Table 111. Spectrophotometric Determination of Boron with 4-Nitropyrocatechol (A = 440 nm) Boron taken, Boron found, PPm Ac - AC.B PPm Error, 0
1.0
L
a
0
2.00 5.00 8.96 8.96 13.90 15.87 20.78 26.64
0.140 0.310 0.490 0.491 0.660 0.716 0.828 0.933
2.00 5.01 8.94 8.98 13.95 15.78 20.73 26.58
0
c
1-0.2 -0.2 +0.2 +0.3 -0.5
-0.2 -0.2
Oo2
~
Table IV. Influence of Interfering Substances on Colorimetric Determination of Boron Substance Mannose, 5 ppm Fructose, 50 ppm Arsenic(III), 110 ppm
Boron, ppm Taken Found 10.0 9.4 10.0 9.3 10.0 9.0
Error,
z
t/ LI
1
I
10
I
I
20
PPm B
30
Figure 4. Standard curve for boric acid when the 4-nitromol/l., the pH 8, and pyrocatechol concentration is 1.5 X the ionic strength 0.1
6 7 10
absorbance measurements was 440 nm and the pH of the chosen buffer solution 8-8.5. The results obtained in control analyses using a calibration curve based on data obtained with a 4-nitropyrocatechol mol/l. at a pH of 8 (phosphate concentration of 1.5 X buffer) (Figure 4) are shown in Table JII. When the influence of ionic strength in the range 0.05-0.2 was studied, the accuracy was found to be the same throughout the range. When the nitropyrocatechol concentration was 5 X 10-5 mol/]. and the pH 8.5, the sensitivity was about 0.015 mg/ml per absorbance unit. All species such as many cations that react with 4-nitropyrocatechol interfere with the determination of boric acid. Interfering species are also any substances such as polyols that react with boric acid. When pyrocatechol violet has been employed as reagent (IO), most interfering cations [except iron(II1) and aluminum] can be chelated with EDTA. The same applies, of course, when 4-nitropyrocatechol is employed. The interference of only mannose, fructose, and arsenic(II1)-ion was studied in the present investigation; the results are presented in Table IV.
When the applicability of the different pyrocatechol derivatives as reagents in the determination of boric acid is considered, it is noted that the sensitivities of pyrocatechol violet, Tiron, pyrocatecholcarboxylic acid, and 4-nitropyrocatechol are of the same order of magnitude. Of these four reagents, pyrocatechol violet and 4-nitropyrocatechol can be used for absorbance measurements in the visible range. This in turn means that these two pyrocatechol derivatives can be used in the determination of boric acid with a conventional colorimeter, A comparison of the stabilities of the complexes produced by pyrocatechol violet and 4-nitropyrocatechol at the pH, 8-8.5, where the determinations were performed reveals that the color produced by pyrocatechol violet is stable for about 10 minutes, whereas the absorbance of a solution containing 4-nitropyrocatechol and boric acid did not change to a noticeable extent over a period of four hours. ACKNOWLEDGMENT
The authors wish to express their gratitude to Miss Marjatta Kuisma for her assistance in the performance of the analyses.
RECEIVED for review December 20, 1971. Accepted March 30,1972.
