Inductively coupled plasma-atomic emission spectrometric determination of rare earth elements in geological materials

Specrrochimica Acta.Vol.418.No. 3. pp.237-242.1986. Printed m GreatBritain

0584-8547/86 s3oo+o.oo 0 1986. Pergamon Prm Ltd

Inductively coupled plasma-atomic emission spectrometric determination of rare earth elements in geological materials S. J. BUCHANAN and L. S. DALE* CSIRODivision of Energy Chemistry, Lucas Heights Research Laboratories, Mail Bag 7, P.O. Sutherland, N.S.W., 2232, Australia (Received 6 March 1985; in revisedform 24 June 1985) Abstract-Inductively coupled plasma-atomic emission spectrometry (ICP-AES) has been applied to the determination of the rare earth elements (REE) lanthanum to lutetium (except terbium) in a range of geological materials. Group separation of the REE is carried out by sintering the sample with sodium peroxide to remove the bulk of the matrix, followed by fluoride precipitation with an yttrium carrier.This minimizes spectral interferences and provides sensitivities that are adequate for concentration levels around crustal abundances. The precision (2~) is 3-5 % for most of the elements and about 10 % for some of the less abundant elements with concentrations that approach the limit of determination. Comparison of results obtained on a range of referencesamples with literature values demonstrates the suitability of the procedures to provide rare earth abundance data for geochemical investigations.

1. INTRODUCTION IN STUDIES of petrogenesis, REE distributions provide valuable data on crustal and mantle evolutionary processes. These data are not only applicable to the study of igneous systems Cl-73 but also to sedimentary processes in which REE patterns in sedimentary rocks show little relative fractionation [3]. Geochemical investigations of these processes have been substantially assisted by the high precision brought the analysis of these elements by isotope dilution mass spectrometry [4]. Neutron activation analysis [S] and spark source mass spectrometry [6] have also been used successfully to provide adequate data. All of these techniques, however, require long analysis times and are of limited availability to geoscientists. The development of ICP-AES has, on the other hand, provided a fairly rapid alternative means of determining the REE at precisions that are generally suitable for geochemie studies. This is as a result of its high element sensitivity and precision and, more particularly, its high sample throughput. Although suitable instruments are now available in many laboratories their application in determining these elements directly in solutions of rocks is complicated by spectral interferences on the analytical lines arising from matrix components and by the lack of sensitivity to some elements whose concentrations approach crustal abundance levels. To overcome these limitations, a number of procedures based on chemical separation using ion-exchange, have been reported [6-lo] which provide analytical data suitable for studies of petrogenesis. *Author to whom correspondence should be sent. [1] L. A. HASKIN,Phys. Chem Earth 11, 175 (1977). [2] L. A. HASKIN and T. P. PASTER, Geochemistry and mineralogy of the rare earths, in Handbook on thePhysics and Chemistry of the Rare Earths, Ed. K. A. G~CHNEIDNER and L. EYRING,Vol. 3, Chap. 21, p. 1. NorthHolland, New York (1979). [3] G. N. HANSON, Ann. Rev. Earth Planet Sci. 8, 371 (1980). [4] G. N. HANSON, Accuracy in trace analysis: sampling, sample handling and analysis. NBS Spec. Pd. 422,937 (1976). [S] G. E. GORWN, K. RANDLE, G. G. COLES,J. B. CORLISS, M. H. BEESON and S. S. OXLEY, Geochim.Cosmochim Acta 32, 369 (1968). [6] S. M. MCLENNAN and S. R. TAYLOR, Chem. Geol. 15, 303 (1975). [7] J. J. CRCKK and F. E. LICME, Anal. Chem. 54, 1329 (1982). [8] S. E. CHURCH,Geostand. News&u. 5, 133 (1981). [9] J. N. WALSH,F. BUCKLEY and J. BARKER, Chem. Geol. 33, 141 (1981). [lo] A. BOLTON, J. HWANGand A. VANDERVOET,Spectrochim. Acta 388, 165 (1983). 237

