Counting, microscopic

A representative sample of the coarse wet-sieved fraction (>125 pm) is mounted on a labeled, glass microscope slide with epoxy resin, having an index of refraction (n) of 1.55 to 1.56, or balsam fir resin n = 1.537) as described below, and the limestone particles are counted microscopically to determine their percentage. Counting techniques are given additional discussion later in this chapter. 

Midgley, H.G., and Dharmadhikari, P.V., "The Point Counting Microscopic Method for the Quantitative Determination of the Silicate Phases in Portland Cement Clinker," Building Research Current Papers, Research Series 13, Building Research Station, London, England, 1964,7 pp. 

Figure 7 Mean infection scores of P. ctm nti cysts in homogenates of lung tissues in our four-day cyst reduction model (41). Immunosuppressed rats were allowed to develop heavy P. carinii infections, they were treated with LY303366 for up to 4 days, and the numbers of cysts in the lungs were counted microscopically. Treatment groups were A, animah receiving a single 10 mg IV dose 4 days prior to necropsy B, animals receiving a single 5 mg/kg IV dose 4 days prior and daily 5 mg/kg oral doses on days 1—4 prior to necropsy C, animals receiving daily oral doses on days 1-4 prior to necropsy. Infected control (Inf. cont.) animals received no therapy for P. carinii pneumonia.

Computerized tomography (CT), different kinds of porosimeter and point counting microscope are used in our experiments. 

The excess heat of solution of sample A of finely divided sodium chloride is 18 cal/g, and that of sample B is 12 cal/g. The area is estimated by making a microscopic count of the number of particles in a known weight of sample, and it is found that sample A contains 22 times more particles per gram than does sample B. Are the specific surface energies the same for the two samples If not, calculate their ratio. 

The direct microscopic count determines the number of viable and dead microorganisms ia a milk sample. A small amount (0.01 mL) of milk is spread over a 1.0 cm area on a microscope sHde and allowed to dry. After staining with an appropriate dye, usually methylene blue, the sHde is examined with the aid of a microscope (oil immersion lens). The number of bacterial cells and clumps of cells per microscopic field is determined and, by appropriate calculations, is expressed as the number of organisms per milliliter of sample. 

With modern detectors and electronics most Enei -Dispersive X-Ray Spectroscopy (EDS) systems can detect X rays from all the elements in the periodic table above beryllium, Z= 4, if present in sufficient quantity. The minimum detection limit (MDL) for elements with atomic numbers greater than Z = 11 is as low as 0.02% wt., if the peaks are isolated and the spectrum has a total of at least 2.5 X 10 counts. In practice, however, with EDS on an electron microscope, the MDL is about 0.1% wt. because of a high background count and broad peaks. Under conditions in which the peaks are severely overlapped, the MDL may be only 1—2% wt. For elements with Z < 10, the MDL is usually around 1—2% wt. under the best conditions, especially in electron-beam instruments. 

Because X-ray counting rates are relatively low, it typically requires 100 seconds or more to accumulate adequate counting statistics for a quantitative analysis. As a result, the usual strategy in applying electron probe microanalysis is to make quantitative measurements at a limited collection of points. Specific analysis locations are selected with the aid of a rapid imaging technique, such as an SEM image prepared with backscattered electrons, which are sensitive to compositional variations, or with the associated optical microscope. 

Fiber counting A microscopic technique which is of particular relevance to asbestos, where the fibers are counted on a filter paper. 

Because physiological deterioration is generally accompanied by an increase in bacterial population, as pointed out by Nielsen, Wolford, and Campbell (33), estimation of bacterial numbers might serve as the basis of a test for condition. Obviously, the plate count method is not adaptable because of the time limitations imposed. Direct microscopic count would be much more appropriate, especially if a positive field count were substituted for cell count as suggested by Wolford (39). 

The instrument has been evaluated by Luster, Whitman, and Fauth (Ref 20). They selected atomized Al, AP and NGu as materials for study that would be representative of proplnt ingredients. They found that only 2000 particles could be counted in 2 hours, a time arbitrarily chosen as feasible for control work. This number is not considered sufficient, as 18,000 particles are required for a 95% confidence level. Statistical analysis of results obtained for AP was impossible because of discrepancies In the data resulting from crystal growth and particle agglomeration. The sample of NGu could not be handled by the instrument because it consisted of a mixt of needles and chunky particles. They concluded that for dimensionally stable materials such as Al or carborundum, excellent agreement was found with other methods such as the Micromerograph or visual microscopic count. But because of the properties peculiar to AP and NGu, the Flying Spot Particle Resolver was not believed suitable for process control of these materials... 

Another method is to count particles between a given range and then sum the counts as in 5.9.1. Alternately, one can purcheise an automatic peurticle counting instrument for about 50-60,000. The instrument consists of a microscope, a scanning device (usually a flying-spot sceumer), a television display and a pre-programmed microprocessor. All of the particles within a... 

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