Underwood (1f ) , the normal contents of selenium and brom ne in biological materials are of the order of 1 to 10 and 10 to 100 p.p.m., respectively. Thus the spurious arsenic contents corresponding to the As76 activities formed from selenium and bromine impurities by Reactions 2 and 3 may be of the order of 10-6 to 10-6 and to lov4 p.p.m., respectively. These sources of error should therefore be considered when very low arsenic contents are to be determined. rl spurious arsenic content of 0.08 p.p.m. in pure germanium dioxide due to Reaction 4 has been determined experimentally after 75 hours of irradiation a t 2.1012n./sq. cm.-second by Smales and Pate ( I O ) . Thus a germanium content of 100 p.p.m. will equal a n arsenic content of p.p.m. at this flux. This source of error can therefore be disregarded unles:; the germanium content is very large or the irradiation is performed in a high neutron flux. The latter condition is valid, because the formation of A4s76activity due to Reaction 4 varies as the square of the flux (10).
The sources of error encountered in neutron activation andysis in generale.g., neutron flux depression, neutron self-shielding, neutron flux topography, statistical counting errors, and instrumental reproducibilitj -have been dis-
cussed thoroughly by Plumb and Lewis ( 7 ) . The resulting maximum error will, however, vary with type of reactor, irradiation position, and measurement technique. It is therefore desirable t o determine the resulting maximum error experimentally under certain conditions by application of the straight-line test according to the procedure of Youden ( I S ) . This test, applied to mercury activation analysis ( l a ) , indicated a standard deviation of about 301,. It therefore seems reasonable to presume that the resulting maximum error, including the errors introduced during the chemical separation, does not exceed &5%, which is usually considered satisfactory in trace analysis. ACKNOWLEDGMENT
The author is indebted to Torbjorn Westermark, who suggested the possibilities of this line of research and who has given valuable advice and suggested certain changes which have been made in the manuscript. Thanks are also due to Roland Christell for kind cooperation i n the development of the arsenic part of the procedure. The neutron irradiations were performed by AB Atomenergi, Stockholm. The author is also indebted t o Ole Lamm for providing laboratory space at the Division of Physical Chemistry.
The manuscript has been linguistically revised by H. G. Peacock, LITERATURE CITED
( 1 ) Bethge, P. O., Anal. Chim. Acta 10, 317 (1954).
(2) Christell, R., Sjostrand, B., Acta Chem. Scand. 16, To. 9, 2123 (1962). (3) Gorsuch, T. T.. Analvst 84, 135 (1959).
(4) Hjortsjo, C. H., ed., “Erik XIV, Gravoppningen 1958 i I’asteras Domkyrka,” P. A. Korstedt & Soner, Stockholm, 1962 (English abstr.). ( 5 ) Inthoff, W., Nucleonics 13, ?io. 11, 67 (1955). (6) Konijn, J., Aktiebolaget Atomenergi, Stockholm, Rept. AE-58 (August 1961). ( 7 ) Plumb, R . C., Lewis, J. E., Sucleonics 13, N o . 8, 42 (1955). (8) Roesmer, J., Kruger, P., Natl. Acad.
Sci.-Natl. Research Council, Nuclear Sci. Ser. NAS-NS-3026 (1960). (9) Scott, W. W., “Stpdard Methods of Chemical Analysis, 5th ed., Tol. 1, D. 579. Van Koatrand Kew York. 1939. (10) SmaleB, A. A., Pate, B. D., ’ANAL. CHEM.24, 717 (1952). (11) Underwood, E. J., “Trace Elements in Human and llnimal Sutrition,” DD. .. 361. 379. Academic Press. New York, 1956. ’ (12) Westermark,
T.,
Sjostrand,
B.,
Intern. J . Appl. Radiation Isotopes 9, 1 (1960). (13) Youden, W., J., “Statistical Methods
for Chemists, pp. 40-9, Wiley, New York, 1951. RECEIVEDfor review April 19, 1963. Accepted November 29, 1963. Work supported financiallv by the Swedish Atomic Research, Natural Science Research, and Medical Research Councils.
Determination of Metallic Platinum in P h i num-,Numina Catalysts b y X-Ray Diffraction R . A.
VAN NORDSTRAND,’
S inclair
Research, Inc., Harvey, 111.
A. J. LINCOLN and A. CARNEVALE Research and Development Division, Engelhard Industries, Inc., Newark, N. 1.
b A method is described for direct determination of the amount and the crystallite size of metallic platinum in catalysts comprising about 0.5% total platinum supported an activated alumina. The method is based on the magnitude and the brseadth of the 31 1 platinum diffraction line of Bragg spacing 1.1 8 A. This line is superimposed on a nonlewel background which is the diffraction pattern of the alumina support. Stepwise counting i s used to define the diffraction pattern in the vicinity of this line. The resultant intensity vs. angle graph i s then compared to a series of reference graphs which provide for platinum content up to 0.71%, in five size steps from 50 to 300 A. The reference graphs were constructed numerically from
diffraction data on three standard materials, a platinum black of 100 A. crystallite size and two activated aluminas. The lower limit of detection varies from about 0.1% of 50 A. platinum to 0.01% of 300 A. An abbreviated procedure provides a practical screening method for presence of metallic platinum in the catalyst. The limits of detection extend down to 35 A., at which size 0.5% is the least detectable.
T
role played by about platinum in the platinum-alumina reforming catalyst has prompted much study of the chemical nature and the degree of dispersion of this platinum. The agglomeration of all the platinum into HE IMPORTANT
0.5Q/, total
metal crystallites larger than a certain size is recognized 8s one cause of failure of this catalyst. Thus, the study of dispersion and of factors causing agglomeration and redispersion is important both for the making and for the use of the catalyst. Two general types of measurement are used to determine platinum metal dispersion-those which sense surface area of the platinum and those which sense platinum particle or crystallite size. The first t-vpe includes the methods of surface chemistry-hydrogen-deuterium exchange ( 8 ) , adsorption of hydrogen (1, 6), of carbon monoxide (6, 7 ) ,and of benzene ( I S ), and platinum Present address, Sinclair Research, Inc., Tulsa, Okla. VOL. 36,
NO. 4, APRIL 1964
819
SPECTROMETER ANGLE-
Figure 1. X-ray diffraction patterns of eta and gamma aluminas with indices, based on a cubic spinel lattice, shown in parentheses Superimposed are full vertical lines marking the centers of the first five diffraction lines of metallic platinum; the Pt indices are shown at the foot of each vertical line
solubility (8, 10, 11). The second type includes radiation methods-x-ray diffraction line broadening (1, 2, 4, 6, 11, 18, 14, 16), electron microscopy ( 1 , 4 , and light scattering (3). Results of the various methods are in general agreement although each method may be useful over but a limited range of platinum dispersion. The x-ray diffraction method provides a quantitative measure of platinum metal larger than about 35 A. and is sensitive to crystallite size over the range 35 to lo00 A. The lower part of this range, perhaps 50 to 150 A., may be the critical range with respect to catalyst performance ( l a , 18). The x-ray diffraction method may be the simplest of the methods cited from the standpoint of experimental technique and also from the standpoint of interpretation of results. The method described here applies to the catalyst directly, without chemical treatment, without chance for platinum alteration. I n this sense it is referred to as a direct method. An alternative procedure was attempted, usin? acids to dissolve the alumina, examining the residue by x-ray diffraction. The indirect method was abandoned due to some erratic results with a few used
820
ANALYTICAL CHEMISTRY
