Determination of some rare-earth elements by plasma-jet emission spectrometry

Analytica ChimictJ Acta, 65 (1973) 303-309 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

303

D E T E R M I N A T I O N O F S O M E R A R E - E A R T H E L E M E N T S BY PLASMA-JET EMISSION SPECTROMETRY

I K U O ATSUYA and H I D E H I R O G O T O *

Kitami Institute of Technology, Kitami (Japan) (Received 9th N o v e m b e r 1972)

M a n y m e t h o d s h a v e been r e p o r t e d for the d e t e r m i n a t i o n o f r a r e - e a r t h e l e m e n t s by m e a n s o f s p e c t r o p h o t o m e t r i c m e t h o d s ~ o r f l a m e emission s p e c t r o m e t r i c m e t h o d s z based o n solvent extraction procedures. H o w e v e r , s p e c t r o p h o t o m e t r i c m e t h o d s necessitate p r i o r isolation o f e a c h o f the r a r e - e a r t h elements, a n d flame emission m e t h o d s are not sufficiently sensitive. E m i s s i o n s p e c t r o g r a p h i c m e t h o d s 3, X-ray fluorescence m e t h o d s * and a c t i v a t i o n analyses w i t h t h e r m a l n e u t r o n s s have been developed, but these various m e t h o d s require c u m b e r s o m e a n d expensive devices in o r d e r to o b t a i n accurate results. A t o m i c a b s o r p t i o n a n d emission m e t h o d s with n i t r o u s o x i d e - a c e t y l e n e flames 6-s a n d h i g h - f r e q u e n c y p l a s m a t o r c h spectrometric m e t h o d s 9'x° h a v e been p r o p o s e d in recent y e a r s for the d e t e r m i n a t i o n of r a r e - e a r t h elements. Several studies o f plasma-jet s p e c t r o m e t r y based o n an arc-jet p l a s m a generator as a light s o u r c e have recently b e e n described 1~-~3. In the w o r k discussed here, the stabilizing effect of the magnetic field on the plasma j e t by the L o r e n t z force was e x a m i n e d , a n d d e t e c t i o n limits for s o m e rare e a r t h e l e m e n t s were established. Plasmajet s p e c t r o m e t r y was then applied to the d e t e r m i n a t i o n o f l a n t h a n u m , yttriun5 a n d g a d o l i n i u m in a m o n a z i t e sample f r o m w h i c h t h o r i u m h a d been separated. W h e n the c a l i b r a t i o n c u r v e s were prepared, the internal s t a n d a r d a n d b a c k g r o u n d c o m p e n s a t i o n m e t h o d ~3 was applied. EXPERIMENTAL

Reagents Standard stock solutions of rare-earth elements. A c c u r a t e l y weighed, spectroscopically pure, r a r e - e a r t h oxides ( R e s e a r c h Institute o f I r o n , Steel a n d O t h e r Metals, T o h o k u U n i v e r s i t y ) were dissolved in nitric acid; c e r i u m oxide was dissolved in sulphuric acid. T h e solutions were e v a p o r a t e d in o r d e r to eliminate excess o f acid, t r a n s f e r r e d to 100-ml v o l u m e t r i c flasks, a n d diluted with distilled water to the mark. These stock solutions c o n t a i n e d 5.00 m g m l - 1 of the r a r e - e a r t h metal, a n d were diluted to the desired c o n c e n t r a t i o n a t the time o f use. Standard magnesium solution. M a g n e s i u m m e t a l (99.9Y/o) was dissolved in h y d r o c h l o r i c acid, the solution was e v a p o r a t e d to dryness, a n d t h e residue was dissolved in w a t e r to m a k e a solution c o n t a i n i n g 1 m g M g m l - ~ , which was diluted to the desired c o n c e n t r a t i o n as required. All acids w e r e o f analytical grade. * Present address: T o y a m a University, "l'oyama, Japan.

304

I. ATSUYA, H. GOTO

Apparatus T h e p l a s m a - j e t g e n e r a t o r a n d H i t a c h i 139 a u t o - r e c o r d i n g s p e c t r o m e t e r u s e d w e r e a s d e s c r i b e d p r e v i o u s l y it A m a g n e t a n d g a u s s m e t e r ( D e n s h i - J i k i K o g y o M o d e l 4 2 0 ) w e r e also used.

