Table I. Experimental Values of R for Some Organic Oxidation Processes n Compound Solvent Ferrocene Methylene chloride 1 Tetramethylbenzidine Methylene chloride 1 Dimethyl-p-anisidine pH = 2.3 1 Dirnethyl-p-anisidine Acetonitrile 1 o-Dianisidine 1M HS04 2 Benzidine pH = 1.0 2
for some well-characterized, reversible organic oxidation processes. The experimentally observed values of R agree t o within 6 of the theoretical value. This method appears quite applicable t o determination of n-values of electrode reactions. It eliminates any need t o know diffusion coefficients or electrode area. Its experimental simplicity offers an advantage over thin-layer electrolysis. We are presently investigating the possibility of applying this method t o determining n-values and electrode mechanisms in systems involving kinetic complications and will report on this in the future,
R 4.80 4.64 4.74 5.06 7.02 7.12
Experimentally, one obtains a series of linear sweep voltammograms at various sweep rates and calculates ip/ul,”. Then, using the same electrode and solution of electroactive species, a potentiostatic current-time curve is obtained and i t 1 / ?calculated. Equation 3 requires, of course, that the electrode be constant. If process be reversible and that ip/ul‘z and the electrode process under consideration involves a oneelectron transfer, R = 4.92; if a two-electron process is involved, R = 6.96. Table I presents some experimentally observed values of R
ACKNOWLEDGMENT
We thank Prof. Ralph Adams of the University of Kansas for his interest in this work and for the contribution of some of the experimental data presented here.
PAULA. MALACHESKY Tyco Laboratories, Inc., Waltharn, Mass. 02154 RECEIVED for review May 21, 1969. Accepted June 25, 1969.
Flame Emission Detection and Determination Limits for the Rare Earth Elements in the Nitrous Oxide-Acetylene Flame ~~
SIR: The striking similarity in the chemical properties of the rare earth or lanthanide elements has precluded the use of classical techniques for the quantitative analysis of most rare earth mixtures. Thus, the analyst has turned to spectrometric techniques, including optical emission spectrometry, in order to obtain analytical data on these mixtures ( I ) . However, the conventiona1 arc or spark emission spectra of most of the rare earth elements are very complex, possessing thousands of lines of rather uniform intensity and often lacking the characteristically intense lines found in the spectra of other elements. Even under high dispersion the probability of line interference is high and difficulties are frequently encountered in locating interference free lines. A major advance in the spectrometric analysis of rare earth mixtures resulted from the discovery that the fuel-rich oxyacetylene flame could produce and excite free atoms of the rare earth elements (2). The resulting line spectra are very simple compared t o arc or spark spectra, and it is possible to achieve adequate spectral resolution with small table-model spectrometers. The application of fuel-rich acetylene flames to the analysis of rare earth mixtures has been recently reviewed (3). Pickett and Koirtyohann (4) have suggested the use of the premixed nitrous oxide-acetylene flame, burning on a conventional atomic absorption slot burner, as an excitation source for emission spectrometry. These authors published (1) V. A. Fassel, ANAL.CHEM., 32 (11) 19A (1960). (2) V. A. Fassel, R. H. Curry, and R. N. Kniseley, Spectrochim. Acra, 18, 1127 (1962). (3) R. N. Kniseley, V. A. Fassel, and C. C. Butler in “Analytical Flame Spectroscopy,” R. Mavrodineanu, Ed. N. V. Philips
Gloeilampenfabrieken, Einhoven, in press. (4) E. E. Pickett and S. R. Koirtyohann, Spectrochim. Acta, 23B, 235 (1968). 1494
ANALYTICAL CHEMISTRY
~~~
Table I. Instrumental Conditions Jarrell Ash Model 82000, 0.5 meter Spectrometer grating spectrometer 1200 ru!ings/ mm grating blazed for 5000 A. Entrance and exit slit were set at 20 p . Detector EM1 6256 B photomultiplier Amplifier Keithley Model 417 picoammeter with remote preamplifier. Time constant was adjusted to 2.0 sec Recorder Texas Instrument Model FWD strip chart recorder Gas control system Rotameter flow control system described in reference (5).
an extensive list of emission detection limits utilizing this flame but only three of the rare earth elements were included in their study. Because most of their detection limits were significantly better than any which have previously been reported, it appeared appropriate to complete their table by determining detection limits for all of the rare earth elements in the nitrous oxide-acetylene flame and to appraise the applicability of this flame as an emission source for the analysis of rare earth mixtures. The question of the value of detection limits as useful quantities is one which frequently arises. Detection limits are not physical constants which remain unchanged but instead they are physical measurements which reflect the “state of the art.” They therefore provide the analyst with numerical values for making useful comparisons of the potential (5) J. A. Fiorino, R. N. Kniseley, and V. A. Fassel, Spectrochim. Acta, 23B, 413 (1968).
