1028
A N A L Y T I C A L CHEMISTRY
,
hap been slioivii that comparison of spectra can be made between qolid films and solutions for some steroids and not for others. There are some shifts in going from solution to solid states, and although this did not interfere with identification of a number of steroids, variations among other hydrocarbon curves may introduce serious doubt as to identity. Jones (6) has suggested that variations in the spectra of closely related structures-e.g., replacement of a methyl by an ethyl group-at times are of the same magnitude as variations observed in changes of state. It would be entirely misleading to compare spectra obtained on different states of such similar compounds. The steroids of low or moderately high melting point have been shown to be structurally unaltered during preparation of melted films. The spectra of the melted films discussed here were identical with solution spectra, except in molecules which had profound tendency for hydrogen bonding. Such bonding was the major cause of spectral differences and occurred between 9 and 10 microns. Since absorption between 8 and 13 microns characterizes a molecule specifically, an extensive similarity of curves between 8 and 9 and 10 and 13 microns, irrespective of discrepancies between 9 and 10 microns, would be strongly suggestive of identity. Examination of Figure 2 reveals that the 10- to 13-micron regions of the different preparations of A5-androstene-3p-ol were comparable. Despite the dissimilarities between 9 and 10 microns, it is suggested that the curves are interchangeable on the basis of the 10- to 13-micron region and could be used for identification. The curve of corticosterone (Figure 6) obtained from the melt preparation was shown to arise from a change in structure and this was reflected not only in the 9- to 10-micron region but also between 10 and 13 microns. Intensification and merging of absorption bands clearly indicated the spectra Lvere not comparable. Therefore, if band positions and relative intensities are apparently the same in the fingerprint region, then despite variations between 9 and 10 microns, where bonding plays a significant role, it would appear that curves obtained from different preparative methods could be compared successfully in many cases. Preparation of unknoivn compounds for comparison with a group of known steroids via their infrared spectra should be performed in an identical manner. The melt technique has the advantage of permitting the recording of a complete spectrum on a small sample ( I to 3 mg.) of material with a minimum of handling. Infrared studies of solutions permit analysis of microquantities of steroidal material only nhen a microcell is available ( 3 ) .
Transfer of a sample from one solvent to another in order to obtain a complete spectrum is inconvenient and not' always is n suitable solvent available. Carbon disulfide has proved esceedingly useful for Cis-steroids (6). Josien, Fuson, and Cary ( 7 ) have applied Xujol mulls for their observations on normal and isosteroids. ;Ibsorption bands of good int,ensities are obtained on mineral oil mulls containing 3 mg. of substance. Preparation of a mull is simple, but recovery of the steroid from the mineral oil may be troublesome. The most general technique for preparing a steroid for spectroscopic analysis is deposition of a film. Empirical trials with different solvents usually yield crystalline or glassy films which are suitable for analysis and a complete spectrum can be obtained 0110.5 to 2 mg. of substance. ACKNOWLEDGMENT
The aut,hors wish to express their gratit,ude to the following for samples of crystalline steroids: The Ciba Pharmaceutical Co., Summit, S.J., for androstane, Aj-andaostene-3/3-ol, and A'-androstene-3, li-dione, and Oscar Hecht'er, Robert Jacobseii, and Frank Ungar, Worcester Foundation for Esperimental Biology, dhrewsbury, Mass., for desosycorticosteroiie, dehydroepiandrosterone, and corticosterone. LITER4TURE CITED
(1) Furchgott, R. F., Rosenkranta, H., and Shorr, E., J . B i d . C'bem., 163.375 (1946). (2) Ibid., 171, 523 (1947). (3) Hardy, ,J. D., Wilson, H., and Dobriner, K., Federation Proc., 8, 204 (1949). (4) Jones, R. N., A p.. p l . Spectroscopu, 6, No. 1 (1951). . (5) Jones, R. N., personal communication. (6) Jones, R. S . , and Dobriner, K., Vitnmins and H O F , ~ L O / 7, ~CS. 293 (1949).
(7) Josien, 31. L., Fuson, S . ,and Cary, A. S., .J. Aiu. C ' h P m . Soc., 73, 4445 (1951). (8) Rosenkrantz, H., Milhorat, A. T., and Farber, AI.. J . B i d . Chem.. 195,509 (1952). RECEIVED for review Kovexnber 8, 1952. Accepted April 27, 19.53. Investigations aided by a grant from the U.S. Public Health (C-321) Siprvice and supported in part b y contract KO.DA-49-007-MD-184, Medical Research and Development Board, Office of the Surgeon General, Drpartment of the Army, and in part by the Permanent Science Fund of t h e .itnerican Academy of Arts and Sciences.
