Table VI. Sample A B C
D E F G
Analysis of Aged Gun Propellants for DPA DPA by DPA by UV bromination, spectroscopy, DPA by oxidation, % % 0.42 0.32 0.40 0.33 0.44 0.41 0.68 0.66 0.65 0.62 0.70 0.68 0.43 0.34 0.27 0.44 0.34 0.25 0.44 0.38 0.24 0.44 0.39 0.24 0.63 0.56 0.48 0.64 0.60 0.52 0.53 0.48 0.42 0.53 0.47 0.41 0.44 0.38 0.33 0.43 0.36 0.29
Table VII. Recovery as DPA from Mixtures with 2-NDPA and CNDPA (all units in micrograms) Found (calculated as DPA) Taken 103.0 100 DPA 5 2-NDPA 106,O 100DPA 10 2-NDPA 113.0 100 DPA 20 2-NDPA 104.0 100DPA 5 4-NDPA 107.4 10 4-NDPA 100 DPA 115.0 100 DPA 20 4-NDPA
+ + + + + +
the ultraviolet procedure was performed on material separated from the gun propellant by steam distillation. While Table VI does not, in some cases, show a good agreement among the three methods, it must be kept in mind that DPA added to single base gun propellants in manufacture is slowly changed into N-nitroso DPA, 2-NDPA, 4-NDPA, and other degradation products as the propellant ages (6). (6) W. A. Schroeder, Earl W. Malmberg, Laura L. Fong, Kenneth N. Trueblood, Janet D. Landed, and Earl Hoerger, Ind. Eng. Chem., 41,2818 (1949).
Thus, all of the data in Table VI can be subjected to the same criticism. There is considerable uncertainty as to what is being measured in each case. Certainly several closely related species are present. INTERFERENCES
It is apparent that strong reducing agents would interfere with the oxidation. Likewise powerful oxidizers would be expected to interfere. The following substances did not interfere with the DPA analysis in the indicated quantities (relative to the DPA). Ethyl centralite Methyl centralite 2,CDinitrotoluene Dibutyl sebacate Ethyl alcohol Methyl alcohol Water Phosphoric acid Sodium chloride 2,4-Dinitrodiphenylamine 2,4’-Dinitrodiphenylamine
lox
lox lox
lox
1 ml per determination 1 ml per determination 5 ml per determination 500X 250X does not oxidize does not oxidize
Interference with the DPA determination was noted from 2-NDPA and 4-NDPA which, of course, oxidize and contribute to the absorption values. In sizable amounts the 2-NDPA and 4-NDPA shift the position of maximum absorbance toward the shorter wavelengths. The extent of the interference may be judged from Table VI1 which shows results obtained from DPA with measured quantities of 2-NDPA and 4-NDPA added. The mixtures were oxidized and calculated as if only DPA was present. Interference was also seen with N-nitroso DPA which oxidizes to the same product as the DPA and shows the same molar absorptivity. Some interference was noted from large amounts of phosphoric acid.
for review November 19, 1969- Accepted February 12, 1970
Direct Reading Emission Spectrometric Determination of Trace Elements in Rare Earth Compounds In,corporating a Rotating Electrode Effects of Alkali Chlorides as Radiation Buffers Yasuaki Osumi, Akihiko Kato, and Yoshizo M i y a k e Gouernment Industrial Research Institute, Osaka, Midorigaoka, Ikeda-shi, Osaka-fu, Japan
DURINGTHE COURSE of preparing high purity rare earth cornpounds, a highly sensitive and reproducible method for analyzing trace rare earth elements was required to evaluate the purified products. Emission spectrometry has been used for the determination of rare earth elements in various samdes - (Z-4). . . (1) J. A. Norris and C. E. Pepper, ANAL.CHEM., 24, 1399 (1952). (2) J. P. McKaveney and G. L. Vassilaros, ibid., 34, 384 (1962). (3) K. Morita, J . Soc. Muter. Sci. (Japan), 13 (124), 1 (1964). (4) K. Morita, Rep. Goc. Ind. Res. Ins;., Nagoya, 14, 215 (1965).
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No one, however, has reported o n the determination of trace elements in rare earth compounds by direct reading emission spectrometry. The present study offers a systematic investigation of solution excitation methods, in the presence of alkali salts, for the quantitative determination of trace elements in rare earth compounds. Sensitivity is enhanced by the addition of alkali salts as radiation buffers. The rotating electrode method and photoelectric measurement are employed.
