ANALYTICAL EDITION
March, 1945
the suthors, no titanium being recovered from potassium pyrosulfate fusions of the ignited residues. Steels of titanium content higher than 0.05% may be analyzed by the method outlined, using proportionately smaller samples. The method may also be used for cast iron, as well as steel of various alloy composition. ACKNOWLEDGMENT
The work done by H. A. Sloviter, of this laboratory, in preparing the spectrophotometric CUNW is acknowledged with appreciation. LITERATURE CITED
Z. anal. Chem., 92, 1-7 (1933). (2) Boyer, W. J., paper presented at 47th Annual Meeting of Am. SOC.Testing Materials, June, 1944, New York. (1) Bendig, M., and Hirschmuller, H.,
145
(3) Cunningham, T. R., IND. ENCI.CHEW,ANAL.ED.,5 , 305-6 (1933). (4) Ibid., 10, 233-5 (1938). ( 5 ) Kenigstul, M. D., Zuvodskayu Lab., 9,1203-5 (1940). (6) Lundell, G. E. F., Hoffman, J. I., and Bright, H. A., “Chemical Analysis of Iron and Steel”, New York, John Wiley & Sons, IYSI.
(7) Silverman, L., IND.ENG.CHEM.,ANAL.ED.,14, 791-2 (1942). ( 8 ) Sokolova, I., Amhpromyshlennost, 1939,No. 3, 52-5. (9) Terrat, S., Documentation sci., 4, 17-19 (1935). (10) Thomas, A., paper presented at 47th Annual Meeting of Am. SOC. Testing Materials, June, 1944, New York. (11) Thornton, W. M., and Roseman, R., Am. J. Sci., (5), 20, 14-16 (1930). (12) U. S. Steel Corp. Chemists, “Sampling and Analysis of Carbon and Alloy Steels”, pp. 188-9, New York, Reinhold Publishing Corp., 1938. THEopinions expreased are those of the authors and are not as reeecting the 05cial views of the Navy Department.
to
be construed
Quantitative Spectrographic Method for Analysis of
Small
Samples of Powders
ELMORE J. FlTZ
AND
WILLIAM M. MURRAY
General Electric Co., Piftsfield, Mass.
A method i s described for the quantitative spectrographic analysis of inorganic substances using only 0.1 to 1.0 mg. of powdered sample. When applied to such small samples, the method gives sufficient accuracy for many purposes, and the saving in time is considerable, Calibration curves have been prepared for ten elements and it should be possible to extend this number to include most of the common elements which are excited in the direct current arc. The technique has been found satisfactory for complex rocks, slags/ refractories, corrosion products, boiloc scale, ash from ignition of organic materials, and various small samples of inorganic nonmetallic powders.
HE development of the technique described was prompted
T
by the need of a rapid method for the quantitative analysis of minute samples of inorganic powders such as corrosion products, nonmetallic inclusions, and ash from organic niaterials. It has &en the authors’ experience that the usual quantitative microchemical analysis of such materials is tedious and discouraging. A powder method employing a ball mill for mixing the samples has recently been described by Oshry, Ballard, and Schrenk (3). The common desire to secure speed a t the expense of accuracy led to the present method, which eliminates several of the time-consuming operations involved in mechanical grinding. In the authors’ case the time elapsed between the receipt of the sample in the laboratory and the delivery of the analysis may at most be 2 hours. This allows for preparation of the sample, making any necessary spectrographic adjustments, exposure, processing, photometering, and calculation. A quick analysis of this kind is a very useful preliminary to a complete silicate analysis. The method is a n adaptation of the ratio quantitative powder technique of Lewis ( 2 ) , who synthesized powder mixtures until the resulting line intensities of a known mixture matched those of the unknown powder. The general technique consists of mixing the sample to be ana1y;ed with a large excess of a pure powder mixture which serves as a b d e r and internal standard. Weighed proportions of this mixture are then pressed into pellets which are burned in a direct carrent arc and the spectra prepared. Line intensity ratios between selected spectrum lines of the sample constituents and the
internal standard are determined photometrically and converted to weight of constituent by reference to calibration curves. EQUIPMENT
A Xeiss QU-24 spectrograph and external lens system was used
for recording the spectra. This lens system forms an intermediate image of the source so that any part of the arc may be screened off by the choice of a suitable diaphragm. Kational Carbon Company specially purified graphiteelectrodes0.47 cm. (3/la inch) in diameter were used throughout this work. All photographic plates were Eastman Kodak Company Process plates which were developed in Eastman D-11 solution. A Zeiss spectrum line photometer was used for all line intensity measurements. PREPARATION OF SAMPLE
Barium nitrate was selected as an internal standard because it is not a common constituent of the materials to be considered it will not vaporize too rapidly in the Crc i t can be obtained readily as a pure salt, and i t makes afirmer pellet than other barium salts. The internal standard-buffer mixture finally chosen was composed of equal parts by weight of barium nitrate and ammonium sulfate. The possibility of the formation of ammonium nitrate in mixing these salts together should not be overlooked. Bearing this in mind, it is not’ advisable, when preparing pellets, to employ salts of the unknown which have stron reducing properties. The optimum weight of a standard-%der was found t o be 20 mg. and of sample 1 mg. In certain cases where extremes of concentration were encountered, the sample weight was varied between 0.1 and 5.0 mg. pvith the regular 20 mg. of standard-buffer. The sample and standard-buffer were weighed into an agate mortar and ground into a well-mixed fine powder. This was then pressed into a small pellet, using a micropress which forms a cylindrical pill 2 mm. in diameter. This spectrographic method has been applied to samples weighing 0.1 to 5.0 mg. Although the method has been applied to the determination of ten elements only, there seems to be no reason why this number cannot be expanded to include most of the elements excited by the direct current arc. Work along this line and on other applications is in progress. EXPOSURE CONDITIONS
A diaphragm, C, with a small aperture (0.5 X 10 mm.) was used a t the point of intermediate image formation in the external
lens system. This aperture was of such size that the optimum
INDUSTRIAL AND ENGINEERING CHEMISTRY
146
Figure 1.