Table 11. Flame Emission Detection Limits for the Nitrous Oxide-Acetylene Flame Minimum determinable concentration in original sample) Detection limit (pg/ml) Flame Flame Wavelength Emission Absorption4 emission absorption (A) (HzOsoh) (Alcohol soln) (Hz0 soh) (H20 soln) (Alcohol soln) (HzO soln) Element Ce 3801.53 28 5699.23 16 n.d. 2.8 1.6 n.d. DY 3531.70 0.07 0.02 4045.99 0.002 0.007 4186.78 0.4 0.04 Er 3906.34 4007.97 0.04’ 0.02c 0.1 0.004 0.002 0.01 Eu 3819.67 4594.03 0.0006 0.001 0.2 O.ooOo6 0.0001 0.02 Gd 3684.13 4 0.4 3768.39 4401.86 2 1 0.2 0.1 Ho 3891.02 0.01c 0.002 0.001 4053.93 0.02c 0.3 0.03 4163.03 La 3927.56 2 0.2 4086.72 5501.34 8 4 0.8 0.4 Lu 2615.42 3312.11 3 0.3 4518.57 1 0.4 0.1 0.04 Nd 4061 ,09 4634.24 2 0.2 0.3 0.02 0.03 4924.53 0.2 Pr 3908.41 4939.74 10 1 4951.36 1 0.5 0.1 0.05 sc 3613.84 3907.49 0 .03c 0.03c 0.003 0.003 3911.81 0.03< 0.01c 0.2 0.003 0.001 0.02 4020.40 0 .05c 0.005 Sm 3609.49 4296.74 5 0.5 4760.27 0.2 0.05 0.02 0.005 4783.10 0.06 0.006 4883,77 0.05 0.005 4883.97) Tb 3509.17 4318.85 0.4 0.2 0.04 0.02 4326.47 0.5 0.4 2 0.05 0.04 0.2 Tm 3462.20 3717.92 0.02” 0.01” 0.002 0.001 4094.19 0.1 0.01 Y 3620.94 0.04 0.3 0.04 0.03 4077.38 0.3 0.03 Yb 3694.19 3987.98 0.002 0.0003 0.04 0.0002 O.ooOo3 0.004 From reference (6), * Calculated from values in reference (7). Emission detection limits were run using second order to improve resolution of line from band structure.
(x
Emissionb spectrographic (dc arc) 0.2 0.005 0.003 0.005 0.002 0.02
0.003 0.0003 0.01 0.01
0.0003
0.01
I
0.08
0.03
0.003
EXPERIMENTAL
value of 4.8 liters/min to provide the optimum signal-tonoise ratio for each element. As a result, the stoichiometry of the flame varied from near stoichiometric for those elements which d o not form stable metal monoxides-e.g., Eu and Yb-to very fuel-rich for elements which form very stable monoxide molecules-e.g., La and Ce. The nebulizer was adjusted t o provide a solution uptake rate of 4.1 ml/min. A few detection limits were also determined using a PerkinElmer nitrous oxide-acetylene slot burner for comparison purposes. The results for the two different burners agreed within a factor of two. The instrumental conditions used are summarized in Table I.
The burner used for these studies was a standard Bausch and Lomb nitrous oxide-acetylene burner (Bausch and Lomb, Inc.). The nitrous oxide flow was held constant at 8.4 litersimin and the acetylene was varied around a nominal
(6) W. ‘Slavin, “Atomic Absorption Spectroscopy,” Interscience, New Yolk, N. Y . , 1968,pp 60-61. (7) D. L. Nash, Appl. Spectros., 22, 101 (1968).
capabilities of techniques for determining trace elements. Detection limit comparisons between flame atomic emission and absorption techniques are particularly useful, because the flames normally used are the limiting factors in both systems. Thus, if the same basic burner and nebulizing system is utilized for both flame emission and absorption measurements, and if the other instrumental conditions are optimized, then detection limit data permit a direct comparison of the capabilities of these techniques.