Table I. Apparatus Spectrometer Shimazu, 3.4-meter Ebert G;ating Spectrometer GEM-340 (Dispersion 5 A/mm, Rulings 600 lines/mm, First order). Power supply Shimazu, Modular Source, High Voltage Spark, 280000 type. Photoelectric Shimazu, 241000 type. equipment Electrodes Rotating disk electrode, Nippon Carbon Corp., 12.5 @ X 3.2 4 X 5 mm. Counter electrode, graphite rod, Nippon Carbon Corp., 4.6 mm diam X 38 mm, 1/32 R. EXPERIMENTAL Apparatus. The apparatus used are listed in Table I. Reagents. The materials used were ceric oxide, 99.99% (Johnson, Matthey Co., Ltd.); lanthanum oxide, 99.99x ; yttrium oxide, 99.99%; and other rare earth oxides, 99.9% (Spex Industries Inc.). Stock solutions, containing rare earths 2 mg/ml, were prepared by dissolving the rare earth oxides (except cerium) in hydrochloric acid or in hydrochloric acid with hydrogen peroxide (ceric oxide). Alkali metal chlorides were used as a source for alkali elements. All other solutions were prepared from analytical grade reagents. Analytical Lines and Excitation Conditions. Analytical lines and excitation conditions are summarized in Tables I1 and 111. The rotating electrode method was employed for sample feeding. The discharge of graphite electrodes was always carried out in argon using the enclosed combination analyzer. Measurement of Spectral Line Intensity. The measurement of spectral line intensities was carried out photoelectrically. Background intensities for the spectral lines were estimated from the radiation emitted by solutions not containing the elements to be analyzed, but having a similar composition. Background corrections were made by subtracting the chart reading of the background from that of the spectral line. Rate of variation of emission intensity in the presence of alkali elements is calculated by the following equation :
Table 111. Excitation Conditions Excitation source: High voltage spark Spark voltage, KV 10 Capacitance, p F 0.007 X 2 Inductance, gH 450 Resistance Residue Analytical gap, mm 3 Slit width: inlet, p 60 outlet, p 100 2 Atmosphere argon (99.998 %), l./min. Pre-spark time, sec 60 Integration time, sec 20 Rotating velocity of electrode, rpm 10
and ZC+M give the emission intensities of the elements to be analyzed and the background in the presence of alkali elements, respectively. Experimental results listed in the tables are the average values of 3 to 5 measurements. Measurement of Rate of Vaporization of Sample Solution. One-half milliliter of sample solution was placed in a 1-ml porcelain cell and excited by electrode rotation. Rate of vaporization of the sample solution during discharge was calculated from the time required to mark the sudden change of the intensity in the intensity-time curve for a certain spectral line. Experimental results shown are the average of three measurements. RESULTS AND DISCUSSION
Effects of Alkali Chlorides. I n the powder excitation method, remarkable effects of matrix materials have been observed. I n the solution excitation method, the effect is generally not as remarkable, although a few papers have discussed the problem. The authors have investigated the efftcts of matrix materials, lanthanum, yttrium, neodymium, and europium, for the emission of analytical elements using a rotating electrode. The emission intensity of the element depends o n the change in matrix content. Because the emission intensities of the analytical elements were greatly lowered with rare earth elements as matrix materials, the authors investigated the effects of alkali chlorides as radiation buffers to enhance the emission intensities of the elements. By using yttrium as a matrix material, the relation between the emission intensities of the analytical elements and ZRE+M - Zo+nr I = the concentration of alkali chlorides was investigated. ApIRE- I o preciable variations were observed in the emission intensities of the atomic and ionic lines by changing the character and the where IRE and ZO give the emission intensities of the elements t o be analyzed and the background, respectively, and IRE+II concentration of the alkali elements (Figure 1). The emission Table 11. Analytical Lines” Wavelength, A I1 3949.10 I1 4186.59 I 4012.38 I1 4100.74 I 4222.98 I1 4303.57 I1 4424.34 I 4594.02 Eu Eub I1 4205.04 Gd I1 3654.63 Gdb I 4346.46 Y I1 3710.29 From “M.I.T. Wavelength Tables.” Movable slit. Elements La Ce Ceb Pr Prb Nd Sm
-
Intensity
Arc 1000
Spark
80
25 20
60 200
125 100 300 500 R 200 R 200 w 150 80
800 50
40 40 300 200 50 200 60 150
Excitation potential, eV 3.54 3.34 3.64
...
