Page:NIOSH Manual of Analytical Methods - 7601.pdf/3

 SILICA, CR YST ALLINE : MET HO D 760 1, Issue 3, dated 15 M arch 200 3 - Page 3 o f 6 SPECIAL PRECAUTIONS: Avoid inhaling silica dust [5]. Use pers ona l protec tive equipm ent to prevent contact of acids with the skin. HF dissolves glass. Use plastic labware for all solutions containing HF. Concentrated acids are corrosive.

SAMPLING: 1. Calibrate each personal sampling pump with a representative sampler in line. 2. Sa m ple at 2.2 ± 5% L/min with HD cyclone and 1.7 ± 5% L/min with nylon cyclone for a total sam ple size of 400 to 800 L. Do no t exceed 2 m g dust loading on the filter. NOTE 1: Do not allow the sampler assembly to be inverted at any time wh en u sing a cyclone. Turning the cyclone to anything other than a horizontal orientation may deposit oversized material from the cyclone body onto the filter. NOT E 2: A single sampler/flow rate should be used for a given application. Sampling for both crystalline silica and coal mine dust should be done in accordance with the ISO/CEN/ACGIH/ASTM respirable aerosol sampling con vention. Flow rates of 1.7 L/min for the Dorr-Oliver nylon cyclone and 2.2 L/min for the Higgins-Dewell cyclone have been found optimal for this purpose. O uts ide of c oal m ine dust sampling, the regulatory agencies currently use these flo w rate s with th e D orr-O liver cyclone in the United States and the Higgins-Dew ell sampler in the United Kingdom. Though the sampling recomm endations presented in a NIOSH Criteria Document have been formally accepted by MSHA for coal mine dust sampling, the Dorr-Oliver cyclon e at 2.0 L/m in with 1.38 conversion fa cto r is currently used in the United States for the purpose of matching an earlier sampling convention [6]. In any cas e, a single sampler/flow rate should be used in any given application so as to eliminate bias introduced by differences between sampler types and sampler conventions [5,7]

SAMPLE PREPARATION:

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NOTE: Successful use of this m etho d de pen ds o n clos e attention to details and uniform ity in sam ple handling procedures. Place a we ighed sa m ple co ntaining n o m ore th an 2 .5 m g SiO 2, or a m em brane filter sam ple, in a 250-m L Phillips beaker. Add 3 to 4 mL conc. HNO 3 to ash m em brane filters. Heat on a hotplate to absence of brown fumes. Add an additional 2 mL conc. HNO 3 and heat to dryness. Repeat this step until any remaining residue is white. NOTE: As h PVC filters using 2 m L conc. H ClO 4. Heat slowly until the filter shrinks, turns brown and is dissolved. A dd additional portions of H ClO 4 if necessary. If a perchloric acid hood is not available, see APP EN DIX B for alternate ashing procedures for PVC filters. Pretreatment proc edu res to rem ove variou s co ntam inants from bulk sam ples are o utlined in referenc e [7]. Add 25 m L 85 % H3PO 4 to the beaker. Cover the beaker with a bent-stem funnel. Make sure that the tip of the funnel stem touches the wall of the flask. Start reagent blanks at this time. NOTE: The bent-stem funnel is used to prevent too rapid dehydration of the sample and to prevent spattering. Place the 550-wa tt he ate r on the rotato r in a fume hood. Preheat the heater and adjust to maintain a tem perature of 240°C. Heat each sample and blank for exactly 8 min, swirling it by the action of the variable speed rotator. NOTE: The solution in the flask should reach 240°C during this procedure. Rem ove the beaker from the heater with padded tongs and swirl it for 1 min. Allow the be aker to cool. Ad d about 125 m L warm (60 to 70°C) de ionized water and swirl to m ix com pletely. Filte r the sam ple with suctio n through a 47-m m filter. W ash thoroughly with 1:10 H Cl. Place the 47-m m filter in the bottom of a 1 50-m L po lyethylene bea ker. Add 0.5 mL 48% HF to the filter surface. Float a thin polyethylene disc of about 50-m m diameter over the 47-mm filter and cover the beaker. Allow to stand for 30 min.

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition