# Arsenic and Compounds, as As (7900)

SAMPLING
ACCURACY
MEASUREMENT
Background absorption is overcome by the use of D ₂ or H₂ continuum.
This revises P&CAM 139 [1]); a similar method appears in the criteria document [2]. Method 7901 uses a sampler designed to collect As ₂O₃ vapor and an alternate measurement technique (graphite furnace-AAS). Method 7300 (ICP-AES) also gives an alternate measurement technique.
​REAGENTS:
* Nitric acid, conc.
* Hydrochloric acid, conc.
* Sulfuric acid, conc.
* Perchloric acid, conc.*
* Calibration stock solution, 1000 mg/mL.* Commercially available or dissolve 1.320 g primary standard As 2O3 in 25 mL 20% (w/v) KOH. Neutralize with 20% (v/v) HNO 3 to a phenolphthalein endpoint. Add 10 mL conc. HNO3 and dilute to 1 L with distilled or deionized water.
* Ashing acid, 3 volumes HNO 3, 1 volume H2SO4, and 1 volume HClO 4.
* Hydrogen.
* Argon.
* Distilled or deionized water.
* Sodium borohydride, pellets.
* Air, compressed.
EQUIPMENT:
* Sampler: cellulose ester filter, 0.8-μm pore size, 37-mm diameter; in cassette filter holder.
* Personal sampling pump, 1 to 3 L/min, with flexible connecting tubing.
* Atomic absorption spectrophotometer with appropriate hydrogen burner head or quartz tube furnace, and arsenic hollow cathode lamp or EDL and arsine generation system.
* Regulators, two-stage, for air, hydrogen and argon.
* Beakers, Phillips, 125-mL, or Griffin, 50-mL, with watchglass covers.*
* Volumetric flasks, 25- and 100-mL.*
* Pipets, volumetric, as needed.*
* Hotplate, surface temperature 140 °C.
SAMPLING:
* Calibrate each personal sampling pump with a representative sampler in line.
* Sample at an accurately known flow rate between 1 and 3 L/min for a total sample size of 30 to 1000 L. Do not exceed ca. 2 mg total dust loading on the filter.
SAMPLE PREPARATION:
* Open the cassette filter holders and transfer the samples and blanks to clean beakers. NOTE: Analyze the backup pad separately if qualitative indication of As 2O3 vapor is desired. Use Method 7901 if quantitative collection of As 2O3 vapor is desired.
* Add 5 mL ashing acid and cover with a watchglass.
* Heat on hotplate (140 °C) until the solution is colorless.
* Add 1 mL conc. HNO 3 and/or 70% HClO 4 drop by drop as needed to complete the ashing.
* Remove the watchglass.
* Heat on 140 °C hotplate until dense SO 3 fumes appear.
* Allow the mixture to cool.
* Transfer the solution quantitatively to a 25-mL volumetric flask.
* Dilute to volume with distilled or deionized water.
CALIBRATION AND QUALITY CONTROL:
* Analyze working standards together with the blanks and samples (steps 18 through 25).
* Prepare calibration graph (absorbance vs. solution concentration, μg/mL).
* Analyze a standard for every 10 samples.
* Check analytical recoveries with at least one spiked media blank per 10 samples.
* Use method of standard additions occasionally to check for interferences.
* Set spectrophotometer according to manufacturer's recommendations and to conditions on page 7900-1.
* Set up arsine generator per manufacturer's instructions.
* Pipet 5 mL aliquot of the 25-mL sample into the arsine generation flask.
* Add 25 mL distilled or deionized water, 3 mL conc. HCl, and mix well.
* Connect the flask to the generation system.
* Introduce a single sodium borohydride pellet or sodium borohydride solution to the sample solution.
* Allow the gases to flush into the flame of the atomic absorption instrument.
* Record the absorbance readings. NOTE: If the absorbance values of the samples are above the linear range of the standards, dilute the solutions, or use a smaller aliquot, reanalyze, and use the appropriate dilution factor in calculations.
This method was evaluated in July, 1976, over the range 0.02 to 3 mg per sample by laboratory testing with spiked filters. Precision and accuracy data are given on page 7900-1 [1].
[1] NIOSH Manual of Analytical Methods, 2nd. ed., V. 1, P&CAM 139, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 77-157-A (1977).
[2] Criteria for a Recommended Standard...Occupational Exposure to Inorganic Arsenic, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 75-149 (1975).
Mark Millson, NIOSH/DPSE.