Oxygen, nitrogen, methane, and carbon monoxide are eluted in 34, 53, 101, and 154 seconds, respectively. A standard calibration curve is prepared by plotting the ratio of C H I / N ~peak heights us. the volume % MMH. RESULTS AND DISCUSSION
A typical chromatogram is shown in Figure 1. The relationship b e x e e n the ratio of methane to nitrogen and the MMH concentration was determined on six different occasions and had the following slopes: 1.70, 1.75, 1.70, 1.64, 1.64, 1.69. The data are tabulated in Table I, together with the stand,trd deviations for those concentrations where there were enough determination3 for a statistical calculation. It was fe t that four or less determinations were insufficient. The ratio of CH4 to Nz has been plotted, rather than the CHh value itself. This eliminates the va aiations in day-today instrument respor se and any variations in the size of the 0.50-cc. gas sample which is injected. The calibration curve intersects the x-axis a t a concentration of 0.3y0, which possibly is
the limit of detectability under these conditions. Nitrogen is produced in the reaction; however, it is negligible compared to that present in the air in the serum bottle. Potassium chloride is used in the reaction to decrease the solubility of methane in the liquid phase. The molar ratio of sodium hypochlorite to M M H on the average is 5 to 1, so that there is always an excess of hypochlorite present. The stoichiometry of the reaction of M M H and sodium hypochlorite in aqueous solution, as well as the mechanism for the production of both carbon monoxide and methane is not known. Possibly under the conditions of the reaction, dealkylation occurs to produce a methyl radical which can abstract hydrogen to form methane. Methane in turn may be oxidized to carbon monoxide. The presence of alkyl chlorides certainly would lend credence to this type of reaction. No attempt has been made to study this reaction further. Under the conditions used in this
Table 1.
MMH,
%
0.4 0.5
Reproducibility Data
No. of
detns.
CHdN2 X lo2, average
Std. dev.
0.17 0.34
...
0.8
3 4 3
0.82
4.0 4.5
4 2
6.53 7.05
e . .
...
method, the maximum concentration of MMH permissible would be of the order of 5%. It certainly would not be suitable as a method of assay for MMH. LITERATURE CITED
(1) Clark, J. D., Smith, J. R., ANAL.
CHEW33,1186 (1961). RECEIVED for review October 14, 1963. Accepted December 6,1963. The authors thank the Olin Mathieson Chemical COT. for permission to publish, especially the personnel of the Chemicals Division, who supported the study.
A Simplified Routine Method for X-Ray Absorption Edge Spectrometric Analysis EUGENE P. BERTIN, RITA J. LONGOBUCCO, and RITA J. CARVER Radio Corporation of America, Electron Tube Division, Harrison, N.
b A
simplified routine method for x-ray absorption edge spectrometric analysis was developed for use on standard flat-crystal x-ray spectrometers with only minor modifications. The only x-ray measurements required are the intensities transmitted by the sampIe(s), an empty sample cell, a correction standard, and the open x-ray secondary beain tunnel at each of two wavelengths bracketing an x-ray absorption edge of the element determined. Sample concentration is calculated from these measurements and from certain other data derived from the literature, Tables of such data were compiled and are presented. No standards or calibration are required, and matrix effects are usually absent. The bracketing wavelengths are x-ray spec:traI lines excited in secondary targets placed in the sample drawer. The procedure is most readily applied to solutions, but is also useful with pciwders, briquets, thin slices, foils, and films. Cells for all these sample fcirms and other accessories were designed and made, and are described. The simplified
J.
procedure was evaluated for 10 elements having a wide range of atomic number in a variety of sample forms. The results are only a little less satisfactory lhan those reported in the literature for the more elaborate procedures.
T
HE ANALYTICAL CHEMIST is most frequently called upon to strive to increase the accuracy of his methods. However, there are still many types of analysis for which some accuracy may be sacrificed in the interest of increased speed and convenience. The objectives of this project were to eliminate the preliminary experimental work and some of the refinements involved in the x-ray absorption edge procedures described by Barieau (1) and Dunn ( 6 ) , to incorporate certain techniques reported by various other workers, and to evaluate the resulting simplified procedure for elements having a wide range of atomic number in a variety of sample forms. X-ray absorption edge spectrometry has been known for nearly 40 years (IC), and reviews of its progressive
development have appeared regularly ($0, 21). Routine procedures of general applicability and high precision and accuracy have been described by Barieau ( I ) , Dunn (6), Hakkila and Waterbury (15), Knapp, Lindahl, and Mabis (I@, and others. In view of the many advantages of absorption edge spectrometry, it is difficult to understand why the method is not more widely used. The preparation of standards and calibration curves for x-ray fluorescence spectrometric analysis of a multicomponent system is warranted if there are many samples to be analyzed from time to time, but usually not for one or a few samples on a single occasion. Absorption edge spectrometry eliminates the need for preparation and storage of standards and for their measurement each time an analysis is made. The method is applicable to liquids, solids, briquets, powders, foils, thin films, and even gases. Liquid volumes as small as 0.2 ml. are usually sufficient. Matrix effects are usually absent because only the element determined undergoes a marked change in absorption coefVOL. 36, NO. 3, MARCH 1 9 6 4
641
ficient at the wavelengths measured. Since the calculations involve ratios of the intensities transmitted by the empty cell and the sample-filled cell, the method enjoys many of the advantages of ratio methods in general. The disproportionate intensity-concentration relationship for an element having short-wavelength spectral lines in a light matrix and the overlap of higherorder lines of matrix elements with firstorder lines of the element determined are well-known difficulties in x-ray fluorescence spectrometry; they do not apply to absorption edge spectrometry. The limitations of the method include the following. Compared with x-ray fluorescence spectrometry, the method is not as readily applicable to elements of low atomic number (Mg through Ti) because of high absorption of the long wavelengths in the matrix and cell windows. It is not readily made as sensitive to low concentrations. Sensitivity is reduced by the presence in the matrix of elements having high atomic number, and the reduction is more severe the higher the atomic number and concentration of the interfering element. The principal source of specific interference is the infrequent occurrence of coincidence or near coincidence of absorption edges. The routine procedures described thus far require more or less elaborate preliminary experiments for each element determined for the first time. Finally, the calculations are rather elaborate. The Simplified Method. DESCRIPTION. The x-ray measurements are made on a standard flat-crystal x-ray fluorescence spectrometer (7, 16, 18), rather than a diffractometer, and are made a t only one wavelength on each side of the edge. The bracketing wavelengths are derived from secondary targets external to the x-ray tube (6, 8,9, 15, 18). The targets are chosen so that one has a characteristic spectral line close to the short-wavelength side of the edge, the other a line close to the long-wavelength side. Alternatively, a single target may provide both lines (18). Correction for the difference in matrix absorption at the two wavelengths is made in a manner similar to the one worked out by Barieau (1) from measurements a t the two wavelengths on a "correction standard" containing none of the element determined. No preliminary experiments are required, and all other data necessary for calculation of concentration is derived from the literature. Tables of such data have been compiled and are presented below. ADVANTAGES. The simplified procedure has all the advantages already cited and several additional ones as follows. Preliminary experimental evaluation of absorption coefficients, mathematical 642
ANALYTICAL CHEMISTRY
tm
3-4 a2
m M
2
* m3 m 3m
N
ieg
n
s
00
0
00
CD
? oq 0
0
0
000
c 3 0 0
00000000000000000
0
0
00000000~~0 ej
x U
Eob M
VOL. 36, NO. 3, MARCH 1964
643
A
A m
AE
A’2m A’
Lt:&dJ WAVELENGTH A
Figure 1. Idealized x-ray absorption edge, showing wavelengths and absorption coefficients involved in x-ray absorption edge spectrometry X and A’ ore bracketing line pair. AXZCOis wavelength interval equivalent to 200 volts
constants, or other parameters is not required. The simpler experimental techniques involved and the elimination of the preliminary work and some of the refinements of previously reported procedures result in greater speed, simplicity, and convenience. Most x-ray absorption edge procedures (1, 6, 16) are carried out on diffractometers. However, determination of chemical elements is more the concern of x-ray fluorescence spectrometric laboratories, many of which are not equipped for diffractometry, or a t least cannot afford the time required to rearrange their instruments from time to time for diffractometric operation. The total x-ray intensity incident upon the sample is only a small fraction of that in the more common method because the sample is not subjected to the primary beam. Thus the possibility of heating and chemical action in liquid samples with consequent expansion, gas bubble formation, precipitation, and deposition of solid on cell windows is minimized. Moreover, secondary (“fluorescence”) emission by the constituent elements in the sample is likely to be minimized by the reduced total incident x-ray intensity. The x-radiation passing through the sample is more nearly monochromatic and contains a minimum of radiation of other wavelengths scattered by the sample into the spectrometer at the 28 settings used. The two lines bracketing the absorption edge are usually supplied by different secondary targets and are then not coexistent in the incident radiation. Moreover, the incident radiation consists not of a continuous spectrum, but of fairly well separated spectral lines. Consequently, the resolution require644
0
ANALYTICAL CHEMISTRY
