Mercury cannot be separated from cadmium using an anion exchange resin. However, it can be eliminated prior to anion exchange by using a shiny copper foil, the copper being removed by the resin. All considered interfering ions can be eliminated with less than 15-ml elution volume. Various elution schemes for cadmium using ammonium hydroxide were unsatisfactory for our purposes because of the interference of the Cd(NH3)d2+complex. The elution of micro amounts of cadmium after separation of other cations is facilitated by converting the column to the hydroxide form, followed by the addition of dilute nitric acid. This treatment allows recoveries better than 95% when compared to standards which did not have ion exchange treatment. Somewhat lower results can be attrib-
uted to the elution of dissolved resin. Standards which are treated in the same manner as the mixtures agree very well with the values found for the cadmium. Values found for cadmium standards and for the mixtures are shown in Table I. The detection limit for samples and standards after elution is 2 X 10-sM Cd2+ (0.2 ml of 5 x 10-6M Cd2+ diluted to 50 ml after elution). This corresponds to 2.24 X mg Cd2+/ml or 2.24 ng/ml. The detection limits can be decreased by using smaller columns and smaller elution volumes or by keeping the final dilution less than 50 ml. RECEIVEDfor review December 10, 1973. Accepted July 8, 1974.
Precision and Detection Limits of Rare-Earth Elements in Synthetic Glass Standards by Electron Probe Analysis Robert H. Heidel U.S. Geological Survey, Denver, Golo. 80225
In the past, comparison standards for rare-earth analyses by the electron probe usually consisted of natural minerals, sintered pure rare-earth oxides, or glasses with a lithium metaborate base ( I ) . Problems inherent in the use of these materials and the need for improved standards for the determination of relatively high concentrations of rare earths in terrestrial and lunar materials led to the development of silicate glass standards by M. J. Drake and D. F. Weill a t the Center of Volcanology a t the University of Oregon ( I ) . The four synthetic rare-earth element glass standards (REEI-4) which they made up (Table I) were prepared from a base mixture of SiO2, A1203, and CaO in ranges of 27-28% SiOz, 30-32% A1203, and 25-26% CaO with 4-4.5% rare-earth oxides. In order to eliminate as many spectral interferences as possible, rare-earth oxides were grouped as shown in Table I. Shown also are mean atomic numbers (2) of the component elements in the standards because spectral background, which enters into calculations for minimum detectability limits (CDL'S),is atomic-number-dependent. Backscattering, which affects X-ray intensity, is also atomic-number-dependent. EXPERIMENTAL Data were obtained on an Applied Research Laboratories, Inc. (ARL-EMX-SM) electron microprobe operated at 15 kV with specimen current set to 30 nA on benitoite (BaTiSi3Oy), as reported in previous CDLstudies (2, 3 ) . T o minimize possible inhomogeneities in the standards, a beam diameter of approximately 5 gm was used. Integrated beam current termination taking about 20 seconds per reading was also used. Ten separate areas on each smndard were analyzed to further compensate for inhomogeneities. This study was restricted to the use of the La1 lines of the rare earths for obtaining detectability limits. These La1 lines were diffracted with ADP crystals and detected on sealed-window propor-
tional counters, except for the Y L a l line which was diffracted with a KAP crystal and measured on a gas-flow proportional counter. Birk's criterion ( 4 ) was used for CDL calculations; Le., the value for which the line exceeds background by 3 u (99% confidence level) where u equals the square root of N counts averaged from background readings of several different compounds of similar mean atomic number.
RESULTS AND DISCUSSION Summarized in Table I1 are atomic numbers, wavelengths of the respective lines, overvoltage ratios (operating voltage, E,, to the critical absorption voltage, -Ec),line and background intensities, sensitivities, and CDL'S for each rare-earth in the REE synthetic glass standards. ~
Standard
2038
~
~~~
~~
REEl
REE2
REE3
REM
20.8
18.5
18.2
18.7
...
...
... ... ... ... ...
... ... ...
4.08 4.28 4.00 4.44
... ...
4.26 4.26 ...
... ...
Mean Atomic KO.
< ZJ
YZ03 La203
Ce203 Pr'203 Nd203
SmzOu EuO Gd203
Tb203
Dy203 Ho203
EI'203 (1) M. J. Drake and D. F. Weill, Chem. Geol., I O , 179 (1972). (2) R. H. Heidel, Anal. Chem., 43, 1907 (1971). (3) R. H. Heidel, Anal. Chem., 44, 1860 (1972). (4) L. S. Birks, "X-Ray Spectrochemical Analysis," 2nd ed., Wiley-lnterscience, New York, N.Y., 1969, p 81.
~
Table I. R a r e E a r t h Oxides in Four Synthetic RareE a r t h Element Glass Standards (Weight Percent)
Tm'203 ybZ03
Lu203
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
4.20 4.46 4.35
... ... ... 4.35
... ...
