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46 CFR Ch. I (10–1–10 Edition) Pt. 197, Subpt. C, App. D

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46 CFR Ch. I (10–1–10 Edition) Pt. 197, Subpt. C, App. D Pt. 197, Subpt. C, App. D 46 CFR Ch. I (10–1–10 Edition) APPENDIX D TO SUBPART C TO PART 1. Principle of the method 197—SAMPLING AND ANALYTICAL 1.1. A known volume of air is drawn METHODS FOR BENZENE MONI- through a charcoal tube to trap the organic TORING—MEASUREMENT PROCE- vapors present. DURES 1.2. The charcoal in the tube is transferred to a small, stoppered vial and the analyte is Measurements taken for the purpose of de- desorbed with carbon disulfide. termining employee exposure to benzene are 1.3. An aliquot of the desorbed sample is in- best taken so that the representative aver- jected into a gas chromatograph. age eight-hour exposure may be determined 1.4. The area of the resulting peak is deter- from a single eight-hour sample or two four- mined and compared with areas obtained hour samples. Short-time interval samples from standards. (or grab samples) may also be used to deter- mine average exposure level if a minimum of 2. Advantages and disadvantages of the five measurements are taken in a random method manner over the eight-hour work shift. In 2.1. The sampling device is small, portable, random sampling, any portion of the work and involves no liquids. Interferences are shift has the same chance of being sampled minimal and most of those which do occur as any other. The arithmetic average of all can be eliminated by altering random samples taken on one work shift is chromatographic conditions. The samples an estimate of an employee’s average level of are analyzed by means of a quick, instru- exposure for that work shift. Air samples mental method. should be taken in the employee’s breathing 2.2. The amount of sample which can be zone (i.e., air that would most nearly rep- taken is limited by the number of milli- resent that inhaled by the employee). Sam- grams that the tube will hold before over- pling and analysis must be performed with loading. When the sample value obtained for procedures meeting the requirements of 46 the backup section of the charcoal tube ex- CFR part 197, subpart C. ceeds 25 percent of that found on the front There are a number of methods available section, the possibility of sample loss exists. for monitoring employee exposures to ben- zene. The sampling and analysis may be per- 3. Apparatus formed by collection of the benzene vapor on 3.1. A calibrated personal sampling pump charcoal adsorption tubes, with subsequent having a flow that can be determined within chemical analysis by gas chromatography. ±five percent at the recommended flow rate. Sampling and analysis also may be per- 3.2. Charcoal tubes: Glass with both ends formed by portable direct reading instru- flame sealed, seven cm long with a six mm ments, real-time continuous monitoring sys- O.D. and a four mm I.D., containing two sec- tems, passive dosimeters, or other suitable tions of 20/40 mesh activated charcoal sepa- methods. The employer is required to select rated by a two mm portion of urethane foam. a monitoring method which meets the accu- The activated charcoal is prepared from co- racy and precision requirements of 46 CFR conut shells and is fired at 600 °C before 197.540(a)(6) for the weather conditions ex- packing. The adsorbing section contains 100 pected. Section 197.540(a)(6) requires that mg of charcoal and the back-up section 50 monitoring must have an accuracy, to a 95 mg. A three mm portion of urethane foam is percent confidence level, of not less than placed between the outlet end of the tube plus or minus 25 percent for concentrations and the back-up section. A plug of silanized of benzene greater than or equal to 0.5 ppm. glass wool is placed in front of the adsorbing In developing the following analytical pro- section. The pressure drop across the tube cedures, the OSHA Laboratory modified must be less than one inch of mercury at a NIOSH Method S311 and evaluated it at a flow rate of one liter per minute. benzene air concentration of one ppm. A pro- 3.3. Gas chromatograph equipped with a cedure for determining the benzene con- flame ionization detector. centration in bulk material samples was also 3.4. Column (10 ft.