA for the crystallites was calculated from the Arrhenius relation. The optical absorption edge is shifted towards higher wavelengths with increasing annealing temperature. Other optical parameters such as absorption coefficient alpha , band gap Eg, refractive index n, dielectric constant epsilon and extinction coefficient k were computed from transmittance spectra. The re-growth study was also carried out from micrography obtained from optical phase contrast microscopy at different annealing temperatures and results are interpreted on the basis of grain boundaries, defects and vacancies. The fully annealed films have a hexagonal crystal structure for x<or=0.4 and a cubic structure for x>or=0.6 with varying lattice constants and grain size.

High-Resolution Bright-Field and Dark-Field Hollow Cone Illumination

Hollow cone illumination using a large C2 blocked-aperture (bl apt) in the conventional TEM (CTEM) can remove the beams within the zero-order Laue zone (ZOLZ) thereby making lattice images more simply interpretable. Dark-field (DF) hollow cone illumination has the added advantage of enhancing the Z-contrast within the lattice image, since the electrons contributing to the image must be scattered over a large angle (approximately 10 mrad). Both of these imaging methods have been explored, using a 600 um C2 bl apt and objective aperture sizes of 70, 20 and 10 um, and are reported in this paper.Much interest has been generated by the report of Pennycook [1] on STEM Z-contrast imaging using annular dark-field. In earlier work ,it was noted that CTEM hollow cone imaging and STEM annular dark-field imaging are related via reciprocity [2] (Fig. 1). In addition, Zernike has shown the advantages of hollow cone illumination in optical phase-contrast microscopy [3]. The electron-optical analogues to these optical techniques are now possible because of the low Cs values achieved in modern TEMs.

Comparison of Phase Contrast and Electron Microscopic Methods for Evaluation of Occupational Asbestos Exposures

Abstract Historic air samples collected in the asbestos textile, the friction products, and the cement products industries using chrysotile asbestos during 1964–1971 were analyzed by counting and sizing asbestos fibers by phase contrast optical microscopy and transmission electron microscopy. The proportion of airborne fibers longer than 5 μm in length was found to be significantly different for different industries and operations with a range of 5.0–20.4 percent. Multiple linear regression analyses relating fiber concentrations by phase contrast and electron microscopy methods demonstrated a good correlation for fibers longer than 5 μm. The phase contrast method was found to be a poor predictor of total fiber exposures (all lengths) as measured by transmission electron microscopy. Airborne fibers in the textile industry were found to have longer lengths than those in the cement pipe or the friction products industries. These differences may be important when comparing morbidity and mortality data from di...

IMAGO: a complete system for acquisition, processing, two/three-dimensional and temporal display of microscopic bio-images

This work describes IMAGO, an integrated bio-imaging system developed in our laboratory. The whole system consists of a personal computer, a commercially available frame grabber directly plugged into a personal computer, video input/output modules, specific hardware for z-axis movement and light shuttering, and a software package. IMAGO is user-friendly, menu driven and enables one to perform image acquisition with different methods: optical sectioning, flashing epifluorescence, transmitted and phase contrast microscopy. It makes various functions possible, including: image transfer, gray scale processing, conventional and advanced filtering, logical operations, look-up table management, three-dimensional (3D) editing, 3D representation and auto-correlation techniques. More than 100 image processing functions have been implemented and can be easily managed through IMAGO. Examples are given in the area of biophysical research, like 3D representation of nuclei and of electron microscopic images, in situ microscopy of living cells. IMAGO processes information in an x, y, z, t space.

