Application of Powdered Samples to Graphite Electrodes for Spectrochemical Analysis RAY C . HUGHES, Philips Laboratories, Znc., Irvington-on-Hudson, N . Y . In spectrochemical analysis of miscellaneous r n a terials, samples that cannot be put into solution are frequently encountered. For excitation by the alternating current arc, such samples may be finely powdered and dispersed in glycerol, with the addition of buffers, if desired. The sample may then be applied to flat-end graphite electrodes by dipping the end of the electrode in the dispersion. The glycerol is driven from the electrode by heating. This results in the deposition on the electrode face of a thin, adherent sample coating which burns in a smooth and reproducible manner in the alternating current arc. The determination of silica in iron oxide and a mixed iron, manganese, and zinc oxide product by this method gave results having a probable error for a single determination of 3 to 67” of the quantity present over the range of 0.02 to 0 . 3 q ~ silica. This technique permits alternating current arc excitation, with its superior precision and sensitivity, to be extended to the analysis of solid, insoluble. nonconducting samples.
I
N SPECTROCHEMICAL analysis of miscellaneous ma-
terials, samples which cannot be completely dissolved by any known solvent or flux are frequently encountered. Such samples are excited without difficulty if a direct current arc is employed, but are not convenient for high voltage alternating current arc excitation. Flat-topped graphite electrodes spaced rather closely (1 mm. or less) are generally used with the high voltage alternating current arc. Samples are dissolved, and the solution is applied to the electrode faces, and dried to form a more or less uniform, adherent coating. Attempts to spread a powdered solid sample mechanically on the electrode are usually unsuccessful; uniform coverage of the electrode face cannot be obtained, and the powder is largely blown away when the arc is initiated, or may fuse to form a few large globules incompletely covering the face of the electrode and perhaps shorting out the arc. Under these conditions, satisfactory reproducibility of excitation is not obtained. Adherent, uniform coatings can be obtained with insoluble powders, if the sample is applied as a Buspension in a suitable fluid. Glyercol has been found very suitable for preparing such suspensions. Measured quantities of the powdered sample and C.P. glycerol are ground together in a mortar to form a uniform dispersion having the consistency of paint. The dispersion may be applied to the electrodes by dipping the face of the electrode into it, or, if better control of quantity applied is required, by dropping the dispersion onto the face of the electrode to give a standard weight increase. Volumetric control is not satisfactory, because of the high viscosity of the dispersion, The viscosity of glycerol prevents rapid settling of the powder. The dispersion wets the electrode readily to form a thin uniform coating. The coated electrodes are placed in a suitable holder and permitted to stand at room or slightly elevated temperature until the glycerol is absorbed by the graphite. The entire sample is deposited on the face of the electrode; thus the uncontrolled penetration of the electrode by the sample which may occur with solutions is avoided. Glycerol is removed from the electrodes by heating for a few minutes a t a temperature just below its boiling
point. Spectroscopic buffers such as graphite or ammonium sulfate may readily be incorporated in the dispersion. The photographs of Figure 1 illustrate results obtained by this procedure. A illustrates the best results which could be obtained by mechanically spreading the powdered sample. The electrode shown has sustained an arc. B shows the wet coating obtained by dipping the electrode in the sample dispersion, while C is the dried sample coating, and D is the same coating after it h w sustained an arc. D illustrates the uniform manner in which the sample is consumed in the arc. In the illustrations shown, the sample contained oxides of iron, manganese, zinc, and silicon and could not be completely dissolved by any single solvent or flux. The sample was mixed with equal weights of graphite and ammonium sulfate and dispersed in glycerol. This method of preparing and applying the sample to electrodes is applicable to a wide variety of solid samples not readily convertible to a soluble form. Minerals, rocks, soils, ceramics, insoluble inorganic materials, and miscellaneous deposits and residues might advantageously be handled by this technique. h detailed investigation of the method has been made for t h e determination of silica (SiOn) in a ceramic product composed of the oxides of zinc, manganese, and iron, and in reagent grade iron oxide (FenOa)employed in its manufacture. APPARATUS