238

S. J.

BUCHANAN and L. S. DALE

Although ion-exchange separation effectively isolates the REE, we investigated a procedure in which the samples were sintered with sodium peroxide then precipitated with fluoride. A similar procedure had been used previously in this laboratory for the determination of REE in rocks by neutron activation analysis [ 111. Previous work [ 12,133 has also demonstrated the suitability of the sodium peroxide sinter procedure for the decomposition of a wide range of rock types. Other published work [14] has shown that although ion-exchange separation gave a high chemical yield, more reproducible results can be obtained with fluoride precipitation although the chemical yield is lower. The procedure adopted here uses yttrium both as a carrier and as a means of determining the recoveries. Results obtained on a number of geological materials demonstrate its suitability for providing REE analytical data for geochemical investigations. 2. EXPERIMENTAL 2.1. Instrumentation The instrument consisted of a Labtest 2000 plasma source and matchbox. Light from the plasma was focussed onto the entrance slit of a 0.5 m Ebert monochromator (Jarrell-Ash Co.) fitted with a 2160 grooves/mm grating blazed at 260 nm. The reciprocal linear dispersion was 0.8 nm mn- l. Emission signals were measured with an EM1 62568 photomultiplier and displayed on a Keithley 414s picoammeter. The output from the photomultiplier was connected to a locally built STD bus-based microcomputer with a Motorola 6809 microprocessor. Wavelength selection was carried out manually and the data acquisition and processing were controlled by microcomputer. A concentric glass nebulizer (Meinhard Associates Inc.) was used for sample introduction. Operating conditions are given in Table 1. 2.2. Reagents Standard solutions of the REE and yttrium were prepared from Specpure oxides (Johnson and Matthey). Sodium peroxide was Merck G. R. grade. All acids were A.R. grade. Water of 18 MR quality was obtained from a Millipore-Q purification system. 2.3. Procedure Samples were ground to pass 325 mesh (B.S.). A 0.5-g sample was mixed with 4.0 g of Na?O, in a platinum crucible using a glass rod. A further 2.0 g of NazOz was added to cover the mixture which was then heated at 380°C for 1 h in a muflIe furnace. After removal, it was allowed to cool and 5-10 ml of water added dropwise with a watchglass covering the crucible to prevent losses. A glass rod was used to break up the sinter cake during this operation. After the reaction was complete, the contents of the crucible were transferred, by washing with water, to a 5@ml polythene centrifuge tube. The remainder of the sinter cake in the crucible was dissolved in 2 ml of 10 M HCl and transferred to the centrifuge tube. The sample was then made up to 20 ml with water; it was then shaken, centrifuged and the liquid discarded. A further 20 ml of water was added to the tube containing the residue. It was shaken, centrifuged and again the liquid discarded. The residue was then dissolved in 10 ml of 10 M HCI to

Table Power Gas flow-outer

-carrier Sample uptake Slits--entrance --exit Observation height

Entrance aperture Integration time

1. Operating conditions 1ooow 15 Imin-’

1 Imin-’ 2Smlmin-’ 0.015mm 0.020 mm 16 mm above load coil 2mm, equal to 14-18 mm in plasma.

6s

R. E. J. PORRIITand P. M. PORRITT,Rudiochem. Radioanal. ht. 31,265 (1977). [12] T. A. RAFTER,Analyst 75,485, (1950). [13] Z. SULCEK,P. POVONDRAand J. DOLEZAL,CRC Crit. Rev. Anal. Chem. 6,255, (1977). [14] S. MELONI, M. ODWNE, A. CECCHIand G. POLI, J. RudioamI. Chem. 71,429 (1982). [ll]

ICP-AES determination of rare elements

239

which was added 2 ml of 10 g I- 1 yttrium carrier solution. Ten millilitres of 40 % HF was then added and the solution shaken, warmed and allowed to stand for about 10 min. After centrifuging, the liquid was discarded. This process was repeated after adding 10 ml of 10 M HCI and 10 ml of HF. The solid residue was transferred to a platinum dish using 10 ml of water. Two millilitres of 50 VOLy0 sulphuric acid was then added and the solution fumed to near dryness. This was repeated using a further 1 ml of 18 M sulphuric acid. After cooling, 10 ml of water was added and the solution evaporated to approx. 3 ml. The solution was left to stand to allow precipitation of any excess calcium sulphate. It was filtered and washed twice with 1 ml of water and the combined filtrates diluted to 25 ml. The final solution was then used for analysis.