Fiber exposure reassessed with the New Indices

The concentration of airborne fibers longer than 5 microns, thinner than 3 microns, and with an aspect ratio exceeding 3 as counted by phase contrast optical microscopy is the most widely used fiber exposure index. Recently, more adequate, specific exposure indices for asbestosis, lung cancer, and mesothelioma risk have been suggested by Lippmann (1988, Environ. Res., 46, 86-106). The consequences of using these indices are examined on the basis of calculations for a broad range of theoretical and published size distributions. Optical microscopy appears to be a good predictor of the exposure indices for asbestosis and for lung cancer after scaling. Only fibers longer than about 3 microns need to be counted in a transmission electron microscope. The lung cancer index still cannot explain the large differences of risk among chrysotile exposures. Both the mesothelioma exposure index and the ratio mesothelioma to lung cancer index ranks in order of increasing risk: wollastonite, glass and mineral wool, amosite, glass microfibers, chrysotile, and crocidolite. Amosite is thus not ranked according to epidemiological evidence. Detailed size information should be made available so that the size criteria can be adjusted. It may still prove necessary to use fiber type specific concentration limits.

Levels of airborne man-made mineral fibres in U.K. dwellings. II—Fibre levels during and after some disturbance of loft insulation

In this study airborne levels of MMMF were determined in 11 dwellings during and after disturbance of loft insulation. The disturbance was in two stages. 1. (a) Minor disturbance. Household items (such as suitcases) stored in the lofts were moved around for about 10 min. The disturbance to the insulation material was kept to a minimum. 2. (b) Major disturbance. Insulation wools were physically handled for about 30 min, simulating the disturbance associated with activities such as rewiring or plumbing. About 250 samples of air were collected on membrane filters and analyzed by transmission electron microscopy (TEM) in conjunction with energy dispersive X-ray analysis. A few selected samples were analyzed by phase contrast optical microscopy (PCOM). During physical disturbance of the insulation, fibre levels in lofts measured over a 4-h period were up to 0.03 f ml −1. Personal exposure during the disturbance activity was up to 0.2 f ml −1. Contamination of the living space was not detected even though the access door to the loft remained open throughout the disturbance and sampling period.

Levels of airborne man-made mineral fibres in U.K. dwellings. I—Fibre levels during and after installation of insulation

This paper gives the details of airborne levels of man-made mineral fibres (MMMF) found during installation of loft insulation in 12 dwellings. About 250 samples of air were collected and analyzed by transmission electron microscopy (TEM). Respirable fibre levels measured in static samples collected in the lofts during installation generally were < 0.1 f ml −1. In living spaces respirable fibre levels were< 0.006 f ml −1. Personal samples on the installers gave < 1 f ml −1, with the exception of a fine glass fibre blanket. A few selected samples were also analyzed by phase contrast optical microscopy (PCOM) which showed lower values than those recorded by TEM. Static gravimetric concentrations in the lofts were in the 0.3–6.5 mg m −3 range, and in the living spaces 0.11–0.44 mg m −3, but in both environments most of this dust was not MMMF.

CHARACTERIZATION OF AIRBORNE DUST IN A SOFT PAPER MILL

The characteristics of airborne dust in a soft paper production plant have been characterized by scanning and transmission electron microscopy. A combination of X-ray diffraction, electron diffraction and energy-dispersive X-ray flouresence spectroscopy (EDX) was used to determine the structure and composition of the different components. Size distribution determination and phase identification were carried out. Besides the cellulose fibres, fibres of kaolinite, wollastonite, talc and other silicates were also identified. Gravimetric analysis and fibre counting by optical phase contrast microscopy were used to determine total dust and fibre concentrations. Total dust exposure at the plant was generally below 3 mg m-3. The respirable fraction of the total dust concentrations varied from 15 to 70%. The inorganic dust was 36 +/- 15% of the total dust. The ratio of inorganic fibres to total fibre concentration at the plant varied between 10 and 15%.

Experimental studies of misfit dependence of critical layer thickness in pseudomorphic InGaAs single-strained quantum-well structures

The dependence of critical layer thickness on a misfit in pseudomorphic GaAs/InGaAs/GaAs single quantum-well structures grown by molecular-beam epitaxy is studied using a phase-contrast optical microscope and photoluminescence spectroscopy. The results show that the critical layer thickness as a function of misfit follows more closely with Matthews and Blakeslee’s mechanical equilibrium model [J. Cryst. Growth 27, 118 (1974)] than People and Bean’s energy balance model [Appl. Phys. Lett. 49, 229 (1986)]. However, the observed critical layer thickness is slightly less than that predicted by the mechanical equilibrium model for single quantum-well structures.

