Document 7RVGJzoNX144J2qw2x7raN5gV

Hf (5) mass t of L ield a cation on the . of an permit is pro- 'Pattn's ve>t_ 3 SAMPLING AND ANALYSIS 747 ducing a stoichiometric relationship between the volume of stained indicating gel and the quantity of the absorbed test gas. Such equilibrium conditions may be assumed to exist when stain lengths are directly proportional to the volume of sampled air and are not affected by the sampling flowrate. With this situation a log-log plot of stain length versus concentrations for a fixed sample volume may be prepared in the calibration of a given batch of tubes. From the preceding discussion of the complexity of the heterogeneous phase kinetics of indicator tube reactions, the quality control problems associated with their manufac ture and storage, and the difficulties posed by interfering substances, It is obvious that frequent, periodic calibration of these devices should be made by the user. Dynamic dilution systems for the reliable preparation of low concentrations of a test gas or vapor are recommended for this purpose. The Department of Health, Education and Welfare has issued regulations for the certification of gas detector tube units (18). The perform ance requirements for these units were developed by the NIOSH Division of Labora tories and Criteria Development, with the cooperation and assistance of members of the Joint Direct Reading Gas Detecting Systems Committee of the AIHA and the ACGIH, Table 17.4 lists the detector tubes that have been certified to date (March, 1977) by this program, and the manufacturer or supplier. 4.3.4 Passive Personal Monitors In recent years significant progress has been made in technology for monitoring of gases and vapors using binary diffusion. The monitors utilize Brownian motion to control the sampling process into a collection media. This technology is particularly well suited for personal monitoring devices, resulting in lightweight, low cost monitors that require no power source. The monitors rely on a concentration gradient across a static or placid layer of air to induce a mass transfer. The following equation, based on Fick's law, gives the steady state relationship for the rate of mass transfer. W-. (C, - Co) (6) where W = mass transfer rate D = diffusion coefficient A = frontal area of static layer L = length or depth of static layer C, = ambient concentration C0 = concentration at collection surface It can be seen that by choosing an effective collection surface, such that C, is essentially zero, the mass transfer or collection rate is proportional to the ambient vapor concentration Ct. It may also be noted that the units of D(A/L) are volume per unit time, the same as for the volumetric flow in a pump monitoring system. The rate of 3M 109620 748 ROVERT 0. SOULE Table 17.4 NIOSH-Certified Detector Tubes* SUBSTANCE OrSgor Modol 31 manufacturer Gaatae Kitagawa Modal 400 Modol 400 Ao*ton* Ammonia Banian* Carbon DloalOa Carbon Monoildt Carbon TatracblorM* CMorin* Hydros** Cyanld* Hydrogon Sulfld* Nitric Olid* Nitrogen Oloiid* Sulfur Oloxld* Toluon* TrlcMoroathylan* - CM 20501 57-25541 CH 23501 CH 25501, CH20B0I CH 25701 67-19001 CH 31001 CH 30001 CH3I70I CH 23001 CH 24401 3M 121 2L 1 La (34 BLa * 4U 10 9L SLa 122 I32H I03SO, 1056 I26SO, 1250 toes, too (125b 120b * I03S4, I03d * MSA UMvorsol 450423 450103 55*75 91229 450225 45005B 93099 92523 " " G Names and addresses of manufacturers and distributors of detector tubes: Bendix Corporation 1400 Taylor Avenue Baltimore, Md. 21204 (Gastec distributor) Matheson Gas Products 1275 Valley Brook Avenue Lyndhurst, N.J. 07071 (Kitagawa Distributor) Mine Safety Appliances Company 400 Penn Center Boulevard Pittsburgh, Pa. 15235 National Mine Service Company 3000 Koppers Building Pittsburgh, Pa. 15219 (Drager distributor) sampling of the contaminant is then the product of the D{A/L) term and the average ambient concentration. Figure 17.10 illustrates the construction of a monitor for mercury vapor in which the collection surface is a gold layer (19), A similar configuration (Figure 17.11) has been developed for nitrogen dioxide (20). In the case of the mercury monitor, mercury vapor in the atmosphere passes through the microporous barrier film and continues through SAMPLING / the static a collection si For exan when the d Thus the at The acci exposure ti reported cc analytical t tinguish th All other fa Regardii surface of the monito L term is between 5< higher or tions to be I This tyj total expo These dat. the time-v are chosen Corrobt parison o 3H 109621 SAMPLING AND ANALYSIS 749 the static air column according to Fick's law. The average time required to reach the collection surface is T = ~ seconds (7) For example, the average time for a mercury atom to progress to the collection surface when the depth L is 0.65 cm can be shown, using D = 0.12 cm'/sec, to be 1.75 seconds. Thus the sampling occurs rapidly when thin static air columns are used. The accuracy and precision of the sampling process are functions of the measured exposure time, velocity effects, and temperature effects. The accuracy and precision of the reported