Laser Emission Spectrography of Rare Earth Elements Toshio Ishizuka Government Industrial Research Institute, Nagoya 1-1, Hirate-machi, Kita-ku, Nagoya, Japan Emission spectrography of rare earth elements in several matrixes is studied by using a Q-switched ruby laser as an exciting source for macro analysis. The emission intensities of rare earth elements are considerably affected by the matrixes of the samples. The emission intensities of rare earth elements are most intense in a sodium chloride matrix, but fairly suppressed in a sodium sulfate matrix. Scandium, .yttrium, europium, and ytterbium in a sodium chloride matrix are detected down to concentrations of ppm order with one laser shot. The emission intensities of rare earth elements in refractory oxides, such as aluminum oxide or rare earth oxides, are about two orders of magnitude less than those in a sodium chloride matrix. SOLID-STATE LASERS, such as ruby lasers or neodymium lasers, have been used as exciting sources for emission spectrography. Laser excited emission spectrographic studies have been performed on conductive and nonconductive solid samples, such as biological materials (1-3), metals (4, 5), ceramics ( I ) , minerals (5, 6), semiconductor materials (7), powder materials (8), etc. Because of the excellent focusing characteristics of laser light and the practicality of microsampling on a sample surface by laser shot, most of the work has been performed for microprobe analysis. In laser microprobe analysis, a very small part of the sample is vaporized by laser shot, and an auxiliary spark excitation is used to increase the spectral emission intensity (7). However, disadvantages of the spark excitation are that the graphite electrodes must be changed for every spark excitation and a cyanogen band is observed. Laser excited emission spectrography without auxiliary spark excitation has been studied as a means of macro analysis by several investigators (9-13). They reported the following disadvantages of laser excited emission spectra : weak spectral intensity, intense background, spectral broadening, and remarkable self-absorption (7, 12,14,15). ( I ) Y . Katuno, H. Sunahara, K . Morita, and T. Takeuchi, Bunko Keukyir, 16, 151 (1967). (2) R . C . Rosan, M. K. Healy, and W. F. McNary. Jr.. Science, 142,236 (1953). (3) S . F. Brokeshoulder and F. R. Robinson, Appl. Spectrosc., 22, 758 (1968). (4) S. D. Rasberry, B. F. Scribner, and M. Margoshes, Appl. Opt., 6, 87 (1967). (5) A. Petrakiev, G. Dimitron, and L. Georgierva, Spectrosc. Lett., 2,97 (1969). (6) W. H. Blackburn, Y . J. A. Pelletier. and W. H . Dennen. Appl. Spectrosc., 22,278 (1968). (7) S. D. Rasberry, B. F. Scribner, and M. Margoshes, Appl. Opt., 6 , 81 (1967). (8) A. B. Whitehead and H. H. Heady, Appl. Spectrosc.. 22, 7 (1968). (9) E. F. Runge, S. R. W. Minck, and F. R. Bryan, Spectrocliim. Actu, 20, 733 (1964). (10) E. F. Runge, S. Bontiglio, and F. R. Bryan, ibid.. 22, 1678 (1966). ( I 1) A. Felske, W. D. Hagenah, and K. Laqua, Fresetiius' Z . A m / . Cliem., 216, 50 (1966). (12) E. H. Piepmeier and H . V. Malmstadt, ANAL.CHEM., 41, 700 (1969). (13) R. H . Scott and A. Strasheim, Spectrocliim. Acta, 25B, 311 (1970). (14) E. Archbold, D. W. Harper, and T. P. Hughes, Brit. J . Appl. Pliys., 15, 1321 (1964). 538
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
Ruby Laser
Prism ILaser Light
Spectrograph
Sample 12cm+-24crn
-+I
Figure 1. Schematic diagram of optical system However, in the case of laser excitation w i t h p t auxiliary spark excitation, the cyanogen band (3500-4800 A) is not found, because the graphite electrode for ordinary emission spectrographic analysis is not used. In general, emission spectrographic analysis of rare earth elements is performed in the spectral range of 3500 to 4500 The interference of the cyanogen band on the analysis has been considered, and an atmosphere of inert gas or another device has been used. Therefore, the present author has employed laser excited emission spectrographic analysis using no graphite electodes for auxiliary spark excitation. In this work, samples containing rare earth elements in rare earth oxides, aluminum oxide, and sodium salts are prepared, and the matrix effects on the emission intensities of rare earth elements, the detection limits, and the analytical curves are studied by using a Q-switched ruby laser as an exciting source.