Effect of" magnetic field on the spectral line intensity T h e p r o p e r t i e s a n d p h y s i c a l c h a r a c t e r i s t i c s o f the p l a s m a - j e t s o u r c e u s e d in this w o r k h a v e b e e n d i s c u s s e d in d e t a i l b y Y a m a m o t o 14, w h o p o i n t e d o u t t h a t the p l a s m a a r c c a n b e fixed b y m e a n s o f a m a g n e t i c field so t h a t t h e p l a s m a - j e t flame c a n b e s t a b i l i z e d w h e n a s a m p l e s o l u t i o n is b l o w n i n t o the p l a s m a arc. T h e stabilizat i o n o f t h e p l a s m a - j e t f l a m e b y m e a n s o f ' t h e m a g n e t i c field is s h o w n s c h e m a t i c a l l y in F i g . 1. In t h e w o r k d e s c r i b e d here, the i m p r o v e d n e b u l i z e r d e s c r i b e d in the e a r l i e r p a p e r 12 w a s used, a n d t h e effect o f t h e m a g n e t i c field o n s p e c t r a l line i n t e n s i t i e s a n d b a c k g r o u n d intensities w a s i n v e s t i g a t e d f r o m t h e v i e w - p o i n t s o f the s t a b i l i z a t i o n o f t h e p l a s m a - j e t flame, a n d o f a n a l y t i c a l sensitivity. W o r k i n g c o n d i t i o n s f o r the p l a s m a g e n e r a t o r a n d the s p e c t r o m e t e r , e x c e p t f o r the u s e o f the m a g n e t , w e r e t h e s a m e as d e s c r i b e d p r e v i o u s l y 13, i.e., 400 A f o r t h e a r c current, 1 0 - 1 5 1 r a i n - 1 ( A r - H e ) for the t a n g e n t i a l g a s f l o w - r a t e a n d 5 1 r a i n - 1 ( A r ) for the carrier gas flow-rate. A s t h e m a g n e t i c f o r c e in t h e p l a s m a - j e t g e n e r a t o r v a r i e d w i t h the p o s i t i o n o f the nozzle of the c o p p e r anode, the magnetic force was measured. Results are p r e s e n t e d in Fig. 2. T h e effects o f t h e m a g n e t i c fiel d on s p e c t r a l line a n d b a c k g r o u n d i n t e n s i t i e s w e r e e x a m i n e d w i t h settings o n t h e b a s i s o f t h e p o s i t i o n o f p o i n t c in F i g . 2. As s h o w n in T a b l e I, t h e b a c k g r o u n d intensities d e c r e a s e d w i t h increasing m a g n e t i c force, a n d t h e r a t i o o f t h e spectral i n t e n s i t y o f t h e Y II 3 6 0 . l - r i m line to

....

A f o m l z e ~

j

M - ,ome F

Atomizer

/ v -F,ome

Anode

le piece

Anode

Pole piece

. / I " ~Cathode Argon" L::~"

(A

_ / Argon"

I I~'----Cathode ~=J --

(B)

e

[c)

Fig. 1. Schematic diagram of the plasma-arc stabilization. (A) When the sample solution is blown into the plasma arc without the magnetic field, the plasma jet moves to the opposite side from the aperture of the sample injection. (B) The plasma arc is fixed to the aperture of the sample injection by means of the magnetic field. (C) When the sample solution is blown into the plasma arc, the plasma-jet flame becomes ideal owing to the magnetic field. Fig. 2. Relationship between the magnetic force and the position of the copper anode nozzle (a) 3 I0, (b) 420, (c) 440, (d) 360 gauss.

( Thrltun

305

RARE EARTHS BY PLASMA-JET SPECTROMETRY TABLE I EFFECT OF MAGNETIC

FIELD

ON SPECTRAL

LINE INTENSITY

Magnetic force at point C (Gauss)

Backgroundintensity (chart scale)

/vna~ol ° I8G3~oI

0 330 380 440 490

45 40 28 15 6

1.0 1.1 1.2 1.6 2.0

° Ivu30ol a n d lnGa6ol are, respectively, the s p e c t r a l intensities o f the Y II 360.073-nm a n d Y II 36().192n m lines, w h i c h w e r e not s e p a r a t e d by m e a n s o f the s p e c t r o p h o t o m e t e r used, a n d the b a c k g r o u n d intensity at 360.1 rim.

