copper and the reproducibility is estimated to be 97%. The precision is about 2 to 3 in the ranges of 0.01 to 500 p.p.m. Effect of Interfering Ions. The estimation of copper by this method is unique, since as far as it’ is known no other elements or ions will activate the phosphor a t the conditions used. However, oxidizing agents in solution would destroy t h e phosphor by being absorbed and acting on t h e phosphor during the firing step. The presence of iiilrate, brcliilidc: chloride, sulfate, and hj-drogen p r o s i d e below 0.5% (based on weight of silicate in solution) could bc tolerated, but above this level, noticeable interference occurred. DISCUSSION
Copper may be estimated in the rttnges of 0.010 to 500 p.p.m. by the use of the act,ivation analysis procedure. The advantages of t’his test lie not primarily in its ektreme accuracy at any given copper content’, but in its sensitivity at copper concentrations lower than the sensitivity of the usual colorimetric methods a,nd in its utility and convenience. Copper is removed from solution during the settling and filtering process, only in the presence of silicate, and not quantitatively, except in the presence of both silicate and acetic acid. I t is possible that the mechanism of copper removal from solution follows
that proposed for the formation of television tube screens, particularly since a duplication of the settling system has been employed. Hazel and coworkers (6, 7 , 8) have shown that the silicate adsorbs on the phosphor surface, which acquires a negative surface charge. The silicate-bearing phosphor particles settle to form a screen, but polymerization and bonding of the silicate to form zt stable film do not occur until the zeta potentials of the glass and phosphor surfaces are lowered to a sufficient degree by the presence of a n ion such as acetate. The removal of copper from solution in the presence of silicate could be explained by formation of a coppersilicate complex which is adsorbed on the phosphor surface. Experimentally, no soluble copper was found in the solution after removal of the phosphor particles. However, copper was detected in the settling solution, where no silicate was employed: The luminescence-activation analysis was developed and applied as a routine method for analyzing a commercial grade of potassium silicate solution. It has, however, been demonstrated that the method may be used to determine soluble copper in any solution, provided the copper concentration and/ or electrolyte concentration is not too high.
ACKNOWLEDGMENT
The authors are particularly indebted to A. B. Davis (deceased) and G. V Potter for their advice and encouragement. REFERENCES
(1) Butler,
K. H., Mooney, R. W., Sylvania Technologist 9, 121 (1956). (2) Chilton, J. M., ANAL. CHEM. 25,
1274 (1953); 26, 940 (1954). (3) Delavault, R. E., Zbid., 24, 1229 (1952) (abstryt). (4) DuvaI, C., Inorganic Thermogravimetric Analysis,” p. 237, Elsevier, Amsterdam, 1953. (5) Edelberg, R., Hazel, F. Trans. Electrochem. SOC.96, 13 (1949). (6) Feigl, F., Caldas, A., Anal. Chim. Acta 8 , 117 (1953). (7) Hazel, F., Schnable, G. L., J . Electrochem. SOC.100, 65 (1953). (8) Hazel, F., Schnable, G. L., J . Phys. C h a . 5 8 , 812 (1954). (9) Leverenz, H., “Introduction to the Luminescence of Solids,” p. 246, W h y , New York, 1950. (10) Norwitz, G., A N A L . CHEM. 21, 523 (1949). (11) Parks, T. D., Lykken, L., Zbid., 22, 1503 (1950). (12) Pringsheim, P., “Fluorescence and Phosphorescence,” p. 522, Interscience, New York, 1949. (13) Reynolds, C. A,, Rogers, L. B., AXAL.CHEM.2 1 , 176 (1949). (14) Smith, G. F., McCurdy, W. H., Zbid., 24, 371 (1952). for review December 9, 1960. RECEIVED Accepted May 5, 1961.
Spectrochemical Detection of Nonmetallic Elements C. E. HARVEY C. E . Harvey Associates, P . 0. Box 175, Pullman, Wash.
J. W. MELLICHAMP
U. S.
Army Signal Research and Development laboratory,
A basic technique is given that will permit the simultaneous detection of the halogens, carbon, sulfur, phosphorus, and selenium in a single sample. Strong emphasis is placed on the use of standard instrumentation and on holding auxiliary equipment to a minimum of cost and complexity. In addition, a minimum amount of sample preparation and over-all operational time is sought. The method involves high-voltage spark excitation at reduced pressures. The sample is pressed into a pellet with silver powder and a silver rod is used for the counter-electrode. The technique is adaptable to both macro samples for trace impurities ond micro samples for major constituents.
1242
0
ANALYTICAL CHEMISTRY
N
Fort Monmouth, N. 1.