catalysts. Acetyl acetone, the solvent for alumina recommended by Plank (14) and McHenry ( l l ) ,was not tried. This direct x-ray method has been developed and used over the past 10 years in both laboratories represented by the authors. Methods of surface chemistry were also used (8), primarily to define the condition of that platinum which was too highly dispersed to be seen by x-ray. Results by the direct x-ray method and those by surface chemistry methods were compatible. The application of x-ray diffraction in a direct study of platinum in a platinum-alumina reforming catalyst is made difficult by three factors: The total platinum concentration is low, major diffraction peaks of alumina nearly coincide with those of platinum, and the diffraction pattern of alumina is a function of its thermal experience. The second and third of these factors are responsible for much of the complexity of the procedure herein described. The handicap of low platinum concentration of 0.3 to 0.7% by weight is partially compensated by the strong scattering power of the platinum atom and by the high symmetry of the metal crystal. The conflict of alumina and platinum
diffraction patterns is shown in Figure 1. The activated alumina in most reforming catalysts is of either the gamma or eta form, using the Alcoa nomenclature of Stumpf, Russell, et al. (16). The detailed patterns of the gamma and eta forms of alumina (19) are shown with spinel lattice indices in parentheses, Superimposed are five vertical lines extending through the locations of the centers of the 111, 200, 220, 311, and 222 platinum diffraction peaks. The near coincidence of the alumina and platinum patterns is caused by a structural similarity. Platinum metal is a cubic close-packed array of platinum atoms 2.78 A. in diameter. The alumina may be considered a similar array of oxygen ions of 2.80 A. diameter. This structural similarity or “lattice fit” has been cited (20) as a reason why alumina is a good support for platinum. It is apparent from Figure 1 that the 311 platinum line was selected for the present purpose to minimize conflict with the alumina diffraction lines. The 311 platinum line is about one third as intense as the strongest platinum line, the 111 line (17). The alumina peak with which the 311 platinum line coinpetes is about one tenth the intensity of the one with which the 111 platinum
line competes. F'robably more important are the relative slopes of the alumina backgrounds a t the centers of these two platinur peaks, which likewise greatly favor the 311 line. I n three recent papers brief reference is made to methcds of direct x-ray diffraction study of platinum-alumina reforming catalyst. Spenadel and Boudart (15) use the 311 platinum line to determine the size of platinum crystallites but not their amount. Adler and Keavney (2) use the 111 platinum line for both size and amount in spite of alumina interference. Herrmann et al. (6) used the 311 line for platinum crystallite size and "relative" amounts of platinum. It should be possible to subtract a standard alumina background pattern from the pattern of the platinumalumina catalyst, thereby obtaining the platinum diffraction pattern by difference. The immediate difficulty with this lies in the v a r i a d i t y of the alumina pattern. This pattern has been shown (19) to vary with tEe type of aluminum hydroxide from wEich the catalyst is made, Rith time and temperature of both calcination a n j use, and with the partial pressure of steam a t elevated temperature. The well known Scherrer equation provides the relationship between crystallite size and diffraction line breadth. The equation as used in its simplest form and without corrections is: B L = ucos
e
The symbol L is the mean diameter of the crystal, and X is wavelength; both are expressed in A. units. The Bragg angle, e, is one ha:f the spectrometer angle. B is the breadth in radians of the diffraction line expressed as a function of the spectrometer angle, 28. The breadth used in here, commonly called the "half-breadth," is measured from one side of the diffraction line to the other at the level midway between base and peak. Broadening of the diffraction line by factors other than crystallite size is not of importance in the method described here. The correction procedure outlined by Klug and Alexander (9) changes the values used, 50, XO, 100, and 150, to the values 51, 82, 104, and 160, respectively. EXPERIMENTAL
The equipment used consisted of a Norelco Geiger detector diffractometer with the standard voltage regulator and milliampere st2,bilizer. A copper target x-ray tube was used with 40 kilovolts full wave rectified potential. T h e amperage was adjusted in the range 10 to 20 ma. to give a standard intensity with a reference sample. A set of 4degree divergence slits with a
52 -
54
800
81'
820
830
80.
81.