2.4. Wavelength selection The final solution contained approx. 680 lg ml- ’ Y (based on 85 % recovery) as well as the rare earths, calcium and small amounts of other elements such as iron, strontium and zirconium. It was therefore necessary to check for spectra1 interferences from both line overlap and scattered radiation. This was done by recording scans at about 0.5 nm intervals over a selection of the most sensitive lines while nebulixing solutions of each of the rare earths, 680 pg ml- ‘Y and 100 pg ml-’ Ca. Calcium produced a substantial scattered background in the region of many of the rare earth lines and it was not completely removed during the separation procedure. The most suitable lines were selected from line coincidence tables [ 151 and are listed in Table 2. With the exception of Nd, all of the lines were free from spectral interference from other rare earths. However, they suffered interference from other elements. Also included are the detection limits expressed as pg g- ’ in the original sample and based on 2a of the background counts. A more practical approach is to use the lower limit of determination which is taken to be five times the detection limit. All determinations were made relative to a standard containing appropriate levels of the REE in the presence of 680 pgml-‘Y. A blank Y solution of the same concentration was used to correct for spectral interferences and compensate for the presence of small amounts of rare earths in the carrier solution. It was also used to determine the chemical yield. This concentration of Y was equivalent to the average recovery of 85 % obtained for a range of samples processed. Standard solutions of Ca, Fe and Zr were also used to enable corrections to be made for line and scattered background interferences. 2.5. Rejiience samples analysed The six reference materials selected for analysis were the USGS BCR-1 (basalt), GSP-1 (granodiorite) and SGR-1 (oil shale), the South African NIM-G (granite) and the NBS coal ashes SRM Nos 1633 and Table 2. Analytical lines, detection limits and interferences

Element La

ce Pr Nd Sm EU : Ho Er Tm Yb Lu Ca Fe Zr Y

Wavelength (mm) 408.7 418.7 422.3 401.2 442.4 413.0 342.2 353.2 345.6 326.5 346.2 328.9 261.5 422.6 238.2 257.1 374.8

Detection limit (pgg-r) (based on 0.5-g sample in 25 ml)

Interference

0.2 I 1 0.4

background* ca, Y, Ce

-

& Y Y Y Y, Zr Y Y Y,Ca,Fe Nil Nil background* Nil

QY Y

*Background measured adjacent to peak. [ 151 P. W. J. M. BOUMANS, Line Coincidence Tables for Inductively-Coupled Plasma Atomic EmissionSpectrometry, Vols 1 and 2. Pergamon Press, Oxford (1980).

240

s. J. &%XANANalld

L.

s. DALE!

1633a.Of these, a great deal of published data were avaiI&le for BCR-1 and GSP-I which enabled the procedure to be assessed.Although data for the oil shale and coal ash samplesare somewhat sparse, they were selected because of their interest to gecchemical studies of energy resources. It is hoped that the results will be useful in future compilations of data for these materials.