the b a c k g r o u n d intensity increased. O n e r e a s o n for t h e d e c r e a s e o f b a c k g r o u n d intensity is t h a t t h e t e m p e r a t u r e o f the p l a s m a j e t d e c r e a s e d , b e c a u s e l a r g e r a m o u n t s of the s a m p l e s o l u t i o n were m i x e d w i t h t h e p l a s m a t h a n in t h e a b s e n c e o f t h e m a g n e t i c field, o w i n g to t h e s t a b i l i z a t i o n of t h e p l a s m a arc by m e a n s o f the m a g n e t i c field. A c c o r d i n g l y , analytical sensitivities were i n c r e a s e d by this use of m a g n e t i c fields in p l a s m a - j e t s p e c t r o m e t r y . B a s e d o n these e x a m i n a t i o n s , all s u b s e q u e n t tests were c a r r i e d o u t u n d e r the w o r k i n g c o n d i t i o n s w i t h 490 g a u s s at p o i n t c.

80

Lo 0 ~g/ml

La

5 0 .;Jg/ml

70 60 o

.=- 50 "¢3

~ I

40

8 3O 20

I0 I

I

I

, I

Wave length(nm) Wave length(nm) Fig. 3, L a n t h a n u m s p e c t r u m f r o m the p l a s m a - j e t flame.

l

306

I. ATSUYA, H. G O T O

D e t e c t i o n limits J o r some rare-earth e l e m e n t s A s t h e t e m p e r a t u r e o f t h e p l a s m a j e t u s e d w a s 7 2 0 0 - 6 2 0 0 ° K 11, h i g h s e n s i t i v i t y for the determination of the rare-earth elements was expected. However, the background intensities from the plasma jet were strong and the background variation o n s c a n n i n g t h e w a v e l e n g t h w a s l a r g e ( F i g . 3 ) ; it w a s a l s o n e c e s s a r y t o c o n s i d e r the spectral lines of argon which overlap with the spectral lines of the rare-earth e l e m e n t s in o r d e r t o s e l e c t a p p r o p r i a t e a n a l y t i c a l lines. T h e d e t e c t i o n l i m i t s f o r s o m e rare-earth elements were measured under optimal conditions (Table II); the detection l i m i t is d e f i n e d a s t h e v a l u e w h e r e t h e s i g n a l i n t e n s i t y o f t h e p e a k is t h r e e t i m e s the background fluctuation. As can be seen, the analytical sensitivities for the rareearth elements were quite good. TABLE I1 D E T E C T I O N LIMITS FOR RARE-EARTH E L E M E N T S

Element

Wavelen[Ith (.m)

Detection limit (Itg ml- i ) Pj.s."

Se a Y La Pr Nd Sm Gd Ce

II II II II II II " II II II II 1I II I1 II 1I II

424.6 360. I 361.1 398.8 399.5 403.1 440.8 446.8 449.6 383.6 445.1 428.0 442.4 443.3 396.6 371.2 218.0 222.2 222.5

0.4 0.1 0.3 0.6 0.5 0.5 2.7 5.3 4.0 3.3 3.3 3.3 2.7 2.0 4.0 3.0 3.7 7.3 3.7

A.a.s? 0.8 5.0

F.e.s/ 0.07 0.3

110

1

72

1

35

I

21

0.6

38

2

--

10

'~ Plasma-jet spectrometry. b Atomic absorption spectrometry with pre-mixed flame 7. Flame enaission spectrometry with pre-mixcd flame 6. a The detection limit for scandium was also measured because of the similarity of chemical properties.

A p p l i c a t i o n to the de t e r m i nat i on o f L a , Y and G d in m o n a z i t e A monazite sample from which thorium had been separated was analysed. T h e s t a n d a r d a d d i t i o n m e t h o d w a s u s e d in o r d e r t o e s t a b l i s h t h e a c c u r a c y o f t h e proposed method. For measurements of spectral line intensities, the internal standard a n d b a c k g r o u n d c o m p e n s a t i o n m e t h o d la w a s a g a i n u s e d . T h e M g I I 2 7 9 . 5 - n m l i n e w a s t a k e n a s t h e i n t e r n a l s t a n d a r d , b e c a u s e t h i s l i n e w a s n o t d e t e c t e d in t h e s a m p l e analysed by plasma-jet spectrometry.

