Chemical Analysis of Thin Films by X-Ray Emission Spectrography T. N. RHODlN Engineering Research Laboratory,
E. 1. du
Pont de Nemours & Co, Inc., Wilmington, D e l .
X-ray emission spectrography has been applied to the quantitative analysis of thin (approximately 100 A.) evaporated films of iron, nickel, chromium, and Types 304, 316, and 347 stainless steels deposited on Mylar polyester backing. Oxide films associated with passivity and atmospheric oxidation, isolated from Types 304,316, and 347 stainless steels, were also studied. The corresponding surface densities of the sample films varied from 1 to 100 X 10+ gram per sq. cm. Analytical sensitivities for the three metals corresponding to a counting rate of 1 count per second &;yo are 0.037, gram per sq. cm. for nickel, iron, 0.061, and 0.175 X and chromium, respectively. Comparison of the metallic compositional data obtained by x-ray methods for metal and oxide films 300 A. in thickness or less to those obtained independently by microcolorimetric methods indicates excellent agreement. The conclusion is made that accuracies within 2% or better can be obtained for the chemical analysis of such highly dispersed samples by x-rays without the application of corrections for adsorption deviations.
In spite of the very small fluorescent x-radiation excited in microgram samples of metals, alloys, and related compounds, the sensitivity and relative simplicity of this method recommend it strongly as a tool in metallurgical research ( 2 , 4, 6). Some interesting nonroutine metallurgical applications of x-ray chemical analysis have already been surveyed by Koh and Caugherty ( 5 ) . Preliminary calculations by the author indicated that reliable measurements might be achieved utilizing specially prepared samples and low background counting conditions. A n extension of the approach of Koh and Caugherty to the analysis of ultrathin films (30 to 150 A.) of metals and oxides by x-ray emission spectroscopy was undertaken. Results were very gratifying, once the difficulties of isolating and mounting film samples and measuring accurately very low counting rates were resolved.
C
HEMICAL analysis of surface films, surface residues, and corrosion products on metals and alloys is often essential to an understanding of mechanisms associated with surface reactions. Difficulties in handling samples on the microgram scale by classical microchemical analytical techniques have limited this approach to problems in surface metallurgy (11). B
A
GEIGER COUNTER
-----.A-
\X-RAY
TUBE
1
-LIT~IUM
CRYST4L
FILM S4MPLE M Y L 4 R SUPPOR SUPPORT MOUN
OL-ER
SLOT
FLUORIDE SINGLE
4N4LYZER
CCLLIMATOR
Figure 2. X-ray optics-flat crystal, reflection method
C
A . Bromine-methanol stripping, total solids collected B . Mylar tape stripping, surface fragment3 collected C . Bromine-methanol stripping, sections of film collected
The experimental survey was made on films of pure metals and austenitic stainless steels deposited by evaporation onto Mylar polyester substrates. Oxide films isolated from austenitic stainless steels were also studied. Effort was directed to clarification of the influence of the highly dispersed nature of the samples (1.0 to 100 y per sq. em.) on deviations from absorption and secondary emission interactions, characteristic of x-ray analysis of normal size samples (1, 2, 6).
Reliable sampling, in common with microanalytical schemes, is particularly difficult in the isolation of surface layers. Microanalysis of polycomponent samples such as oxide films from polycomponent alloys by wet chemical means is further complicated by requirements of chemical conversion and separation of microgram quantities. Furthermore, prior knowledge of approximate sample composition is generally required for precision microchemistry. Some effective surface sampling and microchemical schemes for the analysis of thin films and surface residues have been reported ( 7 , 12). One such scheme consists of a series of consecutive microchemical operations in which the isolated film or residue is washed, ignited, weighed, fused, leached, and finally analyzed spectrophotometrically (10). Although the reliability of the microchemical procedure is established, the method is timeconsuming and limited to those elements for which reliable microcolorimetric procedures have been established. A physical technique yielding complete quantitative data on all metallic components by a single evaluation with a minimum amount of manipulation would be extremely valuable in the characterization of thin films and microgram residues.