ELEMENTS present in chemical compounds, usually as a constituent of the negative radical or anion, can be determined by spectrochemical techniques. However, these elements are not determined by conventional spectrochemical methods and some of these methods require e q u i p ment not normally found in the laboratory. Other preparations and conditions are necessary before these elements can be excited and their emitted spectra identified and evaluated. An important consideration of all methods is the selection of proper excitation conditions sufficient to obtain emission spectral lines in a n accessible analytical region of the spectrum. Sparks with high average excitation (6,8), powerful low-tension sparks (7, 10, 11), hollow cathode excitation (f, 9), and highfrequency excitation at low pressures ONMETALLIC
(3, 13) have all been used successfully. Spectrographs operated in a vacuum or inert atmosphere permit the use of sensitive spectral lines below 2000 A. (6, 12). A general discussion of the determination of nonmetallic elements has been given (2). Elements considered to be nonmetallic from the standpoint of spectroscopy are those with excitation potentials greater than 6 e v. Those commonly encountered are the halogens, carbon, sulfur, selenium, phosphorus, and gaseous elements such as hydrogen, oxygen, and nitrogen. I n the technique described in this article the additional equipment needed is a relatively simple system whereby sample materials can be excited at reduced pressures. Sample preparation has been kept to a minimum. The practicality of this method makes
available to spectrographic laboratories the detection of nonmetallic elements with little additional cost and effort. The principal purpose of this investigation was to establish analytical conditions whereby optimum sensitivities of the elements sought could be obtained simultaneously in a single sample. EXPERIMENTAL
Apparatus. A standard Applied Research Laboratories spark unit is used for the excitation of nonmetallic elements. This unit has variable inductance values and fixed capacity settings of 0.007, 0.014, and 0.021 pf. Maximum sensitivities for this unit are obtained with 45 ph. and 0.021 pf. Previous publications suggest other sta'ndard units that can be used (4, 8). A spectrograph with a dispersion of 8 A. per mm. or better, with development and densitometry accessories, completes the conventional equipment needed. The system for excitation consists of a chamber to enclose the discharge a t a reduced pressure or in an inert atmosphere, or a combination of the two. Closed chamber discharges present several problems that are considered in the design of the apparatus. These include: ease of changing samples and subsequent re-evacuation; deposition of vaporized materials on optically critical surfaces; ease of disassembly for cleaning purposes or the possibility of successive use without cleaning; original cost and deterioration rate. The system for excitation is shown in Figure 1. The discharge chamber, F, is made from a 5-inch diameter Lucite tube, 4l/* inches long with l/r-inch wall thickness. The chamber is closed a t the top and bottom by insertion in '/*-inch deep circular grooves cut in '/a-inch aluminum plates, G and H . The grooves contain a thin coating of silicone seal. The lower plate, H , is rigidly mounted to the optical bench of the spectrograph while the upper plate, G, is held in place by the circular groove and is removable for changing electrodes. The sample electrode grip, D , is adjustable by gears, E, externally to the chamber while the counterelectrode grip, C, is fixed in the chamber. The Lucite side arm, J , cut from a rod rather than a tube, houses the quartz optical window. The inside of the side arm tapers from the window a t an angle to accommodate the focal path of the spherical lens, K , to the discharge center which is fixed by A and B. This permits a minimum-sized aperture in the chamber, and with the window offset from the chamber itself, the deposition of vaporized material on the window is negligible. Only occasional cleaning is necessary. Deposition on the walls of the main chamber can be fairly heavy with no interference to the analysis. The main chamber is easily cleaned and realigned without removal of the side arm. Both side arm and quartz window are held in place by a thin silicone seal. The assembly tightens under reduced pressure and is
a-
J--u
c.