820
830
I
I
I
I
I
I
I
I
- 54 -
-- 5 0
-48
- 48 44
42
--
I 800
I 810
I 820
-
I 800
I
830
ANGLE, 2 0
I
I
BID
820
I 830
42
Figure 2. X-ray diffraction reference patterns for 80 A. and 150 A. Pt on low temperature (480' C.) alumina
0.006-inch receiving slit and a nickel foil filter were used. The samples of catalyst or alumina were ground to approximately 200 mesh and pressed into standard aluminum frame diffraction sample holders to form self-supporting sheets 1 cm. x 2 cm. X 0.15 cm. The platinum black was spread on a frosted glass surface over the same area. The 311 platinum peak occurs a t the spectrometer angle 81.3'. The diffraction patterns were obtained by scanning manually stepwise over the interval 79' to 84'. Intensity readings were obtained by measuring the time required for a fixed number of counts. For reasonable statistical significance, 6400 or more counts are registered a t each angle setting, the intervals being 0.10' between 81' and 82', and 0.25' elsewhere. Total counting time for one pattern is about one hour. Stepwise counting, either manual or automatic (6), has proved definitely superior to continuous scanning using a scaling circuit and strip chart recorder. The slowest scanning speed available,
however, was 7.5' per hour. Adams (1) found continuous scanning satisfactory a t a speed of 2.4' per hour for determining platinum dispersion on silica gel. To permit determination of the amount and crystallite size of metallic platinum in a catalyst sample, the intensity data in graphical form are compared to a set of reference patterns. With experience, a single or a limited number of reference patterns may be selected t o give the best match t o the unknown. et al.
REFERENCE PATTERNS
The reference patterns were constructed by a numerical procedure using diffraction data from three pure materials, platinum black, a n activated alumina obtained by calcining aluminum hvdroxide a t 480' C . and a second activakd alumina obtained by calcining a t 650' C. The platinum black gave a diffraction pattern of platinum metal, all of the VOL. 36, NO. 4, APRIL 1964
821
4 Figure 3. X-ray diffraction reference patterns for 0.25% Pt metal of various crystallite sizes on 480" C. alumina
84'
80'
ANGLE, 2 8
lines indicating a crystallite size of about 100 A. The two samples of activated alumina were both 7-alumina. The 480' C. alumina gives a diffraction pattern like that of a n unused platinum-alumina reforming catalyst. The 650" C. alumina gives a diffraction pattern of somewhat sharper lines, comparable to that of a catalyst after long use in a commercial reformer. The reference patterns for 100 A. platinum were constructed by superimposing the diffraction data for the platinum black (smoothed and corrected to zero background) with various multipliers onto the smoothed diffraction d a t a for the 480' C. alumina. Both sets of data were obtained a t the previously specified angle settings. The arbitrary multipliers were then converted to percentages by a calibration experiment on a known blend of the alumina and platinum black. A similar set of reference patterns was then constructed utilizing the 650" C. alumina data. No additional calibration was required as the same platinum data multipliers were used. To obtain reference patterns for platinum of crystallite size less than or greater than 100 A., the diffraction data for the 100 A. platinum black were expanded or contracted in accordance with the Scherrer equation before addition to the alumina data. The shift in the data must retain the peak center at 81.3' and retain the area under the peak constant for a given percentage platinum. The angle scale (20) of the original platinum data is shifted from 2eIwto 20L, where 28L = (28iw- 81.3) X (lOO/L) 81.3. The intensity scale is shifted from 1100 to I L , where
+
822
ANALYTICAL CHEMISTRY
IL
= 1100 X L/lOO. These shifted platinum data are then added to the alumina diffraction data to synthesize the new reference patterns. It is useful to have patterns for 50, 80, 100, 150, and 300 A. platinum crystallite sizes. Typical reference patterns developed by the above procedure are shown in Figure 2. Figure 2 contains the 80 A. and 150 A. platinum on the 480" C. alumina. Figure 3 contains a comparison of patterns for "zero", 50, 80, 100, and 150 A. sizes a t the 0.25Oj, metallic platinum level. DISCUSSION OF PATTERN MATCHING
The diffraction data for the sample under study are plotted on transparent coordinate paper on the same scale as the reference patterns. By superposition it is possible to select a few of the reference patterns as "best fits.'' The uncertainty as to which reference pattern most nearly matches the pattern in question is inherent in the method. I n the first place, the crystallite size distribution may not be so simple as that of the platinum black used as reference. This means the shape of the diffraction peak may be complex. Much of this complexity, however, is obscured by a second factor, statistical fluctuation of each of the recorded experimental points. The diffraction patterns extending from 79" to 84" may be considered in four separate sections. From 79' to 80" the pattern is sensitive primarily to the alumina, changing as the crystallinity of the alumina changes. From 80" to 81" is a region of confusing overlap of alumina and platinum contributions. From 81' to 83" the patI