Schematic Diagram of Lens System
exposure was obtained when the sample was burned completely. The lens system employed has been described by Kaiser (I),and is shown diagrammatically in Figure 1. An image of the arc, A is formed on diaphragm C by means of an 80-mm. focus quartz \ens, B. The diaphragm is a rotatable disk containing openings of various heights. By employing the proper openin any portion of the arc column may be used for illumination. %he portion of the arc selected for illumination is projected by means of a uartz lens, D, of 227-mm. focus onto the collimator lens, E. T%e diaphragm is mounted on a lens of 200-mm. focus which forms an image - of lens B on the slit of the spectrograph. The sample pellet was placed in a small crater in the lower positive electrode and arced until i t was burned completely. A 220volt direct current supply was used. With a current of 6 amperes (direct current) between the bare graphite electrodes which were s aced 4 mm. apart, the current Fose to about 7 amperes while tge pellet was burning, and then dropped back to 6 amperes when the pellet was exhausted. This usually required an exposure of about 2 minutes.
Table
I.
Vol. 17, No. 3
for comparison with a line of each of the constituent elements. The wave lengths of the spectrum lines used are given in Table I. The variations of average line-ratio values for a given concentration of an element were usually less than 20% from plate to plate. When several plates were used to get an average line-ratio value for a Iarge number of determinations, the plot of these averages against concentration yielded working curves with all points falling very near a straight line. The straight lines drawn through the points were calculated by the method of least squares. Several of these curves are illustrated in Figures 2 and 3. A second set of working curves was prepared by another operator one year after the first set. The old and new curves did not differ by more than 5%, and they coincided in most instances.
Wave Length of Spectrum Line Used for Each Element
A. Ba 8n Si A1 Fe Mg
2771.4 2429.6 2436.2 2862.6 2723.8 2783.0
A. Cu
2824.4 Mn 2933.1 NI 2943.9 Ca 3168.9 Ti 3188.6
DEVELOPMENT AND PHOTOMETRY OF PLATES Both the developer and fixer tanks were kept in a water bath at 65" F. A mechanical developer powered by an electric clock motor was used to move the plate back and forth through a 2.5cm. (I-inch) path in the solution once each minute. The line intensities were measured on the Zeiss photometer using a galvanometer scale of 1000-ml. divisions. The Leeds & Xorthru galvanometer was placed 3 meters from the I-meter scale. %e ratio of the galvanometer deflections for internal standard line and unknown line were plotted against weight of oxide on log-log coordinate paper.
This procedure differs from the more general one of converting the galvanometer deflections to relative intensities by use of a plate calibration curve and plotting the calculated intensity ratios against concentration. The technique employed has been found to save time, since preparation of a plate characteristic curve and conversion of galvanometer deflections to relative intensities are obviated. This saving is at the expenrje of some precision. The theory and limitations of the method have been adequately covered by Twyman (4). PREPARATION
I
0.I 0.001
I
I
I
I
IIIII
I
I
I I 1 1 1 1
0.01
0.10
I
I
I 1 1 1 1 1
LOO
MILLIGRAMSOXIOE
Figure
P
P
i
OF WORKING CURVES
Reagent grade oxides of the following elements were 'used in preparing synthetic standard mixtures for the determination of working curves: silicon, aluminum, magnesium, iron, calcium, titanium, tin, copper, manganese, and nickel (calcium was used as carbonate but calculated as oxide). The first five oxides were varied through the range 0.01 to 1.0 mg., while the last five were considered only in the range 0.01 to 0.1 mg. The composition of each standard mixture was varied widely from the others, so that any effect of one element upon another should show up readily. No such effect wasobserved. Each spectrographic plate consisted of several exposures of the five synthetic standards used. A single barium line was chosen
0.1 0.001
OJO
001
MILLIGRWS OxloC
Figure 3
LDO
A N A L Y T I C A L EDITION
March, 1945 Table
11.
% Si01
Application of Method to Analysis of Bureau of Standards !Sample5
% A1:O:
Chem.
Spec.
Chem.
Spec.
Chem.
Spec.
Chem.
B.S. 1, argillaceous limestone B.S. 27a. Sibley iron ore B.S. 69,bauxite
18.0
5.7
6.6
1.7
...
5.3
56.0
67.0
B.8. 76, burnt refractory B.S. 77, burnt refractory B.S. 78, burnt re-
55.0
93 81 17 12 37 35 62 59 96 69
1.8 1.6 48 32 7.8 7.2
