ANALYTICAL CHEMISTRY
14 (16) Garlock, E. A., Jr., and Grove, D. C., J . Am. Pharm. Assoc., 39, 398 (1950). (17) Golumbic, c., ANAL.CHEM.,22, 579 (1950). (18) Grant, H. S.,and Jones, J. H., Ibid., 22, 679 (1950). (19) Hardy, A. C., and Young, F. hl., J . Optical SOC.Am., 39, 265 (1949). (20) Haslett, F. P., Hannan, R. B., Jr., and Wells, J. H., ..~NAL. CHEM.,22, 1132 (1950). (21) Holaman, G., and Niemann, C., J . Am. Chem. SOC.,72, 2044 (1950). (22) Kinsey, V. E., ANAL.CHEM.,22, 362 (1950). (23) Loofbourow, J. R., J . Optical SOC.,Am., 40, 317 (1950). (24) Loofbourow, J. R., Gould, B. S.,and Siaer, I. W., Arch. Biochem., 22,406 (1949). (25) Lundgren, P., and Wallen. O., Svensk. Farm. Tid., 54, 273 (1950). -4., ~ A L CHEM., . 22, 494 (1950). (26) RlcGillivray, (27) RIellon, M. G., ed., “Analytical Absorption Spectroscopy,” S e w York, John Wiley & Sons, 1950. (28) hfoeller, T., and Brantley, J. C., ANAL. CIIEM.,22, 433 (1950). (29) Moeller, T., and Cohen, A. J., Ibid., 22, 686 (1950).
mr.
(30)
Afoeller, T., and Cohen, -4.J., J. Am. Chem. SOC.,72, 3546
(1950). (31) Munch, R. H., Ind. Eng. Chem., 39, 75,4 (April 1947). (32) Perry, J. A., Sutherland, R. G., and Hadden, N., ANAL. CHEM., 22, 1122 (1950). (33) Racker, E., Biochem. et Biophys. Acfn,4, 211 (1950). (34) Schauenstein, E., and Treiber, E., J . Polvmer Sei., 5 , 145 (1950). (35) Shugar, D., Bull. soc. chirn. b i d , 31, 1659 (1949). C m x , 22, 1030 (1950). (36) Telep, G., and Boltz, D. F., ANAL. (37) Trenner, K.R., Ibid., 22, 405 (1950). (38) Vacher, hl., and Faucquembergue, D., BUZZ.SOC. chim. biol., 31, 1419 (1949). (39) Vacher, M., and Tounichon, 0,,Ibid., 31, 1430 (1950). (40) S’arshavskiI,Y. hl., Zavodskaya Lab., 15, 1476 (1949). (41) Warshowsky, B., and Schants, E. J., ANAL.CHEM.,22, 460 (1950). (42) Killits, C. O., Swain, M. L., Connelly, J. A,, and Brice, B. A , , Ibid., 22, 430 (1950). (43) Zaffaroni, A., J . Am. Chem. SOC.,72, 3828 (1950). RECEIVED November 10, 1950.
X-RAY ABSORPTION H E K M i N i. LIEBH-IFSKY, General Electric Co., Schenectady, Y. Y .
