ASBESTOS by TEM: METHOD 7402, Issue 2, dated 15 August 1994 - Page 6 of 7 20.
Counting and Sizing: a. Insert the sample grid into the specimen grid holder and scan the grid at zero tilt at low magnification (ca. 300 to 500X). Ensure that the carbon film is intact and unbroken over ca. 75% of the grid openings. b. In order to determine how the grids should be sampled, estimate the number of fibers per grid opening during a low-magnification scan (500 to 1000X). This will allow the analyst to cover most of the area of the grids during the fiber count and analysis. Use the following rules when picking grid openings to count [7,8]: (1) Light loading (<5 fibers per grid opening): count total of 40 grid openings. (2) Moderate loading (5 to 25 fibers per grid opening): count minimum of 40 grid openings or 100 fibers. (3) Heavy loading (>25 fibers per opening): count a minimum of 100 fibers and at least 6 grid openings. Note that these grid openings should be selected approximately equally among the three grid preparations and as randomly as possible from each grid. c. Count only grid openings that have the carbon film intact. At 500 to 1000X magnification, begin counting at one end of the grid and systematically traverse the grid by rows, reversing direction at row ends. Select the number of fields per traverse based on the loading indicated in the initial scan. Count at least 2 field blanks per sample set to document possible contamination of the samples. Count fibers using the following rules: (1) Count all particles with diameter greater than 0.25 µm that meet the definition of a fiber (aspect ratio ≥3:1, longer than 5 µm). Use the guideline of counting all fibers that would have been counted under phase contrast light microscopy (Method 7400). Use higher magnification (10000X) to determine fiber dimensions and countability under the acceptance criteria. Analyze a minimum of 10% of the fibers, and at least 3 asbestos fibers, by EDX and SAED to confirm the presence of asbestos. Fibers of similar morphology under high magnification can be identified as asbestos without SAED. Particles which are of questionable morphology should be analyzed by SAED and EDX to aid in identification. (2) Count fibers which are partially obscured by the grid as half fibers. NOTE: If a fiber is partially obscured by the grid bar at the edge of the field of view, count it as a half fiber only if more than 2.5 µm of fiber is visible. (3) Size each fiber as it is counted and record the diameter and length: (a) Move the fiber to the center of the screen. Read the length of the fiber directly from the scale on the screen. NOTE 1: Data can be recorded directly off the screen in µm and later converted to µm by computer. NOTE 2: For fibers which extend beyond the field of view, the fiber must be moved and superimposed upon the scale until its entire length has been measured. (b) When a fiber has been sized, return to the lower magnification and continue the traverse of the grid area to the next fiber. d. Record the following fiber counts: (1) fs, f b = number of asbestos fibers in the grid openings analyzed on the sample filter and corresponding field blank, respectively. (2) Fs, F b = number of fibers, regardless of identification, in the grid openings analyzed on the sample filter and corresponding field blank, respectively.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