VOL. 41, NO. 11, SEPTEMBER 1969
1495
RESULTS AND DISCUSSION
The detection limits which were determined for the rare earth elements are listed in Table 11. These detection limits are defined as that concentration of the element which produces a signal equal to twice the root mean square value of the noise. Complete details on the exact method used for measuring the detection limits have been previously published (8). Emission detection limits are listed for both water and absolute alcohol solutions. For most elements the use of alcohol solutions improved the detection limits by a factor between two and four. The most pronounced enhancement was observed for Yb where the use of an alcohol solution improved the detection limit by a factor of ten. Detection limit values for the lanthanides can be improved by factors of 2 to 3 by adding high concentrations (-1000 pg/ml) of an easily ionized element-e.g., K-to reduce ionization of the rare earth atoms. A more interesting comparison is between the atomic emission and atomic absorption detection limits. As shown in Table 11, in both water and alcohol solutions, the detection limits in flame emission are significantly superior for 13 elements, essentially equivalent for G d and Y, and Ce has not even been detected in absorption. The lowest determinable concentrations, based on the original sample composition, for flame emission, atomic absorption, and dc arc emission spectrographic techniques are found in the last four columns of Table 11. These concentrations were calculated assuming that quantitative measurements can be made at concentrations five times the detection limit (signal/noise (rms) = 10) and that the total sample concentration in solution is 5000 pg/ml. A comparison of the dc arc and flame emission values reveals that the latter shows superior capabilities for four of the rare earths (Eu, Ho, Tm, and Yb) and is competitive for five others. Dc carbon arc excitation shows a definite superiority for Ce, Gd, La, Lu, and Sc. However, the values given by Nash were determined using controlled atmosphere excitation and the second order of a 1200-groove/mm grating in a 3.4-meter Ebert mounting spectrograph. It is important to emphasize that in many cases flame emission possesses the capability of determining trace impurities (from 100 ppm down to fractional ppm amounts) in highly purified rare earths. In order to ascertain if the calculated lowest determinable concentration values summarized in Table I1 are realistic, a Y 2 0 3 sample containing Tb, Gd, Dy, Eu, La, and Yb as impurities at concentration levels equal to twice the calculated minimum determinable concentration values was examined. The total rare earth concentration in the solution was 5000 pg-ml. Figure 1 shows typical recordings of the line intensities at the wavelengths of peak line emission compared with the background from the yttrium matrix at the same wavelengths. It is readily apparent from Figure 1 that the predicted determination limits can be achieved. All of the elements, with the exception of Eu, in this artificial sample could easily be determined with a relative standard deviation of 7 3 In the case of Eu the presence of a relatively strong Y O band component under the most sensitive line caused difficulties and a ten-fold increase in the Eu concentration (to 0.006x) was necessary in order to achieve the precision listed above.
x.
(8) V. A. Fassel and D. W. Golightly, ANAL. CHEM.,39, 466 (1967).
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ANALYTICAL CHEMISTRY
w Tb 4326.47
0.1 %Tb in Y
c
i
La 5501.34 1.57 Yo La in Y
Dy 4045.99 i 0.014Yo Dy in Y
Figure 1. Typical recorder deflections at twice the calculated minimum determinable concentrations
While this manuscript was in preparation, Christian (9) reported detection limits for most of the rare earth elements when a N20-C2H2flame was employed as an emission source. A comparison of Christian’s values for atomic line spectra with those reported in the present communication show that Christian’s powers of detection ranged from factors of 80 to 4500 inferior for water solutions and from 10 to 600 fold inferior when organic solvents were employed. Christian’s data indicate, and he has, in fact, concluded that under his experimental conditions “atomic absorption spectroscopy appears to still offer greater detection ability than does emission in this flame for most elements.” On the other hand, the data reported by us in this note and by Pickett and Koirtyohann ( 4 ) establish that just the exact converse is true-Le., flame emission shows superior powers of detection for most elements. Both our data and those of Pickett and Koirtyohann were observed in the same flame with spectroscopic facilities similar to those employed by Christian. It is appropriate to note that Christian did not optimize flame conditions to enhance the emission intensities and did not filter his photomultiplier signals to improve signal/noise ratios. In Pickett and Koirtyohann’s study as well as in ours, flame conditions were optimized for each element and the photomultiplier signal was appropriately filtered. The greatly superior powers of detection observed under these conditions suggests that Christian’s measurements were severely limited by these and perhaps other instrumental and/or experimental factors. RICHARD N. KNISELEY CONSTANCE C. BUTLER VELMERA. FASSEL Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Iowa RECEIVED for review January 13, 1969. Accepted June 11, 1969. Work performed in the Ames Laboratory of the US. Atomic Energy Commission. (9) G . D. Christian, Anal. Left.,1, 845 (1968).