concentration are functions of the calibration standards, collection Media, and analytical method used. Of these factors, the potential velocity and temperature effects dis tinguish this type of monitoring device from the conventional dynamic or flow monitor. All other factors are common to both methods. Regarding velocity effects, the thickness of the attached boundary layer on the outer surface of the barrier film is a function of the velocity of air movement over the face of the monitor. Sampling that is independent of this air velocity can be achieved when the L term is large compared to the average boundary layer thickness. For temperatures between 50 and 88F, the temperature factor is constrained to 1.8 percent. For use at higher or lower temperatures, the temperature data may be rerecorded to allow correc tions to be made. This type of personal monitor is attached to the worker in his breathing zone. The total exposure time is noted, and analysis results give the amount of vapor collected. These data provide an average mass collection rate, which can then be used to calculate the time-weighted average concentration. The physical parameters of the sampler design are chosen according to desired exposure time and the substance to be monitored. Corroborative testing of both the mercury and nitrogen dioxide monitors and com parison of results to reference methods have shown excellent agreement. With the Atmotphere being templed Figure 17.10 Cross-sectional view of mercury detector. 750 RO*MT D. SOUiE Polypropylene cep plug Figure 17.11 Exploded view of nitrogen dioxide monitor. increasing emphasis on development of specific methods for monitoring workers' exposures to contaminants, it is likely that the passive monitor concept will become the vital basis for a new generation of industrial hygiene monitoring equipment. 5 SAMPLING FOR PARTICULATES In classifying airborne particulates, the term "aerosol" normally refers to any system of liquid droplets or solid particles dispersed in a stable aerial suspension. This requires that the particles remain suspended for significant periods of time. Liquid particulates usually are classified into two subgroups, mists and fogs, depend ing on particle size. The larger particles generally are referred to as mists, whereas small particle sizes result in fogs. Liquid droplets are normally produced by such processes as condensation, atomization, and entrainment of liquid by gases. Solid particulates usually arc subdivided into three categories: dusts, fumes, and smoke, the distinction among them being primarily related to particle size. Dusts are formed from solid organic or inorganic materials by reducing their size through some mechanical process such as crushing, drilling, or grinding. Dusts vary in size from the JLE ind t of vil! :nt, its. onits, ' an na idy t in are New ass.. and .tmit iccu. 75tmrDSH final Cinttrial ncin- .hers, ton of SAMPUNG AND ANALYSIS 769 14. C. D. YafTe, D. H. Byers, and A. D. Hosey, Encyclopedia of Instrumentation for Industrial Hygiene, University of Michigan Press, Ann Arbor, 1956. 15. B. Witten and A. Prostak, "Sensitive Detector Crayons for Phosgene, Hydrogen Cyanide, Cyanogen Chloride, and Lewisite," Anal. Chem., 29,885-887 (1957). 16. American Industrial Hygiene Association, Direct Reading Colorimetric Indicator Tubes Manual, A1HA, Akron, Ohio, 1976. 17. B. E., Saltrman. Direct Reading Colorimetric Indicators, Section S, in: Air Sampling Instruments for Evaluation of Atmospheric Contaminants, 4th ed., American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio 1972. 18. Fed. Reg., 3S, 11458 (May 8, 1973), incorporated into Code of Federal Regulations as Title 42, CFR Part 84. 19. D. L. Braun, "Personal Monitoring of Mercury Vapor Exposure," paper presented at the International Conference on Environmental Sensing and Assessment, Las Vegas, 1975. (to be published.) 20. G. H. Schnakenberg, "A Passive Personal Sampler for Nitrogen Dioxide," U.S. Department of the Interior, Bureau of Mines, Technical Progress Report, Pittsburgh, 1976. 21. National Institute for Occupational Safety and Health, Criteria for a Recommended Standard . . Cot ton Dust, U.S. Department of Health, Education and Welfare, Cincinnati, Ohio, 1973. 22. A. Linch, Biological Monitoring for Industrial Chemical Exposure Control, CRC Press, Cleveland, 1974. 23. National Institute for Occupational Safety and Health, Criteria for a Recommended Standard .. Bentene, U.S. Department of Health, Education and Welfare, Cincinnati, Ohio, 1974. 24. National Institute for Occupational Safety and Health, Criteria for a Recommended Standard. . . Inor ganic Lead, U.S. Department of Health, Education and Welfare, Cincinnati, Ohio, 1972. 25. National Institute for Occupational Safety and Health, Criteria for a Recommended Standard.. . Carbon Monoxide, U.S. Department of Health, Education and Welfare, Cincinnati, Ohio, 1972. 26. R. D. Stewart et al., Biological Standards for the Industrial Worker by Breath Analysis: Tri chloroethylene, U.S. Department of Health, Education and Welfare, Cincinnati, Ohio, 1974. 3H 109624