_ _ ~
Figure 1 . Atmospheric control system spark excitation of nonmetallic
for
elements released a t atmospheric pressure for disassembly and sample loading which can be accomplished in less than a minute. When an inert gas atmosphere is desired a large vacuum reserve chamber, R, with connecting valves and tubing, L , is used to facilitate evacuation and flushing of the discharge chamber. The reserve chamber is made from a 6-inch diameter brass tube approximately 24 inches long with permanent plates soldered to each end. -4vacuum pump is connected a t Q and a manometer a t P . An additional pressure gage, not shown in the diagram, is mounted to read pressures outside the range of the manometer. Toggle valves, M and N , are held by a tee fitting, the side arm of which is connected to the discharge chamber. Valve M leads to a tank of inert gas whose regulation valve remains slightly open to maintain a positive pressure in connecting tubing. The valve on upper plate, G, is to release the reduced pressure in the discharge chamber. In practice, the sample electrode is placed in the discharge chamber and the entire system is pulled to the desired vacuum with valve N open and Jl closed. This requires only a few seconds when running a series of samples since N is closed when the vacuum is released in the discharge chamber and the reserve chamber remains a t the desired pressure. By alternately opening and closing N and M the discharge chamber can be filled with inert gas and reevacuated several times in succession to reduce the original air content to a negligible amount. Because the reserve chamber is large as compared with the discharge chamber, the pressure in the reserve chamber changes by only a few millimeters of Hg with each flushing. After sufficient flushing, the entire system can be restored to the desired pressure in a matter of seconds. Preparation. The technique wa8
developed using transistor grade silicon as a matrix. Since silicon is not a good conductor a t room temperature, a metallic powder is necessary to bind the silicon to form a conducting pellet. Both silver and gold meet the requirements for a binder because they are good conductors and are obtainable in a high purity powder form. These metals are infrequently sought in analysis and are usable both as a buffer and as a n internal standard. Their spark spectra are not overly complex and produce a minimum interference with analytical spectral lines. Preliminary studies indicated little preference between the two, thus silver was selected as the binder because of its lower cost. Silver powder of sufficient purity and mesh is obtainable from most chemical companies. Chlorine and sulfur are present as impurities in some of the powders. Materials to be determined for trace amounts of the nonmetals and which are available in sufficient quantities are first formed into sample Fellets with a '/4-inch mold from a mixture of 1 part of minus 100-mesh sample and 2 parts of minus 200-mesh silver. A mixture of 50 mg. of sample with 100 mg. of silver is sufficient to form a surface for sparking. However, to give bulk and ease of handling to the sample electrode, the sample mix is pressed into one face of a '/(-inch disk that has approximately 0.002 inch removed from the surface to facilitate insertion into the pellet mold. This also reduces the amount of sample required without changing attainable sensitivities. A recess is cut in the face of the disk, Figure 2, to anchor the sample pellet. In practice, the siIver disk is placed in the mold, the sample mix added, and the pellet pressed a t around 96,000 p.s.i. The counter-electrode is a hemisphericallytipped, '/(-inch silver rod. The tip is resurfaced after each burn and reused. For the determination of the major constituents in micro samples, 1 mg. and less, a special 1/8-inch mold was constructed so that the sample material can be pressed into the tip portion of a l/s-inch silver pellet. A '/*-inch silver rod is used as the counterelectrode. Standards. All the preliminary studies were made with the same set of standards in order t o make effective comparison of other variables in the procedure. Other synthetic standards with different matrices were made for further investigations. A large amount of transistor grade silicon, which had no impurities detectable by spectrochemical analysis, was crushed in a Plattner-type mortar and screened through a stainless steel sieve to minus 100 mesh. The powdered silicon, maintained under an inert (argon) atmosphere as much as practical, was used as the base material for the synthetic standards. Semiconductor grade silicon is widely used in the electronic industries and is available in high purity crystalline form that can be powdered in the laboratory. Silver compounds of the elements VOL. 33, NO. 9, AUGUST 1961
1243
sought were added to the base material to cover the concentration range from 0.01 t o 1%. Silver compounds were chosen in view of the silver binder and also because these compounds are stable and nondeliquescent. Since several levels of concentration were needed, the amounts of the various compounds added were varied in different directions so that the total weight added to each sample was about the same. The elements sought were also separated into several groups. Carbon was added as S i c because of the silicon base material. Discharge Conditions. A range of pressures from atmospheric t o 5 mm. of H g were investigated using t h e spark conditions mentioned previously for excitation. The pressure effect appeared t o be a t a minimum between 10 and 40 nim. of Hg, thus 20 mm. of Hg was the final choice. There was no marked advantage of different conditions for different elements between the 10 and 40 mm. of Hg. Samples were excited in both air and argon at 20 mni. of Hg. While there was no great difference noted between air and argon, the sensitivities were generally better in argon. The discharge system is so arranged that gases can be used interchangeably with little complication to the technique. Other analytical conditions include a 2-mm. gap and a 60-second exposure without pre-spark. I n actual practice, the 60 seconds are taken in two periods with a 30-second "off" period between. I n this way a cooler pellet is maintained with less tendency for the spark to remain fixed a t one point on the sample surface. Eastman SA2 film provides adequate speed and sensitivity in the 3500- to 5000-A. region where the analytical spectral lines used (Table I) are located.
