A Cooled Sample Holder for the X-Ray Spectrograph C. W. Dwiggins, Jr., J. R. Lindley, and 8. H. Eccleston, Petroleum Experiment Station, Bureau of Mines, U. S. Department of the Interior, Bartlesville, Okla.
A c a o l e d sample holder for the x-ray spectrograph often gives more reproducible intensity measurements. A sample holder with a fixed cover (8) is Batisfactory for many liquids, but it is difficult to fill and clean, when working with very viscous liquids. The sample holder described was designed to overcome this difficulty. Although it has been used primarily for internal standard methods such as the determination of trace metals in petroleum (S),provisions are incorporated for filling to a reproducible level when used with external standard methods. The sample holder was designed so that all parts exposed to the x-ray beam were of pure aluminum; the sample well had as small a volume as practical without undue reduction of intensity; a thin plastic film could be attached to the sample well when a sample cover was necessary ; the sample holder was easily cleaned; thermal equilibrium was reached rapidly; and the height of the sample was adjustable with respect to the x-ray beam. A new x-ray tube shield (Figure 1) allows better use of the space in the sample chamber and prevents leakage of radiation when the door of the sample chamber is open. When the sample holder is inserted in the sample chamber, a pin activates the lever attached to the tube shield and swings it upward. The construction details of the sample holder are shown in Figure 2. By measuring the sample holder supplied with the spectrometer and the sample chamber, the correct size may be decided. The sample should be positioned in the most intense region of the x-ray SECTION THRU SHIELDHANGER BEARING
beam (1, 4). A sample well approximately 1 inch in diameter and */B inch deep wm used for the Norelco spectrograph, because this size utilized nearly all of the incident x-ray beam without requiring a large samp!e cup. The walls of the cup should be as thin as possible, yet consistent with good design and adequate strength, so that thermal equilibrium is reached rapidly. The parts not exposed to the x-ray beam were machined from brass. Aluminum of the highest purity was used for the parts exposed to the x-ray beam, because impurities cause interfering fluorescence. The two wedges attached to the micrometer allow the sample well and water jacket to be moved up or down for optimum intensity The sample cup is closely fitted to the holder housing to prevent tilting. Drift in x-ray emission can be Compensated by adjustment of the micrometer, but large drifts are best compensated by adjustment of the x-ray tube voltage or of the current.
COOLANT
Figure 2. Sample tray with variableheight, water-cooled sample cup X-RAY TUBE IATION S H I E L D
A thin Mylar or other suitable plastic film attached to the top of the sample cup with heavy grease was a satisfactory sample cover. It was discarded after use and the grease was removed with chloroform. A small bevel was machined a t the top of the sample well. For most work, adequate reproducibility in height of the sample was attained by filling to the bottom of this bevel. This was advantageous in work with viscous liquids that could not be measured accurately by volume and are difficult to introduce into sample holders with a fixed cover. The micrometer should be adjusted so that the sample cell can never strike the x-ray tube when a sample cover is used. A cover should be used whenever there is danger of splattering if the sample well is placed very near the x-ray tube. It is possible to work much closer to the x-ray tube with this sample holder than with the standard model. This results in increased intensity and lower practical detection limits for analysis. The sample holder may readily be modified for work with powdered samples. Thermal equilibrium is attained within 2 to 5 minutes for samples, if water a t 10' C., for example, is circulated through the holder a t 2 liters per minute. The sample holder should remain in the sample housing, except when it is being filled or cleaned, so that rapid thermal equilibrium may be reached. Water hose disconnects (8) may be used if it is necessary to remove the water connections frequently. If a helium path is used, the flow of helium should be well stabilized, and the rate of flow should always be the same for a particular standardization. Flow stability may be attained readily with the usual helium regulator, if a back pressure of about 2 p.s.i. is used. A small orifice following the flowmeter may be used to product. thr dcsired back pressure.
LITERATURE CITED
(1) Davis, E. RT., Hoeck, B. C., A ~ a r . . CHEM.27, 1880 (1955). (2) Doughman, R. R., Hirt, R. C'., Langdon, R. T., Norelco Reptr. 4,No.2 . Figure 1 . Adaptation of swinging radiation shield to tube housing on Norelco x-ray spectrograph
1928
ANALYTICAL CHEMISTRY
(1957). (3) Dwiggins, C . W., Dunning, H. S . , ANAL.CHEM.31, 1040 (1959). (4) Hirt, It. C., Doughman, W. I t . , Gisclarti, J. B., Ibid., 28, 1649 (1950).
