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Determination of rare-earth elements in rocks by liquid chromatography
Chemical Geology, 67 (1988) 185-195 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
185
[41
DETERMINATION OF RARE-EARTH ELEMENTS IN ROCKS BY LIQUID CHROMATOGRAPHY R.M. CASSIDY
Max-Planck-Institut fi~r Chemie, D-6500 Mainz (Federal Republic of Germany) (Received July 1, 1987; revised and accepted October 20, 1987)
Abstract Cassidy, R.M., 1988. Determination of rare-earth elements in rocks by liquid chromatography. Chem. Geol., 67: 185-195. A high-performance liquid chromatographic (HPLC) procedure has been developed and tested for the determination of rare-earth elements (REE) in rocks. The REE in a ~ 50-mg rock sample were first separated as a group on a 3-ml bed of cation-exchange resin with a combination of oxalic and nitric acids, and then separated from one another by dynamic ion-exchange HPLC on a reversed-phase column. Eluted REE were monitored by visible spectrophotometry at 658 nm after an on-line postcolumn reaction with Arsenazo III. Detection limits for the HPLC procedure were in the range of 0.1-0.5 ng ( ~ 0.01 ttg g- 1 for a 50-mg rock sample) and repeatabilities were _+0.5-2.0% for samples with concentrations > 10 X detection limit. The REE results obtained for a series of rock standards showed good agreement with recommended values and values obtained by isotope-dilution techniques. For the nine rocks studied the concentrations of the REE covered the range of 0.09-470 ttg g- 1,and the average value of the ratio, (HPLC result) / (reference result), was 0.983. The average % r.s.d, value of this ratio for individual REE was 4.1, which is the value expected for the comparison of two analytical techniques each having a r.s.d, of 2.9 %. Rocks used for this evaluation included the international rock standards BCR1, W-2, DR-N and BR, and a granite, a komatiite, a mid-ocean ridge basalt (MORB) basalt, a Hawaii Kilauea basalt and a begalith.
1. I n t r o d u c t i o n T h e g e o c h e m i c a l b e h a v i o r of t h e r a r e - e a r t h e l e m e n t s ( R E E ) h a s b e c o m e a n i m p o r t a n t tool for t h e d e v e l o p m e n t of o u r k n o w l e d g e o f a n u m b e r of p e t r o l o g i c a l p r o c e s s e s . T h e m a j o r a n a l y t ical t e c h n i q u e s t h a t h a v e b e e n u s e d to d e t e r m i n e R E E in r o c k s include n e u t r o n a c t i v a t i o n (NAA) (Joron andOttonello, 1985),isotopedilution m a s s s p e c t r o m e t r y ( I D M S ) ( S t r e l o w a n d J a c k s o n , 1974), s p a r k - s o u r c e m a s s spec-
*Address for correspondence: Atomic Energy of Canada Ltd., Chalk River, Ont. K0J 1J0, Canada.
t r o m e t r y ( S S M S ) ( T a y l o r a n d G o r t o n , 1977; J o c h u m et al., 1981), X - r a y f l u o r e s c e n c e ( X R F ) ( R o b i n s o n et al., 1986), i n d u c t i v e l y coupled plasma (ICP) emission spectroscopy (Aulis et al., 1985) a n d D C - p l a s m a e m i s s i o n s p e c t r o s c o p y ( F e i g e n s o n a n d Carr, 1985 ). All of these techniques can provide adequate analytical r e s u l t s for m o s t R E E , b u t suffer f r o m a n u m b e r of p r o b l e m s , s u c h as long a n a l y s i s t i m e s , high costs for i n s t r u m e n t a t i o n , a n d t h e n e e d for highly skilled o p e r a t o r s for t h e i n s t r u mentation. Recently ICP mass spectrometry h a s b e g u n to a t t r a c t i n t e r e s t s for R E E determ i n a t i o n s ( D a t e a n d G r a y , 1985; D o h e r t y a n d
186 VanderVoet, 1985), but this method also suf . fers from some of these disadvantages. Consequently there is still a need for methods that are fast, less costly, and simpler to implement. High-performance liquid chromatography ( HPLC ) has been used for sample cleanup prior to ICP analysis ( Yoshida and Haraguchi, 1985; Aulis et al., 1985), but with only moderate success. HPLC with a bonded-phase ion exchanger has been briefly examined for the determination of REE in rocks (Mazzucotelli et al., 1985), however this method suffered from incomplete REE separations and poor sensitivity. The analytical data presented by these authors for two rock standards also showed large discrepancies between the HPLC results and certified REE concentrations. The present author's experience with dynamic ion-exchange techniques has shown that it can provide rapid and accurate methods for the determination of La in nuclear fuels and REE in refining process streams (Barkley et al., 1986; Cassidy et al., 1986). In these techniques a hydrophobic ion, which is present in the eluent, is dynamically sorbed onto the surface of a hydrophobic support to provide a charged surface that can be used for ion-exchange separations. The advantages of this approach relative to conventional ion exchangers includes faster exchange, good reproducibility, and a wide range of ion-exchange capacities, which can be used to adjust the selectivity of the separation. With an on-line postcolumn reaction, detection limits are in the nanogram range and analysis times, after a group REE separation, of 10-20 min. are possible for all REE. Consequently the potential of this approach for the determination of REE in rocks and minerals was examined and this paper summarizes the results of this study. 2. E x p e r i m e n t a l 2.1. HPLC apparatus The HPLC instrumentation used in these studies is shown schematically in Fig. 1. The
.
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SAMPLE
. L0~,~ ' AR REGULATOR C~ ....... ~ A R
COLUMN
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~c:~ ', r
......
