ANALYTICAL CHEMISTRY
222
at 8.70 microns where o-nitrotoluene is measured. (The concentration of 2,4dinitrotoluene in actual samples, however, is expected to be very low.) The results listed in Table I show that, when an ingredient is present in amounts greater than 2 or 3%, the relative error and the coefficient of variation are rather small considering the nature of the method which involves the analysis of a four component mixture of isomers and related compounds. When the ingredients are 2% or less, as expected, the relative error and the coefficient of variation are in some cases rather high. This is caused by the fact that the relative error and the coefficient of variation are adversely affected by the small concentration. However, even in these cases, as judged by the absolute error and the standard deviation, the results obtained using the developed method are very useful. I n spectrophotometric measurements maximum precision is obtained at 37% transmittance ( 3 ) ; however, the precision does not suffer very much unless the measurement is made a t much greater or smaller values. For the purpose of obtaining good precision the developed method prescribes the preparation, if necessary, of more than one sample solution so that the absorbance measurement will be made near the optimum point. I n general, i t was found that two separate solutions of the sample were sufficient to obtain reasonably good measurements. I t has been known (3) and confirmed (1) that absorptivities vary with slit width. This fact makes it imperative that the slit width used in the analysis of samples be exactly the same as the one used in obtaining the absorptivity constants using standards. The analysis of the mixture of 0-,m-, and p-nitrotoluene and 2,4dinitrotoluene involves the making of absorbance measurements at four wave lengths corresponding t o the four selected infrared bands (one for each ingredient). The four absorption bands were chosen taking into consideration reasonably strong absorbance and freedom from interference from absorption bands of other components. The four selected bands are not completely ideal, as a t 8.70 microns (used for the o-nitrotoluene) both onitrotoluene and 2,4dinitrotoluene have bands whose peaks are
a t exactly the same nave length. However, because 2,4-dinitrotoluene is usually present in very small amounts in actual samples and also because the two bands are a t exactly the same wave length the use of the 8.70-micron band has proved to be satisfactory. Cyclohexane was selected as the solvent because of its relatively small absorbance a t the four points of measurement and also because the four components were reasonably soluble in this solvent (2,4dinitrotoIuene is not too soluble in cyclohexane, but in the presence of 0-, m-,and p-nitrotoluene, such as present in actual samples, its solubility is adequate for the analysis). I n the process of obtaining absorbance measurements it is sometimes difficult to be certain that the measurement is being made exactly a t the point where the maximum absorbance of the particular band occurs. This is especially true when the cell and solvent absorbances are being determined or when the absorbance of the sample is such as to produce a shoulder alongside of another major band instead of producing a well-defined band. This difficulty was surmounted by first locating the position of the true peak, using a reference cell containing a solution of about 5 % of each of the ingredients in q-rlohexane. ACKNOWLEDGMEYT
The authors wish to express their appreciation t o -1.J. Clear, C. J. Bain, R. Frye, and J. C. -4rmitage of Picatinny hrsenal for help rendered in the publication of this report. -4ppreciation is further expressed to the Ordnance Corps for permission to publish this paper. LITERATURE CITED (1) (2)
Pristera, F., A p p l . Spectroscopy, 7, S o . 3, 1 1 S 2 1 (1953). Pristera, F., Perkzi~-ElmerI n s t r u m e n t S e w s , 2, Xo. 2 (winter 1951).
(3) Robinson, D. Z . , . & s ~ L . CHEM.,24, 619 (1952). (4) Smith, D. C., and Miller, E. C., .I. O p t . Soc. A m e r . , 24, 130-4 (1944). (5) Soc. A p p l . Spectroscopy Bull., 4, S o . 2 (January 1949) RECEIVED for review April 13. I954
rlccepted October 21, 1954
Application of the logarithmic Sector to Quaatitative Spectrographic Analysis of Petroleum Ash Residues E. B. CHILDS and J. A. KANEHANN Technical Service Department, Socony-Vacuum O i l Co., Inc.,
A large number of deposits, sludges, and used oil samples are analyzed by petroleum laboratories to furnish clues of proper or improper operation of engines and machines. This investigation was undertaken to provide a rapid quantitative spectrographic procedure for the analysis of these samples. The logarithmic sector method is fast and obviates the need for an expensive densitometer. Analytical procedures, data, and results on 13 different samples are presented. The average standard deviation is 30%. Working time per sample is about 73 minutes for ashing and quantitative determination of eleven elements.
