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V O L U M E 28, NO. 2, F E B R U A R Y 1 9 5 6 Mulliple Exposure Film Method of X-Ray Diffraction Powder Analysis J. T. Quam, U. 5. Nova1 Radiological Defense Loborofary, San Froncirco 24. Calif.
qualitative analysis of a powder material by x-ray Iof accurat.ely diffraction, the practice of combining a standard substance known lattice spacing with a t.est saqple to obtain N THE
convenient calibration lines has the disadvantage of possible overlap of lines of the standard with those of the sample. Furthermore, comparing lines between two diffraction patterns for tlre complete identification of compounds in a mixture can be rather tedious when numerous lines are involved. Although the desirable features of both these practices without their associated disadvantages may be bbtained by the use of multiple-expasure cameras (1, 4, 5), it has been found that an adapter similar in function to that of Frevel (e, 3) but of simpler construction, can produce equally effective results in a conventional camera.
A composite of the two appears a t the center where the background is slightly higher because of the multiple exposure. The composite pattern looks the same as that of an intimately mixed standard material and test sample. When used for identifying lines, this method permits a direct elimination of the lines of compounds already identified, since the lines of the known standard are continuous with those of an identical constituent, in the mixture. Therefore, if a oombinatian of all known compounds is used as the standard, any discontinuous lines of the test sample on the film represent unidentified components. Equatorid 28 measurements of these lines are possible when the background is not excessive. This technique has been found partioularly effective in identifying fine impurities in clays and sails.
EQUIPMENT AND TECHNIQUE
The adapter is a brass ring (Figure 1,A) with diametric cutouts which fit partly around the collimator and beam trap of a n x-raj camera. Slotted adapter mounting braokets a t the collimatoi and beam trap supports permit more or less concentric mounting of the ring. A small shielding tab added to the beam trap mount, ing bracket completely masks the x-rays diffracted a t low angles. The diameter of the adapter is large enough to permit the use of an ascillatingwedgesamplemount. Theadapter shownin Figuri 1,B, suffices for small cylindrical-shaped samples. I n use the adapter is situated between the irradiated sample and th< film (Figure 2). I n this position it intercepts slightly less thsr half of the x-rays diffracted along the longitudinal film axis. d.
Figure 2. Mounted adapter ring; for cylindrical samples with sample mount removed
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Figure 1. Adapter rings
1 and reletivelv transD&ent to x-rays.
A powder sample to b i f
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this powder is replaced w i t h a sta'ndard material which i8 the: given its proper exposure. At this stage, a series of standard 1 oomponnds either singly or in combination
In the study of effects of physicochemical processes (a) on various compounds, this method sllows small changes in lattice spacings to be more easily detected Lines from each edge 6%tion of the photograph, though unresolved a t the center section, are perceptible as lines of different &spacings if they are fzirly sharp, moderately intense, and if their centers are very slightly separated. A test pattern of 300-mesh quartz and halite, each exposed with filtered copper Ka radiation for an equal period of time on opposite edges of a. film in a 14.32-cm. diameter camera, showed that in the composite, the 220-line of halite, completely obscured the 201-line of quarts. Rawever, 8. closer examinat,ion of each section of the film a t the adjoining edges revealed that the centers of these two lines did not actually coincide. ADVANTAGES AND DISADVANTAGES
There are two obvious drawbacks to the use of this technique: the longer time to register two patterns on one film and the increased background in the center section of the film.
..~-.. .. .-.~, ...~ ..._ film showing overlap of exposures at center to give a oomposite
ANALYTICAL CHEMISTRY
276 The inherent simplicity of the method is an advantage. Because no basic change is made in the existing camera design, the diffraction characteristics remain the same. The additional time spent in making a second exposure on the same film is compensated for, because the analyst is better able to resolve uncertainties that may exist in the relative line spacings of test sample and standard material. The problem of nonuniform shrinkage between two films is eliminated, and reference lines are readily available for calibration. ACKNOWLEDGMENT
The author wishes to thank Jarvis Todd for assistance in the preparation of this paper, the Engineering Services Section for the fabrication of the device, and the Photographic Section and Illustrating Section for the preparation of photographs and dram-ings. LITERATURE CITED
Davey, W. P., "Study of Crystal Structure and Its dpplications," 1st ed., pp. 112-14, McGraw-Hill, New Tork, 1934. Frevel, L. K., Ret. Sci. Instr. 6 , 214-15, (1936). Frevel, L. K., and Anderson, H. C., Acta Cryst. 4 , 186 (1981). Ubbelohde, A. R., J . Sci.Instr. 16, 155-61 (1939). Wolff, P. hl. de, Acta Cryst. 1, 207-11 (1948).