Spectrographic Determination of Residual Impurities J. E(. IIURWITZ Division of Physical Metallurgy, Department of Mines and Technical Surceys, Ottawa, Ontario, Canada
S
PECTROGRAPHIC analyses are often requested for materials for which no standards are available because of the lack of other accurate chemical methods. The general procedure is to prepare standard solutions or powders which resemble the smiples in chemical composltion. However, sometimes the materials used in preparing synthetic standards contain unknown residual amounts of the elements to be determined. The presence of these elements will introduce errors in the quantitative determinations if they are not accounted for. Several methods of background and blank corrections have I)een proposed (1, 3-6). This paper stresses that background wri ections are important when residual impurities are determined. Furthermore, these determinations may be made even though the residual concentrations are below the visual limits of tion of which the spectrographic technique is capable. tl(5tc.c
B4CKGROUND CORRECTIONS
The apparent intensity of any spectral line is composed of two parts-the background intensity and the real intensity of the spectral line. The real intensity may be emitted by two sources of atoms of the same element: atoms that are present as residual impurities and atoms deliberatelk added. Only the background and apparent intensities in the form of ratios can be derived inimediately from niicrophotonieter readings and the emulsion calibration curve without further calculation. Because background intensity may cause a serious nonlinearity of the working curve, coirections for this effect must be made. The methods of background corrections described by Honerjager-Sohm and Kaiser (4)and Pierce and Sachtrieb ( 5 )are both mathematically and experimentally sound. Since the usual procedure at the hlineq Branch i- to use the logarithm o f the in-
V O L U M E 25, NO. 7, J U L Y 1 9 5 3
1029 ~~
~~
The spectrographic procedures for the determination of residual impurities or blanks and for background corrections were examined from both mathematical and experimental points of view. Background corrections were found t o be significant whenever the ratio of spectral line intensity to background intensity was less than approximately 80 to 1. A modification of t h e usual procedure for background corrections has been developed. The importance of those corrections was stressed because the background intensity was shown to have exactly the same effect on the working curve as the intensity contributed by a residual inipurity. Concentrations of residual impurities as low as 20% of the risual limit of detectability ma) be determined.
tensity r:tther than intensit); the correction method of the lormrr authors was followed Iyith a small modification which made the calculations somewhat simpler. If no vorrection is made, the logarithm of t,he intensity ratio of the anal~-ticulspectral line to the internal staridard spectral line is given by log
1'z ~
Is
+
+
iib
isb
where I ' , is the intensity of the analytical spectral line, I , is t.he intensity of the internal standard spectral line, and i z b arid j a b are the background intensities of the analyt'ical and internal standard spectral lines, respectively. This ratio may be rewritt,en
9.5
P.0
///
7
-u
1.5
Y
0 1.0
A
LOG?
L O G Y
LOG INTENSITY RATIO 0.5
Figure 2. Typical Working Curve Corrected for Bachground Intensities hut Not for Residual Impurit?
-P.O
1 .o
-0.0 LOG / t ' i
-1.0
2.0
and
I / i = 83.3
Figure 1. Background Correction Curve for Determining Log I l i
+
=\ gralih (Figure 1) inay be plotted between log ( I i)ji which is known and log I / i which is to be determined, where I is the intensityof the spectral lineantl i is its background intensity. Therefore, when log I'ziid, and log Za/& have been obtained, the following final calculation is possible:
log I'z!Ia
= log
I ' ~ / i r b- log Ip/iSb
+ log
irb/isb
The question concerning the magnitude of spectral line intensity ivhich is not affected by background intensity arises at. this point. I n practice, the ratio of spect,ral line to background inteiisity must be such that the background intensity will not affert the logarithm of the intensity ratio of the hoinologous spectral line pair in the second decimal place. This ratio may IF determined by means of the following equations:
RESIDUAL IMPURITY DETERMINATIONS
T h e linear portion of the working curve may be expresped. i n general. by the following equation log C ' J C ,
=
A log l',i'I,
+B
where C', and C. are the concentrations of the element whose analysis is desired and of the intrriinl standard elenlent, respectively, and A and B are constants. Suppose that the residual impurity concentration is c and i is t,hP intensity cont,ributed b,~. this concentration. i may he so sinall that i t will produce littlr. or no detectable blackening on the photographic ernulsioii. Therefore, the above equatioii hecomcs log ( C ,
+ c ) c, = '4 log (I, + i ) i I , + H
where C, is bhe added voncentration of the impurity rlexnent and I , is the corresponding intensity contributed by Tliiequation may be remitten i n the form: (Iz.