P
XPERS presented a t scientific meetings since the last review
show that analytical methods based upon x-ray absorption and related phenomena are being put increasingly to use. Among these meetings was a Symposium on Instrumental Methods for the Determination of Tetraethyllead in Gasoline, held by a subdivision of Committee D-2, American Society for Testing Materials. Of the four papers on x-ray methods given on that occasion, only the introductory paper (22) was published 29) have now appeared in time for this review. The others (4,6, in ANALYTICAL CHEMISTRY,and the four will be reviewed as a group later. Other papers as yet unpublished are listed chronologically in Table I. Two of these deal with the determination of uranium, a logical field for methods of the kind considered here. Clark has described the field under review in the excellent first chapter of a recent treatise on physical methods in chemical analysis ( 2 ) . The arrangement and nomenclature oE the previous reviews (20, 81) are retained here, and a discussion of important developments in the analysis of biological materials is included. X-RAY ABSORPTION SPECTROMETRY
the determination of minute amounts of calcium and phosphorus in biological material. His subsequent work, both with monochromatic and with filtered beams, warrants discussion not only because of its great importance in biological problems, but because the analytical chemist may find it profitable to adapt Engstrom’s techniques in special cases. Usually the method of Glocker and Frohnmayer (n-hich might be called differential absorptiometry with monochromatic beams) consists in making absorbance measurements a t several wave lengths above and several below an absorption edge of the element sought. An x-ray spectrometer is consequently required. Engstrom’s “spectrometer” is novel and simple, and appears to give radiant energy that is satisfactory for microanalyses where the utmost precision is not sought. Collimated x-rays sufficiently low in wave length fall upon a sheet of an element mounted (asone of 16) on a disk that can be rotated. The K-lines of this element are excited and passed (after filtering, if desired) through the sample and to the detector, usually a photographic film, though others will do. In this way, intensity measurements are made a t one wave length above, and one below, that corresponding to an absorption edge of the element being determined. The example below (compare with Equations 6 to 10, 20) will serve in lieu of further discussion.
The first review (20) mentioned that Engstrom (13) had successfully applied the method of Glocker and Frohnmayer (18) to
Table I. duthors
Iron has its K-absorption edge a t 1.74 A., and for its determination Engstrom used the K a lines of nickel (1.66 A.) and cobalt (1.79 A.‘). Suppose z is the proportion of iron in a s a m p l e weighing m grams. The fundamental absorption Recent Unpublished Papers on X-Ray Absorption equations are:
Liebhafsky H. A and Winaluw, E . H.. GenLral Electric Co. Hughes, H. K.. and Wilczewski. J. W , Socony-Vacuum Co. Levine, S. W.. Atlantic Refining Co. Lambert, M. C., General Electric co. Lamb, F. W.. and Niebylski, L. >I.E t h y l Corp. Bartlett T. W Union Carbide SCarbdn Corp.”
Title Differential .kbsorbance Rleasurements on Polychromatic X-Ray Beams Rapid X-Ray Determination of Sulfur in Distillate Fuels
Determination of Sulfur in Petroleum Fraction b y X-Ray d b sorption X-Ray Photometric Determination of Uranium Determination of Total Chlorine in Chlorinated Benzenes b y X-Ray Bbsorptiometry X-Ray Photometric Deterruination of Uranium in Solution
Meeting CHEMIClt SOCIETY, 117th Meeting, Houston, Tex., April 13, 1950 Symposium on Analytical Research, American Petroleum Institute, Cleveland, Ohio, May 1, 1950 Spplied Spectroscopy Society meeting, S e w York. May 26-27, 1 _win ”-Ai\fERICAN CHEMICAL SOCIETY, Pacific Northwest Regional Meeting, Richland, Wash., June 9, 1950Gordon Research Conference Colby Junior College New London, N. H., 4ug. 2, i950 -4XERICAS CHEMICAL SOCIETY 118th Meeting, Chicago, 111.:
A\lERICAS
Bept. 7, 1950
and
+
log IL/I’ = kb,mx kgm ( 1 - 2 ) ( A = 1.64 A.) (2) when the k’s are proportionality constants ( 2 0 ) and the subscript S refers to the sample less iron, free or combined.