... ... ... ... ... ... 4.26 4.26
...
...
...
...
... ... ...
... ... ... ... ... ... ...
4.36 4.41 4.36
...
...
...
... ...
Table 11. Atomic Numbers, La1 Wavelengths, Overvoltage Ratios, Line and Background Intensities, Sensitivities, a n d CI)I,’sfor R E E Synthetic Glass S t a n d a r d s
Race earth oxide contain-
Atomic
in!l
Z
Y La Ce Pr Nd Sm Eu
Yb
39 57 58 59 60 62 63 64 65 66 67 68 69 70
Lu
71
Gd
Tb DY Ho Er Tm
NO.
Wavelength
Lei Line, i
6.449 2.665 2.561 2.463 2.370 2.199 2.121 2.046 1.975 1.909 1.845 1.784 1.726 1.672 1.619
Intensity
Overvoltage ratio,
Sensitivity
(cts/20 seconds)
-S,
EoIEc
Line
7.22 2.73 2.62 2.51 2.41 2.23 2.15 2.07 1.99 1.91 1.86 1.80 1.73 1.68 1.62
300 1600 1400 2100 1600 1900 1600 1600 1400 1600 1200 1600 1100 1300 760
The CDL’Sare illustrative of values obtained under the instrumental and experimental conditions outlined in Table 11. These do not necessarily represent the best possible CDL’Sobtainable, but they would be somewhat poorer should Lp1 lines (first or second order) be used for entirely interference-free spectra. I t is of interest to discuss the above conditions and X-ray generation factors because they affect line and background intensities. Regardless of the intrinsic intensities of X-ray lines, measured intensities are also dependent on crystal reflectivity and/or detector efficiency. The spectrometer channel equipped with the KAP crystal (having lower reflectivity) and a gas flow proportional counter (having a lower quantum counting efficiency) was used for the long wavelength measurements of the yttrium L a line. Yttrium also has an inherently lower spectral intensity because of a small fluorescent yield-even when excited with an electron beam energy much higher than the critical energy (E,/E, = 7.22). Hence, the somewhat lower intensity and CDLas compared with the CDL’Sfor the other rare-earths except thulium, ytterbium, and lutetium. I t is also apparent that as one progresses toward higher atomic-numbered (2)rare earths, line intensities decrease because of decreased crystal reflectivity for smaller 28 spectrometer settings. In addition, line intensity decreases with lower overvoltages (E,/E,) even though fluorescent yields increase with increasing atomic number. Background increases as more white radiation (bremsstrahlung) enters the detector a t lower 28 spectrometer settings. Background also increases with increasing 2. A tabulation of spectral lines adjacent to the L a l lines for each of the rare-earth elements reveals possible spectral interferences. Values of CDL’Smay be biased by these interferences, as, for example, a higher apparent sensitivity (and better CDL) for praseodymium (PrLal, h = 2.4626 A) en-
a
Background
cts/wt %
i 16
28 105 109 130 125 174 160 197 161 221 173 235 2 04 258 221
90 4 10 410 530 400 460 450 360 315 370 280 350 230 280 140
f
* 34 * 36
i 20
i 24 i 50 &
30
i 50
i 30 i 35 i 30
* 35
i 40 i 40 i 40
Minimum detectability l i m i t - cDL 3 a above background, PPm
1700 750 770 650 840 850 850 1200 1200 1200 1430 1330 1900 1720 3 130
hanced by lanthanum (LaLP1, h = 2.4582 &, both contained in REE3. The effect of interferences also shows up in higher-than-normal background readings; i.e., GdLPl a t 1.8462 A from gadolinium in R E E l contributing significant intensit when the spectrometer is set for the HoLal line a t Only yttrium and neodyminum are interference1.8447 free. Care must be exercised, therefore, if these standards are to be used for quantitative determinations a t lower concentrations. The best approach in these cases might be to prepare synthetic glass standards for the individual rare-earth elements, but standards which contain the interfering elements as well. Intensities for the rare-earth metals for 20-second counting periods were for YLal, 9950 counts; for DyLal, 40100 counts; for ErLal, 40700 counts; and for YbLal, 32600 counts. Background intensities were somewhat higher than for REE glass standards because the metals have a higher atomic number. However, metallic rare-earth standards and the synthetic glass standards had approximately the same sensitivity limits.
1.
ACKNOWLEDGMENT The author gratefully acknowledges the permission of M. J. Drake and D. F. Weill to use their REE synthetic glass standards and thanks them for furnishing the standards for this study. The assistance of George A. Desborough with this investigation and his helpful comments in the preparation of the manuscript are also gratefully acknowledged. RECEIVEDfor review March 7, 1974. Accepted July 1, 1974. Presented in part a t the 15th Annual Rocky Mountain Spectroscopy Conference, August 20-21, 1973, Brown Palace Hotel, Denver, Colo. Publication authorized by the Director, U S . Geological Survey.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
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