Excitation Source. A 2200-volt alternating current arc source (1) is employed for excitation of sample spectra. T h e current is adjusted to 2.5 amperes by a bank of resistors in series with the arc. Spectrograph. A 2-meter grating spectrograph with 15,000line-per-inch grating and giving a reciprocal dispersion of 8.34 A. per mm. is used. Densitometer. A Hilger nonrecording densitometer is used for measuring transmittances of spectrum lines. Developing Equipment. Plates are developed in an open tray a t a temperature of 20’ f 1” C. The tray is rocked by hand throughout development. SPECTROGRAPHIC PROCEDURE
Sample Preparation. The sample is weighed out (0.100 gram) into a boron carbide mortar, and is finely ground. Spectroscopically pure graphite powder (300-mesh) and ammonium sulfate (0.100 gram of each) are added and ground together with the sample. Ten drops of C.P. glycerol, redistilled if necessary to free it of metallic contaminants, is added, and the mixture is ground to give a uniform dispersion having the consistency of paint. Flat-topped graphite electrodes are dipped into the sample dispersion to give a coating covering the face of the electrode and extending slightly (ca. inch) up the side of the electrode. The electrodes are loaded onto a metal holder and allowed t o stand a t room temperature or slightly higher until the glycerol has been absorbed into the graphite. The electrode holder is then transferred to a hot plate adjusted to produce a temperature which will evaporate the glycerol rapidly, but without boiling. The dried, loaded electrodes may be used immediately or may be stored under a dust cover until re uired for use. Preparation of Standards. %tandards are prepared from analyzed and selected chemicals found to be low in silica. Silica is weighed out and added to the base in the required amount to produce a content of 1.00% silica, mixed thoroughly, and ground together in a boron carbide mortar and pestle. An agate mortar is unsuitable, having been found to give significant silica contamination. The 1% standard and the base material are transferred to pure alumina crucibles with covers (Morganite, Inc.) and fired a t an appropriate temperature for 4 hours. Firing 1406
V O L U M E 24, N O . 9, S E P T E M B E R 1 9 5 2
t h i k pksical coGditian h l he Bimila t b thiEt of the samples. The fired materials are re round, and more dilute standards in the range of 0.01% to 0.32 silica ire made by grinding together appropriate proportions of the 1% standard and the fired base material.
1407 Photography. EMuLsIoN CALIBRATION.Calibration patterns are imposed by recording spectra of a standard sample through an eight-step, 1.5 to 1 ratsting sector. The Si 2881 line is meat ured and a plot of density us. relative intensity is made. This calibration is made on five plates from the lot of plates employed throughout. Photographic Processing. Emulsion. Eastman Spectrum Analvqip No. 1 .
1)
mded with Sample
Develovment. Esstman D-19, 5 minutrs 20" C
Drvine." Room tekverature Phhometry. Densities of the analytical and
comparison
lines
:o&osition &ne of 0.02 t o 0.3% silica. The workine curve was ~~~~~~
~
~
~.~~~ ~
ines and conversion t o relative intensities 'by reference t o the ,late calibration; and this in turn is referred to the working :uwe t o obtain the percentage of silica in the sample. The nean of five replioate determinations is reported 3s the silica :ontent. Analytical Line Pairs. The Si 2881.576 line is employed as ,he anslytied line. Far the analysis of iron oxide samples Fe
ANALYTICAL CHEMISTRY
1408
2877.301 serves as the comparison line. For the mixed oxide s.%mplesMn 2830.794 is the comparison line. Use of this line is permissible because the manganese, zinc, and iron Contents Of the sample are closely controlled. These line pairs were chosen on the basis of preliminary measurements which showed the intensity ratios to be relatively insensitive to changing excitation conditions.