V O L U M E 23, N O . 1, J A N U A R Y 1 9 5 1
15
The second term on the right-hand side of these equations can be eliminated by introducing a factor ( l J 4 / 1 79)p to compensate the increased absorbance due to S a t 1.79 A. as compared with 1.64 A. (The value of P , which is usually near 3, is taken from tables of known data.) Multiplying both sides of Equation 1 by ( 1.64/1.79)p, and then subtracting Equation 2 gives (*.fi4/1
7Q)p
log Io/I
- log Ii/I‘
=
(1.64/1.79)Pk~em -5 kk,mx
(3)
provided P has been properly chosen. When the four intensities ( 1 ’ s ) have been measured, 5 is the only remaining unknown. Equation 3 is thus analogous to the simpler Equation 10 ( 2 0 ) and is equivalent to the expression given by Engstrom ( 1 3 ) . He tested the method on aqueous solutions of known iron content near 1 cu. mm. in volume. Results apparently precise to about 5 % were obtained for iron contents in the microgram range. Engstrom (12) has determined calcium and phosphorus in the dentine cells of a guinea pig’s tooth in substantially this way. Details of this and other applications are given in the original literature (12), and Figure 1 contains experimental results in units that emphasize the scale of operations, Cauchois and Mac Taggart ( 7 )have applied this type of differential absorptiometry to the zirconium-hafnium system, Several possibilities were explored, but no detailed results are given. Most of the work seems to have been done on the determination of zirconium, the Kal (0.708 A.)-or Kaz-and the Kpl (0.631 A.) lines of molybdenum being used to bracket the K-absorption edge of zirconium (0.687 A.). A large range of zirconium contents (ranging down to 1 mg. per sq. em. in a cell several centimeters long) was covered and results precise to several per cent were obtained. The zirconium-hafnium problem may be regarded as a proving ground for x-ray methods (3, IO, 29). As sufficiently extensive results become available, these could well serve as a basis for evaluating methods and equipment
used to distinguish this energy from the fluorescence used, for example, by Birks and Brooks ( S ) . ] This background, which is a nuisance in diffraction investigations, has been cleverly used by Beeghly (1) to measure the thickness of tin coating on steel. His scheme will find other applications. The primary x-ray beam is chosen to give a maximum intensity of secondary radiant energy from the base metal and a minimum from the coating. Absorption by a coating reduces the intensity (power) of both the primary beam and the secondary radiant energy excited in the base metal. The sum of these effects is a function of the coating thickness, such that a simple empirical calibration curve is obtained. The results of determinations based upon this curve are given in Table I1 (taken from I ) , together with coating weights found by a standard (chemical) method.
Table 11. Coating Weights Obtained by Standard and X-Ray Methods Sample 1
Side A
B
Av.O
2
B
6
0.52
.4 B AV.
5
0.42
A
Av. 4
0.52
A B Av.
3
Coatin Wei ht, P o u n d s b a s e %ox Rtd. method X - r a y method
0.50
0.50
A
B Av. A
B Av.
0.70 0.83
A
B Av.
0.49 0.54 0.52 0.41 0.39 0.40 0.54 0.51 0.625
1.03
0.47 0.485 0.73 0.70 0.715 0.81 0.83 0.82 1.21 1.08 1.145 1.55 1.31 1.43
A B Av. 1.25 a Values obtained b y standard method are true averages. Averages shown for x-ray method are averages of two readings, one on each side of test coupon, and are not necessarily true averages. First six coatings electrolytic; others, hot dip. 8
0.4
0,e DISTANCE
1.2
FROM
16 .
APEX, MM.
Figure 1. Determination of Calcium and Phosphorus in Dentine Cells of Tooth from Guinea Pig ( 1 2 )
The Dow automatic x-ray absorption spectrometer (80) continues in successful operation and is used for about 40 chlorine and/or bromine determinations each month (17). ABSORPTIOiMETRY
Analytical methods based upon x-ray absorption are proliferating to an extent that makes simple classification difficult. The methods discussed in this section employ beams considerably removed from monochromatic. Secondary (Fluorescent) X-Rays. In x-ray diffraction work, the diffraction lines are superimposed on a background composed largely of “5uorescent” radiant energy. [Quotation marks are