Precision and Detection Limits of Cadmium, Manganese, Cobalt, and Nickel in Sulfides by Electron Microprobe Analysis Robert H. Heidel US.Geological Survey, Denver, Colo. 80225 SULFIDES COMPOSE an economically important group of minerals because many metals occur in the form of sulfide ores. The mineralogical residence of metals-that is, whether they are contained in a silicate, sulfide, simple oxide, or other mineral-is of importance in guiding extraction and smelting in ore processing. This information cannot be obtained from bulk analysis when mixtures of these minerals or phases are found on a microscopic scale. 1860
The electron microprobe offers the unique advantage of analyzing these phases rapidly and accurately where conventional chemical methods are not possible or feasible. Detectability limits for electron microprobe analysis are comparatively poor when considered in terms of the minimum amount of an element detected relative to the total sample in a homogeneous mixture. However, the ultimate sensitivity is very high considering the actual number of atoms
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
Table I. Average Atomic Number 2 and Weight Per Cent Composition for FeS and ZnS Standards Compositions, weight Average Mn Fe co Ni Zn Cd Sulfide standard atomic No. 2 [In FeS] ... 0.98 ... ... NiFeS ... 60.02 22.11 ... 5.79 ... ... NiFeS ... 55.59 22.24 NiFeS ... 9.89 ... ... ... 51.87 22.37 0.53 ... ... ... ... 60.74 22.13 CoFeS 4.09 ... ... ... ... 57.50 CoFeS 22.12 [In ZnS] 11.52 ... ... ... 54.85 ... MnZnS 24.71 17.48 ... ... ... 48.52 ... MnZnS 24.37 ... ... ... ... 66.10 1.15 CdZnS 25.62 ... ... ... ... 65.12 2.29 CdZnS 25.84 ... ,.. *.. ... 62.23 5.63 CdZnS 29.22
required to produce a detectable signal above background. For example, by depositing a thin film of chromium on a copper substrate and calculating the volume of that element from its thickness and electron beam diameter, one can excite approximately 10” molecules resulting in 30 cps above 10 cps background at a beam current of 600 nanoamperes ( I ) . In terms of mass sensitivity, on the order of 10-14 to gram of an element may be detected (2). Many variables affect sensitivity; therefore a general expression for sensitivity cannot be derived ( I ) . Sensitivity is affected by excitation voltage and beam current, which are selected from a wide range of permissible values depending on analytical requirements. Sensitivity is also affected by the composition of the target, which governs the depth of penetration of electrons and the degree of absorption of Xrays emerging from the excited volume, and by the inherent intensity of the particular line of an element of a given atomic number. Instrumental design factors such as the solid angle of X-rays impinging on the crystal, fraction of X-rays diffracted by the crystal, absorption by counter windows, and counter efficiency also influence the intensity obtained. White radiation contributing to background intensity limits the reliability with which a minimum detectability limit may be calculated. Precision and detectability limits by electron microprobe analysis for certain minor and trace elements in silicatesnamely, titanium, vanadium, chromium, manganese, cobalt, nickel, copper, and zinc-have been reported previously (3). EXPERIMENTAL
Instrumentation. Data in this study were obtained with an ARL-EMX-SM microprobe operated at 15 and 20 kV with a specimen current of 30 nanoamperes on benitoite (BaTiSi30s). Readings requiring approximately 10 seconds (for integrated beam current termination) were taken on 10 separate areas of each sample which resulted in a 100-second total counting period. Since each reading is terminated when a constant amount of current has impinged on the sample and other analytical conditions have remained the same, data may be considered in terms of total count and a conversion of intensities to count rate is not necessary. (1) E. C. Buschmann and J. F. Norton, General Electric Report No. 57GL201, General Electric Company, Schenectady, N. Y., 1957. (2) P. Duncumb, Revue Universelie des Mines, XVII, I(1961). (3) R. H. Heidel, ANAL.CHEM., 43,1907 (1971).
S 39.00 38.62 38.24 38.73 38.41 33.63 34.00 32.75 32.59 32.14
Table 11. X-Ray Line and Wavelength Excitation Potential, and Background for Minimum Detectability Limit (CDL)Calculations X-Ray line Excitation Background for and way- potential Cm, Cts/lO sec 15 kV 20 kV Element (Z) length, A kV 700 917 CoKa 7.710 [In FeS] Cobalt (27) 1 ,7902 NiKa 8.333 567 692 [In FeS] 1.659 Nickel (28) 6.539 272 398 MnKa [In ZnS] 2.103 Manganese (25) 157 281 CdLa 2.34 [In ZnS] 3,9562 Cadmium (,48)
