Internal Differential X-Ray Absorption Edge Spectrometry THOMAS J. CULLEN'
U. S. Metals Refining Co., Carteret,
N. J .
b Differential x-ray absorption edge spectrometry is used to determine the amount of an element present in a sample by measuring the transmitted x-ray beam's intensity on both sides of an element's x-ray absorption edge. If a sample contains another element whose characteristic radiations fall on both sides of the absorption edge, the ratio of the characteristic line intensities will b e proportional to the concentration of the element associated with the absorption edge. The element providing the necessary bracketing lines may b e present in the original sample or may b e added intentionally. Because the intensities measured originate within the sample and not from an independent source, this method is called internal differential x-ray absorption edge spectrometry. Another technique is also described in which the two wavelengths bracketing the analytical absorption edge are derived from the continuous spectrum scattered by the sample. These methods are independent of the concentration of the elements used and sample geometry or surface condition. An x-ray emission spectrograph is used for these methods without alteration. Examples of such analyses are given and limitations discussed.
x
ABSORPTION methods of analysis can be divided into two main types: measurement of either the amount of polychromatic or the amount of monochromatic x-ray radiation absorbed by a sample. Likewise, x-ray absor1)tion edge methods can be so divided. .\bsorption edge methods involve the measurement of the amount of absorbed radiation a t the short wavelength side of the absorption edge of the element of interest and are therefore more sliecific than the first method. If measurements are made on both the short and long wavelength sides of the absorption edge, then the method is known as differential x-ray absorption edge spectromet,ry. All these methods involve the use of x-ray radiation generated by an external source. Discussions of these me1 hods are found in several sources 4). Hertin's recent -RAY
Present
Xutley, N. J.
addresi;, 07110
Sel-Rex
paper ( 1 ) in which secondary target materials are used to generate the, absorbed x-ray radiation lists many characteristic lines of elements which bracket absorption edges of elements to be determined. This paper describes methods t,hat utilize x-ray radiation either generat,ed within or scattered by the sample itself. Application of these methods is as follows. Case I. An element having spectral lines bracketing the analytical absorption edge is already present in the sample. This case is illustrated by 'the determination of Xi in Cu-Xi binary alloys, where the C u K a and -KOlines bracket the nickel K absorption edge. Case 11. The element having t,he bracketing lines is added to t,he sample. This case is illustrated by the addition of Br t'o Se samples. The RrKa and -K/3 lines bracket the SeK absorption edge. Case 111. The two bracketing lines are derived from the continuous spectrum originating a t the x-ray tube target and scattered by t'he sample. This case is illustrated by the determination of Alg in solution samples. Characterist,ic lines of the x-ray tube target material may be used in this method if the wavelengths are suitable. The K, L, 11, etc., line series is generated by the removal of an orbiting electron by either a high energy photon or electron. The x-ray charact,erist,ic line is produced by an outer shell electron falling into the orbit vacated by the ejected electron. Thus, each series of lines is produced by the single event of an ejected electron. The number of x-ray photons produced in this process which give rise to the different lines of the series is governed by the selection rule. In the simplest case, the K series, the x-ray phot,ons K a and Kp are produced in the following manner by an atom.
K state -+ L state K state + A I state
+ K a photon + KO photon
(1) (2)
T h a t is, the K state, a K shell electron ejected, can undergo two processes, replacement by either an L or AI shell electron. The L state process is about five times more likely to occur according to the selection rule. The intensity ratio of Ka/'KP radiations is therefore about 5. This ratio is experimentally
obtained for pure elements. If a second element is added to the pure element, the ratio will be changed to a greater or lesser extent, depending on the location of the second element's x-ray absorption edge and physical density. I f the second element has an absorption edge between the K a and Kp lines, then the ratio is affected to the greatest degree. The I@ photons will be of sufficient energy to be absorbed by the added element and the K a photons will not be of sufficient energy, and thus will not be absorbed. The intensity ratio of Ka,'Kp will then increase in value as the amount of the element associated with the absorption edge increases. In principle, the absorption edge does not have to be of the same series as the characteristic lines of the ratio. Scattered radiation which originates from the x-ray tube decreases in intensity on the short wavelength side of an absorption edge of an element present in a sample specimen. The physical phenomena of x-ray scatter, both coherent and incoherent (Compton), are different from those of characteristic line generation ; however, the intensity of the scattered x-ray photons is dependent on mass absorption laws and density. INSTRUMENTAL DATA
The experimental data presented in this paper were obtained using General Electric XRD-5 and -6 x-ray fluorescence spectrometers. Tungsten target x-ray tubes were used to excite the fluorescent spectra or generate the radiation scattered by the sample. Lithium fluoride analyzing crystals, 0.010- x 3inch Soller slits, and scintillation-type counter tubes were used. Preset count times of 400 seconds were used for most measurements. Power parameters on the x-ray tubes were varied, but were selected to give a maximum count in each sample system. The solution samples were Idaced in a Spex Industries solution cell and covered with lj4-mil Mylar film. Coppernickel alloys were used as briquetted drillings and were rotated to minimize possible segregation effects (3). The ratio of characteristic lines is independent o f surface effects. Thus the sample shape or size is not important so long as the sample geometry is such that it does not affect the optics of the instrument used, and a usable count rate is obtained. VOL. 37, NO. 6, MAY 1965
0
71 1
Table I.