Deuterated polystyrene-poly(α-methylstyrene) blends: range of miscibility and determination of the interaction parameter by small angle neutron scattering

The range of miscibility of deuterated polystyrene-poly(α-methylstyrene) blends (PSD-PαMS) as a function of the molecular weights of the two polymeric components has been established by differential scanning calorimetry and phase contrast optical microscopy. The analysis of small angle neutron scattering measurements (1) in terms of an apparent radius of gyration, (2) from the intensity of scattering extrapolated to zero angle and (3) from the method which consists in replacing part of the PSD chains by PSH chains has allowed the determination of the dimensions of deuterated polystyrene dispersed in poly(α-methylstyrene) and of the interaction parameter χ between the two polymers ( χ=5.0 × 10 −3).

THE USEFULNESS OF VOID-COUNTING FOR FIBRE CONCENTRATION ESTIMATION BY OPTICAL PHASE CONTRAST MICROSCOPY

The void-counting technique for evaluation of the concentration of airborne respirable fibrous dust by phase contrast optical microscopy is assessed with respect to accuracy, reproducibihty, ease and saving of counting time compared with the conventional method of fibre counting. The method is found to have several advantages over the latter. These are outlined. Based on our own data and those of 10 other Swedish laboratories, it is concluded that more reliable and accurate estimates are obtained in the routine evaluation of airborne fibrous dust concentrations by using the void-counting method, provided the sampling is conducted properly

An investigation of airborne asbestos concentration in two UK buildings: before, during and after the removal of asbestos

Some 185 air samples were taken in two buildings before, during and after the removal of sprayed/trowelled amosite asbestos insulation from the ceiling void. The samples were analysed by optical phase contrast microscopy (PCM) and transmission electron microscopy (TEM) for fibres > 5 μm long. The analytical capabilities of the TEM were used to identify the fibres counted, and at higher magnifications to count asbestos fibres of all lengths. Aggressive sampling methods were applied after the asbestos had been removed and the increased fibre concentrations produced from the settled dusts (x7.8‐X69) were a function of the degree of disturbance.

Comparison of the Size Distributions of Occupational Asbestos and Man-Made Mineral Fibres Determined By Transmission Electron Microscopy

Abstract The transmission electron microscope has been used to determine the size distributions of airborne chrysotile, and a range of man-made mineral fibres in the workplace. Personal samples and static samples have been collected on membrane filters and prepared for the transmission electron microscope using a direct transfer technique. Measurement of fibre length and diameter has been made directly from the screen using a novel light-spot technique linked to a microcomputer. Phase contrast optical microscopy has also been used on selected samples. Size distributions from bulk samples have also been obtained for comparison on two synthetic materials routinely used in industry. One particular class of man-made fibres has a diameter distribution similar to asbestos, and these fibres have been termed ‘superfine’. A fibre exposure limit based on the phase-contrast optical microscope is being introduced for this group in the United Kingdom. A comparison of the transmission electron microscope results with those from the optical microscope for a range of airborne samples taken in industry is presented.

Application of Fibre Retention and Carcinogenicity Curves to Fibre Size Distributions of Asbestos

Abstract The use of a direct method of sample preparation for transmission electron microscopy (TEM) analysis has allowed accurate measurement of airborne asbestos fibre size distributions. In this paper, a fibre size dependent model for the carcinogenic potency of fibres by POTT (1978) and another for lung retention by TIMBRELL (1982) have been applied to asbestos size distributions from the occupational and non-occupational environment. The results have been used to assess the effectiveness of current optical phase contrast microscope (PCM) and scanning electron microscope (SEM) procedures. A wide spread of values was found for airborne distributions, with the PCM analysis measuring about 70–80% of the potential carcinogenic hazard for amosite and 20–60% for chrysotile and crocidolite.