2.98 2.87 3.28 2.69 2.93
...
...
3.52
Ionization potential, eV 5.61 6.54 6.54 5.8 5.8 6.3 5.6 5.67 5.67 6.16 6.16 6.38
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r
1
I
I
I
I
I
1
60
50
0
0.1 0.2 CONCENTRATION
0.3 OF
0.4 CESIUM
0.5
0.6
[mole/ I)
-
40
D
a w
F LL
30
a r
0
20
IO I
0
z w
c z
2-4 -
c w 2 9 a
t
I
I
I
I
0.2 0.4 0.6 0.8 I. 0 CONCENTRATION OF POTASSIUM (mole/ I
I
I
1
1.2 I)
0
0
0
40
60
DISCHARGE TIME
I
1
0.2 0.4 CONCENTRATION
0.2 0.4 CONCENTRATION
1
I
0.6
0.8
OF S O D I U M
0.6
0.8
OF LITHIUM
I
I
1.0
1.2
(mole/ I )
I. 0 (mole/ I)
1.2
Figure 1. Relation between relative intensity of spectral line and concentration of alkali chlorides Matrix material: Y103 lye, each element to be analyzed 25 ppm. Ionization potential, Alkali elements eV 3.89 Cesium 4.33 Potassium 5.13 Sodium 5.39 Lithium 0 Ce I1 4186.59, 3 La 11 3949.10, Nd I1 4303.570 0 Eu I 4594.02, 0 Gd I1 3654.63, X Eu I1 4205.04 A
of spectral lines was more intensified in the presence of cesium chloride. The atomic line of euroqium is affected remarkably, while the intensity of Eu 14594.0 A increases in order of Li ( N a ( K ( Cs upon addition of the alkali elements. The relation between the emission intensities of the spectral lines of rare earth elements and the rate of vaporization of the sample solution in the absence of the matrix yttrium as a function of sodium concentration was investigated. In the range of 0 to 0.17 mole/liter of sodium concentration, the spectral line emission is enhanced, while the rate of vaporization remains constant. These results indicate that the enhancement of the spectral line emission is not affected by the rate of vaporization of the sample solution. Because the emission intensity of the spectral line from the discharge column, under constant conditions, is proportional 666
20
Figure 2.
I
t
0
I
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
80
100
120
(sec)
Intensity-time curves of europium
Curves
Matrix material
1 2 3 4
yzo3 1% yzo3 1% Yzo3 1% yzo3 1%
Cesium
-
-0.15 mole/l
--0.15 mole/l
Spectral lines
4
Eu I1 4205.0 Eu I1 4205.0 A Eu 1 4 5 9 4 . 0 4 Eu 14594.OA
to the concentration of the analytical elements in the column, the intensity-time curves for the spectral line indicate the relative concentration of the element in the column a t different stages during the discharge. Figure 2 shows the intensitytime curves obtained with europium using cesium chloride buffer. The enhancement of Eu 14594.0 A was greatest in solutions with 0.15 mole/liter of cesium in contrast to the enhancement in yttrium solution without cesium. O n the other hand, appreciable enhancement of the ionic line emission in the presence of cesium was observed. Appreciable enhancement of the ionic line emission, based on the addition of alkali elements, may be caused by the increase of localization of the emitted ionic species in the center part of the discharge column, as has been observed by Vainstein and Belyaev (5). The emission intensity of the atomic line of the analytical element should change in some degree, for the lowering of the ionization is a major cause of alkali enhancement effects. In the present investigation simultaneous changes in the concentration of the neutral atoms and ions, i.e., degree of ionization, were observed for the analytical elements. As can be seen in Figures 1 and 2, the enhancement of E u 14594.0 A !as greater in solutions with cesium than that of Eu I1 4205.0 A in the same solutions. Consequently, the intensity ratio (Eu I 4594.O/Eu I1 4205.0) was enhanced. Figure 3 shows the relation between the intensity ratio of the atomic lines t o the ionic lines of the various analytical elements and the ionization potential of the alkali elements as radiation buffers. The intensity ratio increases linearly in proportion to the lowering of the ionization potential of the alkali elements. A n increase in the intensity ratio indicates a decrease in the temperature of the plasma, as has been observed by Samsonova ( 5 ) E. E. Vainstein and Yu. I. Belyaev, Intern. J. Appl. Radiut. Isotopes, 4, 179 (1959).