Preparation of Film Samples. Isolation and mounting of the sample were most critical in obtaining good precision and low background intensity. Many plastic and metallic film materials were investigated as sample supports. Mylar polyester film was chosen because of the high strength of thin sections (0.00025 inch) and its very low x-ray scattering power. The design of the Mylar support and sample holder contributed considerably toward obtaining a low stable background. In view of the transparency of the samples to both the incident and emitted x-rays, care was taken to ensure that radiation from the interior of the sample holder itself did not get back to the detector. The metal and alloy films were deposited by vaporization of high purity material in a vacuum ( 1 X mm. of mercury) directly onto the Mylar substrate. The oxide films were formed by floating metal films reinforced with Formvar, onto a solution of 5.0'% nitric a~id-O.57~ potassium dichromate a t 60' C. for periods from 30 to 100 minutes. The reinforced films were then scooped onto Mylar, and the metal and then the Formvar were dissolved away. The immersion time in the oxidizing arid determined the thickness of the oxide film formed. The oxide film was isolated from the metal by chemical-stripping techniques using anhydrous 1% bromine-methanol in an inert atmosphere. Film samples were also obtained from metal coupons using similar techniques. The area over which the sample was dispersed varied, but was approximately 4 to 6 sq. em.
ME
IONS
Figure 1.
FORMVlR BACKING
METAL
Comparison of surface-sampling methods
EXPERIMENTAL PROCEDURE
1857
I
1858
ANALYTICAL CHEMISTRY
Three different variations of this technique (Figure 1) determined whether the sample consisted of film and inclusions, fragmented oxide film, or large sections of film. Both the metal and oxide films were weighed on an analytical microbalance. From these data and accurate measurement of geometric area, the surface film densities were calculated. Approximate film thickness w m also calculated from measured weights and areas, and estimated film densities. Microcolorimetric techniques were used for establishing film standards in the calibration of the x-ray method. Chemical microanalysis was also used to evaluate x-ray compositional data obtained on films of unknown composition. Sources of error from sampling and chemical microanalysis have been discussed and shown to contribute a maximum standard deviation of approximately 5% of the elements determined ( I O ) . Measurement of X-Ray Intensities. The x-ray data were obtained on a North American Philips plane crystal spectrograph with a hlachlett OEG-50 tungsten target tube. The x-ray tube was operated a t 50 kv. and 35 ma. unless otherwise specified. I n addition, several x-ray tubes were checked to get one with not only a low dispersed background, but also a low background a t wave lengths corresponding to the metals of analytical interest.
Table I.
Lletal Chromium Iron Nickel
1
NICKEL 26 9 I C O U N T S I S E C
IIIISO
:
CMI
I/
f
IRON 16 4 l C O U N T S l S E C l I
Approx. Film Thickness, A .
...
.. .
7.7 6.3
Total Intensity, Counts/Sec.
...
sprcific Intensity, (Counts/Sec.)/ f-0 (Sq.Cm.)
97 70
128 170
i6,6 26.9
12,O 12.1 11.1
169 153 125
102 200 298
8.5 16.5 26.8
Chromium Iron Nickel
20 5 19 0 17 8
289 240 200
174 314 455
8 5 16 5 26.7
Chromium 31 0 437 264 430 Iron 26 2 332 h-ickel .,. a Obtained by dividing weight of film by its are%. b Estimated from surface concn. by using bulk density.
Component Metal Pure Metal Chromium Iron Nickel
240-
Surfaceo Concn., ?/Sa, Cm.
Chromium Iron Nickel
Table 11.
2
Effect of Film Thickness of Pure Metals on Specific Radiation Intensity
8.5 16.4
Effect of Film Thickness of Type 304 Stainless Steel on Specific Radiation Intensity Snerifir .. ,.- ....y/Sq. Cm.
Surface Concn.a.
Apnrox. Film Thickness. .%.h
Total Intensity. Counts/Sec.
Intensity. (Counts/Sec.)/ (y) (Sq.Crn.)
31.0 26.2 17.8
417 332 200
264.0 430.0 486.0
8.5 16.4 26.7
53.8 6.2
...