The results for the six reference materials are presented in Table 3. For comparison, literature values are also given and include mean, usable or consensus values which have been compiled from a number of publications (see Table 3 footnote). For BCR-1 and GSP-1 the recent ~mpi~tion of GLnnNsv et nf. [16-J was used. For the other ~pl~~mpi~tions were used but these are based on limited data. IndividuaI reported results were therefore inch&d to indicate the variability of the available data. In SGR-I and 1633a data for some etements were sparse and values obtained at this laboratory using instrumental neutron activation analysis and spark source mass spectrometry have also been included. The agreement between the results obtained by this procedure and the other indicated values is excellent. For example, with the exception ofDy which is low, our results for BCR-1 and the consensus values given by GLWNEY et al. [16] (“a” values in Tabie 3) are in good agreement. The reason for the exception is not known since the Dy values obtained on the other samples analysed do not show any bias. Other possible anomalies are the results for Ho and Er on SGR-1. The precision was assessed on the basis of the number and precision of the individual measurements required to obtain a value for the net peak counts. As a further indication of the precision obtainable with ICP-AES, eight determinations of Er and Nd were made on processed laboratory samples. Er and Nd were selected because each of their lines required corrections for interferences. For Er at a concentration of 6.73 kg g - ’ (based on the solid), the precision (24 was 0.12 pg &I-’ (1.8 % relative standard deviation, RSD). At 1.35 pg g - *, it was O.lfipgg-’ (12% RSD). For Nd at 32.8pgg-‘, it was O.!53pgg-i (1.6% RSD). This demonstrates the high precision of measurement attainablq considering that the Er concentration of 1.35 pg g- ’ is only about three times the detection limit. The procedure is relatively fast and simple. No insoluble residues remained after the hydrochloric acid dissolution indicating that all the samples analysed were successfully decomposed. However the suitability of the procedure to process samples containing minerals such as zircon has not been veri&d. Very bigh decontamination factors are obtained for the major constituents of rocks (Si, Al, Fe, Mg, IQ The only sign&ant trace elements separated with the rare earths are Ba, Sr, Zr, Ca and a small amount of Fe. As previously mentioned the bulk of the Ca is removed during the final concentration step. The recoveries have been very consistent and in the range 85-87 %. Initially, Tb was used to determine the recovery and, although this was satisfactory, Y was equally as good. The agreement between our results and the published data verifies that the recoveries for all the REE are similar using the yttrium carrier. Because of the limitations imposed by the need to measure each element sequentially with our instrument, the concentrations of some of the less abundant rare earth elements at levels near crustal abundance approach the detection limit with a corresponding decline in precision. Terbium at these leveis was below the detection limit and could not therefore be determined. Using a multichannel instrument for simultaneous analysis, the final sample volume could be reduced to 5 ml with a subsequent increase in analysis levels and hence precision for less abundant elements.

[t6] E. S. GUDXEY,~. E. RUKNSand I, ~~EL~~~,~~~s~~~ Newsktr,7,3 (1983). [ 173 S. ABBEy, Gemmed. Hewlett. 6.47 (1982). [NJ E. S. GLADNEY,Anal. Wm. Acca 118, 385 (1980). 1193 R. A. NADKARNIand G. H. MORRISON,J. Radioad. Ckm. 43,347 (1978). [20] P. J. POITS, 0. W. THORPEand J. S. WATSON,Chem Geol. 34,331 (1981).

87i2

(8) (j) 80*8

(f)

79*7 83 79*1.6

(g) (h) (i)

85*3

20+ 21 fl 79*3

(e) (f)

160*4 164*12 180 184f20

53f2 53.7*0.8 415*12 406*20 197*6 210 201 34*1 38.1 38’ 34*4 144*4 152fll 146 157*3.2

Ce

22fl 24f3

6.6*0.7 6.9*0.6 55*2 51*8 20.0*0.6 21.4* 19.4 3.5*0.4 4.12 6 23*1 <100 -

Pr

64f8 65.6 76*8

77*2

64f8 63.4 94* 19

27.3*0.8 28.7i0.6 215f6 190*17 80*2 76.4 83 14.7f0.5 15.4 15’ 69*2

Nd

16.9fO.S 15fl 16.0 17*2

7.1 *to.7 6.58~tO.17 30fl 26.8f2.5 16.2f0.5 16.1 15.7 2.9*0.3 2.66 28* 2.59*0.17 15.9f0.5 12.4fl.l 13.8 15.8f0.3