×1/8 in. stainless steel) evaluated. This work, as reported in OSHA packed with 80/100 Supelcoport coated with Laboratory Method No. 12, includes the fol- 20 percent SP 2100 and 0.1 percent CW 1500. lowing two analytical procedures: 3.5. An electronic integrator or some other suitable method for measuring peak area. I. OSHA Method 12 for Air Samples 3.6. Two-milliliter sample vials with Tef- Analyte: Benzene. lon-lined caps. Matrix: Air. 3.7. Microliter syringes: ten microliter (ten Procedure: Adsorption on charcoal, μl) syringe, and other convenient sizes for desorption with carbon disulfide, analysis by making standards. One μl syringe for sample gas chromatograph. injections. Detection limit: 0.04 ppm. 3.8. Pipets: 1.0 ml delivery pipets. Recommended air volume and sampling 3.9. Volumetric flasks: convenient sizes for rate: 10 liter at 0.2 liter/min. making standard solutions. 412 VerDate Mar<15>2010 12:01 Nov 15, 2010 Jkt 220197 PO 00000 Frm 00422 Fmt 8010 Sfmt 8002 Y:\SGML\220197.XXX 220197 jdjones on DSK8KYBLC1PROD with CFR Coast Guard, DHS Pt. 197, Subpt. C, App. D 4. Reagents 5.3.8. Take necessary shipping and packing precautions to minimize breakage of sam- 4.1. Chromatographic quality carbon disul- ples. fide (CS ). Most commercially available car- 2 5.4. Analysis of samples. bon disulfide contains a trace of benzene 5.4.1. Preparation of samples. In prepara- which must be removed. It can be removed tion for analysis, each charcoal tube is with the following procedure. Heat, under scored with a file in front of the first section reflux for two to three hours, 500 ml of car- of charcoal and broken open. The glass wool bon disulfide, ten ml concentrated sulfuric is removed and discarded. The charcoal in acid, and five drops of concentrated nitric the first (larger) section is transferred to a acid. The benzene is converted to two ml vial. The separating section of foam nitrobenzene. The carbon disulfide layer is is removed and discarded and the second sec- removed, dried with anhydrous sodium sul- tion is transferred to another capped vial. fate, and distilled. The recovered carbon di- These two sections are analyzed separately. sulfide should be benzene free. (It has re- 5.4.2. Desorption of samples. Before anal- cently been determined that benzene can ysis, 1.0 ml of desorbing solution is pipetted also be removed by passing the carbon disul- into each sample container. The desorbing fide through a 13x molecular sieve). solution consists of 0.05 μl internal standard 4.2. Benzene, reagent grade. per milliliter of carbon disulfide. The sample 4.3. p-Cymene, reagent grade, (internal vials are capped as soon as the solvent is standard). added. Desorption should be done for 30 min- 4.4. Desorbing reagent. The desorbing rea- utes with occasional shaking. gent is prepared by adding 0.05 ml of p-cy- 5.4.3. GC conditions. Typical operating con- mene per milliliter of carbon disulfide. (The ditions for the gas chromatograph are as fol- internal standard offers a convenient means lows: correcting analytical response for slight in- 1. 30 ml/min (60 psig) helium carrier gas consistencies in the size of sample injec- flow. tions. If the external standard technique is 2. 30 ml/min (40 psig) hydrogen gas flow to preferred, the internal standard can be elimi- detector. nated.) 3. 240 ml/min (40 psig) air flow to detector. 4.5. Purified GC grade helium, hydrogen, 4. 150 °C injector temperature. and air. 5. 250 °C detector temperature. 6. 100 °C column temperature. 5. Procedure 5.4.4. Injection size. One μl. 5.1. Cleaning of equipment. All glassware 5.4.5. Measurement of area. The peak areas used for the laboratory analysis should be are measured by an electronic integrator or properly cleaned and free of organics which some other suitable form of area measure- could interfere in the analysis. ment. 5.2. Calibration of personal pumps. Each 5.4.6. An internal standard procedure is pump must be calibrated with a representa- used. The integrator is calibrated to report tive charcoal tube in the line. results in ppm for a 10 liter air sample after 5.3. Collection and shipping of samples. correction for desorption efficiency. 5.3.1. Immediately before sampling, break 5.5. Determination of desorption efficiency. the ends of the tube to provide an opening at 5.5.1. Importance of determination. The least one-half the internal diameter of the desorption efficiency of a particular com- tube (two mm). pound may vary from one laboratory to an- 5.3.2. The smaller section of the charcoal is other and from one lot of chemical to an- used as the backup and should be placed other. Thus, it is necessary to determine, at nearest the sampling pump. least once, the percentage of the specific 5.3.3. The charcoal tube should be placed in compound that is removed in the desorption a vertical position during sampling to mini- process, provided the same batch of charcoal mize channeling through the charcoal. is used. 5.3.4. Air being sampled should not be 5.5.2. Procedure for determining desorption passed through any hose or tubing before en- efficiency.
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