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The simultaneous determination of the rare-earth elements in rocks using inductively coupled plasma source spectrometry
Chemical Geology, 33 (1981) 141 --153 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
14 l
THE SIMULTANEOUS DETERMINATION OF THE RARE-EARTH ELEMENTS IN ROCKS USING INDUCTIVELY COUPLED PLASMA SOURCE SPECTROMETRY
J.N. WALSH', F. BUCKLEY: and J. B A R K E R '
'Department of Geology, King's College, University of London, London WC2R LS (Great Britain) :Department of Earth Sciences, University of Leeds, Leeds LS2 9JT (Great Britain) (Received February 18, 1981 ; accepted for publication June 2, 1981 )
ABSTRACT
Walsh, J.N., Buckley, F. and Barker, J., 1981. The simultaneous determination of the rare-earth elements in rocks using inductively coupled plasma source spectrometry. Chem. Geol., 33: 141--153. A method has been developed that enables the rare-earth elements (REE's) in terrestrial rocks to be measured simultaneously using the inductively coupled plasma (ICP) source spectrometer. The use of conventional rock dissolution procedures, followed by a simple cation separation enables REE concentrations to be determined down to, or below, the chondritic abundance level. The choice of spectral lines for the ICP determination of the REE's is evaluated. A precision of 1--2% (relative standard deviation) was attained and this, together with the new values obtained for six international standard rocks, suggests that the method is well suited to the routine simultaneous determination of all the REE's, excluding only Tb and Tm. The extension of the use of the ICP source spectrometer to the determination of the REE's, from its existing established use in routine analysis of silicates for major and trace elements, suggests that ICP spectrometry may now be considered as a standard technique in modern geochemical analysis, enabling a greater range of element determinations to be more readily available.
INTRODUCTION The advancement of our understanding of the processes by which rocks have formed depends upon the availability of accurate analytical data. These m u s t b e a v a i l a b l e in s u f f i c i e n t q u a n t i t y t o e n a b l e m o d e l s t o b e p r o p o s e d f o r , and constraints placed upon, possible modes of origin for the rocks. Relative isotopic abundances for particular elements, the concentrations of major- or trace-element constituents or specifically the rare-earth element (REE) p a t t e r n m a y b e u s e d . I n f o r m a t i o n o n all o f t h e s e m a y c o n t r i b u t e a n d i d e a l l y all s h o u l d b e a v a i l a b l e ; in p r a c t i c e t h i s is r a r e l y p o s s i b l e . I n m a n y i n s t a n c e s a k n o w l e d g e o f t h e R E E a b u n d a n c e s is e s p e c i a l l y v a l u a b l e . T h e u s e f u l n e s s o f t h e s e e l e m e n t s d e r i v e s f r o m t h e c l o s e s i m i l a r i t i e s in c h e m i c a l p r o p e r t i e s a n d t h e g r a d u a l c h a n g e s in i o n i c r a d i i f o r t h e t r i v a l e n t
0009-2541/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
142
cations in octahedral coordination. In addition to the progressive decrease in radius from 1.03 A for La to 0.861 A for Lu (Shannon, 1976), there is the special feature of Eu geochemistry, the ability to exist in the divalent as well as the more common trivalent state. Ce can be trivalent or tetravelent, depending upon the oxidising conditions. The analytical problems in the determination of the REE's in terrestrial rocks are considerable, especially for the "heavy" rare earths (HREE's) where it is necessary to measure at, or below, 0.1 ppm in the rock. The most widely used method at present would appear to be instrumental neutron activation (Gordon et al., 1968; Voider and Haerdi, 1978; Whitley et al., 1979). The alternatives to neutron activation analysis include isotope dilutionmass spectrometry (Hooker et al., 1975) and spark-source mass spectrography (Strelow and Jackson, 1974; Taylor and Gorton, 1977) and possibly proton-induced X-ray emission (PIXE). All these techniques have both advantages and disadvantages, but few would dispute the desirability of investigating alternative methods of analysis. Inductively coupled argon plasma atomic emission spectrometry has been shown to have considerable sensitivity for the measurement of REE's (Broekaert et al., 1979; Nikdel et al., 1979), and the extension of the inductively coupled plasma (ICP) source spectrometry technique to the routine determination of these elements is a logical development from the established use of ICP spectrometry in the analysis of rocks and minerals for major and trace constituents (Brenner et al., 1980; Walsh, 1980; Walsh and Howie, 1980). ICP spectrometry thus offers the possibility of determining major, conventional trace, and REE concentrations on a single instrument, with consequent savings in time and costs. It is necessary to separate and concentrate the REE's before measurement, but it is possible to combine this with the same sample decomposition procedure which is used for the determination of the traces and almost all of the major elements. This, together with the exceptionally small amount of instrument time required, typically 1--2 rain. to acquire the simultaneous output of results directly as concentrations, can extend the range and productivity of the geochemical laboratory. The present paper describes the development of a procedure to enable the routine and rapid determination of the REE's which offers significant improvements in comparison with alternative methods. EXPERIMENTAL
ICP spectrometry is a solution technique and requires that the elements to be measured are taken into solution quantitatively. The standard HF-HC104 attack (Riley, 1958) was used followed by dissolution in HC1. This procedure may fail to completely dissolve certain resistant minerals which can contain significant quantities of REE's (notably the mineral zircon) and it is advisable to take the residue into solution by fusion with NaOH or KHF2.
1,13 To obtain the low detection limits required for the HREE's a simple cation-exchange procedure, described by Strelow and Jackson (1974), was used to separate the REE's from the bulk constituents of the rock. The REE's from the rock are concentrated into a small volume of solution and measured simultaneously on the ICP spectrometer. Comparative tests were carried out following the procedure given by Strelow and Jackson (1974), using 1-g rock portions and resin beds 19 cm long, 2 cm diameter; and a variation using 0.5 g of rock and beds 10 cm long, 2 cm diameter. Satisfactory results were obtained with the shorter columns and this method is described here.
Dissolution procedure 0.5 g of powdered rock is weighed accurately into a Pt crucible or dish, 4 ml HClO4 and 15 ml HF (40%} added and the solution evaporated to incipient dryness. The residue is dissolved in 20 ml warm 25% HCI, filtered and washed through Whatman ® No. 42 filter paper. The precipitate is ignited to 800°C in a Ag crucible, fused for 20 min. with 0.5 g NaOH and after cooling dissolved in 20 ml HC1 {25%). Alternatively, the precipitate may be ignited in a Pt crucible, and then fused with KHF~ on a Bunsen, gradually increasing the flame temperature. The fused mixture is then evaporated to dryness on a sand-bath after adding 4 ml HCIO4 (to remove HF) and then dissolved in 20 ml of warm 25% HC1. The most important REE-bearing mineral which is not completely attacked by the HF--HClO4 treatment is zircon, and both KHF2 and NaOH fusions will attack this mineral. Satisfactory REE results were obtained with either fusion technique for a wide range of rock compositions; the NaOH fusion is somewhat more rapid than the KHF2 fusion. If required the solutions obtained after the filtration and the fusion stages can be diluted to 50 ml and aliquots removed and used for the determination of the trace- and major-element concentrations, excluding SiO2 (Walsh and Howie, 1980).