A.
EXPERIMENTAL
Apparatus. A model JLR-O2A Giant Pulse Laser with a ruby rod 10 cm long and 10 mm in diameter (Japan Electron Optics Lab. Co., Ltd.) is used. The maximum peak power is about 70 MW; the maximum output energy, about 1.5 Joule; the pulse-width, less than 20 nsec; and the reproThe ruby rod and the ducibility of the laser energy, +5%. xenon flash lamp are cooled at 18 O C by distilled water. The laser beam is directed downward by a 45" prism, and is focused on a sample surface by using a 5-cm focal length lens (13 mm in diameter). The diameter of the laser beam is about 1 mm on the sample surface. For the photographic measurement of spectra, a Nippon Jarrell Ash f/22, 1.5-m Wadsworthmount, grating spectrograph (grating, 600 grooves/mma; reciprocal linear dispersion, 11 A/mm; blazed for 4000 A) is used. The photographic emulsion is a Kodak TRI-X film (ASA 400). Neutral density filters are not used in this work. The slit width of the spectrograph is 50 pm, and the slit length is 2 mm. The spectral intensities on the photographic emulsion are measured by a Rigaku Denki Model CM-1 microphotometer. As the line image depends on the height above the sample surface and the uniform line image is required for quantitative analysis, the emission source is located 36 cm from the slit of the spectrograph, and a 8-cm focal length quartz lens (4 cm in diameter) is placed at a point of 12 cm (15) A. V. Karyakin and V. A. Kaigorodov, Zli. A m l . Kliim., 22, 504 (1967).
'O'O
t
c I
(3)
Figure 2. Photographs of laser induced plasmas on europium oxide sampIe obtained by varying the capacitor voltage of power supply to xennn flashlamp
1 I
0.03 0.1 0.3 Metal Concentration. *I.
I
1.0
Capacitor voltage: (1) 3850 V, (2)4000 V, (3) 4200 V, (4) 4400 V
Figure 3. Analytical curves for yttrium and europium in aluminum oxide matrix
from the emission source to get 2 times the source image. A schematic diagram of optical system is shown in Figure 1. Reagents and Sample Preparation. The 1 % stock solutions of the rare earth elements are prepared by dissolving 99.9% rare earth oxides (Shin-Etsu Chemical Co.) in 1M hydrochloric acid. The 1 % stock solutions of sodium choride, sodium nitrate, sodium perchlorate, sodium sulfate, potassium dihydrogen phosphate, and aluminum nitrate are prepared from reagent grade chemicals (Wako Pure Chemical Ind., Ltd.). In order to prepare the samples containing 1-1000 ppm rare earth elements in each of the sodium salts (potassium salt is used as phosphate) as matrix, the mixed solutions of each of the rare earth stock solutions and each of the sodium salt stock solutions are evaporated to dryness and powdered. In order to prepare the samples containing 0.03-1 % rare earth elements in aluminum oxide as matrix, the mixed solutions of each of the rare earth stock solutions and the aluminum nitrate stock solution are evaporated to dryness, and the powdered samples are prepared as the oxide by igniting at 1000 "C in an electric furnace. In order to prepare the samples containing 0.03-1 % lanthanum and 0.1-3 europium in yttrium oxide as matrix, and O.l-l% yttrium in lanthanum oxide as matrix, the mixed solutions of lanthanum and yttrium, europium and yttrium, and yttrium and lanthanum are prepared from the stock solutions of lanthanum, europium, and yttrium. These rare earth elements are precipitated as oxalates by addition of oxalic acid, and the precipitates are ignited at 1000 OC in a n electric furnace. The oxides of the rare earth elements are obtained as the powdered samples. The powdered samples are pressed by using a KBr pellet press (Hitachi Co., Ltd.), and pellets of 13-mm diameter and 1- to 2-mm thickness are obtained and used for laser excited emission spectrographic analysis.