866.2 161.2
8.6 16.1 26.5
0
c
4
I ///
0 b
60
120
PU9E M E T A L OXIDE FILM
ao
1 24 0
SURFACE D E N S I T Y O F E A C H C O M P O N E N T . 1 / 5 9 C M
Figure 3. Calibration of x-ray radiation intensities for pure metal and for oxide isolated from Type 304 stainless steel
T h e basic method has been described in the literature ( 3 ) The arrangement of the x-ray optics in Figure 2 illustrates the lithium fluoride analyzing crystal and the Soller blade collimator. The collimator was 6 inches long and had a collimation spacing of 0.005 inch. Except for scouting experiments, fixed-angle counting was done a t the proper Bragg setting using a Xorth American Philips i162019 argon-filled Geiger tube with a bias voltage of 1500 volts. A general indication of composition was obtained by scanning over a range of diffraction angles corresponding to elements of interest a t a chart speed of 0.5' per minute. The first-order Ka-lines were scaled for all the elements except molybdenum and niobium for which the La-lines were also measured. UNCERTAINTIES AND CORRECTIONS
Although precisions one tenth of this limit or less are characteristic of macroanalysis, an over-all uncertainty of 5% is not unreasonable for microanalysis when one considers sampling and contamination errors associated viith manipulations in the microgram range. Most of the uncertainty in the analysis of films results from the sampling procedure. This is indicated by the improved analytical precision obtained on dispersed samples of synthetic microgram mixtures of pure metallic oxides. Microcolorimetric techniques on these materials yielded standard deviations one tenth as large or approximately 0.5%. Scaling was done for a fixed time to yield a probable counting error of 1.0%. Where counting times exceeded 2 minutes the background count was remeasured every 2 minutes to correct for drifting of the counting circuits. Actual counting rates were lms accurate than corrected counting rates because of uncertainties from scattering, background, and blank determinations. By extending counting times up to 3 0 minutes and by making repeated measurements, net-counting rates of 1.0 rtz 5 % counts per second corrected for background, could be measured. The
Chromium Iron Nirkel
...
Type 304 1300 Chromium 18,'s ... i6o.s Iron 75.7 ... 1211.2 h-i ckel 8.5 ... 216.8 Obtained bv dividing weight of film b y its area. b Estimated from surface concn. by using bulk density.
... 8.7 16.0 26.4
accuracy of the net-counting measurement was attributed in part to the low stable background of approximately 2 counts per second for the scaling measurements. Scattering and blank errors were corrected by measurements on untreated Mylar and on Mylar treated identically to that supporting the film sample, except that the sample was absent. The latter measurement provided for possible contamination of the Mylar by the procedures used to deposit the sample. No corrections were made in the x-ray data for the carbon, manganese, and other minor elements knorrn to be present. The uncertainty introduced by this omission is negligible except for the ultrathin ovide films which contained substantial amounts of silicon. I n this case, refinements in the x-ray instrumentation, required to yield data on silicon, n-ere not available. RESULTS AND DISCUSSION
Radiation Characteristics Specific to Thin Films. Two effects become important n-hen x-ray emission samples are scaled down in thickness. The minimum effective thickness for maximum fluorescent yield is not satisfied, and deviations from adsorption and secondary emission decrease. For nickel, chromium, and iron the critical thickness associated with maximum yield has been experimentally established by Koh and Caugherty (6) to be 0.0003 cm. This is in good agreement with the calculated value of Brissey, Liebhafsky, and Pfeiffer ( 2 ) . For the highly dispersed samples of this study, total fluorescent intensities vere accordingly lower than for massive samples of
V O L U M E 2 7 , NO. 12, D E C E M B E R 1 9 5 5
,
I200
-
I
I
I
1859
I
I
I
~
I
VI w VI
1000IRON 16 I (COUNTS
0
ISECIII~IIS0
NICKEL
262ICOUNTSlSECIlIrl~SOCM1 60>--
I /
0
I0
20 SURFACE
Figure 5.
30
/A 40
DENSITY
50 OF E A C H
60
70
COMPONENT. Z I S O
CM
I
.