REE separation Chromatographic glass columns 20 mm I.D., 250 mm length, with a glass sinter disc and a PTFE burette tap at the b o t t o m were used. 20 g of resin (Dowex °~ AG 50W-X8,200--400 mesh from BioRad Laboratories) was loaded onto the columns, giving a settled height of 10 cm. After washing with 4 N HC1, and rinsing with 1 N HCI, the fusion solution, and the filtration solution (both diluted to ensure the acid strength is less than 10%), are loaded onto the columns. The resin is then washed with 400 ml 1.7 N HC1; this fraction which contains all the major constituents and most of the trace elements, is discarded. The REE's are held quantitatively on the resin (together with all the Ba, and some Sr, Zr and Hf) and are eluted with 500 ml of
144 TABLE I Elutions of the major and trace constituents in the simultaneous determination of R E E ' s i n r o c k s using i n d u c t i v e l y c o u p l e d plasma s o u r c e s p e c t r o m e t r y Amount present in rock
P e r c e n t a g e r e c o v e r e d in successive lO0-ml p ortio.ns_ o f 1.7 NH_C~_". . . . . . .
.
100 m]
200 ml
300 ml
400 ml
500 ml
38 <0.2 <0.l <0.2 29 8 77 -
55 54 89 <0.2 67 67 20 100 81
6.5 45.5 10 72 3 16 2 . . 19
1.5 0.6 0.2 27 0.3 8 <0.5 . . . . .
0.2 <0.2 0.1 0.2 <0.2 <2 <0.5 . . . .
Percentage recovered in 500 ml o f 4 : \ HC1
(wt. %): AI203 Fe203 MgO CaO Na20 K20 TiO 2 P20$ MnO
17.4 9.9 7.7 11.9 (,) 0.2 0.8 0.09 0.13
--
<0.1 ~0.1 <0.1 <0.2 <0,2 <2 <0.5
(ppm): Ba Co Cr Cu Ni Sr V
126 45 330 170 104 360 220
. . 15 11 0.2 2 -
.
. 51 32 94 55 -94
.
.
. 22 14 5 30 0.6 4
.
.
. . 11 -42 2 0.2 <0.1 13 <0.2 32 62 . . . .
100 -<0.1 <02 5 .
* R o c k c o n t a i n s 2.4% N a 2 0 ; in a d d i t i o n 0.5 g o f NaOH were a d d e d during the fusion stage.
4 N HC1. The solution is filtered through a Whatman ® No. 42 filter paper, to remove particles of resin, and then evaporated to dryness. The REE's are subsequently redi~olved in 5 ml of 10~o HC1 and injected into the ICP. The efficiency of this ion~exchange separation procedure was tested by loading onto the columns a standard rock sample and collecting 100 ml aliquots of the 1.7 N HCI solution as they were eluted from the columns. The percentages of the major- and selected trace-element constituents in each fraction were determined (using the ICP instrument) and are given in Table I. Table I also gives the amounts recovered in the whole of the 500 ml of the 4 N HC1 portion in which the REE's are eluted, together with the concentration levels originally present in the rock (the amounts in solution can be estimated from the dilutiona given in the above preparation procedure). The results in Table I confirm the efficiency of the ion-exchange procedure. For the major elements, only Ca w u detected in the h a t 100 ml of 1.7 N HCI eluant. F o r the trace elements, Table I shows that Ba is recovered quantitatively with the REE's together with some of the Sr.
145 REE DETERMINATION The background to the development of the ICP spectrometer is well documented (e.g., Fassel and Kniseley, 1974; Boumans and de Boer, 1975; Greenfield et al., 1975; Boumans, 1978; Fassel, 1978), and the application to the determination of major- and trace-element constituents in rocks and minerals has been described (Brenner et al., 1980; Walsh, 1980; Walsh and Howie, 1980). The ICP m e t h o d of analysis is essentially a " f l a m e " technique, and the flame temperature into which the analyte is injected is so high (6000--10,000 K) that not only are many spectral lines excited but also many of these lines are emitted by ionic rather than atomic species. This is important for the determination of the REE's, many of which have complex ionic spectra giving a wide choice of sensitive spectral lines, but also considerable problems with spectral overlaps. Much information is available on the spectral lines emitted by the REE's, and data have been published on the sensitivities of some lines in the ICP (Fassel and Kniseley, 1974; Broekaert et al., 1979; Nickdel et al., 1979; Boumans, 1981) but no detailed study appears to have been made of the spectral lines for geological use. The instrumentation used in this work (the Philips '~) P V 8 2 1 0 1.5-m ICP spectrometer) is well suited to evaluating spectral lines under analytical conditions with its internally m o u n t e d high-resolution "roving detector". This enables potentially usable spectral lines to be evaluated under "analytical" conditions. The Philips ~) plasma system has been described in detail in several publications, including Boumans (1978). Selection of operating parameters (gas flow rates, observation height, power, etc.) proved relatively straightforward. Since the elements are to be TABLE II Operating parameters for REE determination on the ICP spectrometer .
.
.
.
.
.
.
.
.
.
.
.
Plasma gas flow Auxiliary gas flow Carrier gas flow
.
.
.
.
.
.
.
.
.
15 I min.-I 0 I rain.-1 1.2 l min.-1
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Sample uptake rate Observation zone above coil Power (forward power)
2 ml rain. 15--19mm 1.0 kW (50 MHz)
measured simultaneously it is necessary to obtain operating conditions suitable for all elements. This inevitably results in some compromises but sensitivities for all the REE's were sufficient to accept slightly less than "ideal" settings. Table II gives the settings used, these being somewhat different to those used for the major- and trace-element analysis (Walsh and Howie, 1980). Specifically, the lower-power setting used results in improvements in the signal/background ratio (i.e. detection limits) although the rate of improvement in signal/background ratio varies substantially from element to element as power is lowered.
146 SELECTION OF SPECTRAL LINES The spectral lines used were selected after detailed studies on the suitability of the m a n y lines avAilAble for each REE. The factors which influence line selection for ICP studies are complex and limited data are available. It was necessary to take into account: (1) analytical sensitivity, and the geological abundance which varies substantially from element to element; (2) spectral interferences; and (3) complexities of m o u n t i n g the lines into the spectrometer where m a n y other elemental lines were already installed. Although the lines are therefore n o t necessarily the best for all the REE's and for non-geological applications alternative factors might have to be considereal, they do represent the result of a detailed study which does n o t appear to have been previously undertaken. Therefore some general comments for each element m a y be useful. It is worth emphasising that the "interferences", referred to below, are cases of spectral overlap and their effects are both predictable and linear; in all cases tested doubling the concentration of the interfering element doubled the interference. They may therefore be readily corrected for; furthermore in most cases the interferences are small. In general, ICP spectrometry has a greater freedom from interference effects, especially matrix effects than most other analytical techniques. It should, however, be noted that the spectral overlap effects are a feature of the instrument used, and certainly use of a lower-resolution spectrometer would increase the interferences. It is of course also possible that higher resolution would decrease the interferences.