PLUNGER 1/4'MOL!J
SOLID SILVER
DISC
Figure 2. Mold for making conducting pelletsfor spark excitation of nonmetallic elements
the quantities that can be detected. Of the elements sought, fluorine alone r w l d not be satisfactorily determined with the single technique. Standards as high as 5% F show no detectable spectrum. The only nonmetallic element detected in a blank run of the system-a silver pellet without sample -is carbon estimated at equivalent to 0.0370. The results obtained are considered to be semiquantitative since the amounts present are determined by bracketing between synthetic standards and comparing line-to-background ratios (6). No attempt was made to set up working curves; however, more quantitative results can be obtained by the use of a n internal standard line. A number of materials have been analyzed for trace amounts of the nonmetallic elements. Metallic boron samples from various sources were shown to have between 0.1 and 0.4% carbon with none of the other elements detectable. Geological specimens were found to contain various combinations of the elements sought. The application of the technique to micro samples of a nonrepetitious and unusual nature has proved to be especially valuable. The major constituents of samples of biological, geochemical, archaeological, and industrial origin have been identified. The residual, 0.05 mg., concentrated from 1 gram of transistor-grade silicon showed carbon as a major element. Residuals from other high purity
RESULTS AND DISCUSSION
The technique developed is designed t o obtain under the same conditions maximum sensitivities of the various elements sought and is probably not the maximum obtainable for a n individual element under specific conditions. Table I also lists the elements determined with the spectral lines used and Table 1.
-
Spectral Lines of Nonmetallic Elements for Spark Analysis in the 3500- to 5000-A. Region.
c
P
S
c1
Se
Br
I
3876 2 3876 4 3920.7
3556.5 3706.1 3827.4
3497 3 4142.3 4153.1
3498.0 3724 8 3897.3
4475.3
3562.4
4409.0
4076.0
4587.9"
4162.7" (0.07) 4174.3
3637.5 3711.6 8738 7a (0.1) 3800.9
3506.5 3517.4 3540.1
4074 5
3602.1 4781.3 4794.5" (0.01) 4810.1" 4819.5"
4382.9
4452.9"
4896.8
4704.9" (0.01) 4785.5"
4904.8
4816.7"
4602 0" (0.05)
4267 O b
4525.0
4267. 3a8b (0.1)
4512.6" 4666.5" (0.03)
Strongest lines. Unresolved doublet. Figures in parentheses indicate the detection limits in per cent by total weight.
a
1244
ANALYTICAL CHEMISTRY
materials were also identified. Identifications of both cations and anions of micro samples have been accomplished which were feasible by no known other means. The technique is directly applicable t o either nonconducting or conducting materials. I n the case of metallic samples of sufficient conductivity, metal particles can be pressed m a pellet with or without added silver. Preliminary tests of flat metal samples, using a point&-plane spark technique, showed excellent detectability for carbon-less than 0.1%. The discharge chamber is of sufficient size to permit the use of a Petrey stand table to make possible the analysis of large or irregular specimens of metal. Also the gas input is so located that a Stallwood-type (14) arc can be drawn. Another advantage of using silver as a binder is also noted., The general insolubility of silver compounds opens a possible application of chemical concentration of impurities from various materials previous to analysis. While the steps in the concentration procedure will vary greatly with the type of sample, basically i t involves precipitation of various anions from aqueous solutions with silver nitrate. Thus concentrational levels of the nonmetallic elements can be established in samples below those obtainable by direct analysis. LITERATURE CITED
(1) Birks, F. T., Spectrochim. Acta 6, 169 (1954). (2) Clark, G. L., "Encyclopedia of Spectroscopy," p. 230, Reinhold, New York. 1960. (3) Gatterer, A., Spectrochim. Acta 3, 214 (1949). (4) Gunn, E. L., ANAL. CHEM.26, 1815 (1954). (5) Harrison, G. R., Lord, R. C., Loof-
bourow, J. R., "Practical Spectroscopy," pp. 530-46, Prentice-Hall, New York, 1948. (6) Harvey, C. E., "A Method of Semi-
Quantitative Spectrographic Analysis," Applied Research Laboratories, Glendale, Calif., 1947. (7) Koehler. FV.,.~ Spectrochim. Acta 6 , 223 (1945). (8) Mansfield, W. O., Fuhrmeister, J. C., Fry, D. L., J . Opt. SOC.A m . 41, 412 (1951). (9) RlcNally, J. R., Harrison, G. R., Rowe, E. J., Ibid., 37,93 (1947). (10) Pfeilsticker, K., Mikrochim. Acta 1955,358. (11) PFeilsticker, K., Spectrochim. Acta 1,424 (1940). (12) Romand, J., Balloffet G., Vodar,
B., Colloq. intern. spectrog. V I , Amsterdam, Pergamon Press, London, 338 roo",.
i)
Schroll, E., Rockenbauer, W., Ibid., 1956).
;allwood, B. J., J . Opt. SOC.Am. RECEIVED for review February 10, 1961. Accepted May 10, 196:. Work supported by the U. S. Army Signal Research and
Development Laboratory under contract NO.D-4 36-039 SC-78267.