Fig. 1. Schematicof HPLC system.
conventional instrumentation included two Kratos ® Spectroflow 400 pumps (Kratos G.m.b.H., Karlsruhe, F.R.G. ) a model 450 Kratos ® gradient programmer, a Rheodyne ® 7125 injection valve (Rheodyne, Berkeley, California, U.S.A.), a Kratos ® model 783 variablewavelength absorbance detector, and a Spectra Physics ® 4290 computing integrator (Spectra Physics, Santa Clara, California, U.S.A.). The construction of the stainless steel reactor used to mix the Arsenazo III reagent and the eluate has been described elsewhere (Cassidy et al., 1987). The reagent reservoir ( ~ 400 ml) consisted of interconnected Savillex ® (Savillex Corp., Minnetonka, Minnesota, U.S.A.) Teflon ® tubes. A constant He pressure on the reagent solution was used to force this solution into the eluate, and this pressure was maintained with two-stage regulation; a 0-15-bar regulator on the He tank and a second 0-2.5bar regulator (Fig. 1). The reagent solution was delivered to the mixer via a small Teflon ® tube. Two HPLC columns were used for these studies: a 5-#m Supelcosil ® Cls 4.6 mm × 25 cm column (Supelco Inc., Bellefonte, Pennsylvania, U.S.A.) and a 3-#m Nucleosil ® C~s 4.6 m m × 12.5 cm column (Bischoff, Berlin, F.R.G. ). The precolumn filter was a 5-#m Nucleosil ® Cls 4.6 mm × 20 mm cartridge (Fig. 1 ). The strong cation exchanger used for the REE group separation was 200-400 mesh A G 5 0 W X 1 2 Dowex ® resin (Bio-Rad, Miinchen, F.R.G.). The REE standard solutions were prepared from high-purity metals from Ames Laborato-
187
ries (Ames, Iowa, U.S.A. ). All critical volumes were measured with calibrated pipets or liquid syringes; reproducibilities ( r.s.d. ) were < _+1% except for sample injection in the 10-20-~1 range where it was 1-2%. 2.2. Reagents andmaterials All solutions were prepared with water from quartz sub-boiling stills (Moody and Beary, 1982).Eluentswere filteredthrough0.2-~mfilters and kept at 4 ° C when storage periods were longer than 2-3 days. Unless indicated otherwise, all reagents were reagent-grade quality. The ~-hydroxyisobutyric acid (HIBA) was distilled ( ~ 100°C) under vacuum, and 5 M stock solutions were stored at - 15 ° C. The pH o f t h e e l u e n t s w a s a d j u s t e d w i t h 2 M N H 3 . Stock solutions (0.1 M) of sodium n-octanesulfonate ( OS ) were filtered through 0.2-zm filters and stored at - 15 ° C. A stock solution (1 g l- 1) of 2, 7-bis ( (o-arsenophenyl) azo )-1,8-dihydroxynaphtalene-3,6-disulfonic acid (Arsenazo III) was purified by passing it through a strong cation-exchange resin (H + form), and then made 0.1 M in urea. The postcolumn reagent solution was prepared by dilution of 100 ml of the Arsenazo III stock solution and 60 ml of glacial acetic acid to 1000 ml, followed by filtration through a 0.45-]~m filter to remove small particles that could produce restrictions in the reagent flow through the postcolumn mixer. Acids used for sample dissolution were either commercially available high-purity acids or acids prepared by sub-boiling or isothermal distillation (Mattinson, 1972; Moody and Beary, 1982). 2.3. Samples The samples used in this evaluation study are listed in Table I. The reference results for BCR1 and W2 standards are those given by Govindaraju (1984), and for BR and DRN the data compiled by Roelandts and Michel (1986) was used. The reference values for the HUD gran-
ite, KL2 Hawaiian basalt, and DIO MORB were obtained previously "in house" by standard IDMS techniques; some of the data for KL2 and DIO have been published ( Hofmann et al., 1986; Newsom et al., 1986). The reference values for the M664 komatiite and MF217 begalith (a highly undersaturated carbonate-rich dyke rock) were produced "in house"; for M664 by SSMS procedures described by Jochum et al. (1981) and for MF217 by NAA techniques (Wanke er al., 1977). Data from SSMS were also used when reference values were not available for certain REE. 2.4. Sample dissolution All sample preparation procedures and analyses were performed in conventional (noncleanroom) facilities, and the sediment sample was ashed at 350 ° C overnight before weighing. Approximately 50 mg of the ground rock sample were placed in a Teflon ® container, and 0.5 ml of concentrated HN03 and 1 ml of concentrated HF were added; for the granite, the sample was refluxed in HF for 3 days. The samples were then heated in a mixture of HN03 and a small amount of HC1Q until no visible precipitates were present. The samples were evaporated to dryness and dissolved in ~ 2 ml of 2 M H N Q containing 0.5 M oxalic acid. 2.5. REE group separation The sample solution from above was added to a 0.1 c m × 4 cm bed of 200-400 mesh AG50Wx12 ® strong cation-exchange resin, and elution of the Fe present in the rock samples was accomplished with ~ 10 ml of 2 M HN03 containing 0.5 M oxalic acid. The remaining major components were eluted with ~ 20 ml of 2 M HN03. The REE were eluted with 4 ml of 6 M HN03 and 28 ml of 8 M HNO3 in succession. This REE fraction was evaporated to dryness, and dissolved in 0.5-1 ml of the starting eluent used for the HPLC separation (see section 2.6).
188 TABLE I Rock samplesanalyzedby HPLC Sample
BCR-1, basalt W2, diabase HUD, granite DIO, MORB KL2, Hawaii basalt M664, komatiite MF217, begalith BR, basalt DR-N, diorite River sediment
REE concentration range (#g g- 1)
0.5- 52 0.3- 23 0.8- 56 0.6- 18 0.3- 32 0.1- 1.5 0.6-460 0.3-150 0.4- 45 0.3- 72
Major-elementcomposition ( ~ % as oxides) Si
A1
Fe
Mg
Ca
54 53 70 50 50 46 32 38 53 73
14 15 17 17 14 7 13 10 17 17
13 10 14 9 11 10 10 12 9 10
3 6 2 8 7 29 3 13 4 3
7 11 2 12 11 6 17 14 7 4
MORB= mid-ocean ridge basalt.