Lanthanum. This is one of the more abundant REE's with a chondritic abundance (Nakamura, 1974) of 0.329 ppm (the abundance in a chondritic meteorite may be taken as the lowest level which one might reasonably be expected to determine). The La 398.85-nm line was used in this study and other lines (i.e. the more sensitive La 408.67-nm line) were not studied. The 398.85-nm line is satisfactory for geological use with adequate sensitivity. The only interference detected was a very small interference from Ca, which was n o t large enough to be worth correcting for. Cerium. The most sensitive Ce line is at 418.66 nm and although Ce is the most abundant REE in geological samples (chondritic abundance of 0.865 ppm) the element is one of the least sensitive of the REE's in the ICP. The 418.66-nm line gave good results with no significant interferences. Praesodymium. The 422.20-nm line was used although it does have several small spectral interferences, for which corrections were made. 10 ppm Ce gave a signal equivalent to 0.2 ppm Pr, 10 ppm Sr gave a signal equivalent to 0.90 ppm Pr, and 10 ppm Ca (Ca is always f o u n d in trace amounts in the REE solutions) gave a signal equivalent to 0.02 ppm Pr. The adjacent Pr line at 422.54 nm, which is slightly more sensitive, was also investigated but has a serious Sm interference.
147
Neodymium. This is one of the more a b u n d a n t REE (with a chondritic abundance of 0.63 ppm} and its de t e r m i na t ion should present no problem using the preparation procedure described in this work. The Nd 406.11-nm line was used initially with the roving d e t e c t o r assembly, and gave good results. However, it proved more convenient to install the more sensitive Nd 403.36-nm line in the spectrometer. Although this line is quite satisfactory it has a trace interference from Ba, and a higher background, and probably the 406.11-nm line would be preferable.
Samarium. Great difficulties were experienced in the selection of a suitable Sm line. Although m a ny lines have an adequate sensitivity, interferences from Ce or Nd were f o und with almost all the lines tested. The Sm 359.26-nm line was eventually selected, although it has interferences from Nd (10 ppm Nd gives a signal equivalent to 0.38 ppm Sm) and Gd (10 ppm Gd gives a signal equivalent to 0.82 ppm Sm). These interferences must be corrected for. One o f the few ot he r Sm lines t ha t was found to be suitable was the Sm 373.92 nm line; although free from Ce and Nd interferences, it is a less sensitive line and has a Pr interference.
Europium. This element has a chondritic abundance of only 0.077 ppm, but its ability to exist in both di- or tri-valent states and the implications of possible Eu anomalies makes its det er m i nat i on in geological materials essential. F o r t u n a t e l y it is, with the e xc e pt i on of Yb, the most sensitive of all the REE's in the ICP spectrometer. T he Eu line at 381.97 nm was used and this gave excellent results; although o t h e r sensitive Eu lines exist they were not investigated.Eu 381.97-nm has a trace Nd interference (10 ppm Nd gives a signal equivalent to 0.023 ppm Eu} and this should be corrected for.
Gadolinium. The 335.05-nm Gd line was used although it has a small interference from Ca (10 ppm Ca giving a signal equivalent to 0.14 ppm Gd). The o t h e r Gd lines investigated appear to have more serious interferences, Gd 376.85 nm has Ce and Nd interference, Gd 342.25 nm has Ce interference and Gd 336.22 nm has Y interference. Dyprosium. The most sensitive Dy line (353.17 nm) was used and found to be essentially free from interferences giving excellent results.
Holmium. Ho 345.6 nm is the most sensitive line and no significant interferences were recorded. The low chondritic abundance of this element (0.076 ppm} makes the use of this line (preferably in the first order} almost obligatory for geological use.
Erbium. Both the Er 390.63-nm and Er 369.27-nm lines have been used, and either will produce satisfactory results. The sensitivity of Er in the ICP is n o t as high as might be h o p e d for but the results may be regarded as adequate.
148
Ytterbium. This is the most smutitive R E E in the ICP and it is possible to measure d o w n to, and below, chondritic abundances (0.220 ppm) with the described sample preparation method. The Y b 328.94 n m was used satisfactorily. Other lines of Y b were not studied but probably several would have adequate sensitivity.
Lutetium. The chondritic abundance of this element is low (0.0339 ppm), but as the heaviest REE its satisfactory determination would be a real bonus. Fortunately, the Lu 261.5-rim line is very sensitive in the ICP and is free from significant interferences. The other Lu lines are much less sensitive and the use of the Lu 261.54-nm line, in the first order, is most desirable for geological work. EVALUATION This was carried o u t in three stages: (1) Recovery of the elements after the ion~xchange procedure was tested. (2) Precision tests were made with regard to the instrument and to the whole procedure. (3) The accuracy was tested by analysis of international rock standards.
(1) Column recoveries. Column recoveries were tested by preparing two identical sets of standard solutions and submitting one set to the given ionexchange procedure. Following dilution to constant volume, both sets were measured. The percentage recoveries obtained and the amounts used are set out in Table III. These confirm the results of Strelow and Jackson (1974) that the recovery is complete. Where significant deviations occur, these are due to the loss in precision when working at, or in some cases below, the detection limit. (2) Precision. Fig. 1 shows the results of repeated (N = 11) measurements on a mixed standard solution o f some o f the REE and is a plot of relative standard deviation against concentration. Missing values are due to the signal being too high for measurement. For the ranges of concentration most likely to be encountered the precision is ~ 1--2%. Table IV summarises fifteen separate analyses of one rock sample. The results show that precision is significantly better for the HREE's and that it is worse than 5% only in the case of La. The values of 20 in the table may be regarded as estimates of the detection limits o f the instnlment and as such may be considered satisfactory, especially for the " m i d d l e " and " h e a v y " lanthanides. With the exceptions of La and (marginally Pr) all of these "detection limits" are below chondritic abundance values. (3) Accuracy. Assessing the accuracy of an analytical m e t h o d for rock analysis is a problem, especially when dealing with a suite of elements such as the
1,19
T A B L E III Percentage recoveries and a m o u n t s of R E E ' s used Solution
l
2
3
4
Percentage REE recovery: La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
103 99 115 98 99
101 -101 101 101
I03 99 98 102 98
111 -98 102 98
100 101 105 100 100 100
100 101 101 101 101 101
100 107 102 104 100 101
100 106 102 98 102 103
Concentrations (ppm in solutions): La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
10 20 2 10 2 1 2 1 1 1 1
5 0 1 5 1 0.5 1 0.5 0.5 0.5 0.5
1 2 0.2 1 0.2 0.1 0.2 0.1 0.1 0.1 0.1
0.5 0 0.1 0.5 0.1 0.05 0.1 0.05 0.05 0.05 0.05
% RSD l
\
,,. Gd 335 05nm \
6
c Pr
\ I
5--
422 30nm
+ Lu 261 54 nm
\
•
\
Yb 328 93rim
o Eu 381 96nm
3~
\ \~
2_; • ~ ..
I
\
~ ~ ~ ~ ^ \
~
~:,
+
h--
1
T
0 01
0 1
IO ppm
10
in solution
Fig. 1. The plots s h o w the i m p r o v e m e n t in precision with increasing c o n c e n t r a t i o n level for selected rare-earth elements.
150 T A B L E IV Total p r e c i s i o n t e s t - - s a m p l e SD180
La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
M e a n (15) (ppm)
20
5.83 12.3 1.89 9.64 2.48 0.90 2.59 2.92 1.75 1.79 0.28
0.75 0.86 0.16 0.67 0.11 0.02 0.14 0.05 0.08 0.04 0.01
ID--MS
RSD
(%) 6.5 3.5 4.3 3.5 2.2 1.1 2.7 0.9 2.3 1.1 1.8
-13.3 -. 8.99 2.5 0.94 2.94 3.15 1.98 1.88 --
The 15 determinations were m a d e using the 20-cm-length resin beds. R S D = relativestandard deviation; ID--MS = isotope dilution--mass spectrometry.