The footnote considered, Table I1 shows excellent agreement between the two methods. A unit, already in experimental operation, has been built for the continuous recording and control of the amount of electrolytic tin coating on steel. Polychromatic Beams. The introductory paper (22) in the A.S.T.M. symposium on tetraethyllead proved that it is possible t o carry out differential absorptiometry with polychromatic beams, and further progress has been reported but not published (23). The fundamental idea is that turned to use by Glocker and Frohnmayer (18), but the complications possible with polychromatic beams are formidable enough so that the feasibility of a differential absorptiometric method using such beams could not be taken for granted. The reason for attempting the differential measurements was, of course, to eliminate or mitigate the interference to which measurements with a single polychromatic beam are subject. This aim was realized, and it proved possible to determine both tetraethyllead and sulfur in a hydrocarbon medium. Microradiography with Filtered Beams. In radiography, shadowgraphs are made on either a fluorescent screen or a photographic film. When these are evaluated quantitatively (usually by densitometering a film), radiography becomes absorptiometry. The necessity for studying small specimens has led to the development of microradiography, in which shadowgraphs
ANALYTICAL CHEMISTRY
16
on very fine-grained film are enlarged up to several hundred times. Such shadowgraphs can yield chemical information that cannot be obtained by the more conventional methods of microanalysis. Clark (8,9) in 1939 pointed out the advantages of using monochromatic beams in microradiography, and he has obtained outstanding results in this way. Engstrom has recently reviewed the field (18). For many important problems involving biological materials filtered beams suffice, as Engstrom and Lindstrom have recently shown (14). I n their method, a microradiogram is taken of a biological specimen and a standard built up of strips of nitrocellulose. The exposures m-ere made in vacuum because x-rays of long wave length (an 8 to 12 A. band transmitted through an aluminum filter) had to be used. Quantitative evaluation of the radiographs for standard and unknown gave (to within less than 10%) the distribution of mass in the latter. Engstrom and Luthy (15)studied a single nerve fiber (diameter, about l o p ) by this technique. They proved that the sheath of the fiber has a density of 0.3 to 0.4 (10-12) gram per cu. micron, and the center a density of 0.08 to 0.09 (10-12) gram per cu. micron. A reduction in the calcium content of bone tissue is of particular interest in chronic polyarthritis. Engstrijm and Welin (16) estimated the calcium contents of bones in the thumbs and index fingers of men and women by a method resembling that described above. Aluminum step wedges served as standards. I n clinically advanced cases, good correlation was found between the severity of the disease and the degree to which the content of calcium salts in the bones had been reduced. The work of Engstrom and his associates demonstrates that x-ray absorption methods can reach far into the domain of microchemistry and calls the attention of analytical chemists to developments in the analysis of biological materials, a field that is bound to grow as research in the life sciences continues to expand. Oil-Bearing Strata. Boyer, Morgan, and Muskat (5) have measured by an absorptiometric method the oil saturation attained in cores during fluid-flow experiments. A soluble material of high mass absorption coefficient--e.g., iodobenzenewa8 added to the oil. The extent to which this solution permeates a specimen of oil-producing rock was then established by passing a polychromatic x-ray beam through the specimen, an ionization chamber serving as detector. The method is applicable also to the study of flooding by water. It differs little in principle from the “direct method” of chemical analysis with polychromatic x-ray beams (20). X-RAY FLUORESCENCE