I + i
log -
- log
I
:=
0.oo.j
log CJC, f log (C, Therefore log 1+I
= 0.005
i = 1.012
+ c)/C,
-
+ +
log C,IC, = A log Iz;Is R -4log ( I , i)/lL
+
Rut, log C,/C, = A lox
IdL
+R
1030
ANALYTICAL CHEMISTRY
Table I.
Spectroscopic Equipment and Procedure
Spectrograph Optics Wave-length range Condensing lens Slit width Exposure time Current Voltage Counter electrode
Hence, log (C,
Hilger large quartz and glass Quartz 2450-3500 A. Focus on collimator lens 0.015 mm. 60 seconds 5 7 amperes d.c. 300 volts d.c. Negative polarity. High-purity graphite rod I/?inch diameter. shaped to 22' cone with hemisdherical tiD
+ c),C, - log C*
C8
= A log
(I,
+ z),Iz
Consideration of Figure 2 shoa that Q
ED
=
log (1,
+~)/1z
If the concentration ratio axis is in logarithmic coordinates and, hence, the concentration ratios are plotted directly, DF in terms of concentration ratios is c/C,. Since C, is usually very nearly constant, the working curve in Figure 2 is plott'ed as log C, against log Zz/18, and DF is the residual concentration, c. The equations derived in t'his paper show that the influences of background intensity and an unknown blank upon the working curve are exactly the same. Therefore, in order to observe the presence of an unknown residual concentration, background corrections must be made initially to the intensit,y data. PREPARATION OF STAKDARD SOLUTIONS
Experimental data were sought to verify these calculations. Large quantities of high-purity zinc (99.99yo supplied by Hudson Bay Mining and Smelting Co., Ltd.) with less than 0.0003% cadmium were available. Cadmium (supplied b y -4.C. Leslie and Co.) of the same purity as the zinc was also used for additions in small quantities as an impurity. Zinc (62.5 grams) was dissolved in 400 ml. of dilute niti,ic acid (1 to 4) with additions of concentrated nitric acid sufficient to complete solution. The solution was diluted with distilled water to make 500 nil. Then 10 grams of cadmium were divolved in 75 ml. of dilute nitric acid (1 to 4) a i t h additions of concentrated nitric. acid sufficient to complete solution. The bulk waq adjusted with distilled water to make 1000 ml. With these solutions, standards were synthesized to cover the range from 0.005 to 1.0% cadmium in zinc.
I
I 0.5
I 00
-95
I
l a
ICL/ IZn
Figure 3.
Experimental Working Curve
Illustrating influence of background intensity and various concentmtions of cadmium as a residual impurity
5
SPECTROSCOPIC T E C H l I Q U E
The spectroscopic pi ocedure is summarized in Table I. Sample electrodes were prepared hy drilling a hole, 4 mm. in diameter h v 2 mm. deep, into a high-purity graphite rod, 1/4 inch in diameter by 2 inches long. Aliquots of 0.1 ml. of each solution were pipetted t n ice into the shalloa rup and the electrodes were dried under an infrared heat lamp for 5 minutes. Emuliion calibration was obtained from density readings of iron spectral lines of known intensity ratios ( 2 ) . The spectrum of each standard was recorded in triplicate.
4
IL
tI
/ BLANK CONCENTRATIONS :
2
a-
0015%
c-
0005%
b
- o 010%
d - 0000% I
I
WORKING CURVES 0
Figure 3 illustrates working curves for the photometric data uncorrected and corrected for background intensity and other working curves drawn for various assumed residual impurities.
0-0
0.2
04
Figure 4.
0.6
- PERCENTAGE
08
BY WEIGHT Experimental Working Curves Plotted According to Procedure of Cholak and Story CADMIUM
tO
V O L U M E 25, NO. 7, J U L Y 1 9 5 3 Cuive A shous the typical fading of the cadmium spectral line intensit\ into the background intensity. The linearity of curve B (corrected for background intensity) is apparent for all standards donm to 0.0275Y0 cadmium. This was the lowest concentration to give a spectral line density visible above the background on the photographic plate. I n this standard's spectrum, the ratio of spectral line intensity to background intensity was 0.20. Curves C'. D , and E were drawn for assumed blank concentrations of 0.005, 0.010, and 0.01570 cadmium, respectively. The depar ture from linearity was noticeable in all cases. Every assumed blank was less than the concentration necessary to give a visible s1)ectral line on the eniulsion. DISCUSSIONS
Some evaluation of other proposed methods of residual impurity determination ( 1 , 3 . 6 )may be made. The working curves corrected for background intensity were plotted (Figure 4 I according to the procedure of Cholak and Story ( 1 ) . The analysis of the residual impurity by t.his procedure would he unsatisfactory because of the small displacements observed for the low concentrations assumed. The deviations encountered by using the method proposed in this paper are considerably larger, and the presence of a residual impurity is deterted niore readily. The method proposed by Duffendack and JVolfe (9) also depends on the deviation from linearity of the working curve, but no background intensity corrections m r e made. I t is highljprobahle that failure to include such corrections would introdum large errors! partirularly in the very low concentrations in which these aut,hors were interested. Strock (6) reported a method of blank determinations which recognizes the presence of bwkground intensity. but no direct correction was used. Using the phot'onietric data published in his article and the procedure priiposed herein, it. was found that, t,he working curve was a straight line over t h e full range of concentrations considered.