600
~~-
80
90
'00
CM
Calibration of x-ray radiation intensities for Type 304 stainless steel
equivalent area. To make a corresponding comparison, measurements are expressed in units of specific radiation intensity, (counts per second)/(?) (sq. cm.). .-llthough the gross excitation intensity is greater in massive samples, thin films are actually exposed to a higher average x-ray flux per unit thickness. The fact that deviations decrease considerably with the state of dispersion of the sample has already been reported by Pfeiffer and Zemany (9) for a large number of metals. Deviations are defined according to Liebhafsky (6) as the difference between the percentage of an element actually present in an alloy and the percentage that one would calculate using the pure element as an x-ray emission standard. It is evident that ultrathin films of metals or oxides with surface densities in the range from 1 to 100 y per sq. cm. are equivalent to highly dispersed samples. With decreasing sample thickness for films of pure nickel and iron, the specific radiation intensity increases, because of the higher average excitation flux per unit thickness, as well as the lower over-all absorption of the excited radiation (Table I). Effect of Film Thickness for Metals and Alloys. I n the thickness range studied (1 to 31 y per sq. em.), nickel, iron, and chromium indicate some variation in specific radiation intensity for the pure metal. The average specific radiation intensities can be interpolated from the slopes in Figure 3 to be 26.9, 16.4, and 8.50 (counts per second)/(? per sq. em.) for
nickel, iron, and chromium, respectively, for surface densities from 1.0 to 31.0 per sq. em. The corresponding maximum percentage deviation from these values in this dispersion range is approximately 1%. The analytical sensitivities for the three metals, subject to a netcounting error of 5%, are 0.037, 0.061, and 0.175 y per sq. em., for nickel, iron, and chromium, respectively. The effect of film thickness on specific radiation intensity was determined for the same metals in thin films of Types 304, 316, and 347 stainless steels. Data for Type 304 stainless steel in the film thickness range from 100 to 1300 A . are summarized in Table 11. A typical x-ray spectrogram of passive film isolated from Type 304 stainless steel is illustrated in Figure 4. The oxide film was gelatinous with a lack of any I apparent crystallinity. It was approximately 30 A. thick and had a surface density of 1.65 per sq. em. The data are in good agreement with deviations calculated by Noakes (8) for a similar system. For the compositions and film thicknesses studied, the nickel Kcu- and iron Kcu-radiations have negative over-all coefficients and the chromium Karadiation has a positive over-all coefficient. The maximum percentage deviation as a function of alloy film thickness for iron was -2.6% for the conditions studied. Alternatively, the iron content from uncorrected x-ray data for a film of Type 304 stainless thick, would be too low by this percentage. steel, 1300 -4. A distinction is made between deviations for different thicknesses of the same material and deviations for the same corresponding thickness of a given component in different materials (example, pure iron us. iron in stainless steel). Most of the other deviations were substantially less for the relatively thick stainless steel deposits. The average deviations calculated from the calibration plots in Figure 5 would yield average specific intensities of 26.2, 16.1, and .8.6 (counts per second)/(? per sq. cm.) for nickel, iron, and chromium, respectively, in stainless steel films. These values differ from those for corresponding thicknesses of pure metals by -2.6, -1.8, and +1.2%, in the same order. Effect of Film Thickness for Oxides. The effect of film thickness on specific radiation intensity for oxide films up t o 300 A. thick isolated from Type 304 stainless steel was also determined. Data are summarized in Table 111. The data, plotted in Figure 4,are observed to yield average specific radiation intensities identical with those of thin films of the corre-
Table 111. Effect of Film Thickness of Oxides on Specific Radiation Intensity Component Metal
Surface Conon., r/Sq. Cm.
Approx. Film Thickness, A.b
Oxide No. 2 Chromium Iron Nickel
4.95 0.72 2.50 0.24
100
Oxide No. 3 Chromium Iron Nickel
15.00 2.25 7.46 0.75
300
... ... ... ...
...
...
Total Intensity, Counts/Sec.
Specific Intensity, (Counts/Sec.)/ ( y ) (Sq.Cm.)