I n an important paper, Birks and Brooks (3) have shown that quantitative results of acceptable precision (near 4% of the amount present under favorable conditions) can be obtained by using a Geiger counter to m e m e the intensities of the fluorescent linea excited by polychromatic x-ray beams in either the zirconium-hafnium or the tantalum-columbium systems. I n both, the nature of the elements makes chemical separation difficult. Owing to the atomic-number relationships within each pair (Zr 40, Hf 72; Cb 41, T a 73), the two systems present almost identical x-ray problems. The most serious of these problems is the overlapping of the second-order K-lines of the lighter element in each pair and the firsborder Glines of the heavier. I n the zirconium-hafnium system, the overlapping was satisfactorily eliminated by resorting to fine collimation, and i t could be avoided by lowering the tube voltage 80 that the zirconium K-line was not excited. The former measure proved unsuccessful in the tantalum-columbium system. Here, however, quantitative results could be obtained by comparing the integrated intensity of an unresolved tantalum-columbium doublet with that of a resolved columbium line, a method applicable also to the zirconium-hafnium pair. Pr6vot (25) has estimated ruthenium by the older technique of bombarding the sample (as the residue from the evaporation of a solution on a copper target) in an x-ray tube to excite fluorescent
( L ) lines. Amounts as low as 0.05 microgram of the element could be detected under favorable conditions; in the presence of 10 mg. of molybdenum (as molybdate), the threshold was tenfold higher. MISCELLANEOUS
An analysis of 7678 films (67) worn by personnel exposed to x-rays and to radioactive isotopes showed that the permissible dose x a s being approached and exceeded much more frequently in the case of x-rays. Victoreen (28) has given an empirical method of calculating mass absorption coefficients that gives values apparently reliable to about 1%. His extensive tables not only fill gaps in the experimental values but in some cases improve upon them. Gamma-ray absorption by liquid chlorine is the basis of two methods of measuring the level of this liquid (24). Beta-ray absorption is becoming increasingly important in continuous thickness measurements, especially under conditions where x-ray absorption is too small for wave lengths that can be used in air (8,11). Finally, nuclear bombardment is beginning to be used a i an analytical method. Riezler (26) reports that 0.001% ’ of silicon or sodium can be detected in aluminum that is bombarded with deuterons. Perhaps nuclear accelerators will be de rigueur in the analytical laboratorv of the future! ACKNOWLEDGMENT
The author wishes to thank his various correapondehts ah$ hid colleague, E. H. Winslow, for helping to assemble the material for review. LITERATURE CITED
Beeghly, H. F., Trans. Electrochem. Soc., 97, 152 (1950). Berl, W. G., “Physical Methods in Chemical Analysis,” New York, Academic Press, 1950. Birks, L. S., and Brooks, E. J., ANAL.CHEM.,22, 1017 (1950). Birks, L. S., Brooks, E. J., Friedman, H., and Roe, R. M., Ibid., 22, 1258 (1950). Boyer, R. L., Morgan, F., and Muskat, M., Tram. Am. Inst. Mining Met. Engrs., 170, 15 (1947). Calingaert, G., Lamb, F. W., Miller, H. L., and h’oakea, G. E., ANAL.CHEM.,22, 1238 (1950). Cauchois, Y., and Mac Taggart, K., Compt. rend., 228, 1003 (1949). Clapp, C. W., and Bernstein, S., Elec. E w . , 69, 308 (1950). Clark, G. L., and Gross, S. T., IND. EWG. CHEM,,A N A L . ED., 14, 676 (1942). Coster, D., and Hevesy, G. von, Nature, 111, 79 (1932). Crawford, E. A., and Strain, M., Tech. Assoc. Pulp and Paper rnd., 33, 190 (1950). Engstrom, A., Acta Rudiol., 31, 503 (1949). Engstrom, A., Nature, 158, 664 (1946). Engstrom, A., and Lindstrom, B., Biochem. et Bwphys. Acta, 4 , 351 (1950). Engstrom, A,, and Liithy, H., Ezptl. Cell Research, 1, 81 (I@50). Engstrom, A., and Welin, S.,Acta Radiol., 31, 482 (1949). Frevel, L. K., Dow Chemical Co.. Midland, Mich.. lettsr to H.A.L.. Oct. 6. 1950. (18) Glocker, R., and’Frohnmayer, W., Ann. Physik, 76, 369 (1925). (19) Hughes, H. K., and Hochgesang, F. P., ANAL.C H ~ M22, . , 1248 (1950). ~~..., (20) Liebhafsky, H. A., Ibid., 21, 17 (1949). (21) Ibid., 22, 15 (1950). (22) Liebhafsky, H. A., and Winslow,E. H., A.S.T.M. Bull., No. 167, 67 (1950). (23) Liebhafsky, H. A., and Winslow, E. H., Division of Analyticd Chemistry, 117th Meeting, AM. CHEM.SOC.,Houston, Tex., March 29, 1950. (24) McCarthy, F. S., and Rice, G. A., J . Electrocha. Soc., 97, 249 (1950). (25) PrBvot, A., J . chim. phys., 45, 251 (1948). (26) Riesler, V. W., 2.Naturforsch., 4a, 545 (1949). (27) Spalding, C. K., de Amicis, E., and Cowing, R. F., Nucleonice, 5, No. 6, 63 (December 1949). (28) Victoreen, J. A,, J . Applied Phys., 20, 1141 (1950). (29) Zemany, P. D., Winslow,E. H., Poellmits, G. S., and Liebhafsky, H. A . , ANAL.CHEM.,21, 493 (1949). RECEIVED November 6. 1950