2.6. H P L C determination The flow of the starting eluent (0.05 M HIBA, 0.01 M OS, pH 3.8) was adjusted to 1.5 ml m i n . - 1. The eluent was then programmed to 0.5 M HIBA (0.01 M OS, pH 3.8) over 15 min. to remove impurities t h a t may have collected on the column while the system was standing overnight. The He pressure on the Arsenazo III reagent was adjusted (1.5-2.2 bar) to give a detector reading of 0.085 AU (absorbance unit) at 658 nm ( ~ 0.7 ml m i n . - 1). The detector was then zeroed, and the calibration curve was defined by the injection and gradient separation of four REE standard mixtures. The weights ranges of the REE injected varied from 1 to 12 ng for Lu and 30 to 300 ng for Ce; the injection volumes used for calibration were 50 and 100 ~l. The nonlinear calibration curves (based on peak areas) were calculated with the regression program present in the Spectra Physics ®computing integrator. Samples (10-300 ~tl) of the REE fraction from the group separation (as described in Section 2.5 ) were then separated, and the concentrations of the R E E were calculated with the regression equations. After each separation, the gradient was returned to the initial conditions over a 3-min. period, and when the
baseline was stable ( ~ 2 min. ), the next sample was injected. 3. R e s u l t s a n d d i s c u s s i o n 3.1. REE group separation Initial studies of direct injection of unprocessed dissolved-rock solutions onto the H P L C column showed good separations for most of the lanthanides, but some interferences were present and the maximum weight of rock t h a t could be injected was ~ 0.3 mg. With larger sample weights peak distortion was observed due to column overloading and precipitation of rock components when the sample contacted the eluent. Consequently, analysis of REE at concentrations < ~ 15/tg g-1 was not possible with direct injection of the rock solutions, and a separation procedure was developed to isolate the R E E from most of the major rock components. The method used, as described in Section 2.5, was a nitric acid elution procedure similar to t h a t described by Crock et al. (1984), but with an initial eluent containing oxalic acid to remove Fe. This combination of oxalic acid and 2 M nitric acid maintained small Kd-values for all the major components, and helped to pre-
189
0.6 Gd 0.4 sm 02
o , H,o,, 2~,No, 0.sMo×ALC , Aco,' , 20
Y
Ce
40
VOLUME
(m[)
iO
Fig. 2. Loss of REE due to early elution during group separation. Sample, 75 mg of Hawaiian basalt sample; column, 3-ml bed of 200-400 mesh AG 50Wx12®.
%
106
~
< ~o~ <
Ce
104
"k
~-~~,~, ~°~
4
~Gd
L~"
~87~o "K "-
\~
~ "" 12
16
20
24
28
'
32
\--~- -' 40' ~ 44' 35
VOLUME(m[) Fig. 3. Elution of REE from group separation column. Sample, REE standard containing 0.08-2 #g of the REE; column, 3-ml bed of 200-400 mesh AG 50Wx12®; arrows indicate sample loss if elution stopped at that point,
vent overloading of the small columns used for the group separation, The elution behavior of R E E and Y during removal of the major elements is shown in Fig. 2 for a basalt. H P L C analysis of the fractions showed that complete elution of partially retained elements, such as Ca and Mg, was achieved at ~ 30 ml, and that the loss of R E E was negligible at this point. Most R E E were at or below their detection limits for all of the fractions in Fig. 2. For the early eluting REE, Er, this corresponded to a sample loss of ~<0.5% for elution with ~<40 ml. This result was confirmed by the analysis of the 2 M nitric acid
eluent obtained just prior to the collection of the R E E during the analysis of the rock samples. The volumes of 6 and 8 M H N 0 3 eluent required for complete recovery of the R E E group were determined for a dilute "synthetic" standard (containing only R E E ) which had a similar R E E pattern and R E E weight as would be found in ~ 100 mg of a common basalt such as BCR-1. This aqueous standardwas used inplace of a rock solution to ensure the absence of overloading effects which would cause faster elution for R E E in rock samples. The results given in Fig. 3 show that the recovery of the latest eluting REE, La, should be ~>99.2 % if the volume of the eluent is >~28 ml. Analysis of fractions collected after the recovery of the R E E group from several types of rocks showed that slow elution of R E E caused a maximum sample loss of ~<0.1%. Sample blanks were at or below detection limits for all REE, and corresponded to ~<0.5% of that expected for a typical rock sample. For all samples the group separation gave a R E E fraction that was essentially free of major components and other interfering elements, and all R E E (see discussion for Dy on p. 191 ) were essentially well separated by the H P L C method (Fig. 4). For the komatiite, however, the H P L C separation of the R E E fraction from a second sample gave a large peak at the solvent front and some interfering peaks amongst the R E E peaks. These problems may have been related to the large concentration of MgO (29%) present in the sample as no interferences were observed during the analysis of two other komatiites (not reported here) having MgO concentrations of 23% and 20%. Another rock containing large amounts of Ca ( 47% CaO) has also been analyzed with no apparent problems, but the concentrations of the R E E in this rock were much higher than those present in komatiites.
