Page:NIOSH Manual of Analytical Methods - 7400.pdf/4

 ASBESTOS and OTHER FIBERS by PCM: METHOD 7400, Issue 2, dated 15 August 1994 - Page 4 of 15

collection area = 385 mm²) for optimum accuracy. These variables are related to the action level (one-half the current standard), (fibers/cc), of the fibrous aerosol being sampled by: . NOTE 1:	The purpose of adjusting sampling times is to obtain optimum fiber loading on the filter. The collection efficiency does not appear to be a function of flow rate in the range of 0.5 to 16 L/min for asbestos fibers [7]. Relatively large diameter fibers (>3 µm) may exhibit significant aspiration loss and inlet deposition. A sampling rate of 1 to 4 L/min for 8 h is appropriate in atmospheres containing ca. 0.1 fiber/cc in the absence of significant amounts of non-asbestos dust. Dusty atmospheres require smaller sample volumes (≤400 L) to obtain countable samples. In such cases take short, consecutive samples and average the results over the total collection time. For documenting episodic exposures, use high flow rates (7 to 16 L/min) over shorter sampling times. In relatively clean atmospheres, where targeted fiber concentrations are much less than 0.1 fiber/cc, use larger sample volumes (3000 to 10000 L) to achieve quantifiable loadings. Take care, however, not to overload the filter with background dust. If ≥50% of the filter surface is covered with particles, the filter may be too overloaded to count and will bias the measured fiber concentration. NOTE 2:	OSHA regulations specify a minimum sampling volume of 48 L for an excursion measurement, and a maximum sampling rate of 2.5 L/min [3]. 5.	At the end of sampling, replace top cover and end plugs. 6.	Ship samples with conductive cowl attached in a rigid container with packing material to prevent jostling or damage. NOTE:	Do not use untreated polystyrene foam in shipping container because electrostatic forces may cause fiber loss from sample filter. SAMPLE PREPARATION: NOTE 1:	The object is to produce samples with a smooth (non-grainy) background in a medium with refractive index ≤ 1.46. This method collapses the filter for easier focusing and produces permanent (1–10 years) mounts which are useful for quality control and interlaboratory comparison. The aluminum “hot block” or similar flash vaporization techniques may be used outside the laboratory [2]. Other mounting techniques meeting the above criteria may also be used (e.g., the laboratory fume hood procedure for generating acetone vapor as described in Method 7400—revision of 5/15/85, or the non-permanent field mounting technique used in P&CAM 239 [3,7–9]). Unless the effective filtration area is known, determine the area and record the information referenced against the sample ID number [1,9–11]. NOTE 2:	Excessive water in the acetone may slow the clearing of the filter, causing material to be washed off the surface of the filter. Also, filters that have been exposed to high humidities prior to clearing may have a grainy background. 7.	Ensure that the glass slides and cover slips are free of dust and fibers. 8.	Adjust the rheostat to heat the “hot block” to ca. 70 °C [2]. NOTE:	If the “hot block” is not used in a fume hood, it must rest on a ceramic plate and be isolated from any surface susceptible to heat damage. 9.	Mount a wedge cut from the sample filter on a clean glass slide. a.	 Cut wedges of ca. 25% of the filter area with a curved-blade surgical steel knife using a rocking motion to prevent tearing. Place wedge, dust side up, on slide. NOTE:	Static electricity will usually keep the wedge on the slide. b.	 Insert slide with wedge into the receiving slot at base of “hot block”. Immediately place tip of a micropipet containing ca. 250 µL acetone (use the minimum volume needed to consistently clear the filter sections) into the inlet port of the PTFE cap on top of the “hot block” and inject the NIOSH Manual of Analytical Methods (NMAM), Fourth Edition