AIDS FOR ANALYTICAL CHEMISTF
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Dust-Free Sample Grinding Installation for the Silicate Laboratory Patrick L. Beaulieu Nevada Mining AnalyficalLaboratory, University of Nevada, Reno, Neu.
ALTHOUGHNEW DEVEWPMENTS in sample preparation techniques are seldom published, this first step in analysis of geological samples remains very important. This paper does not introduce a new method, but rather makes an old method far more convenient to the analytical laboratory. The large size of most rock crushing and grinding equipment, the high cost of such equipment, and the problem of dust generation appeared to be obstacles preventing convenient and rapid preparation of powders of rock samples in the geochemical analysis laboratory. The problem of controlling the dust, which might otherwise have contaminated the laboratory in which the equipment was housed, was overcome by construction of the benchtop downdraft hood diagrammed in Figure 1. This is suitable for housing all common sample grinding equipment. However, relatively inexpensive miniature jaw crushers and plate pulverizers have recently become available ( I ) . These reduced the importance of the size and cost considerations, and have allowed rapid sample preparation within the laboratory. (See Figures 2 and 3.) The downdraft hood is constructed of plywood, except for the pegboard table surface. The key features of the hood are the double floor and backwall. which form a chamber to conduct the airflow to the normal overhead Rue, and the> pegboard table, which allows the downward flow of air. 1Easily . i.n m. e upper pan. 01?.. . . me imerior rear wau allow openeo ports rapid changeover to updraft u(hen desired for alternative usage of the hood. The front lip is hinged to allow easy removal of any dust which settles in the> air passages. Alternately, the .-mhhnrrrl o i w f - r n o l m n r i hs r n n double floor (plywood and plbvyyLy strutted as a platform and installed in an available laboratory hood. The backwall of the hood should then be adjusted to gt\e maximum draft from the bottom. The usu:11 installation of erindine r'auimncnt in the laboratory is less than satisfactorjI, since the updraft ventilation normally used suspends fine pairticles in the air, a condition :~~.l-ll~>. . . . L in OUI iauu~awnicn invites SIIIC~SLS. AS me h..o. wJ 1s insraiirc~ tory, five overlapping sheets of poiyethy lene hang down over the hood face, forming doors. The ixppropriate panel is raised for access to the equipment. Th,e rush of air past the operator, into the hood, and down past +ha oniiinment Ireen* y.. the room and hood free of dust. The respirator, often neglected in the past, now seems less necessary for maintenance of safety while grinding rock samples. Proper use of the miniature jaw crusher requires that the sample be obtained in pieces approximately 1inch in diameter. The jaw crusher reduces this material to coarse ground material which has a particle size approximately 4 mm in diameter. Approximately K g is easily handled. This is then split repeatedly until approximately 50 cc of the coarse ground material is isolated. The pulverizer, adjusted to reduce this to 200 mesh or smaller, is then used. The hoppers
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(1) Lemaire Instruments, 3800 N. Virginia St., Reno,Ne". 798
Dimensions me 312 cm long X 96 em high X 80 cm deep
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Figure 1. Benchtop downdraft hood (end removed to show double wall construction)
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6 , MAY 1971
Figure 2. Miniature jaw crusher (side view shown) Manufactured by Lemaire Instruments, Reno, Nev. Dimensions are 25 cm wide X 21 cm hi& X 40 cm deep on our pulverizers have been enlarged to allow convenient handling of this much material. Reduction of the sample as described requires approximately 15minutes. The hood was constructed to house six pieces of the miniature grinding equipment, a sample splitter, and the vacuum cleaner which is used in conjunction with compressed air for cleaning the equipment. The four pieces of equipment
Figure 3. Miniature pulverizer (end view shown) Manufactured by Lemaire Instruments, Reno, Nev. Dimensions are 25 em wide X 25 cm high X 46 cm deep currently in use allow preparation of samples using two grinding media. Samples may be prepared which have used ceramic grinding from the coarse crush to analysis. However, the other grinding route uses an iron faced jaw crusher and a
tungsten carbide ( 6 x cobalt binder) pulverizer. Since the grinding surfaces of different composition are interchangeable within the same type of instrument, two pieces of equipment, the appropriate extra working surfaces, and a shorter hood might suffice for laboratories with a space problem. When an iron pulverizer and a tungsten carbide jaw crusher become available in the desired miniature size, three completely separate grinding media will be possible. At present, cobalt and tungsten are determined in samples ground in the ceramic circuit. Aluminum and silicon are determined in samples prepared in the iron-tungsten carbide circuit. The advantage of using dual grinding circuits for X-ray fluorescence analysis has been described (2). Using samples ground in equipment introducing only noninterfering contamination should be an advantage in trace element determinations by other methods of analysis as well. The convenience of the small equipment, the maintenance of dust free laboratory conditions and the accompanying increase in operator safety have made the grinding installation a part of the analytical laboratory facility. The speed with which rather large samples of rock can he prepared for analysis, and the control of laboratory and sample contamination are analytically important features of the installation.
