Document zbkpwrGkgONy6805B1xaa30az

190 PRANK A. PATTY yet to be explored, largely because of the difficulty of generating, controlling, dis-^ persing, and measuring the intensity of these waves. 2. Ultraviolet ...vl Several organic solvents, especially in the aromatic series, have characteristi absorption patterns in the ultraviolet region of the spectrum.86 The patterns can 6' used to identify very small amounts of solvents, while the opacity of the solvent a. specific wave lengths can be used to measure the amounts. As in the case of iri| frared analysis, a sample must be collected from the air by condensations, by .cqjf lection on silica gel (page 182), or by scrubbing air through a tenacious nonvolatil' solvent that has no interfering absorption pattern. It is frequently necessary' refrigerate the scrubber in order to obtain high collection efficiency. 3. Light Absorption Visible light spectrometry86 is similar to infrared and ultraviolet spectrometj except that there are fewer compounds having distinctive absorption patterns. T spectrometry accurate results require a source of radiant energy confined to*1' narrow band of known (calibrated and checked) wave length, recognition a consideration of interfering absorption, and reliable measurement of transmit! energy. . G. THERMAL CONDUCTIVITY The thermal conductivity of gases and vapors~can be used Wmeasure"^ monitor vapor-air mixtures. Commercial instrumentation to fractionate vapors an measure their thermal conductivity is now available. Gas chromatograph^; technique combining fractionation and thermal conductivity, requires quantify of only a few milligrams for identification and measurement of paint and laicdfe. thinners and other volatile solvents of interest to industrial hygienists. OUSTS, FUMES, AND SMOKES I. Sampling .. The" instrument!':t'o *be' employedVforsampling and' :evaluatTng^pafti| matteFdfi1thd-t|s|iMl^^iil:'d;e|ebd:'Upon:the.iaavvaailialabbilii1lity - of instrument^' to be sampled, nature of thecontaminant, and purpose of the investigation. ..... .......... ----------- --- -------A.-IMEINGEMEN.T....................................... There are several dust-sampling instruments that employ the prinpipl "M. E. Maclean, P. J. Jencks, and S. F. Acree, J. Research Natl. Bur. Standards, 34| (1945). " M. G. Mellon, Ind. Eng. Chem.,.Anal. Ed., 17, 81 (1945). SAMPLING AND ANALYSIS OP CONTAMINANTS 191 jjm^mfnt. Dust carried by air at high velocity is impinged against a plate, ggffljijnljpl'aiTested by an agent, such as a film of water or other liquid, gelatin, or SIL. 1. Greenburg-Smith Apparatus i|f||||tJnited States the Greenburg-Smith81 standard impinger and its midget 1 ' ` rfctoe the most widely used instruments for obtaining dust samples for tsEply counting. In the standard instrument air is drawn through an imW0r orifice 2.3 mm. in diameter at the rate of 1 cu. ft. per minute and ainst a glass plate set perpendicular to, and 5 mm. distant from, the [ Orifice must be calibrated and, if necessary, adjusted to this size and |la`te may be the bottom of the sample flask itself. The plate is covered jf. of at least 1 inch with dust-free water or other, suitable liquid, which H&p and retain the dust particles after they strike the plate. The device Equipped with an electrically driven air pump, but may be obtained for ption, or with an ejector utilizing compressed air as the means of |vice is suitable for sampling all air-borne dusts greater than 0.7 p in " t within the range of definition by light-field technique), but at the Ite of flow its efficiency for dusts of smaller size falls off rapidly. When ffetwo impinger flasks in series, however, it can be used for particles smaller in size, Buch as lead fumes and fluoride fumes, the greater part |re-beIow 0.5 p in diameter. Frequently the greater portion of freshly i is caught in the second impinger flask, perhaps because the particles (Ste' moistened in passing through the first flask and so are more easily 6 seoon<^` Another possible explanation is that condensation of water Individual particles takes place as a result of the cooling of the saturated Ig'denly expands upon emerging from the orifice into the second flask. Ifrtshly generated lead fume,88 the efficiency hah been found to range 'per oent| 34 to 37 per cent *n the seond flask; while with fumes 1 to ^|er-generation 60 to 65 per cent was collected in the first impinger and '^Wffient in the second, for a total collection efficiency of 78 to 92 per cent. glplfome collected in a magnesium foundry- the> second^ impinger con- JSPfrer'cent 0f the total collected (see page 193), and the collection lpp5)pared with a commercial electrostatic precipitator was 94 to 120 `SBr^ an<* ''"determined portion of the fluoride contamination was 'fgpeous. ... sHifeurg and G. W. Smith, V. S. Bur. Mines Kept. Invest., No. 2392, 1922; C. E. Baft 8chrenk, V. S. Bur. Mines Circ. No. 7026, 1938. imtleiaeid, Florence Feicht, and H. H. Schrenk, V. S. Bur. Mines Rept. Invest. f938r /