REE's. The conventional comparison between the new measurements and accepted values for international standard rock samples is not always safe in that varying degrees of certainty are ascribed to the compiled results. Table V shows new routine determinations obtained using the described procedure (with NaOH fusion of residues) for the U.S.G.S. standard rocks W-l, G-2, GSP-I, AGV-1 and BCR-1 together with the South African NIM~3 and the quoted values of Flanagan (1973), and Jackson and Strelow (1975). The overall agreement is good, with no evidence of serious or systematic error. The smooth ehondrite normalised patterns obtained for these rocks (Fig. 2) supports the credibility of the ICP results.
"_~,o~I ,...:"-,..
\\ '-..
".9 ...... t . ' c ~
"\
N,~-G
',
~..
-
~'--- - < ~ - - - - ~ -
~t. ~
I
i
L~I
Ce
"
I
Pr
V
Nd
"
I
I
"I
Sm
Eu
" ]
Gd
-'-f
""
" T . . . . . . l- . . . . . T
Dy
Ho
Er
'
~
I
. ,G-2,~
I
Yb
- ~
Lu
Fig. 2. C h o n d r i t e - n o r m a l i s e d p l o t s f o r i n t e r n a t i o n a l s t a n d a r d rocks.
151 TABLE V REE concentrations (in ppm) for selected international standard rock samples (values in parentheses from the compilation by Flanagan 1973 for U.S.G.S. rocks, and Jackson and Strelow 1975 for NIM-G) q
W-1
BCR-1
AGV-I
GSP-1
G-2
NIM-G
La
10.5 (9.8)
26 (26)
37 (35)
180 (191)
83 (96)
109 (105)
Ce
23.7 (23)
53.2 (54)
66.7 (63)
423 (394)
153 (150)
201 (195)
Pr
3.0 (3.4)
6.8 (7)
8.1 (7)
55 (50)
16.0 (19)
19.4 (18.9)
Nd
14.7 (15)
33.5 (29)
39 (39)
234 (188)
65.5 (60)
83 (73)
Sm
3.6 (3.6)
7.1 (6.6)
5.9 (5.9)
27 (27.1)
7.0 (7.3)
15.7 (15.5)
Eu
2.09
1.66
(1.ll) (1.94)
(1.7)
(2.4)
(1.5)
0.41 (0.39)
Gd
3.6 (4)
4.8 (5.5)
12.7 (15)
4.1 (5)
14.6 (10.9)
3.85 6.4 (4) (6.3)
3.56 (3.5)
6.24 (5,4)
2.28 18.3 (2.6) (15.8)
Ho
0.81 1.42 (0.69) (1.2)
0.79 (0.6)
1.33 0.50 4.0 (--) (0.4) (3.3)
Er
2.2 (2.4)
2.2 (1.2)
3.33 (3.0)
Yb
2.26 3.50 (2.1) (3.36)
1.74 (1.7)
1.81 0.84 (1.8) (0.88)
Lu
0.32 0.55 (0.35) (0.55)
0.27 (0.28)
0.27 0.13 1.95 (0.23) (0.11) (2.1)
By
1.13
7.0 (6.6)
3.87 (3.59)
2.51
1.38
1.2 (1.3)
11.1 (10.0) 13.7 (13.3)
CONCLUSIONS T h e m e t h o d f o r l a n t h a n i d e d e t e r m i n a t i o n s d e s c r i b e d in this p a p e r o f f e r s a n u m b e r o f a d v a n t a g e s . All o f the e l e m e n t s ( p r e s e n t l y e x c l u d i n g T b and T m ) are m e a s u r e d s i m u l t a n e o u s l y and in spite o f the t i m e r e q u i r e d for c h e m i c a l p r e - t r e a t m e n t it is a rapid p r o c e d u r e . Up to 30 s a m p l e s per w e e k m a y be a n a l y s e d r o u t i n e l y , d e p e n d i n g on the n u m b e r o f c o l u m n s e m p l o y e d . Ins t r u m e n t t i m e is very small, 1 - - 2 m i n . per s a m p l e , h e n c e costs are low. Samples m a y be p r e p a r e d in a n y r e a s o n a b l y e q u i p p e d l a b o r a t o r y and t a k e n to an ICP installation if o n e is n o t available in the u s e r ' s D e p a r t m e n t . It is t h e r e f o r e possible to have R E E d e t e r m i n a t i o n s as freely available as m a j o r - e l e m e n t analyses. An i m p o r t a n t a s p e c t o f t h e w o r k is t h a t a single p o r t i o n o f dissolved
152
sample m a y be divided to obtain lanthanide, trace- and major-element components (excluding SiO2 and alkalis). The development of ICP spectrometry is important in geological analysis as it is n o w possible to use one piece of capital equipment for an extremely wide range of elements. ACKNOWLEDGEMENTS
The authors are grateful to N.E.R.C. and King's College London for providing the funds for the purchase of the ICP source spectrometer. The development of the rare earth element method was supported by the instrument manufacturers, Philips, and their financial assistance in installingseveral additional spectral lines, and the assistance of several members of Phflips staff, contributed towards the project. They should also like to thank Professor R.A. Howie for his support and advice. Dr. C. Hawkesworth is thanked for sample, and data on, S D 180. Finally, the authors thank Ms W. Everett for typing the manuscript.
REFERENCES Boumans, P.W.J.M., 1978. ICP atomic emission spectrometry - - a multielement analysis method for liquid and dissolved solids. Sci. Ind. (Philips, Eindhoven), No. 12, p. 1. Boumans, P.W.J.M., 1981. Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry. Pergamon, London, 981 pp. Boumans, P.W.J.M. and de Boer, F.J., 1975. Studies of an inductively-coupled high frequency argon plasma for optical emission spectrometry, II. Compromise conditions for simultaneous multielement analysis. Spectrochim. Acta, 30B: 309--334. Brenner, I.B., Watson, A.E., Russell, G.M. and Goncalves, M., 1980. A new approach to the determination of the major and minor constituents in silicate and phosphate rocks. Chem. Geol., 28: 321--330. Broekaert, J.A.C., Leis, F. and Laqua, K., 1979. Application of an inductively coupled plasma to the emission spectroscopic determination of rare earths in mineralogical samples. Spectrochim. Acta, 34B: 73---84. Fassel, V.A., 1978. Quantitative elemental analyses by plasma emission spectroscopy. Science, 202: 183--191. Fassel, V.A. and Kniseley, R.N., 1974. Inductively coupled plasma optical emission spectroscopy. Anal. Chem., 46: 1110A--1121A. Flanagan, F.J., 1973. 1972 values for international geochemical references samples. Geochim. Cmmochim. Acta, 37: 1189--1200. Gordon, G.E., Randle, K., Coles, G.G., Corli~, J.B., Beeson, M.H. and Oxley, S.S., 1968. Instrumental activation a n a l y ~ of standard rocks with high resolution -/-ray detectors. Geochim. Comnochim. Acta, 32: 369--396. Greenfield, S., Jones, I.L., McGeachin, H_M. and Smith, P.B., 1975. Automatic multisample simultaneous multielement analysis with a HF (high frequency) plasma torch and direct reading spectrometer. Anal. Chim. Acta, 74: 225--245. Hooker, P.J., O'Nions, R.K. and Pankhurst, R.J., 1975. Determination of rare~arth elements in USGS standard rocks by mixed-solvent ion exchange and mass-spectrometric isotope dilution. Chem. Geol., 16: 189--196.