tern resulting from alumina alone is relatively flat; hence it is in this region that the pattern of metallic platinum centered a t 81.3" is to be seen. From 83' to 84' the pattern is again sensitive primarily to alumina. I n the matching of patterns the first decision concerns which alumina provides the best background match. Following this, most of the matching will be concentrated in the 81" to 83' region. Three examples of diffraction patterns of used catalysts containing 0.65% total platinum (18) are shown in Figure 4. These examples are selected to illustrate both the range of the method and the possibilities for platinum growth which this catalyst displays. One of these is considered to contain no metallic platinum. Random fluctuations of the data points prevent any decision regarding presence of up to 0.04% of 80 A. platinum, up to 0.1% of 50 A. platinum, and of any amount of 30 A. or smaller platinum crystallites. Two other examples cited as 50 A. and 150 A. metallic platinum are shown in Figure 4. These catalysts, removed from a reformer after some abnormal operations, had lost much of their reforming activity. The growth of metallic platinum has been given as the explanation of this activity loss. From the diffraction data shown, it is concluded that these two catalysts contain, respectively, "0.5 per cent of 50 A. metallic platinum" and "0.25 per cent of 150 A. metallic platinum." These quoted phrases do not include the complex interrelated uncertainties which surround both the per cent and the size figures. One approach to the use of the method
1
I
I
I
1
I
I
I
I
I
I
54
I
b Figure 4. X-ray diirraction patterns of three examples of used platinumalumina catalyst 0
0
0
No metallic Pt 0.5% 50 A. Pt 0.25% 150 A. Pt
0
0 I
I
1
ANGLE, 2
is to assume that the descriptions are accurate as quoted above and that the balance of the platinum is not metallic but exists either 11s oxide, sulfide, carbide, chloride, or arsenide or is atomically dispersed. This approach involves a second assumption, that the metallic platinum has but a single size in a given catalyst. A second approach is to assume that if any metallic platinum appears in the x-ray diffraction pattl2rn of a particular catalyst, all the p1:itinum is in the metallic state. A distribution of crystallite sizes is then invoked which is compatible with the diffraction pattern. A study of the reliability of the method described herein was made using blends of platinum blacks and aluminas. Prior x-rsy measurements indicated crystallite sizes of 110 -4.and 60 A. for the two blacks used. Interpolation methods were employed. Values of 100, 110, and 125 A. were reported for the Mends containing 110 A. platinum and values of 50 A. were reported for all the blends with 60 A. platinum. Percentage values reported, a s shown in Table I, have a n average deviation of about 0.03% from the nominal values. A MODIFIED TIECHNIQUE
The foregoing method has been modified to provide a rapid reliable technique for detecting metallic platinum in reforming catalysts. Lower limits of detection appear to be about 0.01% of 300 A., or larger platinum, 0.1% of 50 A. platinum and about 0.5% of 35 A. platinum. The technique involves determining the ratio of the diffraction intensity a t 81.3' to the difTraci,ion intensity at
0
0
I
I
I
e
83.0'. The intensity a t 81.3' is the most sensitive to changes in platinum metal; that a t 83.0' is least sensitive to changes both in platinum and alumina. Since this technique utilizes measurements a t only two angles, these measurements are made with greater statistical precision than is practical when a complete (79' to 84') stepwise scan is obtained. The ratio of the two diffraction intensities for 480' C. alumina is about 0.99. This same ratio is obtained generally for fresh catalysts in either the oxidized or reduced state. The ratio of intensities for 650" C. alumina is about 1.01. The same ratio is often found with used catalysts. Table I1 shows values of these ratios computed for 0.64% platinum catalysts in which all the platinum is in the form of metallic crystallites of the size indicated. This method extends the size range covered by x-ray diffraction down to 35 A. Results on fresh catalysts and many used catalysts indicate "no metallic platinum" even down to this size range. DISCUSSION
Recently Adams, Benesi, et al. ( I ) , using a direct x-ray diffraction procedure, have shown that a catalyst consisting of 2.5% platinum on silica gel contains the platinum in the form of metal crystallites of about 35 A. size after reduction in hydrogen. Various workers, including the present authors, have found that the platinumalumina reforming catalyst after hydrogen reduction, even after some use in a reactor, is substantially free of metallic
Table 1. Platinum Analyses by X-Ray Method of Three Series of Platinum Black-Alumina Blends
Series 1, 110 A.