suitable for a tipping McLeod gage, as the volume of the bulb is low and the bore diameter is large enough t o prevent sticking of the mercury. Unless the gage is needed to read low pressures, it is desirable to keep the capillary bore diameter larger than 1.5 mm. t o prevent mercury sticking in the capillary. Hence the gage would be constructed by sealing a length of 3-mm. bore capillary tubing somewhat longer than 100 mm. to a bulb of about 14 cc. volume. To calibrate the gage, the procedure is reversed. The capillary bore and thq volume of the bulb are determined in the usual manner. If the experimentally determined bore were 2.5 mm. and the actual bulb volume 12 cc., then these points on the D and V scales would be connected and a new index point marked. The ruler is pivoted about this new index point to connect the P and H scales. A pressure of 0.01 mm. would be 5.0 mm. from the top of the capillary; 5 X mm. pressure a t 3.6 mm.; and 5-mm. pressure at 110 mm. from the top of the capillary. IO
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~Simplified Method for McLeod Gage Design and Calibration
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LIcLeod gage is the most commonly used low-pressure measurement instrument in the laboratory. Its construction is simple and adequately described in numerous sources (1-4)> but it is difficult to find information as to the exact size of capillary bore and bulb volume for a desired pressure range. The nomograph described eliminates this difficulty, as it makes it possible to set specifications p-hich can be follo-ived by the individual or sent to a glass blower. It is usable for any XIcLeod gage where the mercury in the outside arm is brought level to the sealed top of the capillary tube fixed to the bulb of the LIcLeod gage and pressure readings are made by the length of the column of gas compressed
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Suppose it is desired t o construct a McLeod gage to read pressures accurately from 0.01 to 5 mm. The first graduation (0.01 mm.) then should be about 3 mm. from the top of the capillary and, for convenience, the length of the sealed capillary which is attached to the bulb should b.e 100 mm. long. A ruler is then connected from the 5-mm. graduation on the P scale to the 100mm. mark on the H scale. The intersection of the ruler with the index line (unmarked line in the center of the chart) is marked. Next the ruler is placed on the 0.01-mm. mark on the P scale and the previous mark on the index line to see whether the intersection on the H scale is about 3 mm. The actual intersection is 4.7 mm. on the H scale. This shows that a McLeod gage can be constructed for this pressure range with a capillary length of 100 mm. If the intersection were less than 3 mm., the length of the capillary would have to be increased or the pressure range reduced. The ruler is now pivoted about the fixed index point to connect the D and V scales. All intersections of D and B with the pivot will satisfy the conditions for the desired pressure range, such as 4-mm. bore and 26-cc. volume; 7-mm. bore and 75-cc. volume; 3-mm. bore and 14-cc. volume, etc. The last condition would be
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where
4000V P is the pressure in the system in millimeters of mercury, D is the diameter of the capillary tube in millimeters, H is the length of the air column in the capillary tube in millimeters, and V is the volume of the bulb in cubic centimeters ( 1 , 3 , 5 ) . Tipping McLeod gages are also included in this category. The nomograph is designed for pressures between 30 and 10-6 min. and bulb volumes from 10 to 250 cc. As shown in the key on the nomograph, one pairs the P-H scales and the D-V scales.
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in this capillary. This gage follows the formula P =
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Peter Lott, Department of Chemistry, University of Connecticut, Storrs, Conn.
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The calibration of the LIcLeod gage by use of the chart is approximate. The error due t o inaccuracies in the ease of reading the chart up to a pressure of 5 mm. is about 5%. If precision calibration is necessary, it is recommended that the formula be used and one of these references be consulted (1-3, 6). Above 5-mm. pressures, it is necessary to calibrate by the following formula, P = .lrD2H2/4[1000V- ( r D 2 / 4 ) H ] ,which contains a correction term for the volume of mercury displaced from the total bulb volume by the gas in the capillary (1, 3, 6). This correction is not taken into account by the chart, as it will not affect the use of the chart for design purposes but calibration by the chart would be in error in the range above 5 mm. ACKNOWLEDGMENT
The author wishes to thank Ernest R. Kline for his interest. LITERATURE CITED
(1)
Barr, W. E., and Anhorn, V. J., "Scientific and Industrial Glaas Blowing and Laboratory Techniques," Instruments Publishing Co., Pittsburgh, 1949.