153 Jackson, P.F.S. and Strelow, F.W.E., 1975. The rare-earth content of the six international geological reference materials of South African origin. Chem. Geol., 15: 303--307. Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta, 38: 575--775. Nikdel, S., Massoumi, A. and Winefordner, J.D., 1979. Detection limits of rare earths by inductively coupled plasma atomic emission spectroscopy. Microchem. J., 24: 1--7. Riley, J.P., 1958. The rapid analysis of silicate rocks and minerals. Anal. Chim. Acta, 19: 413--428. Shannon, R.D., 1976. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., 32: 751--767. Strelow, F.W.E. and Jackson, P.F.S., 1974. Determination of trace and ultra-trace quantities of rare-earth elements by ion exchange c h r o m a t o g r a p h y ~ a s s spectrography. Anal. Chem., 46: 1481--1486. Taylor, S.R. and Gorton, M.P., 1977. Geochemical application of spark source mass spectrography, III. Element sensitivity, precision and accuracy. Geochim. Cosmochim. Acta, 41: 1375--1380. Voldet, P. and Haerdi, W., 1978. Determination of rare-earth elements in rocks by neutron activation followed by high-resolution X-ray spectrometry or ~-spectrometry. Anal. Chim. Acta, 97: 185--189. Walsh, J.N., 1980. The simultaneous determination of the major, minor and trace constituents of silicate rocks using inductively coupled plasma spectrometry. Spectrochim. Acta, 35B: 107--111. Walsh, J.N. and Howie, R.A., 1980. An evaluation of the performance of an inductively coupled plasma source spectrometer for the determination of the major and trace constituents of silicate rocks and minerals. Mineral. Mag., 43: 967--974. Whitley, J.E., Moyes, A.B. and Bowden, P., 1979. Determination of rare earths in geological samples by neutron activation analysis. J. Radioanal. Chem., 48: 147--158.
14 l
THE SIMULTANEOUS DETERMINATION OF THE RARE-EARTH ELEMENTS IN ROCKS USING INDUCTIVELY COUPLED PLASMA SOURCE SPECTROMETRY
J.N. WALSH', F. BUCKLEY: and J. B A R K E R '
'Department of Geology, King's College, University of London, London WC2R LS (Great Britain) :Department of Earth Sciences, University of Leeds, Leeds LS2 9JT (Great Britain) (Received February 18, 1981 ; accepted for publication June 2, 1981 )
ABSTRACT
Walsh, J.N., Buckley, F. and Barker, J., 1981. The simultaneous determination of the rare-earth elements in rocks using inductively coupled plasma source spectrometry. Chem. Geol., 33: 141--153. A method has been developed that enables the rare-earth elements (REE's) in terrestrial rocks to be measured simultaneously using the inductively coupled plasma (ICP) source spectrometer. The use of conventional rock dissolution procedures, followed by a simple cation separation enables REE concentrations to be determined down to, or below, the chondritic abundance level. The choice of spectral lines for the ICP determination of the REE's is evaluated. A precision of 1--2% (relative standard deviation) was attained and this, together with the new values obtained for six international standard rocks, suggests that the method is well suited to the routine simultaneous determination of all the REE's, excluding only Tb and Tm. The extension of the use of the ICP source spectrometer to the determination of the REE's, from its existing established use in routine analysis of silicates for major and trace elements, suggests that ICP spectrometry may now be considered as a standard technique in modern geochemical analysis, enabling a greater range of element determinations to be more readily available.
INTRODUCTION The advancement of our understanding of the processes by which rocks have formed depends upon the availability of accurate analytical data. These m u s t b e a v a i l a b l e in s u f f i c i e n t q u a n t i t y t o e n a b l e m o d e l s t o b e p r o p o s e d f o r , and constraints placed upon, possible modes of origin for the rocks. Relative isotopic abundances for particular elements, the concentrations of major- or trace-element constituents or specifically the rare-earth element (REE) p a t t e r n m a y b e u s e d . I n f o r m a t i o n o n all o f t h e s e m a y c o n t r i b u t e a n d i d e a l l y all s h o u l d b e a v a i l a b l e ; in p r a c t i c e t h i s is r a r e l y p o s s i b l e . I n m a n y i n s t a n c e s a k n o w l e d g e o f t h e R E E a b u n d a n c e s is e s p e c i a l l y v a l u a b l e . T h e u s e f u l n e s s o f t h e s e e l e m e n t s d e r i v e s f r o m t h e c l o s e s i m i l a r i t i e s in c h e m i c a l p r o p e r t i e s a n d t h e g r a d u a l c h a n g e s in i o n i c r a d i i f o r t h e t r i v a l e n t
0009-2541/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
142
cations in octahedral coordination. In addition to the progressive decrease in radius from 1.03 A for La to 0.861 A for Lu (Shannon, 1976), there is the special feature of Eu geochemistry, the ability to exist in the divalent as well as the more common trivalent state. Ce can be trivalent or tetravelent, depending upon the oxidising conditions. The analytical problems in the determination of the REE's in terrestrial rocks are considerable, especially for the "heavy" rare earths (HREE's) where it is necessary to measure at, or below, 0.1 ppm in the rock. The most widely used method at present would appear to be instrumental neutron activation (Gordon et al., 1968; Voider and Haerdi, 1978; Whitley et al., 1979). The alternatives to neutron activation analysis include isotope dilutionmass spectrometry (Hooker et al., 1975) and spark-source mass spectrography (Strelow and Jackson, 1974; Taylor and Gorton, 1977) and possibly proton-induced X-ray emission (PIXE). All these techniques have both advantages and disadvantages, but few would dispute the desirability of investigating alternative methods of analysis. Inductively coupled argon plasma atomic emission spectrometry has been shown to have considerable sensitivity for the measurement of REE's (Broekaert et al., 1979; Nikdel et al., 1979), and the extension of the inductively coupled plasma (ICP) source spectrometry technique to the routine determination of these elements is a logical development from the established use of ICP spectrometry in the analysis of rocks and minerals for major and trace constituents (Brenner et al., 1980; Walsh, 1980; Walsh and Howie, 1980). ICP spectrometry thus offers the possibility of determining major, conventional trace, and REE concentrations on a single instrument, with consequent savings in time and costs. It is necessary to separate and concentrate the REE's before measurement, but it is possible to combine this with the same sample decomposition procedure which is used for the determination of the traces and almost all of the major elements. This, together with the exceptionally small amount of instrument time required, typically 1--2 rain. to acquire the simultaneous output of results directly as concentrations, can extend the range and productivity of the geochemical laboratory. The present paper describes the development of a procedure to enable the routine and rapid determination of the REE's which offers significant improvements in comparison with alternative methods. EXPERIMENTAL
ICP spectrometry is a solution technique and requires that the elements to be measured are taken into solution quantitatively. The standard HF-HC104 attack (Riley, 1958) was used followed by dissolution in HC1. This procedure may fail to completely dissolve certain resistant minerals which can contain significant quantities of REE's (notably the mineral zircon) and it is advisable to take the residue into solution by fusion with NaOH or KHF2.