The Kcr X-ray lines of manganese, cobalt, and nickel were diffracted with LiF crystals and the La line of cadmium was diffracted with an ADP crystal. Standards. Minimum detectability limit ( C D L )data were based on 10 synthetic sulfide standards containing the elements of interest. Compositions of these standards are shown in Table I. Ideally, in a C D Lstudy the standards should contain the cobalt, nickel, cadmium, and manganese in low concentrations-not greater than 1 % or so. However, not all sulfide compounds can be easily synthesized with homogeneity at low concentrations as was the case for maganese. Available standards contained 11 to 24 weight per cent manganese. The compositions and average atomic numbers for the standards used are also summarized in Table I. The average or so-called “effective” atomic number (2)is the weighted mean of the 2 values of the component elements in the sample, given by n
cizi
2= i=l
where Ciis the weight fraction and 2, the atomic number of the ith element in the mixture. DISCUSSION Detectability Limit. The detectability limit is defined as that minimum elemental concentration for which a signal can be distinguished from background with a specified statistical confidence (4, 5), and thus it must be calculated from back(4) R. Theisen, “Quantitative Electron Microprobe Analysis,” Springer-Verlag,New York, N. Y., 1965. (5) T. 0.Ziebold, ANAL.C H E M . ,859(1967). ~~,
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1 1 , SEPTEMBER 1972
1861
Table 111. Sensitivity, Precision at 1 2 a, and Minimum Detectability Limit at 3 Sensitivity4 (counts/weight %) 15 kV 20 kV
Element (Z) [In FeS] Cobalt (27) Nickel (28) [In ZnS] Manganese (25) Cadmium (48)
Precision ( 1 2 a) lowest concentration standard 15 kV 20 kV
x,
u
above Background Minimum detectability limit(CoL) at 3 cr above background, ppm 15 kV 20 kV
1470 1600
3100 2400
2Z2.70 14.68
2Z5.28 17.74
410 480
300 310
1500 295
3000 550
2Z1.74 15.63
2Z7.09 2Z8.46
380 1380
200 920
ground readings. The expression proposed by Birks (6) is again used for CDL,that is 3 times the standard counting of the background counts. A value in terms error (3 of concentrations is obtained from the sensitivity S, counts per weight per cent (3, 7, 8). The coefficient of variation or fractional standard deviation [dN, NB/(NT- NB)] turns out to be approximately 1 0 . 5 regardless of the total number of counts taken. Background. Table I1 summarizes X-ray lines used, their wavelengths, excitation potentials, and background intensities for CDLcalculations. The background contributions (Table 11) of FeS at the CoKa and NiKa spectrometer wavelength settings were measured on FeS samples free of cobalt and nickel, and these readings were used for CDLcalculations. Only the background from FeS needs to be considered in the determination of cobalt and nickel. The sulfides of zinc, however, may contain FeS (2 = 21), background 25 cjs and CdS (2 = 41), background 37 cis, in addition to ZnS ( Z = 25), background 28 cjs. Therefore a background taken for any one of these sulfides separately would not be representative of a mixture of these compounds. For calculating the MnKa background a typical natural ZnS mineral is assumed to be a mixture containing 2 CdS and 49% each of FeS and ZnS. The calculated weighted mean ( E ) in terms of the background contributions of each of the component minerals in the mixture is given by
ground. Where assumed to be 50
B
is determined for CdLa, the mixture is each of ZnS and FeS.
4%)
+
RESULTS Sensitivities, precision, and minimum detectability limits obtained are summarized in Table 111. The limits for cobalt, nickel, and manganese are higher in the sulfides than for these elements in the silicates, because of higher backgrounds for the sulfides (3). For the cobalt and nickel in FeS, about 20,000 counts total are accumulated at a signal-to-background ratio of 4 for the K a lines. A counting precision of about =k1% at 1 u is obtained for the 100-second counting period. For cadmium, however, using its L a line with an intensity much lower than the K lines, approximately 5,000 counts are accumulated at a much lower signal-to-background ratio, and the calculated counting precision is 2Zt2.5 at 1 u . Improvement in counting precision and correspondingly better analytical results are possible with longer counting periods. In exploratory work such as ours at the US. Geological Survey, however, rapid determinations as reported here are sufficient, and the additional time required to obtain better precision is not warranted. ACKNOWLEDGMENT
where C is the weight fraction and B the measured back-
The author gratefully acknowledges the assistance of George A. Desborough in planning this investigation. The work of G . K. Czamanske, U. S. Geological Survey, Menlo Park, Calif., in preparing the standards and critically reviewing the manuscript is appreciated.
(6) L. S . Birks, “X-ray Spectrochemical Analysis,” Interscience, New York, N.Y., 1959, p 54. (7) G. A. Desborough and R. H. Heidel, Amer. Mineral., 56, 212935 (1971). (8) G. A. Desborough, R. H. Heidel, and G. K. Czamanske, ibid., pp 2136--41.
RECEIVED for review February 10, 1972. Accepted May 1, 1972. Presented in part at the Tenth National Society for Applied Spectroscopy Meeting, St. Louis, Mo., October 18-22, 1971. Publication authorized by the Director, U. S. Geological Survey.
= 2 ( C Z n S ‘ BZnS)
1862
+
(CFeS * BFeS)
+
(CCdS ’ BCdS)
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