3.2. H P L C separation A gradient elution was used to effect the elution of all R E E in a reasonable time. A typical
190 T A B L E II ,
u~
Analysis of s t a n d a r d R E E solution 7 hr. after calibration 3e La
Tb
v J-o
Element
Nanograms injected
Nanograms found
% deviation
La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu
60.4 146.0 22.3 108.9 34.4 13.9 46.9 8.64 57.7 195.8 12.1 37.7 6.16 39.2 5.71
61.0 146.8 22.5 109.1 34.7 14.1 46.9 8.72 58.6 193.1 12.1 37.7 6.24 39.9 5.69
1.0 0.5 0.9 0.1 0.9 1.4 0.0 1.0 1.5 1.0 0.8 0.0 1.3 1.8 0.3
;m
KL2 5.6 mg
s
P'
lo (rain)
Fig. 4. Separation of R E E after group separation from Hawaiian basalt. Conditions: gradient elution from 0.05 M H I B A to 0.5 M H I B A over 15 min. at 1.5 ml min. -1 and p H 3.8 with 0.01 M OS; sample, 100/~l of a 1-ml solution from R E E group separation from 50 mg of Hawaii basalt,
chromatogram obtained for a rock sample after a group REE separation is shown in Fig. 4. The amounts of the REE in this injected sample varied from 1.8 ng for Lu to 183 ng for Ce; this represented approximately one-tenth of the REE present in ~ 50 mg of a rock sample having REE concentrations from a minimum of 0.3 /lg g - 1 for Lu to a maximum of 34/lg g- 1 for Ce. Detection sensitivity decreased ~5-fold from the light REE (LREE) to heavy REE ( HREE ). For the REE with the poorest sensitivity, Lu, the % r.s.d, was 14 for injections of 1.2 ng, but for 2.5-ng samples of Ho the % r.s.d, was 2.6. When the sample sizes were >t 10 times the detection limits ( 0.1-0.5 ng for 2 × baseline noise ), the % r.s.d, values were in the range of 0.5-2.0 (50-/11 injections ). With proper attention to eluent purity, the stability of the HPLC system was good, as illustrated by the data in Table II, which show the analysis results for a standard REE mixture injected 7 hr. after the HPLC had been calibrated and used for the analyses of a number of rock samples. Previous studies (Cassidy et al., 1986) have shown that similar column systems
can be used for several months without serious changes in column behaviour. During the initial part of these studies, however, a gradual and irreversible reduction in retention times was observed, and this appeared to be associated with the presence of bacteria (2-104 living cells/l) in the water obtained from an ion-exchange purification system. Although the system was equipped with a bacterial filter (sorption type), it could not remove the large numbers of bacteria (~2.1061-1) present in the feed water (tap water purified by mixedbed ion exchange). The addition of bacterial inhibitors to the eluent was not considered, because this action would still leave the dead bacteria in the eluent and the chemicals could also affect the separation. However, the use of distilled water and filtration of all eluents through 0.2-/tm filters did eliminate this problem. At one point in this investigation, a rapid loss of column resolution was observed for metal ion separations, but not for the separation of neutral organic molecules. This loss of column resolution was irreversible, and was traced to an impurity (possibly a large anionic organic compound) in the HIBA. This impurity could not be removed from the eluent
191 by ion-exchange purification procedures (Cassidy et al., 1986), but was removed by vacuum distillation of the HIBA. Reliable operation of a postcolumn-reaction system is required for proper baseline stability and reproducible peak areas. The dual regulator system (Fig. 1 ) used in these studies to control the He pressure on the Arsenazo III solution maintained flow rates to _+0.008 ml min.-1. The He pressure over the Arsenazo III solution was adjusted to give a predetermined absorbance (normally 0.085 AU for an Arsenazo III flow rate of 0.7 ml m i n . - 1with an eluent flow rate of 1.5 ml m i n . - 1), and this absorbance value remained constant ( + 0.002 AU) throughout a normal working day. The peak-to-peak baseline noise obtained with this postcolumn reaction system was ~ 3" 10 -5 AU. Because total sample volumes were limited, samples were injected directly into a large sampie loop. A study of the effect of sample volume on peak areas showed that, for a constant amount of REE, peak areas were constant to within + 1% for sample volumes from 8 to 300 /~l. This simplified the preparation of standard curves and the determination of REE in rock samples where the variation in REE concentration was larger t h a n the calibration range,
3.3. HPLC analysis of rock samples The H P L C results obtained for the rock samplesldescribed in Table I are compared to reference values in Table III; the sources of the reference data are described in Section 2.3. These data are also displayed as plots of chondrite-normalized concentrations in Figs. 5-8; the chondritic values used were those determined by N a k u m u r a (1974). Fig. 8 also contains data for a NBS 4350 Columbia River sediment, but no reference data are available for REE. Because of the appreciable overlap between the Dy and Y peaks (Fig. 4), it was not possible to obtain reliable peak areas for these elements; however, some studies with more efficient columns have shown that it should be
possible to determine these two elements with an accuracy of _+3-5%. These data show that there was good agreement between HPLC and reference results, especially for the L R E E from Tb to La. Although a larger scatter in the data for the H R E E is clearly shown in the chondritenormalized plots (Figs. 5-8), this scatter appears to be as great for the reference values as it is for the HPLC results. For the BR, DR-N and MF217 samples the estimated errors given for several REE reference values were >/_+ 10% and these REE have been indicated in Table III. For some REE the HPLC results were obtained from only one or two injections and the H R E E were not always analyzed at a concentration that was >f 10 X the detection limit. In some samples (DR-N, HUD, BR, MF217, and the sediment) an apparently high value was obtained for Er (Figs. 5 and 7). W h e n second samples of HUD and MF217 were analyzed, the result for Er lay on a smooth curve with the rest of the data (Figs. 5 and 7). Recent results obtained with additional rock samples have suggested a correlation between high Th concentrations and this interference, which appeared as a separated peak between Er and Ho when the column capacity was increased. This is the retention zone expected for Th, and the identity of this peak will be determined during future applications of this method. The HPLC results for all the rock samples are summarized in Table IV. Reference data that were suspected to be in error were not included in Table IV; a data point was considered unacceptable if it had a large r.s.d. (some reference data) and/or showed a large deviation from an otherwise smooth plot of the chondrite-normalized values. The four HPLC values for Er that were known to be high were also discarded. To produce the summary in Table IV, each HPLC REE result was ratioed to the reference value, and for each REE, a mean and r.s.d, value was calculated for these ratios. T h e deviation of the mean value of each ratio, )~r, from 1.0 can be considered to be a measure of the bias (relative to reference value) for a given
192 T A B L E III H P L C analysis {in #g g - 1) of rock samples Element
BCR-I
W2
HPLC
reference
La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu
23.7 52.4 6.7 28.6 6.5 1.93 6.70 1.04 1.31 3.80 0.53 3.50 0.48
25.0 53.7 6.9 28.7 6.6 1.96 6.68 1.05 1.25 3.61 0.59 3.39 0.51
Element
HUD
reference
10.4 23.1 3.0 13.1 3.25 1.07 3.70 0.58 0.76 2.23 0.30 1.98 0.34
10.4 23.4 3.18 13.4 3.3 1.1 4.15 0.63 0.83 2.35 0.4 2.1 0.33
D-IO
HPLC
reference
La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu
28.3 56.6 6.74 25.5 4.17 1.09 2.71 0.31 0.26 0.66 0.075 0.54 0.08
27.6 55.8
Element
DR-N
La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu
HPLC
KL2
25.3 4.2 1.11 2.77 0.68 0.59 0.09
HPLC
0.97 2.65 2.12 0.30
reference
6.29 17.4 2.7
82.0 152 17.3 66.0 12.0 3.5 9.75 1.25 1.05 3.4 0.32 1.91 0.29
83.0 148 5* 65.0* 12.1 3.5 9.67*
4.38 1.54 5,7 0.91 1.33 4.1 3.85 0.56
2.4* 1.75" 0.23*
M664
HPLC
reference
HPLC
reference
21.2 45.6 5.7 23.4 5.3 1.45 5.1 0.78 0.93 3.2 0.39 2.5 0.37
20.7 46* 6.0* 23.4* 5.3 1.42" 4.3*
281 459 44 139 19.6 5.7 14.4 1.93 1.95 5.4 0.67 4.55 0.58
299 492 53* 168 20.8 5.6 16" 2.11 2.5* 0.85* 4.77 0.63
*Estimated errors for referenced value > + 10To.