307

R A R E E A R T H S BY P L A S M A - J E T S P E C T R O M E T R Y

Procedure. Three sub-samples o f a monazite sample were weighed accurately and dissolved in nitric acid, and the s o l u t i o n s were evaporated to dryness. The residues were dissolved in hydrochloric acid and the s o l u t i o n s diluted w i t h distilled water. The s o l u t i o n s were then evaporated, with occasional w a s h i n g o f the beaker walls with distilled water, in o r d e r to r e m o v e excess o f hydrochloric acid. Each solution w a s transferred to a 100-ml volumetric flask, and 5.0 ml o f m a g n e s i u m solution c o n t a i n i n g 20.0 #g o f m a g n e s i u m w a s added as the internal standard. Then definite a m o u n t s o f La, ¥ a n d G d were also added to two solutions, but n o t to the third. All the solutions were then diluted with distilled water to the mark. The spectral line intensities w e r e measured under the c o n d i t i o n s m e n t i o n e d a b o v e . RESULTS AND

DISCUSSION

As s h o w n in Figs. 4-6, the gradients o f the calibration curves for standard solutions w h i c h c o n t a i n e d 1 # g Mg m l - t and definite a m o u n t s of La, Y and G d without s a m p l e w e r e in agreement with the sample solutions prepared as above.

3.0

(i) 2.0

(2) t.5

2.0

(2)

1.0

1.0

, ,~,~"S ~ 1.0

0.5

t p

~P 0 ~- -

¢s

) 60

I 40

!

2

~0

0 La

I ,

20 40 .Ug m l "1

/

60

l

l

eo

I0

1

1.5

/

0.5

/

s.¢

/

¢

0 Y

i0 20 .ug m l "1

I(~0

I00

0 Gd

jig

200 m1-1

Fig. 4. C a l i b r a t i o n curves by the standard a d d i t i o n m e t h o d ( 1 ) a n d by standard s o l u t i o n s (2). Intensity ratio: It.,,,4031/InG4oa t • /Mlll12795/IBG 2795

Fig. 5. Calibration curves by the standard a d d i t i o n m e t h o d (1) a n d b y standard s o l u t i o n s (2). Intensity ratio! l v . 3 6 0 1 / l B o ~t, o t_z

IMglI279s/ InG 279S Fig. 6. Calibration curves by the standard a d d i t i o n m e t h o d (1) a n d b y standard s o l u t i o n s (2). Intensity ratio: lodll 3-/1 :z//InG3 7 t : . IMstla79~/ l n G 2 7 9 5

The g o o d linearity for b o t h graphs indicated a reliable analytical a c c u r a c y o f the proposed m e t h o d . Therefore, the results obtained for the determination o f La, Y and G d in the m o n a z i t e sample are listed in Table III. The precision o f the determination o f l a n t h a n u m in the m o n a z i t e s a m p l e was a l s o established: the coefficient o f variation w a s 1.51% for 5 0 / z g La m l - 1 (11 measurements). Plasma-jet spectrometry has m a d e it possible to obtain g o o d results for the determination o f aluminium, b o r o n t2 a n d rare-earth elements, w h i c h w a s rather difficult by flame emission and a b s o r p t i o n spectrometry with the usual c o m b u s t i o n

308 TABLE

I. A T S U Y A ,

H. GOTO

Ill

ANALYTICAL

RESULTS

Sample taken

La added (~lg)

Intensity ratio

La found ( ltg)

La20 a (% )

0.0378

-20 40

0.500 1.91 2.32

-19.7 39.5

32.9

Y added ( IJg)

Intensity ratio

Y found ( ]tg )

Y~Oa (%)

--10 20

0.215 0.830 1.425

-10.3 19.5

0.59

Gd added ( ltg )

Intensity ratio

Gd found (It.q)

GdzO a (~]/o)

-I00 200

0.394 1.012 1.654

-95 193

0.89

(~)

0.1075

0.1075

flames. F o r the d e t e r m i n a t i o n o f c a l c i u m a n d m a g n e s i u m , high sensitivity was obt a i n e d is. O n the o t h e r h a n d , it m u s t be p o i n t e d o u t that p r o b l e m s remain, e.g. the use o f acids a n d large a m o u n t s o f diverse e l e m e n t s cause a flush o f p r o d u c t s which c o r r o d e the c o p p e r electrode. T h e stabilization o f the plasma-jet flame by m e a n s of the Lorentz force o f the m a g n e t i c field has not essentially settled these problems, but it is c o n s i d e r e d that the p l a s m a - j e t flame will b e c o m e an excellent s o u r c e for emission s p e c t r o m e t r y , if the m e t h o d o f sample injection is improved. SUMMARY