0: Ytlrium(lI), 3710.3 A 0 : Europium(II), 3819.7 A
RESULTS AND DISCUSSION
Conditions of Laser Excitation. The shape and the emission spectra of laser induced plasmas on a sample surface are examined by varying the laser energy. The variation in the laser energy is obtained by varying the capacitor voltage of the power supply to the xenon flash lamp. The photographs of the shapes of laser induced plasmas on a target sample pellet of europium oxide are shown in Figure 2. In Figure 2, the size of the laser induced plasma increases with increasing capacitor voltage. It is about 5 mm in height and
7 mm in width at 3850 V (about 0.5 Joule), about 8 mm in height and 11 mm in width at 4000 V (about 1.0 Joule), about 10 mm in height and 13 mm in width at 4200 V (about 1.2 Joule), and about 11 mm in height and 13 mm in width at 4400 V (about 1.3 Joule). The short spectral length of the laser induced plasma is observed on the plate at a capacitor voltage of 3850 V, near the threshold voltage of the Q-switched ruby laser. At a capacitor voltage over 4400 V, continuous spectra, caused by bremsstrahlung of electrons in the laser induced plasma, are observed remarkably, and the atomic spectra are possible to detect only in the head of the plasma. At a capacitor voltage of 4Mx) to 4200 V, the continuous spectra are observed within a region 2 to 3 mm above the sample surface, and the atomic spectra are observed intensely and clearly over the region. In order to study the spectra of rare earth elements in some matrixes under the conditions described above, sodum chloride, aluminum oxide, and rare earth oxides containing yttrium and europium are used as target samples, and the size, shape, and the spectra of the laser induced plasmas are examined. Similar results for the size, shape, and the spectral length are obtained for the matrixes used. The size and shape of laser induced plasma and thelength of the emission spectra are independent of the matrix type of the samples. When the integrated multi-exposures of emission spectra are performed by a number of laser shots, the atomic spectral intensity increases with superimposing laser shots, and the background intensity increases in proportion to the atomic spectral intensity. The ratio of signal-to-background intensity is not improved. One exposure of an emission spectrum induced with one laser shot is adequate for the spectral intensity measurement. For the experimental results mentioned above, the experimental conditions are as follows: the capacitor voltage of the power supply to the xenon flash lamp is set at 4100 V ; the emission plasma of the region 3 to 4 mm above a sample surface is photographed; the spectral measurement is performed with one exposure of one Laser shot; the laser excitation is performed at the interval of one shot per minute. ANALYTICAL CHEMISTRY, VOL. 45, NO. 3. MARCH 1973
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10.0 I-
I
0.1 0.3 1.0 Metal Concentration, % I
I
I
,
I
I
I
I
,
0.1 0.3 Metal Concentration, %
0.03
,
I
1.0
Figure 6. Analytical curve for yttrium in lanthanum oxide matrix
Figure 4. Analytical cufves for lanthanum and ytterbium in aluminum oxide matrix 0: Lanthanum(II), 4123.2 A 0:
Ytterbium(II), 3694.2 A
10.0
3.0
21
c
'ii
*
1.0
c
c
0.3
0.1
0.03
0.1 0.3 1.0 Metal Concentrat ion, %
3.0
Figure 5. Analytical curves for lanthanum and europium in yttrium oxide matrix 0: Lanthanum(II), 4086.7,A 0 : Europium(II), 3907.1 A
Matrix Effects. In general, laser excited emission spectrography is said to be free of the matrix effect ( 4 , 8, 15) and the emission intensity depends on only the amount of sample vaporized by laser shot. In this paper, the present author has an interest in the laser excitation process, but the process is very complicated to understand. For application of laser excited emission spectrography to the determination of rare earth elements, it is necessary to study not only the laser excited emission spectrographic behavior of rare earth oxides, but also the behavior of rare earth elements in each of several salts, rare earth oxides, and aluminum oxide as matrixes. When a giant pulse laser energy is used to excite the rate earth elements in several matrixes, sonie physicochemical character540