silica in the electrodes by selecting base materials as low in silica as could be found; finally, samples were examined qualitatively by use of silic*free copper electrodes, and the presenoe of silica in the base materials was definitely established. Working curves were adjusted finally, by a method of successive wroximation (I), to give straight lines. Analysis of tho base material, without
th -".,e. The content of silica in thc base material was 0.020% in the ferric oxide and 0.0120%in the mixed oxide base. The arc gap was found to be a most important source of vanability. Increasing gap length caused n decrease in the intensity of the silicon line relative to that of tho comparison lines. Special care wa6 therefore taken to make the clectrode faces securatelg flat andperpendicular t o the axis, and to set the arc gap accurately. Variations which might normally occur in the aro current, in tho order of zk0.2 ampere, were found not t o produce any significant variation in intensity ratio. Variations in arcing time (up t o 90 seconds) had no important effect. Background of the plates, under the exposure conditions employed and analytical range involved, was so small 8.8 to require no correction. Individual plates containing a number of exposures of ampler giving 8. line intensity ratio of approximately 1 were found to give results varying to approximately the same extent 8 s exposures made on different plates. This would indicate that a variation from exposure t o exposure in excitation is largely responsible for the observed nonreproducibility of results. Possible errors due to inhomogeneity of samples was eliminated by prolonged wet ball milling of some of the samples in a steel jar with steel balls. This failed t o give any improvement in precision. Likewise, repeated exposures from a single pair of sample-loaded electrodes failed t o indicate any higher precision. The residual errors in the method me therefore considered to be made up of: "..~
of iron oxide is shown in Figure 1. These spectra illustrate the uniformity of exposure and freedom from background obtained. Working curves for silica in ferric oxide composition are linear over the range of 0.02 t o 0.3% silica, and have slopes close to 1. An intensity ratio of 1 for silica in ferric oxidr? is obtained a t 0.125'% silica. For the mixed oxide samples the intensities of analytical and comparison lines are equal a t O.i00% silica. Typical results obtained in replicate analyses of four m m ganese-zinc-iron oxide samplos follow: Shrn~leNo.
Xvo. of Detns.
579 577
15
Lor" 0.053
10 15 15
0.91
573 671
0.062
0.073
SiOl, % Xieh
-
0.060
Mea"
0,077
0.000 0.009
0.080 0.120
0.108
0.077
Precision of the method has been calculated from the results of 20 replicate determinations on each of three concentrations of
each of the two types of samples (total of 120 individual determinations). For ferric oxide samples, the probable error of a single determination varied from 3.9% at the lower end of the range t o 3.0% at the upper end of the range. The probable error ot a single determination for silica. in the mixed oxide samples varied from 4.9 to 5.9%. Although all sources of error and lack of precision remaining in the method have not been identified, silica in the graphite electrodes and silica in the base material have been found t o be especially important. The effect of silica. in the base has been separated from that of
1_
l.I
.". - ...." ~ .......
lll~"l"...".."
~
1409
V O L U M E 2 4 , NO. 9, S E P T E M B E R 1 9 5 2 1. Variability in excitation. This appears to be the niost important source of vnriability. 2. Errors due t o the plates. 3. Instrumental errors in measuring t,he plate. 4. ,Uncertainty as to the silica cont,ent of electrodes and hase materials. SUhIRIARY
The technique described provides a convenient means for excit,ation of solid samples by the high voltage alternating current arc. The uniform, thin, adherent sample coatings obtained on t,he electrodes burn in a smooth, reproducible manner. ThiP gives more uniform exposures and better reproducibility in line intensity ratios. The tcchnique therefore permits high voltagcl
alternating current arc excitation, with its superior precision and sensitivity, to be extended to the analysis of nonconducting solid samples. ACKNOWLEDGMENT
It is a pleasure to acknowledge the assistance of Josephine Schulz with the experimental work described here. LITERATURE CITED
(1) Duffziidack, 0. S.,and Wolf? R., 10, 161 (1938). RI.Ct:IT-ED
for review J a n u a r y 18, 1951.
ISD. ENG.CHEM.,A N . ~ LED., .
Accepted J u n e 11. 1952.