ments of the spectrometer are not as great as for selection of both monochromatic beams from the continuous spectrum. Dunn and Barieau both cite the desirability of operating the x-ray tube at a potential below that a t which harmonics of the absorption edge wavelength appear (unless pulse-height analysis is used). There is no such restriction on the operating potential in the present method. Finally, a relatively high x-ray intensity is available a t all wavelengths required for bracketing the absorption edge. This is not necessarily true for the primary beam except in the continuous radiation “humpI” particularly when the tube is operated a t reduced potentials to avoid generating harmonics of the absorption edge, as mentioned above. DISADVANTAGES. The same factors that result in speed and convenience inevitably result in certain disadvantages. The precision and accuracy are not as good as those reported for the more refined procedures. The method is limited to samples having relatively high concentration of the element determined] 0.1 or 0.2 mg./ ml. for solutions of elements having absorption edges a t relatively short wavelengths, even higher concentrations for elements of low atomic number. Probably the most serious limitation arises from the method of obtaining absorption coefficients of the element determined a t the bracketing wavelengths. As will be seen below, the coefficients are obtained graphically using values from published tables, and these are mostly calculated rather than measured values. It is well known that substantial discrepancies exist among the various sources of absorption coefficient data in the literature (16). Moreover, the graphic method necessarily disregards the fine structure of the edge and may be expected to give erroneous values, particularly on the short-wavelength side, for wavelengths within -200 volts (equivalent) of the edge. Finally, the writers have experimentally evaluated the procedure using the data in Table I for only 10 elements. It is to be recommended that each time the determination of an element is undertaken with a given line pair for the first time, a synthetic or other sample having known composition be analyzed along with the sample(s). This can usually be done using a solution of the element. EQUATIONS
If a monochromatic x-ray beam of intensity Io is directed through an absorber having mass absorption coefficient p cm.*/gram, density p grams/
emas, and thickness L cm., the transmitted intensity I is given by I = Io exp - p p L (1) or, in common logarithmic form, 2.303 loglo ( I o / l ) = 1pL (2)
For a solution of the element to be determined X in concentration cx g r a m ~ / c m .in ~ a matrix 111 of all the other constituents of the solution,
+
2.303 loglo ( I o / I ) = P X C X L PMCML (3)
Suppose now that the solution is traversed successively by two close x-ray wavelengths X and X‘ near, respectively, the short- and long-wavelength side of the absorption edge of the element determined, as shown in Figure 1. If Equation 3 represents A, a second equation involving I,,’, 1‘, px‘, and pM’ would represent A‘. If X and X’ are so close that p~ = p d , then subtracting the equation for A’ from that for X, solving for cx, substituting C (in mg./ml.) = 10OOcx (in grams/cm.3), and dropping the subscripts from px and px’ gives an equation similar to that derived by Barieau (1) and Dunn (6):
(4)
However, in practice A and X’ are somewhat remote from the absorption edge. Under these conditions if Equation 4 is to give satisfactory accuracy, a correction mechanism must be incorporated to compensate the difference in absorption coefficient of the matrix a t X and A‘. For this purpose, a correction method devised by Barieau (1) was adopted. Barieau notes that in a matrix having components 1, 2, . . . n, the ratios of the mass absorption coefficients pi/pi’ a t A and A‘ are related as follows. p,/p1’
= pz/pz’ = . . . = d p n ’ = (X/X’)” = k
(5)
where k and a are experimentally evaluated constants which are different for each A-X’ pair. Equation 5 assumes that none of the matrix components has an absorption edge between X and A’. Barieau’s correction constant k incorporated in Equation 4 gives
ilnalytical concentrations for liquids are calculated using Equation 6 as given above, and C is in mg./ml. Analytical concentrations for solids are calculated from Equation 6 rearranged to solve for LC in mg./cm.2 The term outside the brackets on the right side of Equation 6 is constant for any given element determined, A-X’ pair, and cell length. Thus simplified equations may be written for liquids and solids, respectively, as follows.
I
secondary targets were excluded: Kr, To, Xe, Pm, At, Fr, Ra, Ac, Pa, and elements having atomic number greater than 92 (U). Lines substantially closer to the absorption edge than -200 volts (equivalent) were excluded insofar as practicable. Table I gives the wavelength interval equivalent to 200 volts Aim a t the K ahsorption edges of elements 22 to 58 (Ti to Ce). The data was calculated from the Duan+Hunt equation as follows.
Figure 2. Accessory for x-ray absorption edge spectrometry in place on x-ray spectrometer
3-cm. liquid sample cell and aperture
12400 A h =A VE
C = &A; K L = 2303/(p - kp’)L (7) LC = KsA; Ks = 2 3 0 3 / ( p - kp’) (8) In Equations 7 and 8, A is the bracketed term on the right side of Equation 6. The parameter k is evaluated experimentally from measurements a t h and A’ on a “correction standard” from which the element determined is absent, and from the bracketed term in Equation 6. For C = 0, . log,,
( M I ) - IC log,, (IO‘/I‘)
=
0 (9)
from which k Equations 7,8, and 10 are the working equations of the procedure reported in this paper. Parameters p and p’ are obtained graphically in the manner described in the next section. L is the cell length or sample thickness. The parameter k is determined experimentally as just described. The intensity values are obtained in the manner described in the Routine Procedure section below. TABLES OF BRACKETING SPECTRAL LINES
Table I is a compilation of data necessary for the application of Equation 6 to absorption edge spectrometric determination of elements 22 to 58 (Ti to Ce) using their K absorption edges, and of Pt, Au, and Pb using their LIII edges. No data are given for Kr, Tc, and Xe. This tahle gives all data required other than the x-ray measurements on the sample, empty cell, correction standard, and open tunnel. Some additional reference data of general interest in x-ray absorption edge spectrometric work are also given. Figure 1 defines most of the quantities given in Table I. All wavelengths of spectral lines and ahsorption edges were taken from (IS));Ah and Ai’, defined in Figure 1,
were calculated from these wievelengths. All mass absorption coefficients were derived from curves of mass absorption coefficient as a function of zm.idnn.4h near the absorption edge of each element listed in column 1 of Table I. The curves were plotted on large log-log paper using data from ($8). At least five points were plotted on each side of the edge. From these graphs were derived PS and p ~ ‘ , respectively the coefficientsa t the short and long-wavelength side (that is, the “top” and “bottom”) of the edge, and p and p’, respectively, the coefficients a t the various spectral lines bracketing the edge on the short and long side. ApB, the absorption edge “jump,” and Ap, the differencebetween the coefficients a t the wavelengths of bracketing line pairs, were calculated from the coefficients derived from the curves. The spectral lines bracketing each absorption edge were found by searching in (IS). Lines having wavelengths close to the edge were considered with the following restrictions. Only Ka, KO, La,, %, LB2, and Lr, lines were considered because they are intense and relatively well separated. Kar refers to the Knl-Ka2 doublet and KO to Kp,. K a lines were excluded when the KarrKa2 doublet brackets the absorption edge, and Lal lines were excluded when the Lar,-Lat doublet brackets the ahsorption edge. Standard flat-crystal x-ray spectrometers do not resolve these doublets adequately for absorption edge work. K lines of elements having atomic number greater than 63 (Eu) were excluded because they are not excited efficiently by a 50-kv. full-wave x-ray unit. A constant-potential power unit would extend this upper limit somewhat, and would also substantially increase the intensities of the K lines of all the heavier elements. Lines of elements impractical as
- V E12,400 f 200
where Tz‘ is the excitation potential equivalent to the absorption edge in practical volts (If). The significance of the 200-volt interval will be discussed below. Table I lists one or two lines which meet the requirements cited on each side of each absorptioi1 edge. The lines are listed in order of iiicreasing distance f-,.... +hn cu6c, ..Amn. -.I+uL1uD > lines are listed in order of decreasing wavelength, X‘ lines in order of increasing wavelength. In most cases there was sufficient latitude of choice to enable avoidance of lines associated with relatively unavailable elements, and to enable inclusion of a Ka line. For each line on the short-wavelength side of hg the table gives the wavelength X, distance Ah from As, and mass absorption coefficient p of the element determined (column 1) at A. Also for each line, starting with the one nearer A, the table lists all K and L absorption edges which fall between that line and A., These edges are listed in order of increasing distance from k,. For simplicity, L edges are designated by arabic rather than the usual roman numerals ( L l , L2, L3, rather than LI, LII, LIII). The elements associated with these edges, if present in the sample matrix, may interfere with a determination using that line. Of course, all such interfering edges entered for the first listed line apply also to the second listed line, if any. If for a listed K a line the Karl-Kars doublet brackets an absorption edge, that edge is listed as an interfering edge. All interfering edges were found by searching in (IS). For each line on the long-wavelength side of AB, the tahle gives the corresponding data k’, Ak’, p’, and interfering edges. Ap, the interval in mass absorption coefficient between two bracketing lines, is given for the line pair most closely bracketing h ~ .If only one line is listed for h, Ap is given for both h’ lines listed with that k line; the same is true if only one line is listed for h‘. Table I1 is a compilation of some of the data necessary for the application of Equation 6 to ahsorption edge spectrometric determination of elements
L~~~~~..~~.. llyIy
Y rl
VOL 36. NO. 3, MARCH 1964
645
3
6
4
x
E
y.