ANALYTICAL CHEMISTRY, VOL. 45, NO. 3, MARCH 1973
Y(II), 3633.1 A
istics of a matrix may show some effects on the spectral behavior of rare earth elements. In order to study the matrix effects on the laser excited emission spectrographic behavior of rare earth elements, sodium chloride, sodium nitrate, sodium perchlorate, sodium sulfate, and potassium dihydrogen phosphate are selected as matrixes. The shape of the crater and the amount displaced by laser shot are observed for sodium salts and potassium salts. The crater produced by laser shot is about 1 mm in diameter regardless of the kind of the matrix, but the depth of the crater depends on the kind of the matrix. The depth of the crater for the sample of the sodium chloride matrix is about 70-80 pm. The depth of the crater for the samples of sodium nitrate, sodium perchlorate, and potassium dihydrogen phosphate matrixes is several times that for the sample of the sodium chloride matrix. The depth of the crater for the sample of sodium sulfate matrix is shallow in comparison with those for the samples dcscribed above. About 100 pg for the sample of the sodium chloride matrix is displaced with one laser shot. About 1.5 to 4 times more than that of the sample of the sodium chloride matrix is displaced for the samples of sodium nitrate, sodium perchlorate, and potassium dihydrogen phosphate matrixes. The amount displaced for the sample of the sodium sulfate matrix is about one half of that for the sodium chloride matrix. For the samples of the oxide matrixes, such as aluminum oxide or rare earth oxides, nearly the same amount as for the sample of the sodium chloride matrix is displaced by laser shot. Keproducibility for the amount of crater displaced by laser shot is good. The precision (coefficient of:variation) of the spectral intensity of yttrium (Y 2+, 3710.3 A) for the sample containing 0.01 %yttrium in a sodium chloride matrix is 7.2 %for eleven measurements. The samples containing 0.01 yttrium, lanthanum, and ytterbium in several sodium salts as matrix (potassium salt as phosphate) are prepared. The spectral intensity and the amount displaced by laser shot are measured for tge above samples. The emission intensities of Y*+ (3710.3 A), Lazt (3949.1 A), and Yb*- (3694.2 A) for the samples of nitrate, perchlorate, sulfate, and phosphate matrixes are compared with those intensities for the sample of sodium chloride matrix, assuming a value of unity for the latter intensity. These values are calculated as the intensity ratio, and the results obtained
are shown in Table I. As the spectral intensity depends on the amount displaced by laser shot, in order to compare the spectral intensities of yttrium, lanthanum, and ytterbium in several salts as matrix, the ratio of the amount displaced by laser shot for several salt matrixes to that for the sodium chloride matrix is calculated, The intensity ratio in Table I is divided by the ratio of the amount displaced. The values are shown in parentheses in Table I. The spectral intensities of yttrium, lanthanum, and ytterbium in the samples of nitrate, perchlorate, sulfate, and phosphate matrixes are weaker than those in the samples of chloride matrix, except for the spectral intensity of lanthanum in the sample of perchlorate matrix. The spectral intensities of yttrium, lanthanum, and ytterbium in the sample of sodium sulfate matrix are remarkably weak, and the spectra for the samples containing 0.01 lanthanum and ytterbium are not detected. It is difficult to dissociate the rare earth elements into atomic vapor