13.7 33.7 4.71 22.3 5.78 1.98 6.16
HPLC
MF217
2.7* 0.39
13.2 32.7 4.64 21.6 5.63 1.91 5.86 0.87 0.99 2.56 0.33 2.03 0.30
reference
4.26 1.46 5.54 0.93 1.29 3.8 0.56 3.52 0.60
reference
BR
7.36 18.7 2.81 -
HPLC
HPLC 0.54 1.43 0.26 1.37 0.54 0.21 0.84 0.145 0.22 0.72 0.1 0.69 0.1
reference 0.54 1.78 0.28 1.49 0.61 0.23 0.84 0.15 0.24 0.69 0.66 0.097
193 100
50
4--4,,
oZ ~
10
Ld J o_~ < cn
5
1
I
l]
KL2
~,
•
o
KL2R HUD
•
HUDR
I
I
~ "o. ~
I
~
I
La Ce Pr Nd
I
I
I
I
I
I
I
I
I
~= ~ 10
A •
~ z 7, ~) ~ ~
00-10 • D-10R
J
5
!
•
M664 M664R
I
I
o
1
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
W2 W2R
L
"~-- ~'~",,~...~ " "~-
I
Lo Ce Pr Nd
Fig. 5. Chonclrite-normalized concentrations for H P L C and reference data. Open points are H P L C data, solidpoints are reference data, and starredpoints are for closely overlapping H P L C and reference data and cross is initial Er value for H U D . ).
I
I
~
I
I
I
L
I
I
1
I
I
t
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 7. Chondriteinormalized concentrations for H P L C and referencedata. Open points are H P L C data, solid points are reference data, and starred points are for closely overlapping H P L C and reference data. 120
1000
500 ~-
1 O0
\.
" 0 , ,
a z
o:=
\ .
z
5o
MF217 MF217R
~
u
BR
I
• ~
"*.•
o •
~
DR-NR
•
:
z
:
# a_
=: I
I
I
I
La Ce Pr Nd
I
I
1
I
-"'~'t~
[3
SED
0
BCR-1
°
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5¢
50
X x t l
I
I
I
I
J
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I
S m Eu Gd Tb Dy Ho Er Tm Yb Lu
__ 10
s
I I I I I I I I I I I I I I I Lo Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig.8. Chondrite-normalized concentrations for H P L C
and
Fig. 6. Chondrite-normalized concentrations for H P L C and reference data. Open points are H P L C data, solidpoints are reference data, starred points are for closely overlapping H P L C and reference data, and cross is initialEr value
referencedata. Open-points are H P L C data, solidpoints are reference data, and starred points are for closely overlapping H P L C and reference data.
forMF21?.
for all R E E was 4.1% which corresponds to a
R E E determination, and the r.s.d,of each -~r-
r.s.d,value of 2.9% for the H P L C assumes that:
value gives a m a x i m u m r.s.d, value for the H P L C determination of the REE; this r.s.d, value is also affected by the r.s.d,value of the analytical technique used to produce the reference value by the relationship: (r.s.d.2)z,= (r.s.d.2)HPLC + (r.s.d."~)re~,~ The r.s.d,data in Table IV show that the best agreement with the reference data was obtained from La to Tb; the average r.s.d,for this range of R E E was 3.4%. The mean r.s.d,value
results if one
r.s.d.HPLC-~r.s.d.~fe ..... The H P L C results obtained for some of the individual rock samples indicated a potential negative bias for the H P L C method. This was confirmed by the pooled data in Table IV where the mean value, .~(z,) of all ratios differed from the expected value of 1.0 b y - 1 . 7 % . With an r.s.d,value of 1.05% for X~z,), this bias was significantat the 95% confidence level.This small
194
TABLEIV
group separation) is fast relative to many other
SummaryofHPLCresultsfor REE
techniques, and sample consumption (1-5 mg rock per analysis) is low. Consequently small columns can be used for the group separation
Element Number Meanratio,)~r % r.s.d. Concentration of (HPLC/reference) range
samples
(gg-1)
La
8
0.981
3.9
0.5 -300
Sm Eu Gd Tb Ho Er Tm Yb
8 0.982 8 0.982 7 0.970 5 0.972 5 0.974 5 0.987 no referencedata available 7 0.992
2.4 2.2 3.7 3.8 5.7 5.4 6.5
0.5 - 20 0.2 - 6 0.8- 16 0.1 - 2 0.2 - 2 0.7- 1 0.8- 0.7 0.6 - 5
5.2
0.0S- 0.6
Ce Pr Nd
au
8 6 6
7
1.005 0.973 0.982
0.994
3.5 3.9 3.5
1 -50o O.3 - 50 1 -150
-~(xr) =0.983, % r.s.d.= 1.05;X (% r.s.d.)=4.1,
bias may be associated with incomplete sample dissolution, or precipitations of insoluble components, a n d / o r an error in the concentration of standards used for calibration; these factors will be examined in more detail as this method is applied to REE determinations on a routine basis. Sample losses during the group separation may also be a contributing factor, but significant losses at this stage should result in larger biases for the LREE, which is not evident from the data in Table IV. For the routine application o f t h i s m e t h o d t o g e o c h e m i c a l s t u d ies the relative concentrations of the REE is most important, and a comparison of the relative concentrations for a series of R E E pairs in the samples and in the reference data showed t h a t the relative concentrations were consist e n t to _+1.6%. 4. C o n c l u s i o n s
and only 0.3 mol HNO3 are required for each sample. The method gives consistent relative concentrations for all REE except Dy, and the small bias of - 1.7% for the H P L C results relative to reference data can possibly be eliminated through minor changes i n t h e analytical procedures. Acknowledgements I would like to t h a n k A1 H o f m a n n for his interest and support throughout these studies. I would also like to t h a n k Peter Pfeiffer for his analysis of my eluents for bacteria, Nick Arndt and Catherine Chauvel for their m a n y suggestions, Steve Elchuk for his technical input, and Chris Roddick for his comments concerning oxalic acid. References Aulis, R., Bolton, A., Doherty, W., VanderVoet, A. and
Wong, P., 1985. Determination of yttrium and selected rare earth elements in geological materials using high performance liquid chromatographic separation and ICP spectrometric detection. Spectrochim. Acta, 40B: 377-387. Barkley, D.J., Blanchette, M. and Cassidy, R.M., 1986. Dynamic chromatographic systems for the determination of rare earth and thorium in samples from uranium ore refining processes. Anal. Chem., 58: 2222-2226. Cassidy, R.M., Elchuk, S., Elliot, N.L., Green, L.W., Knight, C.H. and Recoskie, B.M., 1986. Dynamic ion exchange chromatography for the determination of number of fissions in uranium dioxide fuels. Anal. Chem., 58:
1181-1186. Cassidy, R.M., Elchuk, S. and Dasgupta, P.K., 1987. Per-
The combination of a simplified group REE separation and H P L C analysis is an attractive method for the determination of REE in a wide variety of rocks. The average % r.s.d, of ~ 2.9 is comparable to other techniques and can he iraproved by repetitive sample analysis. The 10-20-min. time required for one analysis (after
formance of annular membrane and screen-tee reactors for postcolumn-reaction detection of metal ions separated by liquid chromatography. Anal. Chem., 59: 85-90. Crock, J.G., Lichte, F.E. and Wildeman, T.R., 1984. The group separation ofthe rare-earth elementsand yttrium from geologic materials by cation-exchange chromatography. Chem. Geol., 45: 149-163. Date, A.R. and Gray, A.L., 1985. Determination of trace elements in geological samples by inductively-coupled-
195 plasma source mass spectrometry. Spectrochim. Acta, 40B: 115-122. Doherty, W. and VanderVoet, A., 1985. The application of inductively-coupled-plasma mass spectrometry to the determination of rare earth elements in geological materials. Can. J. Spectrosc., 30: 135-141. Feigenson, M.D. and Carr, M.J., 1985. Determination of major, trace, and rare-earth elements in rocks by DCP-AES. J. Chem. Geol., 51: 19-27. Govindaraju, K., 1984. Compilation of working values and sample description for 170 international reference samples of mainly silicate rocks and minerals. Geostand. Newsl., 8: 3-16. Hofmann, A.W., Jochum, K.P., Seufert, M. and White, W.M., 1986. Nb and Pb in oceanic basalts: new constraints on mantle evolution. Earth Planet. Sci. Lett., 79: 33-45. Jochum, K.P., Seufert, M. and Knab, H.-J., 1981. Quantitative multielement analysis of geochemical and cosmochemical samples using spark source mass spectrometry. Fresenius Z. Anal. Chem., 309" 285-290. Joron, J.L. and Ottonello, 1985. Radiochemical neutron activation analysis of rare-earth elements in peridotitic rocks. J. Radioanal. Chem., 88: 259-272. Mattinson, J.S., 1972. Preparation of hydrofluoric, hydrochloric, and nitric acids at ultralow lead levels. Anal. Chem., 44: 1715-1716. Mazzucotelli, A., Dadone, A., Frache, R. and Baffi, F., 1985. Determination of trace amounts oflanthanides in rocks and minerals by high-performance liquid chromatography. J. Chromatogr., 349: 137-142. Moody, J.R. and Beary, E.S., 1982. Purified reagents for trace metal analysis. Talanta, 29: 1003-1010.
Nakumura, N., 1974. Determination of rare earth elements, Ba, Fe, Mg, Na, and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta, 38" 757-775. Newsom, H.E., White, W.M., Jochum, K.P. and Hofmann, A.W., 1986. Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core. Earth Planet. Sci. Lett., 80: 299-313. Robinson, P., Higgins, N.C. and Jenner, G.A., 1986. Determination of rare-earth elements, yttrium and scandium in rocks by an ion exchange-X-ray fluorescence technique. Chem. Geol., 55: 121-137. Roelandts, I. and Michel, G., 1986. Sequentially inductively coupled plasma determination of some rare-earth elements in five French geostandards. Geostand. Newsl., 10: 111-120. Strelow, F.W.E. and Jackson, P.F.S., 1974. Determination of trace and ultratrace quantities ofrare-earthelements by ion-exchange chromatography and mass-spectrometry. Anal. Chem., 46: 1481-1486. Taylor, S.R. and Gorton, M.P., 1977. Geochemical applications of spark-source mass spectrography, III. Element sensitivity, precision and accuracy. Geochim. Cosmochim. Acta, 41: 1375-1380. Wanke, H., Kruse, H., Palme, H. and Spettel, B., 1977. Instrumental neutron activation analysis of lunar samples and the identification of primary matter in lunar highlands. J. Radioanal. Chem., 38: 363-378. Yoshida, K. and Haraguchi, H., 1984. Determination ofrareearth elements by liquid chromatography/inductively coupled plasma atomic-emission spectrometry. Anal. Chem., 56: 2580-2585.
185
[41
DETERMINATION OF RARE-EARTH ELEMENTS IN ROCKS BY LIQUID CHROMATOGRAPHY R.M. CASSIDY
Max-Planck-Institut fi~r Chemie, D-6500 Mainz (Federal Republic of Germany) (Received July 1, 1987; revised and accepted October 20, 1987)
Abstract Cassidy, R.M., 1988. Determination of rare-earth elements in rocks by liquid chromatography. Chem. Geol., 67: 185-195. A high-performance liquid chromatographic (HPLC) procedure has been developed and tested for the determination of rare-earth elements (REE) in rocks. The REE in a ~ 50-mg rock sample were first separated as a group on a 3-ml bed of cation-exchange resin with a combination of oxalic and nitric acids, and then separated from one another by dynamic ion-exchange HPLC on a reversed-phase column. Eluted REE were monitored by visible spectrophotometry at 658 nm after an on-line postcolumn reaction with Arsenazo III. Detection limits for the HPLC procedure were in the range of 0.1-0.5 ng ( ~ 0.01 ttg g- 1 for a 50-mg rock sample) and repeatabilities were _+0.5-2.0% for samples with concentrations > 10 X detection limit. The REE results obtained for a series of rock standards showed good agreement with recommended values and values obtained by isotope-dilution techniques. For the nine rocks studied the concentrations of the REE covered the range of 0.09-470 ttg g- 1,and the average value of the ratio, (HPLC result) / (reference result), was 0.983. The average % r.s.d, value of this ratio for individual REE was 4.1, which is the value expected for the comparison of two analytical techniques each having a r.s.d, of 2.9 %. Rocks used for this evaluation included the international rock standards BCR1, W-2, DR-N and BR, and a granite, a komatiite, a mid-ocean ridge basalt (MORB) basalt, a Hawaii Kilauea basalt and a begalith.