0.1 4-
$$
Q)
Bm
xb
81
. . . . . . .
4443334
x
646
ANALYTICAL CHEMISTRY
0000
0000
4343
3333
56 to 92 (Ba to U) using their LIII absorption edges. No data is given for Pm, Po, At, Rn, Fr, Ra, Ac, Pa, and elements having atomic number greater than 92 (U). Substantially the same data are given as in Table I. The wavelengths of the L I I I and LII edges are listed instead of the K edges. The ,511 edges are given to aid in avoiding selection of a bracketing line having wavelength shorter than the LII edge. No absorption coefficients are given because the literature lacks adequate numerical data between ,511 and ,5111 edges to enable plots of the type described above to be made. Absorption coefficients for these elements can be calculated by methods given by Victoreen (26) and Leroux (19). Since adequate data is available for Pt, Au, and Pb, complete data for these elements an given in Table I. SPECTROMETER, ACCESSORIES, A N D SAMPLE CELLS
N
5
Spectrometer. The work was done on a General Electric model XRD-3 x-ray secondary emission (“fluorescence”) spectrometer having flatcrystal optics and conventional (that is, not inverted) geometry, and equipped with a Machlett type AEG50s x-ray tube having a W target, a LiF crystal, a 3.5- X 0.005-inch soller detector collimator, and a G. E. type SPG-2 sealed Kr-filled proportional counter. The original “No. 1” electronic circuitry was retained, including the 14-stage binary scaler, the capacity of which was increased 100-fold by the addition of a count register. The standard x-ray spectrometer sample compartment and drawer were also retained. Accessories. INSTRUMENT MODIFICATION. The only modification required is the replacement of the 1.625- X 0.070-inch sample compartment beam exit soller collimator with an accessory consisting of a beam tunnel and mounting bracket for sample cells and apertures. It was necessary to file a little metal from the under side of the diffractometer beam slit bracket (used only for diffractometric operation) to accommodate the accessory. It is convenient to drill a hole ‘/4 inch in diameter in this bracket to allow screwdriver access to the rear screw securing the beam exit soller collimator or the absorption accessory. These modifications in no way impair the utility of the bracket. Once these changes are made, the exit collimator and absorption accessory can be interchanged without removing the sample compartment, and the instrument can be arranged for absorption edge spectrometry or restored for fluorescence spectrometry in a few minutes. It is necessary to remove the crystal radiation shield when the absorption accessory is mounted. Figure 2 shows the accessory on the spectrometer with an aperture and a 3-cm. liquid sample cell in place. BEAMTUNNEL. Figure 3.4 shows the x-ray beam tunnel, a rectangular tunnel VOL. 36, NO. 3, MARCH 1964
8
647
'/Z-inch wide, 3/8-inch high, and 11/16 inches long in a brass block. For use with certain cells (described below), the tunnel aperture is reduced with a '/z; X 3/8- X 11/16-inch brass insert (Figure 3B) having an axial cylindrical tunnel '/4 inch in diameter. MOUNTING BRACKET.Affixed to the tunnel is the mounting bracket (Figure 3A) for sample cells and apertures. The bracket has channels into which the cells and apertures slide, engaging a stop bar a t the rear. One channel, situated nearest the beam tunnel and having a width of 1 em., is provided for sample cells. Three channels '/16-inch wide are provided for apertures for I-, 2-, and 3-cm. cells. APERTURES.The aperture (Figures 3'2, 3E) consists of a brass plate 1/16-inch thick having a central circular window 3/4 inch in diameter and four holes tapped for 3-56 machine screws. A lead mask '/le-inch thick having a 1/2- X 3/*-inch rectangular window is secured t o the brass plate with four 3-56 machine screws. The screw holes in the lead mask are oversized to enable registering the window with the beam tunnel in the following manner. The plate and lead mask, with the machine screws loose, are inserted in the bracket. A l/*- X 3 / ~ - X 3-inch brass aligning bar (Figure 3F) is inserted through both tunnel and window, thereby holding them in register. The machine screws are then tightened, and the aligning bar is removed. For use with micro and solid cells, an aperture having a circular window inch in diameter (Figure 3 0 ) and the tunnel insert (Figure 3B) are used. The window is registered with a cylindrical brass aligning rod 1/4 inch in diameter (Figure 3F) in the same way as the rectangular window. The apertures are placed on the emergent side of the sample cells and intercept divergent x-radiation scattered in the sample, permitting only direct rays to pass. Sample Cells. LIQUIDCELLS. All liquid cells having length 0.5 em. or more consist basically of Lucite
Figure 4. A. 8.
C. D.
E. F. G. H.
648
Figure 3. Accessories for x-ray absorption edge spectrometry A. X-ray beam tunnel with mounting bracket for sample cells and apertures 8. X-ray beam tunnel insert C. Rectangular aperture D. Circular aperture E. Aperture disassembled, showing brass plate and lead mask F. Aligning bars far rectangular and circular apertures
blocks 11/2-inch square and 1-ern. (0.394-inch) thick. The square faces are carefully machined plane, smooth, and parallel, and the blocks slide smoothly in the 1-em. channels in the cell mounting bracket (Figure 3A). Figure 4A shows 0.5-em. cells made by machining central holes 3/4 inch in diameter through the blocks, and circular recesses, concentric with these holes on both sides of the block, 11/* inch in diameter and 0.25-em. (0.0985inch) deep. The I-em. cells (Figure 4B) are made the same way, except that the concentric recesses are omitted. Figures 4C,40, 4E show 2-, 3-, and 5em. cells which are made by machining central holes 1 inch in diameter in the blocks and cementing into these holes cylindrical Lucite inserts having 1-inch o.d., 3/4-in~hi d . , and lengths respec-
tively 2, 3, and 5 em. The ends of the cylinders are carefully machined plane, smooth, and parallel. They are cemented into the blocks with glacial acetic acid. Micro cells (Figure 4F) are made by machining central holes 3/8 inch in diameter in the Lucite blocks. All these liquid cells are provided with tapered filling ports which are fitted with tapered Lucite plugs (Figure 4G). The plugs may not protrude from the tops of the cells and have axial tapped 4 4 0 holes half way through to receive cap-head machine screws for inserting and extracting the plugs. In general, it is not necessary to plug the cells unless thev are filled with volatile or corrosive liquids. Circular windows of 0.005-inch Mylar 1 inch in diameter are cemented to both sides of the cells with Pliobond rubber cement (Goodyear Tire and Rubber Co.). The cells are pressed under heavy weights for a t least 12 hours to allow the Pliobond to set. Figure 5 d shows cells having shorter lengths for use a t longer wavelengths, particularly with samples having high concentrations of the element determined and/or matrixes of high absorption coefficient. The cells consist of 11/2-inchLucite squares l/*- or 1/16-inch thick, having central holes 3/4 inch in diameter and filling channels joining these holes with the top edges of the cells. Circular windows of 0,001- to 0.005-inch Mylar ll/Binch in diameter are cemented to both sides of the cells with Pliobond, stretched tightly, and pressed for 12 hours as before. These cells, like the others, are inserted in the 1-em. channels in the cell mounting bracket after first being placed in register with Lucite spacers, shown in Figure 5B, having 11/4-inch central holes and thickness to bring the total thickness to 1 em. These cells cannot be plugged and are unsuitable for use with volatile liquids. If properly applied and carefully used, the Mylar windows last a long time. However, eventually the effect of repeated filling and prolonged irradiation may cause the windows to loosen, wrinkle, sag, embrittle, crack, perforate, or become contaminated with deposits that cannot be removed chemically. The windows can then be peeled off and the Pliobond removed by wiping vigorously with a cloth soaked with carbon disulfide. New windows are then cemented on.
Liquid sample cells
0.5-cm. 1.0-cm. 2.0-cm. 3.0-cm. 5.0-cm. Micro cell, 1.0-cm. path length Cell plugs and extraction screws Filling pipet
ANALYTICAL CHEMISTRY
Figure 5. A. 8.