Platinum in blend,
platinum 480" C. alumina, found,
%
70
0.10 0.20 0.30 0.40 0.50 0.60 0.70
0.12 0.17 0.28 0.41 0.47 0.53 0.73
Series 2, 110 A4. platinum 650" C. alumina, found,
Series 3, 60 A. platinum 480" C . alumina, found,
0.12 0.22 0.27 0.44 0.46 0.58 0.72
0.10 0.18 0.32 0.37 0.50 0.55 0.65
7%
c-/ O
Table 11. Computed Ratios of Diffraction Intensities for 0.64% Platinum Catalysts
Crystallite size of platinum
181.3~/183.00 (480" C.
181.30/183.~~0 (650' C.
"Zero" 35 A. 50 A. 100 A.
0.99 1.03 1.12 1.31
1.01 1.05 1.14 1.32
alumina)
alumina)
platinum down to the size limit of the x-ray method, here cited as 35 A. This does not represent a real test of the relative supporting action of silica and alumina, however, as the silica contained four times the platinum content of the alumina. VOL. 36, NO. 4, APRIL 1964
a
823
LITERATURE CITED
(1) Adams, C. R., Benesi, H. A., Curtis, R. M., Meisenheimer, R. G., J . Catalysis 1,336 (1962). (2) Adler, S. F., Keavney, J. J., J . Phys. Chem. 64, 208 (1960). (3) Debye, P., Chu, B., Zbid., 66, 1021 (1962). (4) Eischens, R. P., Francis, S. A,, Pliskin, W. A., Zbid., 60,194 (1956). (5) Gruber, H. L., Zbid., 66, 48 (1962). (6) Herrmann, R. A , , Adler, S. F., Goldstein, M. s., DeBaun, R. M., Ibid., 65,2189 (1961). (7) Hughes, T. R., Houston, R. J., Sieg, R. P., Znd. Eng. Chem. Process Design and Development 1, 96 (1962). (8) Johnson, M. F. L., Keith, C. D., J. Phys. Chem. 67, 200 (1963).
(9) Klug, H. P., Alexander, L. E., “X-
Ray Diffraction Procedures,” p. 504, Wiley, New York, 1954. (10) Kluksdahl, H. E., Houston, R. J., J . Phys. Chem. 65,1469 (1961). (11) McHenry, K. W., Bertolacini, R. J., Brennan, H. M., Wilson, J. L., Seelig, H. S., Actes du Deuxikme Congrks Internationale de Catalyse, Paris, 1960, p. 2295, Editions Technip, 1961. (12) Mills, G. A,, Weller, S., Cornelius, E. B., Actes du Deuxibme CongrBs Zntenationale de Cafalyse, Paris, 1960, p. 2221, fiditions Technip, 1961. (13) Pitkethly, R. C., Goble, A. G., Actes du Deuxikme Congrbs Internalionale de Cafalyse, Paris, 1960, p. 1851, fidi-
tions Technip, 1961. (14) Plank. C. J., Kokotailo, G. T., Drake, L. C., paper presented at 140th Meeting, ACS, Chicago, September 1961.
(15) Spenadel, L., Boudart, M., J . Phys.
Chem. 64,204 (1960). (16) Stumpf, H. C., Russell, A. S., SewBorne, J. W., Tucker, C. M., Znd. Eng. Chem. 42, 1398 (1950). (17) Swanson, H. E., Tatge, E., “Standard X-Ray Diffraction Patterns,” Vol. 1, 1953, Xatl. Bur. Stds., Circ. 539 (quoted in the ASThl X-Ray Powder Data File). (18) Teter, J. W., Gring, J. L., Keith, C. L., U. S. Patent 2,838,444. (19) Van Xordstrand, R. A., Preprints, Division of Petroleum Chemistry, ” . ACS, April, 1956. (20) Yamaguchi, S., J . Chem. Phys. 27, 1114 (1957).
RECEIVEDfor review August 13, 1963. Accepted January 9, 1964.