...
6.1
41.3 6.5
...
19.1 122.9 18.2
Obtained by dividing weight of film b y its area. b Estimated from surface concentration b y using bulk density.
a
...
8.5
16.5 27.0
... 8.5 16.5 26.9
1860
ANALYTICAL CHEMISTRY
sponding pure metals. Oxide films from stainless steels and related alloys apparently can be accurately analyzed by x-ray emission within the net-counting error itself, without making empirical or theoretical corrections for absorption deviations. The dependence of specific radiation intensity on film thickness for films of nickel, iron, chromium, Type 304 stainless steel, and corresponding oxide films is plotted in Figure 6 to illustrate the influence of sample dispersion on absorption deviations. The crit’ical thickness for the occurrence of significant x-ray deviations is observed to depend on film thickness and film material as one would predict (8). A crit’erion of f 5 % error may be assumed as reasonable for sampling errors characteristic of microanalysis. All errors from x-ray deviations observed for dispersed samples of pure metals, stainless alloys, and oxide films, for surface film densities up to 100 y per sq. em., fall within this value. For sufficiently dispersed niet,al, alloy, or oxide films, characteristic of this study, typical error from x-ray analysis is usually less than 2y0. Comparison of X-Ray and Chemical Data. A comparison of x-ray and chemical analytical data for thin films of austenitic stainless ailojrs, and for ultrathin oxide films isolated from their surfaces, was made using the calibration data obtained. The data, grouped in Table IV for comparison under the headings “x-ray” and “chemical,” were determined independently of each other for at least’ t n o samples in each case. Although the x-ray data were determined on the basis of pure metal calibration, there is no significant correlation of the standard deviations with predictable absorption effects. This is attribut’ed to the high state of dispersion of the samples (typical dispersion for any one component, 1 to 10 y per sq. cm.). The uncertainty is actually greater for the chemical than for the x-ray data. This indicates the analytical advantage gained by the simpler sample preparation procedure characterist,icof the x-ray measurements. Comparison of x-ray to microcolorimetric data in general indicates excellent agreement. It is observed that the alloy compositions of the evaporated metal films are typical of austenitic stainless st,eelsexcept that the silicon and chromium contents are somewhat on the high side. The data marked “oxide-1” are charact,eristicof oxide film associated with passivity of these steels in acids under oxidizing conditions. They are characterized by a definite depletion in iron and enrichment in the minor alloying elements, silicon, niobium, and molybdenum. Only microcolorimetric values are available for silicon. At the time this work was done, instrumentation did not permit the
microdetermination of silicon by x-ray emission spectrography. The x-ray data for niobium and molybdenum are poor, because the excitation radiation used did not give sufficient intensity in the best wavelength range for these elements. In addition, use of a krypton-filled Geiger tube instead of an argon-filled Geiger tube would have been more appropriate for measurement of the Ka-wave lengths of niobium and molybdenum (3). The compositional data for the thicker oxide films marked “oxide-2” show the compositional character of these films to be distinctly different from that characteristic of the ultrathin oxide films. Depletion in iron is associated with a marked enrichment in chromium. In addition, nickel is depleted and enrichment in the minor alloying elements is much less marked than for the oxide-1 films. A more complete discussion of the properties of oxide films on stainless steels is covered elsewhere (11). The data summarized in Table IV are presented to illustrate how x-ray analysis can be applied directly to the characterization of protective films on corrosion-resistant alloys. Distinctions between the two methods for the data in Table IV are masked by the over-all uncertainty in sampling, characteristic of both methods. For microgram samples, the observed typical standard deviation of 1 to 5% is considered reasonable. The calibration used in calculating the x-ray compositional data wm
z C
6 6
6 1
E 2
5
6 0
2
9 O
4
B
u 2
e
8 6 8 1
IO
0 , SURFI-E
3EN5 TI
00 OF
E4Ce
COMPOhElT
‘i
SO
LM
Figure 6. Dependence of specific radiation intensity on film thickness for nickel, iron, chromium, Type 304 stainless steel, and isolated oxide films
Table IV, Comparison of X-Ray and Chemical Analytical Data for Films of Austenitic Stainless Steels and for Oxide Films Isolated from Their Surfaces Sample size, 25 t o 100 y Cr Ni ’76 Std. dev. % Std. dev. 20.2 0.2 8.7 0.1 20.6 0.7 9.0 0.1
Type 316 oxide-l thickness, 30 A.