1. I n t r o d u c t i o n T h e g e o c h e m i c a l b e h a v i o r of t h e r a r e - e a r t h e l e m e n t s ( R E E ) h a s b e c o m e a n i m p o r t a n t tool for t h e d e v e l o p m e n t of o u r k n o w l e d g e o f a n u m b e r of p e t r o l o g i c a l p r o c e s s e s . T h e m a j o r a n a l y t ical t e c h n i q u e s t h a t h a v e b e e n u s e d to d e t e r m i n e R E E in r o c k s include n e u t r o n a c t i v a t i o n (NAA) (Joron andOttonello, 1985),isotopedilution m a s s s p e c t r o m e t r y ( I D M S ) ( S t r e l o w a n d J a c k s o n , 1974), s p a r k - s o u r c e m a s s spec-
*Address for correspondence: Atomic Energy of Canada Ltd., Chalk River, Ont. K0J 1J0, Canada.
t r o m e t r y ( S S M S ) ( T a y l o r a n d G o r t o n , 1977; J o c h u m et al., 1981), X - r a y f l u o r e s c e n c e ( X R F ) ( R o b i n s o n et al., 1986), i n d u c t i v e l y coupled plasma (ICP) emission spectroscopy (Aulis et al., 1985) a n d D C - p l a s m a e m i s s i o n s p e c t r o s c o p y ( F e i g e n s o n a n d Carr, 1985 ). All of these techniques can provide adequate analytical r e s u l t s for m o s t R E E , b u t suffer f r o m a n u m b e r of p r o b l e m s , s u c h as long a n a l y s i s t i m e s , high costs for i n s t r u m e n t a t i o n , a n d t h e n e e d for highly skilled o p e r a t o r s for t h e i n s t r u mentation. Recently ICP mass spectrometry h a s b e g u n to a t t r a c t i n t e r e s t s for R E E determ i n a t i o n s ( D a t e a n d G r a y , 1985; D o h e r t y a n d
186 VanderVoet, 1985), but this method also suf . fers from some of these disadvantages. Consequently there is still a need for methods that are fast, less costly, and simpler to implement. High-performance liquid chromatography ( HPLC ) has been used for sample cleanup prior to ICP analysis ( Yoshida and Haraguchi, 1985; Aulis et al., 1985), but with only moderate success. HPLC with a bonded-phase ion exchanger has been briefly examined for the determination of REE in rocks (Mazzucotelli et al., 1985), however this method suffered from incomplete REE separations and poor sensitivity. The analytical data presented by these authors for two rock standards also showed large discrepancies between the HPLC results and certified REE concentrations. The present author's experience with dynamic ion-exchange techniques has shown that it can provide rapid and accurate methods for the determination of La in nuclear fuels and REE in refining process streams (Barkley et al., 1986; Cassidy et al., 1986). In these techniques a hydrophobic ion, which is present in the eluent, is dynamically sorbed onto the surface of a hydrophobic support to provide a charged surface that can be used for ion-exchange separations. The advantages of this approach relative to conventional ion exchangers includes faster exchange, good reproducibility, and a wide range of ion-exchange capacities, which can be used to adjust the selectivity of the separation. With an on-line postcolumn reaction, detection limits are in the nanogram range and analysis times, after a group REE separation, of 10-20 min. are possible for all REE. Consequently the potential of this approach for the determination of REE in rocks and minerals was examined and this paper summarizes the results of this study. 2. E x p e r i m e n t a l 2.1. HPLC apparatus The HPLC instrumentation used in these studies is shown schematically in Fig. 1. The
.
. . ~ ~ ~.~:~,~ ~ ~ -- o~uM( ~ ~ ~ "~ ~ ..... ~_~ ~°~" ] ".
SAMPLE
. L0~,~ ' AR REGULATOR C~ ....... ~ A R
COLUMN
~
~c:~ ', r
......
Fig. 1. Schematicof HPLC system.