1,13 To obtain the low detection limits required for the HREE's a simple cation-exchange procedure, described by Strelow and Jackson (1974), was used to separate the REE's from the bulk constituents of the rock. The REE's from the rock are concentrated into a small volume of solution and measured simultaneously on the ICP spectrometer. Comparative tests were carried out following the procedure given by Strelow and Jackson (1974), using 1-g rock portions and resin beds 19 cm long, 2 cm diameter; and a variation using 0.5 g of rock and beds 10 cm long, 2 cm diameter. Satisfactory results were obtained with the shorter columns and this method is described here.
Dissolution procedure 0.5 g of powdered rock is weighed accurately into a Pt crucible or dish, 4 ml HClO4 and 15 ml HF (40%} added and the solution evaporated to incipient dryness. The residue is dissolved in 20 ml warm 25% HCI, filtered and washed through Whatman ® No. 42 filter paper. The precipitate is ignited to 800°C in a Ag crucible, fused for 20 min. with 0.5 g NaOH and after cooling dissolved in 20 ml HC1 {25%). Alternatively, the precipitate may be ignited in a Pt crucible, and then fused with KHF~ on a Bunsen, gradually increasing the flame temperature. The fused mixture is then evaporated to dryness on a sand-bath after adding 4 ml HCIO4 (to remove HF) and then dissolved in 20 ml of warm 25% HC1. The most important REE-bearing mineral which is not completely attacked by the HF--HClO4 treatment is zircon, and both KHF2 and NaOH fusions will attack this mineral. Satisfactory REE results were obtained with either fusion technique for a wide range of rock compositions; the NaOH fusion is somewhat more rapid than the KHF2 fusion. If required the solutions obtained after the filtration and the fusion stages can be diluted to 50 ml and aliquots removed and used for the determination of the trace- and major-element concentrations, excluding SiO2 (Walsh and Howie, 1980).
REE separation Chromatographic glass columns 20 mm I.D., 250 mm length, with a glass sinter disc and a PTFE burette tap at the b o t t o m were used. 20 g of resin (Dowex °~ AG 50W-X8,200--400 mesh from BioRad Laboratories) was loaded onto the columns, giving a settled height of 10 cm. After washing with 4 N HC1, and rinsing with 1 N HCI, the fusion solution, and the filtration solution (both diluted to ensure the acid strength is less than 10%), are loaded onto the columns. The resin is then washed with 400 ml 1.7 N HC1; this fraction which contains all the major constituents and most of the trace elements, is discarded. The REE's are held quantitatively on the resin (together with all the Ba, and some Sr, Zr and Hf) and are eluted with 500 ml of
144 TABLE I Elutions of the major and trace constituents in the simultaneous determination of R E E ' s i n r o c k s using i n d u c t i v e l y c o u p l e d plasma s o u r c e s p e c t r o m e t r y Amount present in rock
P e r c e n t a g e r e c o v e r e d in successive lO0-ml p ortio.ns_ o f 1.7 NH_C~_". . . . . . .
.
100 m]
200 ml
300 ml
400 ml
500 ml
38 <0.2 <0.l <0.2 29 8 77 -
55 54 89 <0.2 67 67 20 100 81
6.5 45.5 10 72 3 16 2 . . 19
1.5 0.6 0.2 27 0.3 8 <0.5 . . . . .
0.2 <0.2 0.1 0.2 <0.2 <2 <0.5 . . . .
Percentage recovered in 500 ml o f 4 : \ HC1
(wt. %): AI203 Fe203 MgO CaO Na20 K20 TiO 2 P20$ MnO
17.4 9.9 7.7 11.9 (,) 0.2 0.8 0.09 0.13
--
<0.1 ~0.1 <0.1 <0.2 <0,2 <2 <0.5
(ppm): Ba Co Cr Cu Ni Sr V
126 45 330 170 104 360 220
. . 15 11 0.2 2 -
.
. 51 32 94 55 -94
.
.
. 22 14 5 30 0.6 4
.
.
. . 11 -42 2 0.2 <0.1 13 <0.2 32 62 . . . .
100 -<0.1 <02 5 .
* R o c k c o n t a i n s 2.4% N a 2 0 ; in a d d i t i o n 0.5 g o f NaOH were a d d e d during the fusion stage.
4 N HC1. The solution is filtered through a Whatman ® No. 42 filter paper, to remove particles of resin, and then evaporated to dryness. The REE's are subsequently redi~olved in 5 ml of 10~o HC1 and injected into the ICP. The efficiency of this ion~exchange separation procedure was tested by loading onto the columns a standard rock sample and collecting 100 ml aliquots of the 1.7 N HCI solution as they were eluted from the columns. The percentages of the major- and selected trace-element constituents in each fraction were determined (using the ICP instrument) and are given in Table I. Table I also gives the amounts recovered in the whole of the 500 ml of the 4 N HC1 portion in which the REE's are eluted, together with the concentration levels originally present in the rock (the amounts in solution can be estimated from the dilutiona given in the above preparation procedure). The results in Table I confirm the efficiency of the ion-exchange procedure. For the major elements, only Ca w u detected in the h a t 100 ml of 1.7 N HCI eluant. F o r the trace elements, Table I shows that Ba is recovered quantitatively with the REE's together with some of the Sr.
145 REE DETERMINATION The background to the development of the ICP spectrometer is well documented (e.g., Fassel and Kniseley, 1974; Boumans and de Boer, 1975; Greenfield et al., 1975; Boumans, 1978; Fassel, 1978), and the application to the determination of major- and trace-element constituents in rocks and minerals has been described (Brenner et al., 1980; Walsh, 1980; Walsh and Howie, 1980). The ICP m e t h o d of analysis is essentially a " f l a m e " technique, and the flame temperature into which the analyte is injected is so high (6000--10,000 K) that not only are many spectral lines excited but also many of these lines are emitted by ionic rather than atomic species. This is important for the determination of the REE's, many of which have complex ionic spectra giving a wide choice of sensitive spectral lines, but also considerable problems with spectral overlaps. Much information is available on the spectral lines emitted by the REE's, and data have been published on the sensitivities of some lines in the ICP (Fassel and Kniseley, 1974; Broekaert et al., 1979; Nickdel et al., 1979; Boumans, 1981) but no detailed study appears to have been made of the spectral lines for geological use. The instrumentation used in this work (the Philips '~) P V 8 2 1 0 1.5-m ICP spectrometer) is well suited to evaluating spectral lines under analytical conditions with its internally m o u n t e d high-resolution "roving detector". This enables potentially usable spectral lines to be evaluated under "analytical" conditions. The Philips ~) plasma system has been described in detail in several publications, including Boumans (1978). Selection of operating parameters (gas flow rates, observation height, power, etc.) proved relatively straightforward. Since the elements are to be TABLE II Operating parameters for REE determination on the ICP spectrometer .
.
.
.
.
.
.
.
.
.
.
.
Plasma gas flow Auxiliary gas flow Carrier gas flow
.
.
.
.
.
.
.
.
.
15 I min.-I 0 I rain.-1 1.2 l min.-1
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Sample uptake rate Observation zone above coil Power (forward power)
2 ml rain. 15--19mm 1.0 kW (50 MHz)
measured simultaneously it is necessary to obtain operating conditions suitable for all elements. This inevitably results in some compromises but sensitivities for all the REE's were sufficient to accept slightly less than "ideal" settings. Table II gives the settings used, these being somewhat different to those used for the major- and trace-element analysis (Walsh and Howie, 1980). Specifically, the lower-power setting used results in improvements in the signal/background ratio (i.e. detection limits) although the rate of improvement in signal/background ratio varies substantially from element to element as power is lowered.