Liquid sample cells having short path length 1/16- and 1 /&inch cells Spacers
All cells described a,bove except the micro cells are used with the I/z- X 3 / ~ inch beam tunnel (Figure 3 A ) and aperture (Figure 3C). The micro cells are used with the l/rii~ch tunnel insert (Figure 3B) and aperture (Figure 30). For use with liquids that attack Lucite, cells similar to those described above may be made of polyethylene, Teflon, aluminum, stainless steel, or other materials. Demountable cells have been described by others ( I , 6). Filling pipets (Figure 4H) for all cells except those shown in Figure 5A are made from glasri 5 m l . transfer pipets by drawing down the tips, blowing bulbs in the upper part of the chambers, cutting off and flaring the necks, and fitting wiih 10-ml. rubber bulbs. With these pipets, samples are changed in a minute or two by emptying the cell, rinsing cell arid pipet with the next sample, then filling the cell with the next sample. Filling pipets for the cells shown in Figure 5A are made by drawing down the tips of medicine droppers. LIQUIDCELLSHAVLNG VERYSHORT PATHLENGTH. For analyses involvinrr elements of low atomi: number and/& highly absorbing matrixes, cells having very short path length are required, and the shorter the path length, the more precisely the length must be known. For such applications it may be feasible to use demountable short-path cells recently made available for visible and ultraviolet absorption spectrophotometry by Limit Research Corp., Post Office Box 852, Darien, Conn. The cells, designated as types UV-0-1 and UV-0-2, consist of xvo fused silica windows separated by Teflon or lead spacers available in 10 thicknesses from 0.007 to 2.0 mm. Although the writers have done no experimental work with these cells, they may well prove to be suitable for x-ray absorption edge work, particularly if the silica windows are replaced with Mylar. Suitable adapters would be required to mount the cells on the accessories described above. SOLID CELLS. The cell body (Figure 6A) for powder, briquet, foil, and film samples is a brass frame 11/2-inch square and 1-cm. (0.394-inch) thick, having a depression 1li8-inch square and 5/16-inch deep, a centrrtl hole 3/4 inch in diameter, and four 4-40threaded rods as shown. The ‘[floor” of the depression is machined plane and smooth, and parallel t o the outside face of the body. The cell body slides smoothly in the 1-cm. channel in th. cell mounting bracket (Figure 3A). Briquets and foils are sandwiched between brass plates I/sz-inch thick X 3/s-inch) having rectangular or circular (3/s-inch) windows, and holes for the 4-40 threaded rods in the cell body, as shown in Figure 6B. Thin films on substrates reasonably transparent to x-rays are handled the same way. Unsupported thin films are laid on squares of 0.0005- to 0.005-inch Mylar or sandwiched between two such squares, then sandwiched between the plates. The loaded “sandwiches” are lowered into the cell body and secured with 4-40 nuts.
Figure 6. Cells for powders, briquets, foils, and films A. 8. C.
Cell body Frames for briquets, foils, and films Frames of various thicknesses for powders
Cells for powder samples have the same general form as the plates described above (Figure 6B), except that they are made in several thicknesses (‘/32, l/lg,,l/~J and 8//16 inch) and have Ushaped windows, as shown in Figure 6C. Squares of 0.005-inch Mylar are cemented on one side of each plate with Pliobond. The powder is packed into the dish so formed, care being taken that no powder remains outside the depression. The loaded cell is then carefully lowered over the 4-40 posts into the cell body in such a way that when the cell body is later inserted in the mounting bracket, the curved part of the U-shaped window will be down. Sext a square of 0.005-inch Mylar is placed over the powder cell, followed by a plate having a 3/8-inch circular window (Figure 6B). The assembly is then secured with the nuts as before. ROUTINE PROCEDURE
Sample Preparation. Solid samples may be sawed and polished t o thin sections having parallel smooth surfaces, or reduced to powder, or put into solution. Powders may be used in powder cells of appropriate thickness either without preparation or after grinding or admixture with an inert diluent, such as A1203, C, or starch. The more x-ray opaque the powder is, the more advisable such dilution becomes. Alternatively, powders may be put into solution, cast into films, or compressed into briquets, with or without a binder. Thin foils and films may usually be used without preparation. Liquids or solutions may often be used without preparation or after appropriate adjustment of concentration by reduction of volume or dilution with solvent. Solutions may be prepared with little regard for the nature of the reagents used to effect dissolution. However, heavy elements should not be introduced, and the final concentration of the element determined should be favorable. Sample Cells, Blanks, Correction Standards. Cells and supports for
all types of sample have already been described. The higher the absorption coefficient of the sample a t the wavelengths used, the shorter must be the cell length and/or the thinner the cell windows. Conversely, the lower the concentration of element determined, the longer must be the cell length, providing the absorption coefficient of the matrix is not too high. For liquid and powder cells, the empty cells serve as blanks, for briquets, foils, and unsupported films, the open tunnel and aperture, For thin films supported on or sandwiched between Mylar sheets, a sample frame with only the Mylar sheet(s) is used. For thin films on other substrates, clean substrate having the same composition and thickness is used. Aluminum sheet of thickness appropriate to attenuate the A line to to ‘/z its incident intensity serves as correction standard. Highpurity aluminum foil and sheet of thickness 0.001 to 0.125 inch are adequate for the entire wavelength range. The aluminum is mounted in the same type of holder as sample foils. Line Pair. The selection of the two bracketing spectral lines is made from Tables I and 11, or by searching (IS) or (28). In general, that line pair is selected which lies closest to the absorption edge, yet is not subject to the effects of absorption edge fine structure. Fine structure is most marked on the short-wavelength side of the absorption edge where it is usually confined to a wavelength interval equivalent to less than -200 volts ( 3 ) . The closer a line is to AE, especially on the short side, the more likely it is to be subject to fine structure effects. Conversely, the farther both lines are from AE, the less likely the correction standard is to compensate satisfactorily, the smaller is the difference in the values of 10/l and Io’[I’ for the samples, and the greater is the possibility of interposition of an interfering absorption edge. Secondary Targets. The wavelengths bracketing the absorption edge of the element to be determined are generated by excitation of characteristic spectra in appropriate secondary targets placed in the x-ray spectrometer sample drawer in the same manner as samples for x-ray fluorescence spectrometry. The secondary targets may take any of several forms, depending on available forms of each element. The element itself can be used in the form of a coupon cut from plate, bar, or rod stock; sheet or foil cemented to a suitable support; a casting; a plane area ground or machined on an irregularly-shaped lump; a plating on a S i or other coupon; or powder compacted into a briquet or loaded into a Mylarcovered cell ( 2 ) . If the element itself is unavailable or unsuitable, a compound is used as a briquet or in a Mylarcovered cell, or as a slurry coating on a suitable support. Even solutions in plugged Mylar-covered cells may be used (8), but only as a last resort. Only stable compounds are used, and, aside from the secondary target element, only elements of very low atomic VOL. 36, NO. 3, MARCH 1964
0
649
Table 111.
Sample Calculations
unit General data Element determined and absorption edge XEO
Cell length L Bracketing line paira X and A’“ p and p’a Calculation of k (Equation 10) Intensities transmitted by open tunnel and aperture IO and ZO’~ Intensities transmitted by correction standard I and Ifb Zo/I and Zo’/I’ loglo (Io/I) and loglo (Zo’/Z’) k = log10 (Io/Z)/log10 (ZO‘/Z’) Calculation of K L (Equation 7) kP’ P (P
- h’
- kP’)L
A. cm. A. cm.2/gram
650
ANALYTICAL CHEMISTRY
X’
GeKa
ZnKsbs 1.2833 0.5
PtLa1
1.2553 287
1.3130 37
C.P.S.
7474
C.P.S.
2738 2370 2.730 3.131 0.4362 0.4957 0,8800
cm. z/gram cm.’/grarn cm.*/ am mg.Kl.
32 255 128 17.992
K L = 2303/(1 - k p ’ ) L Calculation of C (Equations 6 and 7) Intensities transmitted by empty sample cell I o and Io‘b C.P.S. Intensities transmitted by sample-filled C.P.S. cell Z and I ‘ b Io/Iand ZO’/I’ loglo (ZO/Z) and loglo IO'/^') k log10 (Io‘/I’) log10 ( l o / I ) - k log10 (ZC’/I’) mg./ml. C = KL[lOgio (Zo/I) - k loglo (Io’/I’)] 0 Data derived from Table I or in way described in text. b Measured data.