Q
UANTITATIVE spectrographic methods, used for analyses of many materials, generally rely on densitometric study of the exposed plate to determine line intensities. Another technique for measuring line intensities involves the use of the logarithmic sector. iZ logarithmic sector disk is similar to a step sector, except that the periphery is cut as a smooth curve (Figure 1 ) . It can be thought of as a step sector with an infinite number
472 Greenpoint Ave., Brooklyn 22, N. Y.
of steps. When the sector is rotated in front of the spectrographic slit, light from the source is interrupted. Figure 1 shows that the amount of light will vary a t different locations on the slit. Light interruptions a t the outside of the sector will be short. I n contrast, the blanking of light near the bottom of the V of the sector will be almost continuous. Hence, tapered lines !vi11 be formed on the photographic plate of the spectrograph. Because of the shape of the rotating sector and the response of the photographic emulsion, the length of each line is a logarithmic measure of intensity and the difference in length of two lines is the logarithm of the ratio of the intensities or log I , - log I,, where I , is the intensity of the element line and I , is the intensity of the internal standard line. This intensity ratio is generally independent of conditions of exposure, slit width, development, and plate characteristics (3). Line differences-i.e., intensity ratiosare plotted against per cent concentration to give a working curve. Several factors which must be considered when using a logarithmic sector are (11): The jaws of the slit must be parallel and clean. The sector should b~ placed directly in front of and as close t o the slit as possible.
V O L U M E 27, NO. 2, F E B R U A R Y 1 9 5 5
223
The straight edge of the sector must br parallel to the edges of the slit when it revolves past the slit. The sector should revolve a t least 50 to 100 revolutions during each exposure. Each spectrum line must be uniform along its length when the logarithmic sector is not used. An image of the source should be formed on the prism (or grating), not on the slit. The spectrograph muPt be stigmatic or special arrangements must be made to overcome this difficulty. >lost grating instruments are not stigmatic.
8-CALIBRATION CURVE C - STAN Ob A D DEV IAI I O N
I
-30
0
Figure 2.
-2 0
1
-I 0 00 E L E Y E N I L I N E LENGTH-INDIUM LINE LElGTH
t IO
t 20
Typical analytical curves and standard deviations
The plates are placed on a special viewing box and read with a modified Rausch and I a m b optical magnifier. Other auxiliary equipment includes muffle furnaces, analytical balance, grinders, and general analytical laboratory facilities. Figure 1.
METHOD
Logarithmic sector disk
Scheihe arid Neuhausser (9) are credited \$ith first application of the logarithmic sector to quantitative spectrographic analvsis in 1928. Twyman and Hitchen ( 1 3 ) analyzed dried powders (salts, etc.) by packing directly into crated graphite electrodes. Accuracy of 10 to 20% was claimed. Slavin (10) also used this procedure for determination of cobalt, iron, copper, cadmium, thallium, germanium, and lead in metallic zinc and zinc sulfate solutions. Foster and Horton ( 4 ) used a special optical comparator with a fixed line which is moved along the analysis line until they taper off equally. This method helps define the terminus of the line. Several advantages of the logarithmic sector over the densitometer listed by Green ( 5 ) include speed, self-calibration, and lack of special equipment. Many others ( 1 , 2, 6, 7, 11-15) investigated the application of the logarithmic sector to spectrographic analysis. A complete bibliography of the logarithmic sector applications is given by Ranknma and Joensuu (8). In order to apply the logarithmic sector to analyses of various products, an internal standard (indium) was employed. Indium shows a very simple spectrum in the 2500 to 3500 -4. region. The internal standard line, 2932.6 A., is almost in the middle of the analytical region; thus, any large variations in plate emulsions are eliminated. The use of a logarithmic sector mabes a densitometer and plate calibration unnecessary. Howevei , the other spectrographic techniques remain the same. EQUIPMENT
A Bausch and Lomb large Littrow spectrograph with the camera positioned to photograph the 2500 to 3500 A. range is used. The plates employed are Eastman Kodak, spectrum analysis S o . 1. Exposure time is controlled automatically. All electrodes are high purity spectrographic grade graphite. The anode is 2 inches long and inch in diameter, flat cut. The cathode is 1.5 inches long and '/a inch in diameter, with the end a 30" cone cont,aining an axial 1/16-inch diameter crater, 1/16 inch deep. A standard 5000 r.p.m. motor with a 53/1,-inch logarithmic sector disk (Jarrell Ash Co.) is placed in the optical path. .I direct current motor generator, 115 volts, 60 amperes, is used as a source. The arc is initiated by a radio-frequencygenerated spark. Two current levels are used. The 7.5ampere level is used first to prevent "popping" of the sample. After 5 seconds, the current is increased to maintain sensitivity.