146 SELECTION OF SPECTRAL LINES The spectral lines used were selected after detailed studies on the suitability of the m a n y lines avAilAble for each REE. The factors which influence line selection for ICP studies are complex and limited data are available. It was necessary to take into account: (1) analytical sensitivity, and the geological abundance which varies substantially from element to element; (2) spectral interferences; and (3) complexities of m o u n t i n g the lines into the spectrometer where m a n y other elemental lines were already installed. Although the lines are therefore n o t necessarily the best for all the REE's and for non-geological applications alternative factors might have to be considereal, they do represent the result of a detailed study which does n o t appear to have been previously undertaken. Therefore some general comments for each element m a y be useful. It is worth emphasising that the "interferences", referred to below, are cases of spectral overlap and their effects are both predictable and linear; in all cases tested doubling the concentration of the interfering element doubled the interference. They may therefore be readily corrected for; furthermore in most cases the interferences are small. In general, ICP spectrometry has a greater freedom from interference effects, especially matrix effects than most other analytical techniques. It should, however, be noted that the spectral overlap effects are a feature of the instrument used, and certainly use of a lower-resolution spectrometer would increase the interferences. It is of course also possible that higher resolution would decrease the interferences.
Lanthanum. This is one of the more abundant REE's with a chondritic abundance (Nakamura, 1974) of 0.329 ppm (the abundance in a chondritic meteorite may be taken as the lowest level which one might reasonably be expected to determine). The La 398.85-nm line was used in this study and other lines (i.e. the more sensitive La 408.67-nm line) were not studied. The 398.85-nm line is satisfactory for geological use with adequate sensitivity. The only interference detected was a very small interference from Ca, which was n o t large enough to be worth correcting for. Cerium. The most sensitive Ce line is at 418.66 nm and although Ce is the most abundant REE in geological samples (chondritic abundance of 0.865 ppm) the element is one of the least sensitive of the REE's in the ICP. The 418.66-nm line gave good results with no significant interferences. Praesodymium. The 422.20-nm line was used although it does have several small spectral interferences, for which corrections were made. 10 ppm Ce gave a signal equivalent to 0.2 ppm Pr, 10 ppm Sr gave a signal equivalent to 0.90 ppm Pr, and 10 ppm Ca (Ca is always f o u n d in trace amounts in the REE solutions) gave a signal equivalent to 0.02 ppm Pr. The adjacent Pr line at 422.54 nm, which is slightly more sensitive, was also investigated but has a serious Sm interference.
147
Neodymium. This is one of the more a b u n d a n t REE (with a chondritic abundance of 0.63 ppm} and its de t e r m i na t ion should present no problem using the preparation procedure described in this work. The Nd 406.11-nm line was used initially with the roving d e t e c t o r assembly, and gave good results. However, it proved more convenient to install the more sensitive Nd 403.36-nm line in the spectrometer. Although this line is quite satisfactory it has a trace interference from Ba, and a higher background, and probably the 406.11-nm line would be preferable.
Samarium. Great difficulties were experienced in the selection of a suitable Sm line. Although m a ny lines have an adequate sensitivity, interferences from Ce or Nd were f o und with almost all the lines tested. The Sm 359.26-nm line was eventually selected, although it has interferences from Nd (10 ppm Nd gives a signal equivalent to 0.38 ppm Sm) and Gd (10 ppm Gd gives a signal equivalent to 0.82 ppm Sm). These interferences must be corrected for. One o f the few ot he r Sm lines t ha t was found to be suitable was the Sm 373.92 nm line; although free from Ce and Nd interferences, it is a less sensitive line and has a Pr interference.
Europium. This element has a chondritic abundance of only 0.077 ppm, but its ability to exist in both di- or tri-valent states and the implications of possible Eu anomalies makes its det er m i nat i on in geological materials essential. F o r t u n a t e l y it is, with the e xc e pt i on of Yb, the most sensitive of all the REE's in the ICP spectrometer. T he Eu line at 381.97 nm was used and this gave excellent results; although o t h e r sensitive Eu lines exist they were not investigated.Eu 381.97-nm has a trace Nd interference (10 ppm Nd gives a signal equivalent to 0.023 ppm Eu} and this should be corrected for.
Gadolinium. The 335.05-nm Gd line was used although it has a small interference from Ca (10 ppm Ca giving a signal equivalent to 0.14 ppm Gd). The o t h e r Gd lines investigated appear to have more serious interferences, Gd 376.85 nm has Ce and Nd interference, Gd 342.25 nm has Ce interference and Gd 336.22 nm has Y interference. Dyprosium. The most sensitive Dy line (353.17 nm) was used and found to be essentially free from interferences giving excellent results.
Holmium. Ho 345.6 nm is the most sensitive line and no significant interferences were recorded. The low chondritic abundance of this element (0.076 ppm} makes the use of this line (preferably in the first order} almost obligatory for geological use.
Erbium. Both the Er 390.63-nm and Er 369.27-nm lines have been used, and either will produce satisfactory results. The sensitivity of Er in the ICP is n o t as high as might be h o p e d for but the results may be regarded as adequate.
148
Ytterbium. This is the most smutitive R E E in the ICP and it is possible to measure d o w n to, and below, chondritic abundances (0.220 ppm) with the described sample preparation method. The Y b 328.94 n m was used satisfactorily. Other lines of Y b were not studied but probably several would have adequate sensitivity.
Lutetium. The chondritic abundance of this element is low (0.0339 ppm), but as the heaviest REE its satisfactory determination would be a real bonus. Fortunately, the Lu 261.5-rim line is very sensitive in the ICP and is free from significant interferences. The other Lu lines are much less sensitive and the use of the Lu 261.54-nm line, in the first order, is most desirable for geological work. EVALUATION This was carried o u t in three stages: (1) Recovery of the elements after the ion~xchange procedure was tested. (2) Precision tests were made with regard to the instrument and to the whole procedure. (3) The accuracy was tested by analysis of international rock standards.
(1) Column recoveries. Column recoveries were tested by preparing two identical sets of standard solutions and submitting one set to the given ionexchange procedure. Following dilution to constant volume, both sets were measured. The percentage recoveries obtained and the amounts used are set out in Table III. These confirm the results of Strelow and Jackson (1974) that the recovery is complete. Where significant deviations occur, these are due to the loss in precision when working at, or in some cases below, the detection limit. (2) Precision. Fig. 1 shows the results of repeated (N = 11) measurements on a mixed standard solution o f some o f the REE and is a plot of relative standard deviation against concentration. Missing values are due to the signal being too high for measurement. For the ranges of concentration most likely to be encountered the precision is ~ 1--2%. Table IV summarises fifteen separate analyses of one rock sample. The results show that precision is significantly better for the HREE's and that it is worse than 5% only in the case of La. The values of 20 in the table may be regarded as estimates of the detection limits o f the instnlment and as such may be considered satisfactory, especially for the " m i d d l e " and " h e a v y " lanthanides. With the exceptions of La and (marginally Pr) all of these "detection limits" are below chondritic abundance values. (3) Accuracy. Assessing the accuracy of an analytical m e t h o d for rock analysis is a problem, especially when dealing with a suite of elements such as the
1,19
T A B L E III Percentage recoveries and a m o u n t s of R E E ' s used Solution
l
2
3
4
Percentage REE recovery: La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
103 99 115 98 99
101 -101 101 101
I03 99 98 102 98
111 -98 102 98
100 101 105 100 100 100
100 101 101 101 101 101
100 107 102 104 100 101
100 106 102 98 102 103
Concentrations (ppm in solutions): La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
10 20 2 10 2 1 2 1 1 1 1
5 0 1 5 1 0.5 1 0.5 0.5 0.5 0.5
1 2 0.2 1 0.2 0.1 0.2 0.1 0.1 0.1 0.1
0.5 0 0.1 0.5 0.1 0.05 0.1 0.05 0.05 0.05 0.05
% RSD l
\
,,. Gd 335 05nm \
6
c Pr
\ I
5--
422 30nm
+ Lu 261 54 nm
\
•
\
Yb 328 93rim
o Eu 381 96nm
3~
\ \~
2_; • ~ ..