RECEIVED for review May 18, 1970. Accepted January 28, 1971. (2) A. Volborth, Appl. Specfry.,19,l-7 (1965).
Aluminum Ellipse for Decreasing Limits of Detection in Atomic Fluorescence Flame Spectrometry Marilyn Shull and J. D. Winefordner' Department of Chemisfry, University of Florida, Gainesuille, Fla. 32601
THESIGNAL LEVEL in atomic fluorescence flame spectrometry increases linearly with the radiant flux of exciting radiation incident upon the flame ( I ) . As long as the major source of noise is not principally a result of flicker in scatter of source radiation from unvaporized sample particles, the signal-tonoise ratio should also increase (the limit of detection should decrease) approximately linearly with the source radiant flux (2). With this in mind, an investigation was undertaken to increase the source radiant flux incident upon the flame cell by constructing an elliptical cylinder to reflect radiation from the source at one of the axial foci into the flame at the other focal axis. EXPERIMENTAL An aluminum elliptical cylinder (called an ellipse in subsequent discussion) with a major axis of 13 in., a minor axis of 11 in., and a height of 6 in. was constructed from 0.010-in. aluminum sheet (see Figure 1). The side of the ellipse toward
the monochromator entrance slit and in line witn tne name contained a %in. square opening to allow atomic fluorescence excited in the flame to reach the monochromator. The side of the ellipse directly opposite the monochromator entrance slit and in line with the flame contained a 2-in. opening with a light trap (a small can painted flat black) to avoid direct reflection of source radiation into the monochromator. The sources used were electrodeless discharge tubes (EDT) which were suspended at the proper height by a thermometer clamp at the other focal axis of the ellipse; to avoid ,reflecting, metallic surfaces and opaque materials within the elliptical cavity, the EDT's were operated with an "A" antenna mounted under the lamp as previously described (3). The atomic fluorescence of cadmium (228.8 nm) and mercury (253.7 nm) were investigated; the EDT's were operated as previously described (3) and a premixed Hrair turbulent flame produced with a total-consumption nebulizer burner was optimized as previously discussed (4). The measurement system-except for the source-ellipse-was as previously described by Mansfield et al. (5). Measurements were made ~
Author to whom reprint requests should be sent. (1) J. D. Winefordner, V. Svoboda, and Linda Cline, CRC Crif.
Rev. Anal. Chem., 1,233 (1970). ( 2 ) J. D. Winefordner, M. L. Parsons, J. M. Mansfield, and W. J. McCarthy, ANAL.CHEM., 39,436 (1967).
,(3) K. E. Zacha, M. P. Bratzel, J .D. Wmefordner, and J. M. Mansfield, ibid., 40, 1733 (1968). (4) M. P. Bratzel, R. M. Dagn#all,and J. D. Winefordner, ibid.,
41,1527(1969).
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( 5 ) J. M. Mansfield, C . Veillon, and J. D. Winefordner, ibid., 37,
1049(1965). ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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