number should be present. Oxides or fluorides are used where practicable. The area of element or compound exposed to the primary x-ray beam must be sufficient to fill the sample mask window in the conventional sample drawer, which, in the General Electric instrument, is a 3/4- X 1/2-inch rectangle. In this way the entire exit beam tunnel is illuminated with parallel secondary x-rays. X-Ray Measurements. Unless both bracketing lines come from the same element, i t is convenient to use two x-ray fluorescence spectrometer sample drawers, each loaded with one of the secondary targets. I n this way each target presents an identical surface to the primary x-ray beam each time that target is used. X-ray excitation conditions are selected for each secondary target so that the intensities transmitted by the samples are sufficient to enable measurements of adequate statistical precision in reasonable counting times. It is convenient, but not necessary, to have the excitation conditions the same for both targets. This condition is impracticable when the two lines differ greatly in intensity. In general, four x-ray measurements are required at each of the two bracketing wavelengths: the intensity transmitted by the open beam tunnel (with or without the tunnel insert, as required) and mounting bracket with only the aperture in place, which gives Io and Io’values for calculating k using Equation 10; the intensity transmitted by the correction standard, which gives I
x
7420
6737
6628
243 - _. 27.72 1.4428
282 ~-~ 23.50 1.3711 1.2066 0.2362 4.25
and I’ values for calculating k; the intensity transmitted by the empty sample cell or support, which gives Io and Io’values for calculating sample concentration using Equations 7 or 8; and the intensity transmitted by the sample(s) which gives I and I’ values for calculating sample concentrations. If the sample is unsupported, as is usually the case with briquets and foils, the measurements on the open tunnel and aperture give I Oand lo’ values for calculating sample concentration as well as k. Measurements are made of the intensities transmitted a t one of the two wavelengths by one to three samples, the empty cell, the correction standard, the tunnel and aperture, and one to three more samples, in that order. Then the sample drawer is changed, and the same series of measurements is made a t the other wavelength. Unless the x-ray generator and counting circuitry are very stable, if there are many samples, it is advisable to divide them into groups of not more than six. The measurements are then made in the way outlined above, repeating the measurements on the empty cell, correction standard, and tunnel and aperture with each group. Alternatively, measurements may be made a t both wavelengths on a sample before proceeding to the next one (or to the empty cell, etc.). This method is preferable when the samples requife long counting times. However, in other cases both measurement schemes
seemed to give the same precision, and the scheme described above is more rapid and convenient, and provides less opportunity for error in setting 28 and x-ray tube current. Coincidence Losses. For most of the samples involved in the evaluation of the method, it was practicable to use excitation conditions such that all transmitted intensities were well within the range of linearity of the detector-scaler system. However, samples that are relatively x-rayopaque require very high incident intensities if the transmitted intensities are to be measured with good statistical precision in reasonable counting times. In such cases the intensities transmitted by the open tunnel and empty cell or blank may be so high that the detector-scaler system is no longer linear. This condition arises mostly with the longer wavelengths used for the K edges of the lighter elements. There are two convenient ways of dealing with such cases. The measure ments on the open tunnel and blank may be made with the x-ray tube operating a t reduced current and the transmitted intensities corrected accordingly. Alternatively, the measurements on the tunnel and blank may be made with the same x-ray tube current as the samples, and the transmitted intensities corrected for coincidence losses. Methods for making coincidence corrections are given by Klug and Alexander (17) and are discussed in relation to absorption edge spectrometry by Barieau (1) and Dunn (6). Calculations. Coincidence corrections are applied to the intensity data where required. The value of k is calculated from Equation 10 and the intensity data for the correction standard and tunnel-aperture assembly. Sample concentrations are then calculated from Equations 7 and 8 and the intensity data for the samples and empty cell, sample support, or (for unsupported samples) the open tunnel-aperture assembly. If the samples are analyzed in groups as suggested above, the calculation for each sample is made using the empty cell, correction standard, and tunnel-aperture data taken with its group. Table 111 is a typical series of calculations. EXPERIMENTAL
The simplified method was evaluated for 10 elements distributed throughout the periodic table from atomic number 24 to 82 (Cr to Pb) in the form of solutions, powders, briquets, thin foils, and films. All these elements were determined using their K absorption edges except Pb, for which the ,5111 edge was used. The work was done using the instrument, accessories, cells, and routine procedure given above, and the data in Table I. All work with liquid samples was done with the X 3/8inch rectangular tunnel and aperture, all work with solids with the 1/4-inch circular tunnel insert and aperture. Sample Preparation. SOLUTIONS. Three types of solution sample were used: synthetic solutions prepared
from reagent-grade chemicals; chemically-analyzed plating solutions; and standard solutions for other instrumental methods of chemical analysis. Sr, Nb, Ag, Cd, and Sn solutions were held in 1-cm. liquid cells (Figure 4B), Zn, Ge, and Pb solutions in 0.5-cm. cells (Figure 4A), and Cr and Ni solutions in 1/16-inch (0.159-cm.) cells (Figure 5A). All cells had 0.005-inch Mylar windows. POWDERS.Three substances were used for powder samples: triple emission carbonate, (Ba, Sr, Ca)CO3, having weight percent composition 57.2 BaC03, 38.8 SrCO3, 4.0 CaC03; the superconductive intermet,zllic compound Nb3Sn having very newly the stoichiometric composition; ,and Si-Ge alloys having two compositions, 38.5 and 62.3 wt. yo Ge. The powders were mixed with starch in weight rztio 1 sample to 5 starch. All powder samples were held in cells of the type shown in Figure 6C having 0.005-inch Mylar windows, SiGe in l/l,&ch (0.159-l:m.) cells, P\’bsSn in 1/8-inch (0.318-cm.) cells, and (Ba, Sr, Ca)COs in cells of both sizes. Ge, Sr, Nb, and Sn were determined in the powders. BRIQUETS.The stwch-diluted powders described above were compacted a t -10,000 p s i . into disks having diameter 1/2 inch and thiclrness -l/16 inch. The briquets were mounted between plates having circular open windows (Figure 6B) in the cell body shown in
Element detd. 24-Cr 28-Ni
30-Zn 32-Ge 38-Sr 41-Nb 47-Ag
48-Cd
50-Sn 82-Pb
Figure 6A. The same elements were determined as in the powders. FOILS.Ni was “determined” in pure Ni foil 0.0005-inch thick and Ge in Cu-Ge-Ni alloy foil 0.001-inch thick having -12 wt. % Ge. The foils were mounted in the same cell arrangement as the briquets. THINFILMS.The (Ba, Sr, Ca)COa and Nb3Sn powders were cast into films in the following way (19, 94). One hundred fifty grams of powder, 4 grams of powdered Lucite, 400 mi. of 2% (wt./vol.) solution of nitrocellulose in butyl acetate, and 16 ml. of dibutyl phthallate were mixed thoroughly, and the volume of the mixture was measured. The mixture was then ball-milled for 24 hours. Five milliliters of the slurry was then pipeted on a clean 30- X 12inch glass plate and spread with a Gardner film-casting knife with micrometer blade adjustment (“doctor blade”) , sold by Gardner Laboratory, Inc., Post Office Box 5728, Bethesda 14, Md. The micrometer was set at 0.007 inch to give a final dry film thickness of 0.001 inch. The film was allowed to dry for 24 hours. The area of the film was measured by tracing its periphery on millimeter coordinate paper and counting squares. The area density (mg./cm.2) of element to be determined was calculated from the total volume of the stock slurry, the volume used for casting the film, the weight of powder in the slurry, the known composition of
the powder, and the film area. Pieces of film were removed from the glass by scoring the film in parallel lines about an inch apart, flooding the film with water, and “coaxing” the strips off with a narrow razor blade. The films were mounted in the same cell as the briquets and foils. A single layer of Xb3Sn film was sandwiched between 0.005-inch Mylar sheets. The (Ba, Sr, Ca)C03 film was somewhat stronger and was not supported. Measurements were made on 1, 3, and 6 layers of this film. Sr, Nb, and Sn were determined. Blanks. For liquid and powder samples, the empty cells served as blanks, for briquets, foils, and (unsupported) (Ba, Sr, Ca)C03 films, the open tunnel with insert and circular aperture, and for Nb3Sn films the empty cell holding two 0.005-inch Mylar sheets. Correction Standards. Barieau (1) reports a study of the considerations involved in the choice of correction standards. However, for this work aluminum sheet of high purity was used for all determinations. Different thicknesses were used depending on the wavelength region involved for determination of each element as follows: Cr, 0.002-inch; Ni, 0.003-inch; Zn, Ge, 0.006-inch; Sr, Nb, 1/16-inch; Ag, Cd, Sn, ‘/s-inch; Pb, 0.025-inch. In general, the thickness was chosen so as to reduce the intensity of the X line
Table IV. Bracketing Lines, Instrument Settings, and Cell Lei;gths Used for Evaluation Work X-ray tube current? ma. Powders Cell length L, cm. and Line Solutions briquets Foils Films A, xi 28” Solutions Powders x FeKa 57.52 5 0.159 MnKa X’ 62.97 10 x 2 ZnKa 41.80 45 0.159 X’ 44.41 20 TaLat 20 x GeKa 36.33 3.5 0.5 A’ 38.06 20 PtLa1 x SeKa 31.89 5 10 10 0.5 0.159 A‘ GeKp 32.56 30 45 45 x 21.91 15 45 35 ThLh 1.0 0.159, 0.318 A‘ SrKp 22.42 10 31 25 x RuKa 18.42 15 34 34 1.0 0.318 A‘ NbKp 19.03 13 30 30 x CdKp 13.54 34 1.0 X‘ SnKaC 14.04 10 x SbKa 13.46 15 A’ 14.18 30 AgKB x InKp 12.96 15 A‘ InKa 14.66 9 x TeKa 12.91 20 A’ PdKp 14.85 25 x SnKp 12.41 45 X’ CdKa 15.31 7 IKa x 12.40 30 RhKp A’ 15.57 45 InKp x 12.96 45 1.0 SbKaa A’ 13.46 15 TeKa x 12.91 20 A’ CdKp 13.55 40 SbKp x 11.89 45 45 45 1.0 0.318 IKa A’ 12.40 32 45 45 x BrKp 26.79 25 1.0 A’ 28.22 25 PbLbi ~~
For LiF crystal. For Machlett type AEG-50s x-ray tube having W target and operating at 50 kv. First line pair given for Ag and Cd was used for analytical evaluations, other pair(s) only for evaluation of effect of choice of line pair
(Table VIII).