I
\
~ ~ ~ ~ ^ \
~
~:,
+
h--
1
T
0 01
0 1
IO ppm
10
in solution
Fig. 1. The plots s h o w the i m p r o v e m e n t in precision with increasing c o n c e n t r a t i o n level for selected rare-earth elements.
150 T A B L E IV Total p r e c i s i o n t e s t - - s a m p l e SD180
La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu
M e a n (15) (ppm)
20
5.83 12.3 1.89 9.64 2.48 0.90 2.59 2.92 1.75 1.79 0.28
0.75 0.86 0.16 0.67 0.11 0.02 0.14 0.05 0.08 0.04 0.01
ID--MS
RSD
(%) 6.5 3.5 4.3 3.5 2.2 1.1 2.7 0.9 2.3 1.1 1.8
-13.3 -. 8.99 2.5 0.94 2.94 3.15 1.98 1.88 --
The 15 determinations were m a d e using the 20-cm-length resin beds. R S D = relativestandard deviation; ID--MS = isotope dilution--mass spectrometry.
REE's. The conventional comparison between the new measurements and accepted values for international standard rock samples is not always safe in that varying degrees of certainty are ascribed to the compiled results. Table V shows new routine determinations obtained using the described procedure (with NaOH fusion of residues) for the U.S.G.S. standard rocks W-l, G-2, GSP-I, AGV-1 and BCR-1 together with the South African NIM~3 and the quoted values of Flanagan (1973), and Jackson and Strelow (1975). The overall agreement is good, with no evidence of serious or systematic error. The smooth ehondrite normalised patterns obtained for these rocks (Fig. 2) supports the credibility of the ICP results.
"_~,o~I ,...:"-,..
\\ '-..
".9 ...... t . ' c ~
"\
N,~-G
',
~..
-
~'--- - < ~ - - - - ~ -
~t. ~
I
i
L~I
Ce
"
I
Pr
V
Nd
"
I
I
"I
Sm
Eu
" ]
Gd
-'-f
""
" T . . . . . . l- . . . . . T
Dy
Ho
Er
'
~
I
. ,G-2,~
I
Yb
- ~
Lu
Fig. 2. C h o n d r i t e - n o r m a l i s e d p l o t s f o r i n t e r n a t i o n a l s t a n d a r d rocks.
151 TABLE V REE concentrations (in ppm) for selected international standard rock samples (values in parentheses from the compilation by Flanagan 1973 for U.S.G.S. rocks, and Jackson and Strelow 1975 for NIM-G) q
W-1
BCR-1
AGV-I
GSP-1
G-2
NIM-G
La
10.5 (9.8)
26 (26)
37 (35)
180 (191)
83 (96)
109 (105)
Ce
23.7 (23)
53.2 (54)
66.7 (63)
423 (394)
153 (150)
201 (195)
Pr
3.0 (3.4)
6.8 (7)
8.1 (7)
55 (50)
16.0 (19)
19.4 (18.9)
Nd
14.7 (15)
33.5 (29)
39 (39)
234 (188)
65.5 (60)
83 (73)
Sm
3.6 (3.6)
7.1 (6.6)
5.9 (5.9)
27 (27.1)
7.0 (7.3)
15.7 (15.5)
Eu
2.09
1.66
(1.ll) (1.94)
(1.7)
(2.4)
(1.5)
0.41 (0.39)
Gd
3.6 (4)
4.8 (5.5)
12.7 (15)
4.1 (5)
14.6 (10.9)
3.85 6.4 (4) (6.3)
3.56 (3.5)
6.24 (5,4)
2.28 18.3 (2.6) (15.8)
Ho
0.81 1.42 (0.69) (1.2)
0.79 (0.6)
1.33 0.50 4.0 (--) (0.4) (3.3)
Er
2.2 (2.4)
2.2 (1.2)
3.33 (3.0)
Yb
2.26 3.50 (2.1) (3.36)
1.74 (1.7)
1.81 0.84 (1.8) (0.88)
Lu
0.32 0.55 (0.35) (0.55)
0.27 (0.28)
0.27 0.13 1.95 (0.23) (0.11) (2.1)
By
1.13
7.0 (6.6)
3.87 (3.59)
2.51
1.38
1.2 (1.3)
11.1 (10.0) 13.7 (13.3)
CONCLUSIONS T h e m e t h o d f o r l a n t h a n i d e d e t e r m i n a t i o n s d e s c r i b e d in this p a p e r o f f e r s a n u m b e r o f a d v a n t a g e s . All o f the e l e m e n t s ( p r e s e n t l y e x c l u d i n g T b and T m ) are m e a s u r e d s i m u l t a n e o u s l y and in spite o f the t i m e r e q u i r e d for c h e m i c a l p r e - t r e a t m e n t it is a rapid p r o c e d u r e . Up to 30 s a m p l e s per w e e k m a y be a n a l y s e d r o u t i n e l y , d e p e n d i n g on the n u m b e r o f c o l u m n s e m p l o y e d . Ins t r u m e n t t i m e is very small, 1 - - 2 m i n . per s a m p l e , h e n c e costs are low. Samples m a y be p r e p a r e d in a n y r e a s o n a b l y e q u i p p e d l a b o r a t o r y and t a k e n to an ICP installation if o n e is n o t available in the u s e r ' s D e p a r t m e n t . It is t h e r e f o r e possible to have R E E d e t e r m i n a t i o n s as freely available as m a j o r - e l e m e n t analyses. An i m p o r t a n t a s p e c t o f t h e w o r k is t h a t a single p o r t i o n o f dissolved
152
sample m a y be divided to obtain lanthanide, trace- and major-element components (excluding SiO2 and alkalis). The development of ICP spectrometry is important in geological analysis as it is n o w possible to use one piece of capital equipment for an extremely wide range of elements. ACKNOWLEDGEMENTS
The authors are grateful to N.E.R.C. and King's College London for providing the funds for the purchase of the ICP source spectrometer. The development of the rare earth element method was supported by the instrument manufacturers, Philips, and their financial assistance in installingseveral additional spectral lines, and the assistance of several members of Phflips staff, contributed towards the project. They should also like to thank Professor R.A. Howie for his support and advice. Dr. C. Hawkesworth is thanked for sample, and data on, S D 180. Finally, the authors thank Ms W. Everett for typing the manuscript.
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