X-ray Chemical
Fe ‘76’’ Std. dev. 70.5 0.5 69.6 1.2 69.5 1.0 68.5 1.8 2.0 70.6 69.4 2.1 37.2 4.0 34.0 3.1 42.1 3.2 45.0 4.2
Type 347 oxide-1 thickness, 30 A.
X-ray Chemical
45.4 44.5
4.2 5.0
17.7 20.7
3.0 3.2
7.7 8.2
0.8 2.6
12.9
3.2
Type 304 oxide-2 thickness, 300 A .
X-ray Chemical
15.0 15.1
2.0 2.6
4.55 46.6
3.7 5.0
3.8 4.0
0.2 0.5
... 3.3
0.1
Type 316 oxide-2 thickness, 300 4.
X-ra? Chemical
25.0 25.2 24.8 25.0
3.2 4.0 2.8 3.2
36.8 36.1
3.1 4.3
5.0 5.2
0.3
36.1 35.2
2.1 4.1
2.2 2.2
0.1 0.1
Kature of Film Sample Type 304 alloy thickness, 300 A.
Analytical Method X-ray Chemical
Type 316 alloy thickness, 300 A .
X-ray Chemical
Type 347 alloy thickness, 300 A.
X-ray Chemical
Type 304 oxide-1 thickness, 30 A.
X-ray Chemical
Type 347 oxide-2 X-ray Chemical thickness, 3 0 0 A . a Per cent by weight.
%
... 1.0
... 1.2
Si Std. dev.
...
0.1
...
% ’
...
... ...
19.4 19.6
0.2 0.3
11.3 10.9
0.1 0.2
18.2 18.8
0.2 0.6
9.8 10.2
0.1 0.2
0.9
0.1
0.2
13.4 14.0
2.0 3.4
9.4 10.7
1.2 4.0
... 9.9
2.0
...
...
12.6 13.0
0.7 2.0
4.9 5.4
0.6 2.1
...
14.5
...
...
0.8
0.1
. . . . . .
...
3.2
...
... ... ... ...
(E;)
Nb Std. dev.
... ... ... ... ...
3’%
...
...
7.2 0.4
0.4 0.7
...
...
...
...
...
0.5
0.2
...
... ...
,..
... ...
...
...
(1.2) 4.2
1.1
1 .o
...
(0.1) 0.4
0.1
...
. . I
...
...
...
...
...
4.0
... 0.3
... ...
...
w:;)
... 2.0
... 0.2
... 1.2
...
M0 Std. dev
...
... ... ...
...
...
0.3
...
... ...
0.3
... ...
.
V O L U M E 27, N O . 12, D E C E M B E R 1 9 5 5 based on the specific radiation intensities obtained for pure metals. For microanalytical systems with smaller sampling errors, use of specific radiation intensities calculated from the corresponding alloy or oxide standards would be preferred. APPLICATIONS TO MICROANALYSIS
Application of this technique is not limited to microanalysis of corrosion films and residues, but may be applied to routine analysis of experimental alloys or other metallic materials using suitably dispersed samples. An evaluation of the relationship between absorption deviations and the state of sample dispersion for each new system would determine the minimum sample dispersion required for the achievement of a given analytical uncertainty. Once this is done, need for carefully tailored standards may be eliminated in many cases. In addition to the counting uncertainties characteristic of low intensity x-ray measurements, special care is required for adequate correction of blank and background errors peculiar to highly dispersed samples. Improvement in x-ray tubes and radiation detectors could extend microanalytical applications considerably beyond that described here (4). Finally, this particular application is most suitable for the analysis of dispersed samples where errors from absorption deviations are to be minimized. For trace analysis of large samples, a completely different instrumental approach is indicated ( 4 ) .