The effects o f a m a g n e t i c field o n spectral intensities in p l a s m a - j e t spectrom e t r y were e x a m i n e d , a n d d e t e c t i o n limits for r a r e - e a r t h elements were calculated. Plasma-jet emission s p e c t r o m e t r y was applied to the d e t e r m i n a t i o n o f l a n t h a n u m , y t t r i u m and g a d o l i n i u m in a m o n a z i t e s a m p l e f r o m which t h o r i u m h a d been separated. A s t a n d a r d a d d i t i o n m e t h o d was used in o r d e r to i m p r o v e accuracy, a n d the internal s t a n d a r d a n d b a c k g r o u n d c o m p e n s a t i o n m e t h o d was applied to m e a s u r e m e n t s o f spectral lihe intensities to achieve g o o d precision. T h e coefficient of variation was 1.51% for 5 0 / t g La m1-1. RI~SUMI~

O n e x a m i n e l'influence d ' u n c h a m p m a g n 6 t i q u e sur les intensit6s spectrales de la spectrom6trie .h injection de plasma. Les limites de d6tection des 616ments des terres rares sont calcul6es. C e t t e t e c h n i q u e est appliqu6e au d o s a g e d u lanthane,

RARE E A R T H S BY P L A S M A - J E T S P E C T R O M E T R Y

309

de l'yttrium et du g a d o l i n i u m dans une monazite, apr6s s6paration du thorium. O n proc~de par addition d'6talon afin d'arriver "h une exactitude plus grande. La m 6 t h o d e de l'6talon interne et la c o m p e n s a t i o n du bruit de fond permettent d'arriver /t une b o n n e pr6ciston. Le coefficient de variation est de 1.51% pour 5 0 / z g La m l - a ZUSAMMENFASSUNG

D e r Einfluss eines magnetischen Feldes a u f Spektralintensit~iten bei der Plasrnabrennerspektrometrie wurde untersucht; die N a c h w e i s g r e n z e n ftir SeltenerdE l e m e n t e wurden berechnet. D i e Emissionsspektrometrie mit einem Plasmabrenner wurde a u f d i e B e s t i m m u n g v o n Lanthan, Yttrium und G a d o l i n i u m in einer M o n a z i t •probe angewendet, aus der T h o r i u m abgetrennt w o r d e n war. Es w u r d e eine Stand a r d z u m i s c h m e t h o d e angewendet, um die G e n a u i g k e i t zu verbessern, und die M e t h o d e des inneren Standards und der U n t e r g r u n d k o m p e n s a t i o n w u r d e auf M e s sungen von Spektrallinienintensit~iten angewendet, um eine gute Reproduzierbarkeit zu erreichen. D e r Variationskoeffizient war 1.51~/o bei 50 ~tg La m l - 1 . REFERENCES I See, e.y., E. B. Sandell, Colorimetric Deterlnination o f Traces o f Metals, Intcrscience, N e w York, 3rd Ed., 1959. 2 T. C. Rains, H. P. H o u s e and O. Menis, Anal. Chim. Acta, 22 (1960) 315. 3 V. A. Fassel, H. D. Cook, L. C. Krotz and P. W. Kehres, Spectrochim. Acta, 5 (1952) 207. 4 Y. T a k a h a s h i a n d E. Asada, Jap. Anal., 11 (1962) 926. 5 D. L. Massart and J. Hoste, Anal. Chim. Acta, 42 (1968) 7. 6 V. A. Fassel, Anal. Chem., 41 (1969) 1495. 7 M. D. A m o s and J. M. Willis, Spectrochira. Acta, 22 (1966) 1325. 8 J. C. Van Loon, J. H. Galbraith and H. M. Aardcn, Analyst, 96 (1971) 47. 9 S. M u r a y a m a , Spectrochim. Acta, 25B (1970)791. 10 V. A. Fassel a n d D. W. (3olightly, Anal. Chem., 39 (1967) 466. 11 H. (3ot~ and I. Atsuya, Z. Anal. Chem., 225 (1967) 122. 12 H. (3ot~ and I. Atsuya, Z. Anal. Chem., 240 (1968) 102. 13 I. Atsuya a n d H. GotS, Spectrochim. Acta, 26B (1971) 359. 14 M. Y a m a m o t o , J. Spectrosc. Soc. Jap., 12 (1963) 84.