VOL. 36, NO. 3, MARCH 1964
651
to to '/z of its incident intensity. The aluminum foils or plates were mounted in the same type of cell as foil samples. Secondary Targets. The evaluation work required spectral lines from 22 elements in the following physical forms: metal bar, Zn, Ge, P b ; metal sheet or foil cemented to aluminum plates '/*-inch thick, Nb, Pd, Ag, Cd, In, Sn, Ta, Pt, Th; electrolytic metal polished on one side, Mn, Fe; electroplating on Ni coupon, Ru, Rh; plane area ground on irregularly-shaped lump, Sb, Te; metal powder in Mylarwindowed cell, Se; compound in Mylarwindowed cell, Br (NaBr), Sr (SrFJ, I (CHIa). The secondary targets were mounted in the conventional x-ray spectrometer sample drawer. X-Ray Measurements. Table I V gives for each element determined in
each sample form the bracketing lines, spectrometer settings (20), excitation conditions (kv., ma.), and cell length. The excitation was set to give incident intensities of -7000 count/second, the intensity above which the detectorscaler system noticeably departs from linearity. The preselected count technique was used, and the size of the count was set to give a counting time of 30 seconds to not more than -2 minutes. Each count was scaled 3 to 5 times, the counting times were averaged, and the intensity was calculated from the average time. The preselected count for the open tunnel and empty cell was usually 204,800, for the correction standard 25,600 to 102,400, and for the samples 8192 to 204,800, depending on the transmitted intensity. Calculations. The analytical concentrations of liquid samples were
Table V.
Element deM.4
Sample matrix
5
652
ANALYTICAL CHEMISTRY
RESULTS AND DISCUSSION
Analytical Results. The results of the evaluation work are summarized in two tables, solution samples in Table V, powder, briquet, foil, and
Analytical Results for Solution Samples
Line pair
x,
A'
Concentration of element determined Theoretical Analytical by x-ray or anal. by Error independent absn. edge method Absolute method mg./d. Rel. % mg./ml. mg./ml.
1.0 FeKa, MnKa Dil. "08 5.0 Dil. HNO. HNOs 10.0 Dil. HNO; Dii; "08 25.0 Dil. "03 5.0 5 mg./ml. each: Cu, Ag, Pb 0.96 79. mg. c u / d . , HClO4 soh. 1.0 ZnKa, TaLa1 2&Ni Dil. "08 5.0 Dil. HNOl 10.0 Dil. HNO; 25.0 Dil. "08 1.6 35 mg. Cu/ml.,.5 mg. Ge/ml. 47. 0/4b NiClzHCl plating soln. 52.0/4* Ni2S04-H2S04 platjng soh. 61 .0/4b Ni2SOd-HzSO4 plating soh. 4.0 GeKa, PtLal Dil. HNOa 30-Zn 8.0 Dil. HNOa 4.8 SeKa, G e m 32-Ge 35 mg. Cu/ml., 0.3 mg. Ni/ml. 2.3 ThLpl, SrKB 3&Sr 3.98 mg. Ba/ml. 5.0 Dil. "08 10.0 Dil. HN03 25.0 Dil. HNOa 5.0 RuKa, NbKB 41-Nb 0 . 2 gram KHSOd/ml. 7.0 0 . 2 gram KHSOl/ml. 10.0 0 . 2 gram KHSOl/d. 7.01 0 . 2 gram KHSOd/ml., 2.99 mg. S n / d . 4.39 0 . 2 gram KHS04/ml., 5 . 6 1 mg. Sn/ml. 1.0 CdKp, SnKa Dil. HNOa 47-Ag 5.0 Dil. HNOa 10.0 Dil. HNOa 20.0 Dil. HNOa 35.0 Dil. HNOa 50.0 Dil. "08 5.0 5 mg./ml. each: Cr, Ni, Pb 4.5 KCN, K2COa,-25 mg./ml. 20.4 KCN, KzC08, -25 mg./d. 9.4 InKB, SbKa 48-Cd Plating soln. 1.0 SbKp, IKa 50-Sn 0 . 2 gram KHSOl/ml. 5.0 0 . 2 gram KHSO4/ml. 10.0 0 . 2 gram KHS04/ml. 7.93 0 . 2 gram KHSOd/ml., 2.07 mg. Nb/ml. 5.61 0 . 2 gram KHSOl/ml. , 4.39 mg. Nb/ml. 2.99 0 . 2 gram KHS04/ml., 7 . 0 1 mg. Nb/ml. 1.0 Dil. HNOa 82-Pb 5.0 Dil. HNOa 10.0 Dil. H?rT03 25.0 Dil. HNOs 50.0 Dil. HNOa 5.0 5 mg./ml. each: Cr, Cu, Ag All elements except Pb were determined using their K edges, Pb using its LIII edge. b Plating solutions diluted 1:3 with water.
24Cr
calculated from Equation 7 and are reported in mg./ml. The analytical concentrations of solid samples were calculated from Equation 8. Cbncentrations of foils and films are reported in mg./cm.2 Concentrations of powders and briquets are reported in wt. % calculated from LC; the known area of the powder cell or briquet; the sample weight as determined by weighing the powder cell before and after filling, or by weighing the briquet; and, when applicable, the known proportion of sample and diluent (starch).
0.89 4.47 8.67 22.36 4.62 1.16 0.88 4.82 9.75 24.12 1.48 11.50 12.41 14.73 4.10 8.02 4.74 2.36 4.88 9.87 23.80 4.77 6.95 9.78 7.55 4.36 1.03 5.00 9.89 19.05 32.98 46.18 4.94 4.56 20.06 9.35 1.0 4.64 9.38 7.51 5.67 2.97 1.08 5.18 10.09 25.50 51.10 5.02
-0.11 -0.53 -1.33 -2.64 -0.38 +o. 20 -0.12 -0.18 -0.25 -0.88 -0.12 -0.20 -0.59 -0.47 +0.10 $0.02 -0.06 $0.06 -0.12 -0.13 -1.20 -0.23 -0.05 -0.22 +0.54 -0.03 +0.03 0 -0.11 -0.95 -2.02 -3.82 -0.06 $0.06 -0.34 -0.05 0 -0.36 -0.62 -0.42 +0.06 -0.02 + O . 08 +0.18 +0.09 +O. 50 +1.10 +0.02
-11.0 -10.6 -13.3 -10.6 7.6 f12.5 -12.0 - 3.6 - 2.5 3.5 7.5 - 1.7 4.5 - 3.1 2.5 0.25 - 0.12 0.26 2.4 1.3 - 4.8 - 4.6 - 0.7 2.2 0.77 0.68 3.0 0 1.1 4.8 - 5.8 - 7.6 - 1.2 1.3 - 1.7 - 0.53 0 - 7.2 - 6.2 - 5.3 1.1 0.66 8.0 3.6 0.9 2.0 2.2 0.4
-
-
++ +-
++ -
+
+ ++ ++ + +
Table VI.