with laser shot, since the rare earth sulfates have high melting points and boiling points. The emission intensities of rare earth elements in refractory oxides, such as aluminum oxide or rare earth oxides, are about two orders of magnitude less than those in the sodium chloride matrix. Detection Limits of Rare Earth Elements in Sodium Chloride Matrix. As described in Table I, the spectral intensities of rare earth elements in the sodium chloride matrix show a large intensity ratio ; therefore, the detection limits of the rare earth elements are studied on the sample of the sodium chloride matrix. Samples containing 1, 3, 10, 30, 100, 300, and 1000 ppm of rare earth elements in the sodium chloride matrix are prepared and the spectral intensities are measured. Various criteria have been suggested €or expressing the detection limits in rare earth elements. In order to express these detection limits for the sodium chloride matrix, the spectral intensity is recorded by using a microphotometer, and a spectral intensity having twice the magnitude of the background fluctuation is selected as the detection limit. The results are shown in Table 11. As shown in Table 11, scandium, yttrium, europium, and ytterbium in sodium chloride matrix are detected on the order of ppm. If a detectable amount is expressed as the product of the detection limit shown in Table I1 and the amount displaced by laser shot, the value is in the range of 10-9-10-10 g. The detection limits of lanthanum, dysprosium, and holmium are l0ppm. The detection limits of lanthanum, dysprosium, and holmium are 10 ppm. The detection limits of praseodymium, samarium, terbium, erbium, thulium, and lutetium are in the range of 20 to 60 ppm, and the detection limits of neodymium and gadolinium are 500 ppm and 200 ppm, respectively. Analytical Curves. The analytical curves for rare earth elements in aluminum oxide and rare earth oxide matrixes are examined. The analytical curves for yttrium, lanthanum, europium, and ytterbium in an aluminum oxide matrix are shown in Figure 3 and Figure 4. The concentration ranges are 0.03 to 1.0% for yttrium, europium, and ytterbium, and 0.1 to 1.0% for lanthanum. The analytical curves for lanthanum (0.1-3 .O %) and europium (0.03-1 .O %) in an yttrium
Table I. Matrix Effect of Alkali Salts Intensitv ratio -__ Y2+i0.01Z), La2+(0.01 %), Yb2+(0.01Z) Matrix 3710.3 A 3949.1 A 3694.2 A NaCl 1.oo 1.oo 1 .oo (1 .m)= (1.00) (1 00) NaNO, 0.76 0.28 0.31 (0.27) (0.10) (0.11) NaClO, 0.60 1.04 0.19 (0.52) (0.90) (0.16) Na2SOI 0.15 0.00 0.00 (0.33) (0.00) (0.00) ,KH~POI 0.83 0.50 0.23 (0.45) (0.27) (U. 12) ): corrected by the amount of the sample displaced by a ( laser shot. Table 11. Detection Limits of Rare Earth Elements in Sodium Chloride Matrix Wavelength, Detection limit, Element A ppm sc2+ 3613 8 2 Y2+ 3710 3 -7 LazT 3949 1 10 Pr 2T 4179 4 40 Nd24303 6 500 Sm23568 3 40 EU27 3819 I 5 Gd23671 2 200 TbZT 3848 8 60 Dy 3531 7 10 Ho2+ 3456 0 10 3499 1 30 Er 2+ Tm+ 3883 1 30 YbZ’ 3694 2 2 LU2f 2911 4 20
oxide matrix are shown in Figure 5 , and that for yttrium (0.11.O %) in a lanthanum oxide matrix is shown in Figure 6. In Figures 3-6, each point on the analytical curves represents the average of five individual measurements, and each standard deviation is shown as a vertical line on each point. The analytical curves for lanthanum and europium in yttrium oxide and those for yttrium in lanthanum oxide are also prepared by an internal standard method, using Yz+ (3878.3 and La*+ (3557.3 A) as internal standard lines, respectively. The standard deviations for each point show a tendency to be slightly reduced in comparison with those in Figure 5 and Figure 6.
A)
ACKNOWLEDGMENT
The author acknowledges the invaluable advice of H. Sunahara and the assistance of 0.Ito. RECEIVED for review August 7, 1972. Accepted November 6, 1972.
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