Emission Spectrographic Determination of Lead in Powder Samples with Spark Excitation D. C. HARGIS
and G. W. SMITH
Ethyl Corp. Research Laboratories, Detroit, Mich. ,Spark excitation of a powdered sample is used as the basis for a spectrochemical procedure of general applicability. In this procedure, matrix and chemical effects are eliminated by fusing the sample. lnterelement effects are eliminated by diluting the fusion with graphite powder and exciting the mixture in a high-voltage spark. Ejection of the sample into the spark is controlled by buffering the sample with graphite and by using a high-inductance spark. When applied to the determination of lead in a variety of materials, the procedure gave results that were accurate within 5% for concentrations ranging from 1 to 1 0 0 ~ o .
T
dry powders in spectrochemical analysis has many advantages. As Jaycox has pointed out, standards can easily be prepared in powder form, samples can be converted to a known composition, and radiation buffers can be added readily (6). However, when a n unknown sample is analyzed by the usual d.c. arc techniques, serious errors may result from interelement effects and selective volatiliaation of components from the electrode. These errors may occur even if the sample has been dissolved or fluxed to obtain it in a known form. Addition of radiation buffers does not always eliminate these difficulties. While good detection is usually achieved, quantitative results are sometimes neither as precise nor as accurate as desired. Although spark excitation usually gives more reproducible line intensities than the d.c. arc, it has not been genHE USE OF
824
ANALYTICAL CHEMISTRY
erally used for analyzing powders. When a powder is sparked in a n open cup electrode, the discharge strikes the rim of the electrode and does not pass to the sample directly. Instead, the powder is ejected from the cup through the spark stream. The actual excitation of sample particles is erratic unless the ejection of the sample is controlled. Where spark excitation was required, most investigators have preferred t o handle powders by briquetting them (6),by impregnating them on rods or on tape (2, S), or by using a n arrangement such as the sifter electrode (4). However, Milbourn and Hartley obtained good precision and sensitivity of detection by direct sparking of metallic oxide powders (7), and Steinberg and Belic successfully used direct sparking in the analysis of slags (8). Milbourn and Hartley observed that the method appeared to be free from selective volatilization and interelement effects. This indicated that the technique might be developed into a general procedure for the analysis of unknown materials. Such a procedure has been developed, and has been tested by using it for the determination of lead in inorganic and organometallic lead compounds and in a number of National Bureau of Standards standard samples. I n this method, matrix effects are eliminated by fusing the sample with potassium pyrosulfate to which cadmium as cadmium oxide is added as an internal standard. The melt is ground to a powder, with graphite added to control ejection from the electrode during excitation. The resulting mixture is packed in a graphite cup and excited in a high-voltage spark. Re-
sults show that the method is free from matrix and interelement effects. Lead concentrations from 1 to 100% can be determined within 501, of the amount present. Lower concentrations can be determined by varying the amount of sample and the excitation conditions. EXPERIMENTAL
Equipment and Materials. The equipment consists of a 2-meter grating spectrograph with a reciprocal linear dispersion of 5.2 A. per mm. in the first order, a high-precision excitation source unit, and a comparatordensitometer, all supplied by Applied Research Laboratories, Glendale, Calif. The spectrograph is equipped with a Model 9010 arc-spark stand supplied by Spex Industries, Metuchen, N. J. A Spex Industries Model 8000 mixer mill is used for grinding and mixing samples. The lower electrode is a 0.242-inch diameter erauhite rod with a 0.180-inch diameter Geiked crater (National Carbon Company No. 3903, Type AGKSP). The upper electrode is a 0.242-inch diameter graphite rod with a 30-degree included angle cone tip (Ultra Carbon Corp. No. 5340). Samples are prepared using graphite powder (Sational Carbon Co. Grade SP-2) and carbon powder (Ultra Carbon Corp. Grade UCP-3). Baker and Adamson reagentgrade potassium pyrosulfate is used for the fusions. The cadmium oxide used for the internal standard and the pure lead used in preparing standard samples are supplied by Johnson-Matthey, Ltd. Sample Preparation. A 100-mg. sample is fused in a size 0 porcelain crucible with 5 grams of potassium pyrosulfate containing 0.35y0 of cadmium oxide. For samples containing