Mass Absorption Coefficient Corrections for Iron on CuKa/CuKb Intensity Ratios for Nickel
Table 11. Effect of X-Ray Tube Power Parameters on Intensity Ratios
Power parametersa
Ratio, CuKa/ Intensity * kv. ma. CuKa CuKP CuKP 15 10 39264 4718 8 322 17 10 71230 8477 8 403 18 10 88901 10515 8 454 15 i 28848 3548 8 128 20 5 61846 7291 8 482 a OEG-75 tungsten target x-ray tube. Single sample of copper-nickel alloy, 100 sec. count times.
nickel (8.90) are nearly the same, eliminating this source of nonlinearity. A pure copper briquet was used to determine the 0% nickel calibration point. Addition of another element to the sample will affect the ratio in direct relationship to the ratio of mass absorption coefficients a t the two wavelengths measured. Addition of iron to the copper-nickel system illustrates this
RESULTS A N D DISCUSSION
An example in which the sample already contains the element having lines bracketing the analytical absorption edge is illustrated by the determination of nickel in Cu-Si binary alloy. The C u K a (1 5 4 A) and CuKp (1.39 fall on either side of the nickel K absorpt’ion edge (1.49 A , ) . The intensity ratio CuKalCuKp will increase as the nickel content increases. Figure 1 shows the calibration of a series of copper-nickel alloy samples. The int,ensity ratio of CuKaICuKp is plotted us. the nickel content. A straight line relationship is noted. The densities of copper (8.92) and
10
20
30
40
50
PERCENT NICKEL
Figure 1 . Nickel calibration on briquetted copper-nickel binary alloy
712
ANALYTICAL CHEMISTRY
SELENIUM
MG\ML
Figure 2. Selenium calibration on selenium solutions with bromine added as potassium bromide
point. The mass absorption coefficients of iron a t the C u K a and CuKp wavelengths are reported as 325 and 252, respectively. Hecause the C u K a intensity is affected to the greatest extent, it can be corrected by adding to it an intensity calculated by the following: I C u K a wt’. fraction of iron present ratio (325/252). This correction requires that, the amount of iron present be known. .A conventional x-ray fluorescence spectro metric deterinination is adequate in this case. Table I shows the results obtained froin such corrections. If a sample does not contain an element \\ith the required characteristic lines, it, may be added. This may be used advantageously in powdered and liquid samples. 13ecause of dilution and density effects, a constant amount of this element should be used, although
/ 25
50 75 SILVER M G \ M L
100
Figure 3. Silver calibrations on silver solutions and density-corrected calibration (straight line)
accurate weighing or pipetting is not necessary if the amount added is small. This method is illustrated by the addition of bromine to a solution of selenium. The SeK absorption edge (0.98 -4.)is between the BrKa (1.04 A,) and I3rKp (0.93 .L). Bromine was added as potassium bromide to a chloride solution. Figure 2 shows the calibration obtained by plotting I3rKaj I3rKp intensity ratio cs. the selenium content. The calibration exhibits a negative deviation from the straight line because of density effects. Because background scattered radiation, which originates from the continuum radiation of the x-ray tube, decreases in intensity a t the short wavelength side of the absorption edge of an element in the scattering media, a ratio of the scattered intensities at the high and Ion wavelength sides is also directly proportional to the amount of the element associated with that absorption edge. To illustrate this method, a silver nitrate solution system was chosen because the silver K absorption edge (0.49 &\.) occurs near the maximuni of the x-ray continuum when sufficient x-ray tube voltage is used (in excess of about 35 kv.). The silver nitrate solutions were prepared from the salt and background intensity measurements were made at 0.63 A. and 0.46 d. ‘These wavelengths were chosen because they are free froin interference of the silver characterihtic lines. Figure 3 shows the calibration of the background intensity ratio us. silver content. The straight line relationship is obtained by correcting the curved line for density effects. The curved line calibration is entirely adequate for the silver determination if the hilver content is diluted to be within the range of the calibration
shown. The corrected curve is added to illustrate that the amimption of the rffecat of density is correct. The ~iower ~)aranietersset for the x-ray tube are not critical when the barkground is low in comljarison to the peak intensities or if net intensities are used in the calwlation of the ratio. The intensities used in the ratio calculatiow in this 1)al)er were gross intensities, peak 1)luh backgrourid. Table I1 s h o w the effevt of the power parameters on the intensity ratio of (L'uI