1861 ACKNOWLEDGMENT
The x-ray measurements were made with the assistance of the Research and Control Instrument Division, North American Philips Co., and with the collaboration of F. A. Behr and William Zingaro. The writer also appreciates the assistance of R. H. Fisher and H. W. McGonigal, who assisted in the preparation of the film samples and in the colorimetric analyses. LITERATURE CITED
(1) Birks, L.
S.,and Brooks, E. J., ANAL.CHEX, 27, 437 (1955).
(2) Brissey, R. M., Liebhafsky, H. A., and Pfeiffer, H. G., ASTM, Spec. Bull. 157,43 (1954). (3) Carl, H . F., and Campbell, W. J., Ibid , 157, 63 (1954). riedman, H., Birks, L. S.,and Brooks, E. J., Ibid., 157, 3 (4) 11954) (5) Koh, P. U., and Caugherty, B., J . A p p l . Phgs., 23,427 (1952). (6) Liebhafsky, H. A., ANAL.CHEY.,26, 26 (1954). (7) Mahla, E. M., and Nielsen, N. A., J . Electroehem. SOC.,93, 339 (1948). (8) Noakes, G . E., A S T M , Spec. Bull. 157, 57 (1954). (9) Pfeiffer, H . G . , and Zemany, P. D., Nuture, 174, 397 (1954). (10) Rhodin, T. N., Ann. N . Y . Acud. Sci., 58, 855 (1954). (11) Rhodin, T . N., Corrosion, submitted for publication. (12) Vernon, W. H. J., Wormwell, F..and Nurse, T. J., J. Iron Steel Inst. (London),2,81 (1944). RECEIVED for review July 5, 1955. Accepted September 9, 1955. Pittsburgh Conference on Analytical Chemistry and ilpplied Spectroscopy, P i t t s b u r g h Pa., March 1955.
Flame Spectra of Twenty Metals Using a Recording Flame Spectrophotometer MARVIN WHISMAN and BARTON H. ECCLESTON Petroleum Experiment Station, Bureau of Mines, Bartlesville,
One phase of the Bureau of Mines study of distillatefuel storage stability required determination of a large number of metals present in trace quantities in these fuels. A Beckman Model DU flame spectrophotometer was modified for this work by installing an automatic wave length drive and a recorder, which reduced the time needed for analysis to about 10 minutes and the sample requirements to 10 ml. These modifications are described, and spectrograms of 20 metals are given with tables of the principal flame lines and an estimated detection limit for each of the metals. The data presented can be used in planning the analysis for any particular metal or combination of metals, as well as aiding the identification of lines in flame spectra. A method of analysis for metals in distillate fuels is briefly outlined.
Okla.
estimate of the quantity of each element present was made, which saved time if subsequent quantitative measurements were necessary. INSTRUMENT MODIFICATION
The instrument used in this work was a modified Beckman DU flame spectrophotometer. The electrical modifications consisted of the replacement of the Beckman phototube with a photomultiplier phototube and its associated power supply; the incorporation of a Sorensen voltage stabilizer; a Leeds & Northrup adjustable zero adjustable range Speedomax having a 1second full-scale pen travel; and a wave-length drive. The following block diagram shows the arrangement of these components in the circuits: Damping circuit
T
HE need for determining metals in trace quantities, both qualitatively and quantitatively, in Diesel fuels was the motive for investigating a flame spectrophotometric method in this laboratory. Most metals present in these oils were present in concentrations of less than 1 p.p.m., and it was necessary to concentrate them to within the detection limit of the instrument. A dry-ashing technique was used as a means of concentration, but often the amount of ash obtained provided only a few milliliters of aqueous solution in sufficient concentration for analysis. The use of a recording technique was especially advantageous as the flame spectrum of a sample could be scanned in a few minutes, using about 10 ml. of solution, and providing a permanent record suitable for qualitative analysis. Study of these records frequently revealed unexpected elements that might have been overlooked using a visual scanning technique. An
Bridge balance control Fine sensitivity control
Leeds &
Sorensen
voltage - ax.stabilizer
II
'
1-second recorder
drive motor