Analytical Results for Powder, Briquet, Foil, and Film Samples
Concentration of element determined" AnaTheo- lytical retical or by anal. by x-ray independ- absn. Error Line pair ent edge Element Sample ' A, A' method method Absolute Rel. % Sample Sample matrix detd. form -0.20 -1.7 Ni-F ZnKa, TaLal 11.8 11.60 Pur's 0.0005-inch Ni foil 28-Ni Foil -0.36 SeKa, GeKp 2.78 2.68 -0.10 Ge-F 0,001-inch foil, -88% Cu, 0.25% Ni 32-Ge Foil -3.9 38.5 36.98 -1.52 Ge-P-A Powder 5 grams starch, 1 gram -40% Ge-Si alloy -2.2 62.3 60.92 -1.38 _.5 grams starch, 1 gram ~ 6 0 Ge-Si 7 ~ alloy GR-P-R -3.3 38.5 37.23 -1.27 Ge-B-Alb Briquet Firet powder listed above -3.4 Ge-B-A2b 38.5 37.21 -1.29 First powder listed above -2.3 62.3 60.85 -1.45 Ge-B-Blb Second powder listed above -3.0 Ge-B-B2b 62.3 60.42 -1.88 Second powder listed above $3.7 ThLpl, SrKp 23.0 23.86 $0.86 Sr-P-1" 38-Sr Powder 5 grams starch, 1 gram (Ba, Sr, Ca)COa $0.26 23.0 23.06 $0.06 Sr-p-2~ 5 grams starch, 1 gram (Ba, Sr, Ca)COa 23.0 23.14 +0.14 Sr-B-l* $0.61 Briquet Powder listed above Sr-B-2d $2.95 23.0 23.68 +0.68 Powder listed above $1.4 0.72 0.73 $0.01 Sr-F-16 (Ba, Sr, Ca)COs film, -3 mg./cm.a Film $0.93 Sr-F-2e 2.16 2.18 $0.02 (Ba, Sr, Ca)COa film, -3 mg./cm.2 Rr-B-.?e $3.9 4.32 4.49 $0.17 _. - (Ba, Sr, Ca)COa film, -3 mg./cm.2 -4.1 Nb-Sn-P RuKa, NbKp 70.1 67.20 -2.90 41-Nb Powder 5 grams starch, 1 gram NbrSn -1.9 Nb-Sn-B-lf 70.1 68.80 -1.30 Briquet P&der listed aboqe -1.9 Nb-Sn-B-2f 70.1 68.76 -1.34 Powder listed above 2.16 2.24 $0.08 $3.7 Nb-Sn-F Film NbrSn film, -3 mg./cm.2 -4.3 Nb-Sn-P SbKp, I K a 29.9 28.62 -1.28 50-Sn Powder Nb-Sn-B-11 $5.1 29.9 31.42 $1.52 Same samples as for Nb above Briquet f6.1 Nb-Sn-B-2f 29.9 31.72 +1.82 Nb-Sn-F -8.7 0.92 0.84 -0.08 Film a For powder and briquet samples, concentrations are in wt. %, for foil and film samples, in mg./cm.% b Samples Ge-B-A1 an(i -A2, and Ge-B-B1 and -B2 were duplicate pairs of briquets. c Samples Sr-P-1 and 9-P-2 were duplicate samples in, respectively, 1 / and ~ l/s-inch powder cells. Samples Sr-B-1 and Sr-B-2 were duplicate briquets. e Samples Sr-F-1, -2, rtnd -3 were, respectively, 1, 3, and 6 layers of film. f Samples Nb-Sn-B-1 and -2 were duplicate briquets.
1
film samples in Trtble VI. Both tables give for each sample the matrix constituents, line pair, "true" and analytical concentration of element determined, and the absolute and relative error of the analytical concentration. The relative error data in Tables V and VI is summarized further in Table VII. This table gives the number of detcrminations, the (high) and - (low) numbers of errors, the median, mean, highest, and lowest errors, ttnd the spread between the highest and lowest errors. Three such summaries are given, one for the data in Tahle V, one for the data in Table VI, and one for the data from both tables combined. Table VI1 was compiled from the absolute values of the individual determinations, that is, sign was disregarded. Choice of Line Pair. Table VI11 shows the effect of choice of line pair on the analytical results for a Ag and a Cd solution. For e:xh line pair the table gives the value of AA and AA', A p , the analytical concentration, and the error of the analytical from the true concentration. The first six line pairs given for the Ag determinations are more or less symmetrical with respect to the Ag K absorption edge and are listed in order of increasing distance from the edge. Note that the line pairs include three in which both A and A' lines are derived from the same
+
secondary target (CdKp-CdKa, InK& InKa, and SnQ-SnKa). No generalizations can be drawn from a single study such as this. All the line pairs give reasonably satisfactory accuracy within the aims of this project, but some line pairs are clearly superior to others. However, it is difficult to see how one might predict these most favorable pairs on the basis of proximity to the edge (AA, Ah'), symmetry of the line pair with respect to the edge, Ap, or which, for the Ag K edge is -0.004 A. Matrix. Table IX shows the effect of matrix on the analytical results. Solutions of Ag in concentration 10 mg./ml., with and without various other elements having atomic numbers ranging from 24 (Cr) to 82 (Pb) , were analyzed using an AI plate '/*-inch thick as correction standard. Precision. The precision was evaluated in three ways as follows. Ten determinations were made on the same Ag solution without removing the sample cell from the mounting bracket. A11 10 analytical results were calculated from the same set of measurements on the open tunnel and aperture, blank, and correction standard. Seven other determinations were made with separate measurements on the tunnel, blank, and correction standard for each measurement on the sample. Finally, the long-term re-
Table VII. Summary of Relative Error Data in Tables V and VI
Table V Table VI (liquid (solid samples) samples) Total No. of detns. 48 23 71 No. of $ 14 24 errors 10
No. of
-
errors 32 13 2.5 3.2 Mediana 3.9 3.0 Mean" 8.7 13.3 HighQ 0 0.3 Low5 13.3 8.4 Spreada Disregarding sign (+ or - ).
45 3.0 3.6 13.3 0
13.3
(I
producibility was evaluated by making eight determinations over a period of two months; two operators participated in the analyses. Table X summarizes the precision data. For each series of determination the table gives the number of determinations made; the average, highest, and lowest analytical results; the difference A between the highest and 1 owest values; and the standard deviation U , in absolute and relative terms, calculated from u = dZd2/(n
-
1)
where d is the deviation of an individual value from the mean of the n values. VOL. 34, NO. 3, MARCH 1944
653
Table
Element detd. Ag
VIII.
Evaluation of Effect of Choice of Bracketing Line Pair”
Line pair
AA, AX’b
AMC
Analytical concentration, mg./ml. Run 1 Run 2 Errord
A, X’ A. cm.’/g. CdKp,SnKa 0.0107, 0.0062 44.1 9.87 9.89 -0.11 SbKa, AgKa 0.0140, 0.0112 43.5 9.49 9.56 -0.44 InKp, InKa 0.0313, 0.0278 37.2 10.08 10.20 +0.20 TeKa, PdKp 0.0330, 0.0347 36.4 10.04 10.12 +0.12 SnKp, CdKa 0.0506, 0.0506 30.9 10.83 10.81 $0.81 IKa, RhKp 0.0510, 0.0598 30.1 10.22 10.18 +0.18 CdKp, InKa 0.0107, 0.0278 43.3 9.68 9.59 -0.41 CdKp, CdKa 0.0107, 0.0506 42.0 9.93 9.81 -0.19 InKp, SnKa 0.0313, 0.0062 38.4 9.61 9.81 -0.19 SnKp, SnKa 0.0506, 0.0062 33.4 10.14 10.20 +0.20 Cd InKp, SnKa 0.0096, 0.0077 40.9 9.35 -0.05 TeKa, CdKp 0.0113, 0.0110 40.5 9.36 -0.04 TeKa, SbKa 0.0113, 0.0077 40.7 9.41 f0.01 a For conditions, see Table 111. Concentrations of test solutions were 10 mg. Ag/ml., 9.4 mg. Cd/ml. Compare with AXzm for Ag 0.004 A., for Cd 0.003 A. Compare with APE for Ag 47.3 cm.’/grem, for Cd 44.2 cm.*/gram. d Errors for Ag were calculated from results for run 2.
Other appraisals of the precision can be made by comparing the data in Table VI for duplicate briquets and the data in Table VI11 for the two series of analyses of the Ag solution using different line pairs. The precision, as indicated by Tables X, VI, and VIII, appears to be satisfactory. Accuracy. The accuracy of the evaluation determinations is given in absolute and relative terms for solution samples in Table V and for solid samples in Table VI. Relative error data are summarized in Table VII. The accuracy of the simplified method
Table IX.
Effect of Matrix”
Matrix At. no. symboi None 24-Cr
29-CU
3&Sr
56Ba
Analytical
Concn., mg./ml.
Ag concn.,
10.0 10.0 10.0 5.0 10.0 25.0 10.0
9.88 9.87 9.92 9.83 9.94 9.85 9.85 9.90
mg./ml.
82-Pb Conditions were those given in Table I11 for CdKp-SnKa line pair; Ag concentration in all solutions was 10.0 mg./ml. a
Table X.
No. of measts. made
compares surprisingly well with t h a t given by the more refined methods (1, 4-6, 10, 23, 27). While in general the accuracy is poorer than that reported by other workers by a factor of two to five, in many of the evaluation experiments the accuracy of the simplified method is as good as, and even better than, that reported elsewhere. CONCLUSION
The method of x-ray absorption edge spectrometry has been simplified to the point where an element can be determined rapidly and conveniently without any preliminary experimental work and without standards. The accessories required are simple and can be installed on an ordinary x-ray secondary emission spectrometer in a few minutes. The simplification is realized a t the expense of some accuracy compared with the more refined absorption edge methods, but the accuracy remains acceptable for many types of analysis. One or more elements can be determined in a sample of a type never before dealt with in a particular laboratory, often in one to two hours, with good precision, and accuracy almost certainly better than 10% of the amount present, and averaging about 3%. The sample
Evaluation of Precision”
Anal. Ag concn., mg./ml. U Test Av. High Low A Abs. Rel. % Sample undisturbedb 10 9.58 9.72 9.46 0.26 0.08 0.8 Sample reloadedc 7 9.45 9.57 9.23 0.34 0.11 1.1 Zmonth period 8 9.52 9.89 0.69 0.22 2.2 9.20 4 Conditions were those given in Table I11 for CdKp-SnKa line pair; Ag concentration was 10.0 mg./ml. Measurements were made on open tunnel and aperture, empty cell, and correction standard once for all 10 measurements on sample. c Measurements were made on open tunnel and aperture, empty cell, and correction standard once for each of 7 measurements on sample.