1031 Contrary to Strock's reported blank concentration of 0.00037, strontium, no significant blank was observed. Therefore, it may be concluded that a reasonable doubt exists in Strock's determination. COhCLUSIONS
Background intensity corrections are an integral part of residual impurity determinations. The usefulness of the Korking curve may be extended don-n to as low as 20% of the minimum detectable concentration. Other than background corrections, the only precaution that has to be observed is that a sufficient number of solutions with concentrations up to a t least 50 times the blank concentration is used to ensure that an appreciable straight-line portion of the n-orking curve is obtained. Obtaining blank-free materials for synthetic standards is not of great importance. The only requirement is that the residual impurity's concentration be equal to or less than the lowest amount present in the samples submitted to t'he analytical laboratory. ACKNOWLEDGMENT
The author wishes to thank IT. R. Inman and R. L. Cunningham for their useful suggestions. LITERATURE CITED
Cholak, J., and Story, R. V., J . Opt. SOC.Amer., 32, 502 (1942). 12) Dieke, G. H., and Crosswhite, H. AT., Ibid., 33,425 (1943). 1 3 ) Duffendack, 0. S., and Wolfe, R. A , , IND. ENG.CHEM.,ANAL.ED., (1)
10,161 (1938). (4)
Honerjiiger-Sohm, AI., and Kaiser, H., Spectrochim. A c t a , 2, 396 (1944).
Pierce, W. C., and Sachtrieb, Ii.H., IND.ENO.CHEM.,A X - ~ L . , ED.,13,774(1941). '6) Strock, L. W., J . Opt. SOC.Amer., 32, 103 (1942). RECEIVED for review April 5 , 1952. Accepted April 15, 1953. Published i.5)
by perinission of the Director-General of Scientific Services, Department of
lfines and Technical Surveys, Otran-a, Ont., Canada.
Spectrographic Analysis of 18-8 Stainless Steel For Cobalt, Manganese, and Stabilizing Elements UEIL E. GORDON, JR., K i L P H 31. JACOBS,~ Y IIARILIS D C. RJ(.hEL W e s t i n g h o t i s e Electric Corp., P i t t s b u r g h , P a .
This spectrographic method w a s deieloped to decrease the time required by wet chemical procedures for the determination of niobium, m o l l bdenum, tantalum, cobalt, titanium, dnd manganese in 18-8 stainless steel. 4 salt technique is used with an alternating current arc excitation and a precision nf ='=lo%is obtained. The method is particularly applicable where samples of diiersified shape, size, and nietallurgical histor? are submitted. Standards can be s> nthesized readill from pure metals and analytical grade reagents.
I
S T E R E S T wai. expi essed in the analj-tical control of tantalum (0.003 to cobalt (0.02 to 0.2%), niobium and manganese (0.2 to 2.0Yc), molybdenum (0.09 to O.7yc),and titanium (0.10 to 1.0%) in 18-8 stainless steel. The combined ivet chemical procedures required for these elements are tedious and time consuming; consequently, the development of a spectrographic method was undertaken m-hereby these elements could he determined rapidly. The samples subniitted to the laboratory were of a varied nature, coming from numerous sources, in different forms of fabiication, and with metallurgical history incomplete. Therefore a method utilizing solution or salt techniques afforded the best approach to the problem. Moreover, suitable standards were not available. and Kith such methods they could be synthesized
l.Ooc),
rea(lil>-. Owing to the notably poor spectral sensitivity of tantalum, the salt technique was selected because i t permitted the possibility of incorporating a carrier or buffer if needed.
A set of standards was synthesized by salting aliquots of a solution of sample lOlC from the Sational Bureau of Standards with solutions of the pure elements. The resulting solutions were evaporated to dryness as the sulfates and baked a t 350' C. until all fuming had ceased. The salts were ground to a fine powder and placed in a desiccator until needed. Through a series of moving plate studies ( 3 ) it was determined that satisfactory distillation characteristics for the elements desired could be obtained by burning a salt graphite mixture of 1 : 1 volumetric ratio in an alternating current arc. The 8ame conditions made possible the determination of tantalum down t o a concentration of 0.0570. hdditional measures taken to