Standards, Internal Standard, and Buffer. Prepare two stock synthetic. ashes by combining pure compounds of the various elements. Stock ash A contains four elements: iron, silicon, lead, and barium. Stock ash B contains eight elements: aluminum, magnesium, tin, chromium, calcium, copper, silver, and sodium. Ilse oxides and carbonates in preparing these standards, except where these compounds are hygroscopic or deliquescent. To ensure purity, analyze spectrographically all the compounds used in formulating the standards. hlake blends of stock ashes A and B and dilute with lithium carbonate to give known concentrations of the various metals. The range of concentrations covered is listed in Table I. To the final standards add enough lithium carbonate to equal 10 times the quantity of synthetic standard used. Finally add 1 % of indium as the oxide to the resultant mixture of buffer and standard. Arc the standard samples and develop the plates as drscribed in the section on analysis of samples.
Table I.
.Analytical Lines and Concentration Ranges Wave Length, A. 2660.4 3092.7 2779.8 2795.5 2833.1 2833.1 2840.0 2881.6 2983.3 3014.5 3071.6 3158.9 3179.3 3247.5 3274.0 3280.7 2932.6
Element .Iluminum 3Iagnesium Lead Tin Silicon Iron Chromium Barium Calciri m Copper Silver Indium (intern. std.)
Element i n Ash,
70
0.3-10 0.02-0.5 0 2-10 0.01-0.2 0.2-50 0.2-50 0 02-1.0 0.1-50 0.2-50 0.02-1 .o 1-50 0.5-10 0.5-10 0.04-1.0 0.04-1.0 0 01-1.0
Table 11. Arcing and Exposure Conditions" Type of excitation D.c. arc with radio frequency initiating spark Circuit constants Voltage across electrodes 45-55 Current 5 sec. a t 7.5 amp., 20 sec. a t 16 amp. Capacitance. mf. 0 4 Resistance, ohms Arc gap, mm. Spectral region. A. 2500-3500 Slit width, microns 20 Height, nun. 13 Exposure time, sec. 25 ( 5 a t 7.5 amp., 20 a t 16 amp.) Log sector disk and motor S o condensing lenses, diaphragms, or filters are used.
'jo
224
ANALYTICAL CHEMISTRY
Draw calibration curves by plotting the difference in line lengths of the element line and internel standard line against the concentration (log scale) on semilog paper. Table I gives the element lines used. Two typical calibration curves (with standard deviation) are shown in Figure 2. Analysis of Samples. Thoroughly mix the sample in its original container by vigorous shaking. Viscous samples require heating in addition to shaking. D r y samples must be ground in a mortar. Samples not requiring ash treatment are run directly after mixing. Weigh to the nearest 0.01 gram a sufficient sample to yield about 0.05 gram of ash residue. Ignite the sample in a porcelain crucible and burn gently to a carbon ash. Cool and add one drop of concentrated sulfuric acid. Heat slowly to drive off sulfuric acid fumes and put the sample into a muffle furnace maintained at 1000" f 50' F. until all the carbon is consumed. Cool in a desiccator and weigh the ash to the nearest 0.0001 gram. Thoroughly mix the ash and weigh 0.01 gram into a screw-cap vial. Add 10 times a s much lithium carbonate to the vial and to the final mixture add 1% ' indium a s the oxide. Grind the mixture in a mortar and store in the original vial. Pack the mixture into a cratered electrode by pushing the hollow end of the electrode into mme of the sample contained in the cap of the vial. Set the spectrograph a s indicated in Table SI a.nd ~s...m -..-S.PP -.. the ...-. .~n.l e- .
Develop the Eastman Kodak spectrum analysis No. I plates for 2.5 minutes a t 68" F. in EK-DI9 develomr with conatant agitation. Short stop by a 10-second immerbion in 3% acetic ~~
~~
~~~
~
acid. Fix for 5 minutes in E K rapid liquid fixer with hardener. Wash for 10 minutes in circulating cold water and dry in a drying oven. The ratio of intensities between two lines is measured by their difference in lengths. The length of the line may he measured from any spot that is common to all lines. I n the method used here a hairline was fixed permanently to the 20-micron slit. This hairline served a8 a cemmon base for the measurements. Measurements are performed with a modilied Bausch and Lomb magnifier. The modificttt,ion consists of a small piece of emulsion
containing a typical logarithmic line attached to the face of the magnifier (Figure 3). With the logarithmic line on the eyepiece direct intensity compnrisan is facilitated. Determine the length of the line by moving the eyepiece along the line being measured until the rate of extinrtion of the lines is the same. Estimate the line length to the nearest 0.05 mm., using the ~ c a l e in the magaic---
\
Figure 3. Measuring eyepiece
225
V O L U M E 27, NO. 2, F E B R U A R Y 1 9 5 5
trace analysis and the longer, more complicated densitometric procedure. It is particularly suitable for samples requiring somewhat less precise results than given by chemical analysis. I n this laboratory operator time is about 1.25 hours, in contrast to about 2 hours for the densitometric method or 24 hours for chemical analysis of the 11 elements.