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ANALYTICAL CHEMISTRY
may often be analyzed in the form in which it is received. However, if dissolution is required, little regard need be given to the reagents used. If it is necessary to separate one or more heavy matrix constituents, which it usually is not, such separations need not be quantitative. The method is particularly well suited for such applications as plating solutions, rough assay of minerals and ores, etc., where high accuracy is not essential. The introduction to this paper cites the many advantages of deriving the bracketing wavelengths from characteristic spectral lines excited in secondary targets, as compared with deriving them from the continuous primary spectrum. In the method reported here, much of the value of these advantages is lost because of the simplified techniques used to realize speed and convenience. It follows that more refined techniques, combined with the advantages of using secondary spectral lines, should result in improved precision and accuracy. Among the refinements that might be readily applied are the following. With more sophisticated electronic equipment, much longer counting times and/or higher counting rates are feasible, particularly if resolving time corrections are applied. The Mylar windows used in the cells described above may tend to bulge under certain conditions, thus changing the effective cell length. Polystyrene windows -0.010-inch thick, such as those described by Dunn and others, are much superior in this respect. The most substantial improvement would be realized by experimental measurement of the absorption coefficients; this would be particularly advisable if the method were applied to frequently determined elements. LITERATURE CITED
(1) Barieau, R. E., ANAL. CHEM. 29, 348-52 (1957). (2) Bertin, E. P., Longobucco, R. J., Advan. X-ray Anal. 5,447-56 (1962). (3) Compton, A. H., Allison, S. K., LLX-rays in Theory and Practice,” ed. 2, pp. 662-71, Van Nostrand, New York, 1935. .~ (4) Dietrich, W. C., Barringer, R. E., ~~
~
U. S. Atomic Energy Comm. Rept. Y-1153 (1957). (5) Drahokoupil, J., Czech. J . Phys. 1, 147-54 (1952); C.A. 47, 7313a (1953). (6) ~. Dunn. H. W.. ANAL. CHEM. 34. 116-21 11962). ‘ (7) Eccleston. B. H.. Whisman. M. L.. ‘ h i d . , 28, 545-8 (1956). (8) Ekstrom, A., Acta Radiol. Suppl. 63 106 p. (1946). (9) Ekstrom, A., Rev. Sci. Znstr. 18, 681-2 (1947). (10) Ferro, A., Gallotto, C. P., Ann. Chim. (Rome) 45, 1234-43 (1955). (11) Fine, S., Hendee, C. F., Nucleonics 13, No. 3, 36-7 (1955); Noreko Reptr. 3, 113-15 (1956). (12) Finnegan, J. J., Advan. X-ray Anal. 5, 500-11 (1962).
(13) General Electric ~CO., X-ray Dept., Milwaukee, Wis., “X-ray Wavelengths for Spectrometer,” cat. no. A4961-DA (1959). (14) Glocker, R., Frohnmayer, W., Ann. Physik 76,369-95 (1925). (15) Hakkila, R. A., Waterbury, G. R., Develov. Avvl. Svtctru. 2. 297-307 (1963j. ‘ (16) Hughes, H. K., Hochsgesang, F. P., ANAL.CHEM.22,1243-58 (1950). (17) Klug, H. P., Alexander, L. E.. “X-ray’ Diffraction Procedures for Polycrystalline and Amorphous Materials,” pp. 281-90, Wiley, New York, 1954. (18) Knapp, K. T., Lindahl, R. H., .
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I
Mabis, A. J., Advan. X-ray Anal. 7 , in press (1964). (19) Leroux, J., Ibid., 5, 153-60 (1962). (20).Liebhafsky, H. A,, Pfeiffer, H. G., Winslow, E. H., Zemany, P. D., “X-ray Absorption and Emission in Analytical Chemistry,” pp. 313-17, Wiley, New York. 1960. (21) Liebhafsky, H. A., Winslow, E. H., Pfeiffer, H. G., ANAL.CHEM.34, 282R294R (1962). (22) N. V. PhiliDs GloeilamDenfabrieken. ‘ S‘ci. Equipment Dept., Application Lab.; Eindhoven, Netherlands, “ 9 b l e of Mass Absorption Coefficients, Noreko Reptr. 9, No. 3 (1962). (23) Peed, W. F., Dunn, H. W., U. S.
Atomic Energy Comm. Rept. ORNG 1265 (1952). (24) Raag, V., Bertin, E. P., Longobucco, R. J., Advan. Electron Tube Techniques 2,249-59 (1963). (25) Stainer, H. M.,U. S. Bur. Mines Circ. 8166 (1962). 126) Victoreen. J. A.. .J. A.o.ol. Phus. 20. 1141-7 (1949). ’ (27) Wright, W. B., Jr., Barringer, R. E., U. S. Atomic Energy Comm. Rept. Y-1095 (1955). (28) Zingaro, P. W., Norelco Reptr. 2, 92-5 (1956). ‘ 1
\ - - I
RECEIVED for review September 16, 1963. Accepted November 12, 1963.
Nonaquelous Titration of Acidic Groups of Lignins. Titration in Pyridine Using Potassium Methoxide as Titrant STANLEY 0.THOMF‘SON and GORDON CHESTERS Department o f Soils, University o f Wisconsin, Madison, Wis.
b The acidic properties of dioxane and alkali lignins were investigated by potentiometric titraticn, using a system with pyridine as solvent, potassium methoxide as titrant, and a platinized platinum-calomel electrode combination. Reproducibile results were obtained for all compounds used in evaluating the system, and for all lignin samples. Differentiating titration was possible witii some polybasic compounds. Sharp potential breaks were obtained for lignin samples. Acidic properties varied with lignin source and method of isolation. Some results indicate that the acidic hydrogen content of lignins might be higher than generally reported.
T
H E IMPORTANCE (OF ACIDIC GROUPS
relative to an understanding of the structure of lignin is well recognized, and several methods fcr their qualitative and quantitative decermination have been described (2, 3 ) . Methods that have received greatest emphasis in recent literature are those employing ultraviolet absorption, and conductometric and potentiometric titration. Goldschmid (9) developed a method for determining phenolic hydroxyl groups in lignin based on the difference in ultraviolet absorption of samples dissolved in neutral and ,alkaline solutions. Conductometric titration was employed by Sarkanen and Schuerch ( l a ) for determination of equivalent weights of lignin preparations dissolved in an acetone-ethanol-water mixture. Conductometric titration was also employed by Gaslini and Nahum (8) who investi-
gated the acidic properties of alkali lignins in an aqueous lithium metaborate solution. A conductometric method using ammonia as solvent was successfully employed in the titration of thiolignins (7). Butler and Czepiel ( 4 ) determined the phenolic and enolic hydroxyl groups in lignins by potentiometric titration in a nonaqueous system with dimethyl formamide as solvent and potassium methoxide as titrant. Results obtained by the various methods for similar lignin samples might differ considerably. Consequently, the acidic properties of isolated lignins merit further research either through improvements of existing
i
SUCCINIC
‘7
EXPERIMENTAL
: 5z
I
methods or the development of new ones. This paper presents the results of an investigation of acidic properties of dioxane and alkali lignins in a nonaqueous system that combines features of the method of Butler and Czepiel (4and , that developed by Cundiff and Markunas (5) for the nonaqueous determination of acids. The theory and practice of titration in nonaqueous systems has been reviewed extensively (1, 6, 11, IS). I n the present system, the samples were dissolved in pyridine and titrated with potassium methoxide, using a platinized platinum indicatingmodified methanol calomel reference electrode combination. Use of the method of Cundiff and Markunas as described (6) gave no distinct potential breaks in the titration of lignin preparations. The system of Butler and Czepiel ( 4 ) gave unsteady potentials.
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0 I m.
I
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I
YILLILQUIVALENTS O f TITRANT
Figure 1. Titration of benzoic and some polybasic acids
Apparatus. Beckman Zeromatic p H meter, Model 96; platinized platinum indicating electrode; sleevetype calomel reference electrode in which the saturated aqueous potassium chloride solution was replaced with a saturated methanol solution of potassium chloride ( 5 ) ; 10- or 25-ml. buret; tall-form weighing bottle or any suitable substitute fitted with three-hole rubber stopper; and magnetic stirrer (grounded) and stirring bar. Reagents. Pyridine, reagent grade; potassium methoxide in benzenemethanol mixture, prepared according to Fritz (6) and standardized against benzoic acid; and azo-violet indicator: a saturated solution of resorcinol in dry redistilled benzene. VOL. 36, NO. 3, MARCH 1964
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