Sm

3.8 *0.4

3.6*0.1 3.21*0.18 -

1.96*0.05 2.6 *o,l 2.36*0.22 0.53 *0.04 0.35 0.41 0.53 f0.05 0.50 0.54. 0.38 *0.06 2.9*0.1 2.6*0.2 276 3.0*0.5

1.8*0.1

EU

14.5f0.5 10.0* 1.0 12*1

-

12.1*0.36 15.0*0.5 15.3 17*2

12.4*0.4 8.8 f 2.3 -

5.8 f0.2 6.35f0.12 6.0*%.2 5.4*0.4 17.5fO.S (18.5’) 18.3 1.5fO.l 1.77 2.8,2.3 -

DY

12.7*0.4 11.2 -

6.4*0.2 6.68*0.13 12.7f0.4 13*2 13.4io.4 16.3* 14.6 1.7*0.1 1.93 2.0 -

Gd

2.7 f0.5

2.9 *0.3 -

2.5*0.3 1.94 -

1.3iO.l 1.25f0.14 1.4it.1 1.2*0.5 3.7 f 0.3 (4.2*) 4.0 0.12*0.05 0.39 1 -

Ho

8.0f0.8

-

7.5*0.6

-

6.9 * 0.6 Cl00

(13*) 11.1 1.9f0.2 1.11 1.3 -

4.0*0.3 3.61f0.09 3.3 fO.3 2.5*0.4 12.6iO.4

Er

1.9f0.2

1.6f0.1 -

-

1.4fO.l 1.3 -

0.22rto.05 0.14 0.4,0.17 -

0.56f0.05 0.590*0.035 0.27 *;.OS 0.45f0.12 2.2iO.l 2.3 -

Tm

8.4 f 0.9

7.OiO.2 8.2 f0.3 -

3.3*0.1 3.391tO.08 1.6fO.l 1.7io.4 13.4*0.4 14.4 13.7 0.86 f 0.03 0.97 1.W 1.16*0.08 6.7 f 0.2 5.5*2.6 7.1 6.1*0.18

Yb

-

1.10*0.05 1.21 lto.10

Sl

a = G:

ir

0

ii

s 1.5* 1.2 0.94 -

g. 2 s,

0.11 0.18f0.02 0.96 f 0.05

E % ip”

1.95 2.2 0.16*0.01 0.15

0.512*0.025 0.26 f 0.02 0.22iO.05 1.90f0.07

0.55*0.03

Lu

(1) This work; (2) other values. (a) GLADNEY et al. [16]; (b) Ports et al. [ZO]. Values with asterisk indicates results obtained by interpolation from chondrite normahmd curve. Bracketed values have extra uncertainty. (c) WALSHet al. [9], (d) MCLENNONand TAYLOR[6]; (e) ABBEY[ 17]--*indicates probable values, others are individual laboratory results; (f) instrumental neutron activation analysis values Lucas Heights Research Laboratories (g) GWDNEY [18]; (h) NAD~ARNIand MORR~XIN[19]; (i) CHURCH [8]; (j) Spark source mass spectrometry values at Lucas Heights Research Laboratories.

1633 a

1633

SGR-1

NIM-G

(c) 109 19.8f0.6 (d) 20.5

26.8 f 0.8

(a) 25.0f0.08 191 f6 (a) 186*13 117*4 (b) 114

1

2

BCR-1

GSP-1

La

Sample

Table 3. Rare earth element concentrations found (pg g-t)

242

S. J. BUCHANANand L. S. DALE

4. CONCLUSION The determination of the REE in a range of geological materials by ICP-AES has been facilitated by the use of a chemical separation procedure based on sodium peroxide sintering and fluoride precipitation. Because of the inherent high precision of ICP-AES, the data are satisfactory for studies of petrogenesis. Acknowledgements-The

authors wish to thank J. J. FARDY and I. M. WARNER for neutron activation analysis and

spark source mass spectrometry results respectively.