Figure 4 indicates how the length of the various lines decreases with decreasing concentration. The use of a constant amount of an internal standard in conjunction with the logarithmic sector makes plate calibration unnecessary. DISCUSSION
This method can be applied to any powder or sample of ash residue. It has been successfully applied to many types of samples, such as used engine oils, greases, cracking catalyst,s, and deposits. The analyses of 13 samples are shown in Table 111. The five cracking catalyst samples were supplied by the Subcommittee on Analysis of Cracking Catalysts, Committee on Analytical Research of the American Petroleum Institute. One sample (used oil B ) was supplied by the Pennsylvania Railroad laboratories as a member of the ASTM Committee D-2, Reswrch Division 111, Section E on Spectrographic Analysis of Lubricating Oils. Results obtained by chemical analyses are shown also. These samples were used only as reference samples t o show the universal application of the method. The method is a valuable adjunct to the usual major-minor
Table 111. Sample Grease ash Copper Nagnesium Csed oil (Diesel) ash A Aluminurn Magnesium Silicon Iron Barium Copper Used oil (ash residue) B .4luminum Magnesium Lead Tin Si1icon Iron Barium Copper Calcium Grease ash Aluminum Magnesium Lead Iron
The correlation between standardization data and known elemental concentration is very high (average correlation coefficient for all elements is 0.96). I n Table I11 results are presented on a variety of samples. The accuracy of the method was calculated from the National Bureau of Standards samples and equals zk 22.8’%. However, in two cases-namely, aluminum in XBS 88 and chromium in NBS 97-the errors are very large (72 and SO%, respectively). These errors are noticed in the 0.05% concentration range, which equals about 2.5 p.p.m. in the original oil sample. The accuracy for the National Bureau Standards, disregarding these two questionable results, is within &12.2%. The standard deviation of the mean for all samples is 4=30OJo. In the standard deviation calculations the chemical results were taken as the true value.
Experimental Results Logarithmic Sector, P.P.M.
Ash,
% 0.75
1
Chemical, Av.
3
0.8 4.5
0.9 1.1
1.0 1.5
0.9 2.3
5.3 1.2 5.0 31 2520 1.7
7.3 1.8 5.0 26 2580 1.7
7.9 1.8 7.3 44 2900 1.7
6.8 1.6 5.6 33 2660
5.5 3.7 110 4.1 16 110 3200
56 4 1 105 3 2 20 105 3040 13.8 250 184
3 5 2.8 97 7.3 12 68
4.9 3.6 104 4.8 16 94 3100 16.4 237
.... 22 ....
7.5 30 Too high Too low
8.6 24.6 21,000 4.9
’
0.56
0.46
15.6
2.46
P.P.M.
2
194 346 59 11
20 0
212
9.0 ... 42 . ~ . .,. ... ...
34
..,. ,... 1
Synthetic catalyst, new Aluminum Silicon Copper Synthetic catalyst, contaminated Aluminum Silicon Iron Copper S a t u r a l catalyst, new Aluminum Si1icon Iron Copper S a t u r a l catalyst, contaminated Aluminum Silicon Iron Copper S B S 1A (argillaceous limestone) Aluminum Magnesium Silicon Iron S B S 88 (dolomite) Ahrminum S B S 98 (plastic clay) Magnesium Iron Copper S B S 97 (flint clay) Magnesium Silicon Iron Chromium Xatural synthetic catalyst mix Aluminum Iron Silicon Copper ‘ Maximum calibration value.
ACCURACY AND P R E C I S I O N
1.7
LogarithmicI Sector, %
2
3
3.1
3.2
30 1.6 0.01
7.8 4.8 >30 >30 1.2 0.5 0.01 0.01
6.7 >30 1.1 0.01
7.5 29.8 1.96 0.014
3.7 >30 2.5 0.03
7.4 4.1 >30 >30 1.7 1.8 0.04 0.04
5.4 >30 2.0
3.1 30a 0.01
...
6.0
>30 0 4 0.01
7.4 >30 0.2 0.01
0.7 1.3 4.8 0.5 0.07 0.36 1.8 30