I
2,0t
I
I
I
I
120
100
g0 8 0 a
U
a 60
l-
a Q
I V
3.5 4.0 4 . 5 5.0 5.5 IONIZATION
40
3.5 4.0 4.5 5.0 5.5
POTENTIAL OF L K A L I ELEMENTS ( e V )
20
Figure 3. Relation between intensity ratio of atomic lines to ionic lines and ionization potential of alkali elements as radiation buffers
0 0
Matrix material: YZO3lyO;each alkali element: 0.1 mole/l. Eu 1 4 5 1 9 4 . 0 , ~ G d14346.4 @Ce 14012.3 0 Eu I1 4205.0 Gd I1 3654.6’ Ce I1 4186.5’
400 0 2 0 40 6 0 8 0 I00
200
Eu,Ce,AND N d CONCENTRATIW IN LANTHANUM OXIDE (ppm)
Eu,Ce,ANDGd CONCENTRATION I N YTTRIUM OXIDE (ppm)
Figure 4. Working curves 0
(6), and a decrease in the degree of ionization as explained by the Saha equation (7). The authors claimed that the presence of easily ionizable alkali elements causes a decrease of the temperature in the plasma to lower the degree of ionization of the analytical elements, and that the decrease in ionization shifts the excitation to the atomic lines. Hence, the population of the neutral atom of the analytical elements increases gradually by decreasing the temperature of the plasma. The magnitude of this enhancement, as shown in Figure 3, depends o n the ionization potential of the alkali elements. From the above results, the enhancement of the sensitivity of the analytical rare-earth element by the addition of alkali chloride buffers is due to the additive effects that act simultaneously,-Le., the concentration of the emitting species in the central region of the plasma and an increase in the number of neutral atoms according to Saha’s relationship. Cesium was more satisfactory than any other alkali element for enhancing the sensitivity of the analytical element. In practice, the correct concentration of cesium to be added is 0.06 to 0.15 mole/liter. Precision and Limit of Detection. By using lanthanum and yttrium solutions containing 0.06 mole/liter of cesium and a microamount of the rare earth elements to be analyzed, the working curves were prepared using lanthanum and yttrium as the internal standards. Working curves for the elements were prepared by giving the relation between the concentration of a series of synthetic standards and the ratio of the charges o n the integrators connected to the internal standard and the element photomultiplier tubes. As shown in Figure 4, a linear relationship is obtained in the range of 0.0-400 ppm of the rare earth elements.
Matrix materials: LapO, 5 7 0 Y t 0 3 1%. EuI4594.0,O Ce I1 4186.5, @ Nd 114303.5, o Gd I1 3654.6 A
Table IV. Precision and Limit of Detection Lanthanum oxide Limit of detection, Coefficient of Elements pprn variation,
z
Ce Pr Nd Sm Eu Gd Y
15 40 30 15 20 45 15
11.8 9.2 11.5 10.3 3.4 5.0
2.1
Yttrium oxide Limit of detection, Coefficient of ppm variation, 10 10
12.3
25
9.7 1.3
10 10 15 I . .
3.0 6.6
10.4
...
z, Y2031 z. (z) = 100 u / 3
Sample solution: Laz035 Coefficient of variation: Y
n = 5
Each element to be analyzed 10 ppm.
As can be seen from Table IV, the limits of detection for rare earth elements in lanthanum oxide or yttrium oxide are 10-45 ppm, the coefficients of variation are 2-12 %. ACKNOWLEDGMENT The authors are indebted to Shigero Ikeda, Osaka University, for his guidance.
(6) Z . N. Sarnsonova, Opt. Spectros. (USSR)(English Transl.), 12,
257 (1962). (7) M. N. Saha and N. K. Saha, “A Treatise on Modern Physics,” Vol. 1, p 630, Indian Press, Calcutta, 1934.
RECEIVED for review July 18, 1969. Accepted January 16, 1970.
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