Standardization of Adsorbent Mixtures Used in Vitamin A Chromatography J. B. W ILKIE ARD S. F.JONES Division of aVutrition, Food and Drug idministration, Federal S w i r r i t ) igeitcy, Washington 25, D . C . Cnexplained variations in adsorption chromatography have been attributed to many factors. This paper describes a method for evaluating or standardizing one of the more important factors-the strength or adsorptive capacity of the adsorbent. Quantitative determinations of adsorbent strength (as adsorption indexes in terms of grams of F8;D Butter Yellow No. 4 per gram of adsorbent) have been conducted with magnesia as well as w-ith magnesia-Celite mixtures. The study includes the effects of atmospheric constituents upon adsorbents a5 well as correlation of adsorbent strength with separation of vitamin A and carotene in mixtures. The separation of vitamin A and carotene was found possible through a w-ide range of adsorption indexes. Chromatography may be improved and made more dependable by the use of the proposed standardization technique.
C
HEMISTS generally recognize the advantages gained by the application of chromatography in certain quantitative chemical separations, but frequently avoid the use of this procedure, particularly for routine purposes. Unfortunately, poor pwformance of an adsorption column does not give a definite indication of the major cause of a difficulty. The activity of the atlrorbent, which has been found to vary in an uncertain manner, has often been the h s i s for criticism where such difficulties have been observed. The development of an easily applied method for an independent quantitativc estimation of an adsorbent activity was considered of importance for this reason, and it was felt that such a method would permit standardization of the adsorbent and result in a general improvement of the chromatography. Of particular concern in the determination of the vitamin A pot,ency of margarines was the standardizing of adsorbents used in the separation of vitamin A and carotene from impurities and fromeachother ( f , 2j. JIagnesiuni oxide-Celite adsorbent mixtures are in general chromatographic use and are of special interest because they are readily available. flexible in composition, and well adapted to t,he use of ultraviolet light for viewing bands of fluorescent substances on the adsorption column. The activitJ- of magnesium oxide has been evaluated by the use of an iodine number deter-
niination and found by the manufacturer to be satisfactory for control purposes ( 3 ) . It x a s considered important, however, to have a method more convenient for use by the chromatographer, a method that makes use of the common adsorption tools and usual spectrophotometric equipment. Repeated observations have shown t'hat the certified food dye, F&D Butter Yellow 90.4, is strongly adsorbed by magnesium oxide, and information from the Division of Cosmet,ics, Food and Drug *idministration,indicated that this dye is available with a purity of greater than 99%. These facts led to the experimental work reported in this paper. Various techniques viere tried for determining the amount of this dye adsorbable by various mixtures containing the active adsorbent magnesium oxide. The final technique makes use of an original stock solution containing 5 X gram of FBiD Butter Yellow S o . 4 per ml. in a purified petroleuni ctlier. (This petroleum ether must be substantially free from fluorescence arid have a transmittance of greater than 85% a t 300 mp when measured in a 1-cm. quartz cell against a no-cell blank set at 100% transmittance. A product that neets this specification can he obtained from the J. T. Baker Chemical Co., Phillipsburg, S . ,J.) The authors' purpose w s to correlate the amount of this dye adsorbed viit'h the observed chromatographic behavior, using a petroleum ether solution containing vitamin A and carotene. To do this i t TYas necessary to develop the practical technique that would make the dyestuff adsorbed by a fixed sample, reliably and conveniently measurable. Initially i t was soon found possible to make a rough direct measurement by merely titrating a petroleum ether solution of dye into a sample of adsorbent suspended in petroleum ether until a definite color remained in the supernatant liquid after successive additions with continual stirring. After such an experimental approximation, the means for more precise measurements 1Tere determined experimentally. The method finally chosen depends upon adding under effective adsorption conditions an excess of the dye (as determined by an initial titration approximation above described), and then the amount of unadsorbed dyestuff was determined by spectrophotometric means. From the difference between total added dyestuff and the total excess dyestuff remaining after the adsorption step as calculated from the spectrophotometric adsorptions at 438 nip, the total dye adsorbed is readily obtained, and this value may be designated as the Butter Yellow KO.4 index of