conventional instrumentation included two Kratos ® Spectroflow 400 pumps (Kratos G.m.b.H., Karlsruhe, F.R.G. ) a model 450 Kratos ® gradient programmer, a Rheodyne ® 7125 injection valve (Rheodyne, Berkeley, California, U.S.A.), a Kratos ® model 783 variablewavelength absorbance detector, and a Spectra Physics ® 4290 computing integrator (Spectra Physics, Santa Clara, California, U.S.A.). The construction of the stainless steel reactor used to mix the Arsenazo III reagent and the eluate has been described elsewhere (Cassidy et al., 1987). The reagent reservoir ( ~ 400 ml) consisted of interconnected Savillex ® (Savillex Corp., Minnetonka, Minnesota, U.S.A.) Teflon ® tubes. A constant He pressure on the reagent solution was used to force this solution into the eluate, and this pressure was maintained with two-stage regulation; a 0-15-bar regulator on the He tank and a second 0-2.5bar regulator (Fig. 1). The reagent solution was delivered to the mixer via a small Teflon ® tube. Two HPLC columns were used for these studies: a 5-#m Supelcosil ® Cls 4.6 mm × 25 cm column (Supelco Inc., Bellefonte, Pennsylvania, U.S.A.) and a 3-#m Nucleosil ® C~s 4.6 m m × 12.5 cm column (Bischoff, Berlin, F.R.G. ). The precolumn filter was a 5-#m Nucleosil ® Cls 4.6 mm × 20 mm cartridge (Fig. 1 ). The strong cation exchanger used for the REE group separation was 200-400 mesh A G 5 0 W X 1 2 Dowex ® resin (Bio-Rad, Miinchen, F.R.G.). The REE standard solutions were prepared from high-purity metals from Ames Laborato-
187
ries (Ames, Iowa, U.S.A. ). All critical volumes were measured with calibrated pipets or liquid syringes; reproducibilities ( r.s.d. ) were < _+1% except for sample injection in the 10-20-~1 range where it was 1-2%. 2.2. Reagents andmaterials All solutions were prepared with water from quartz sub-boiling stills (Moody and Beary, 1982).Eluentswere filteredthrough0.2-~mfilters and kept at 4 ° C when storage periods were longer than 2-3 days. Unless indicated otherwise, all reagents were reagent-grade quality. The ~-hydroxyisobutyric acid (HIBA) was distilled ( ~ 100°C) under vacuum, and 5 M stock solutions were stored at - 15 ° C. The pH o f t h e e l u e n t s w a s a d j u s t e d w i t h 2 M N H 3 . Stock solutions (0.1 M) of sodium n-octanesulfonate ( OS ) were filtered through 0.2-zm filters and stored at - 15 ° C. A stock solution (1 g l- 1) of 2, 7-bis ( (o-arsenophenyl) azo )-1,8-dihydroxynaphtalene-3,6-disulfonic acid (Arsenazo III) was purified by passing it through a strong cation-exchange resin (H + form), and then made 0.1 M in urea. The postcolumn reagent solution was prepared by dilution of 100 ml of the Arsenazo III stock solution and 60 ml of glacial acetic acid to 1000 ml, followed by filtration through a 0.45-]~m filter to remove small particles that could produce restrictions in the reagent flow through the postcolumn mixer. Acids used for sample dissolution were either commercially available high-purity acids or acids prepared by sub-boiling or isothermal distillation (Mattinson, 1972; Moody and Beary, 1982). 2.3. Samples The samples used in this evaluation study are listed in Table I. The reference results for BCR1 and W2 standards are those given by Govindaraju (1984), and for BR and DRN the data compiled by Roelandts and Michel (1986) was used. The reference values for the HUD gran-
ite, KL2 Hawaiian basalt, and DIO MORB were obtained previously "in house" by standard IDMS techniques; some of the data for KL2 and DIO have been published ( Hofmann et al., 1986; Newsom et al., 1986). The reference values for the M664 komatiite and MF217 begalith (a highly undersaturated carbonate-rich dyke rock) were produced "in house"; for M664 by SSMS procedures described by Jochum et al. (1981) and for MF217 by NAA techniques (Wanke er al., 1977). Data from SSMS were also used when reference values were not available for certain REE. 2.4. Sample dissolution All sample preparation procedures and analyses were performed in conventional (noncleanroom) facilities, and the sediment sample was ashed at 350 ° C overnight before weighing. Approximately 50 mg of the ground rock sample were placed in a Teflon ® container, and 0.5 ml of concentrated HN03 and 1 ml of concentrated HF were added; for the granite, the sample was refluxed in HF for 3 days. The samples were then heated in a mixture of HN03 and a small amount of HC1Q until no visible precipitates were present. The samples were evaporated to dryness and dissolved in ~ 2 ml of 2 M H N Q containing 0.5 M oxalic acid. 2.5. REE group separation The sample solution from above was added to a 0.1 c m × 4 cm bed of 200-400 mesh AG50Wx12 ® strong cation-exchange resin, and elution of the Fe present in the rock samples was accomplished with ~ 10 ml of 2 M HN03 containing 0.5 M oxalic acid. The remaining major components were eluted with ~ 20 ml of 2 M HN03. The REE were eluted with 4 ml of 6 M HN03 and 28 ml of 8 M HNO3 in succession. This REE fraction was evaporated to dryness, and dissolved in 0.5-1 ml of the starting eluent used for the HPLC separation (see section 2.6).
188 TABLE I Rock samplesanalyzedby HPLC Sample
BCR-1, basalt W2, diabase HUD, granite DIO, MORB KL2, Hawaii basalt M664, komatiite MF217, begalith BR, basalt DR-N, diorite River sediment
REE concentration range (#g g- 1)
0.5- 52 0.3- 23 0.8- 56 0.6- 18 0.3- 32 0.1- 1.5 0.6-460 0.3-150 0.4- 45 0.3- 72
Major-elementcomposition ( ~ % as oxides) Si
A1
Fe
Mg
Ca
54 53 70 50 50 46 32 38 53 73
14 15 17 17 14 7 13 10 17 17
13 10 14 9 11 10 10 12 9 10
3 6 2 8 7 29 3 13 4 3
7 11 2 12 11 6 17 14 7 4
MORB= mid-ocean ridge basalt.
2.6. H P L C determination The flow of the starting eluent (0.05 M HIBA, 0.01 M OS, pH 3.8) was adjusted to 1.5 ml m i n . - 1. The eluent was then programmed to 0.5 M HIBA (0.01 M OS, pH 3.8) over 15 min. to remove impurities t h a t may have collected on the column while the system was standing overnight. The He pressure on the Arsenazo III reagent was adjusted (1.5-2.2 bar) to give a detector reading of 0.085 AU (absorbance unit) at 658 nm ( ~ 0.7 ml m i n . - 1). The detector was then zeroed, and the calibration curve was defined by the injection and gradient separation of four REE standard mixtures. The weights ranges of the REE injected varied from 1 to 12 ng for Lu and 30 to 300 ng for Ce; the injection volumes used for calibration were 50 and 100 ~l. The nonlinear calibration curves (based on peak areas) were calculated with the regression program present in the Spectra Physics ®computing integrator. Samples (10-300 ~tl) of the REE fraction from the group separation (as described in Section 2.5 ) were then separated, and the concentrations of the R E E were calculated with the regression equations. After each separation, the gradient was returned to the initial conditions over a 3-min. period, and when the
baseline was stable ( ~ 2 min. ), the next sample was injected. 3. R e s u l t s a n d d i s c u s s i o n 3.1. REE group separation Initial studies of direct injection of unprocessed dissolved-rock solutions onto the H P L C column showed good separations for most of the lanthanides, but some interferences were present and the maximum weight of rock t h a t could be injected was ~ 0.3 mg. With larger sample weights peak distortion was observed due to column overloading and precipitation of rock components when the sample contacted the eluent. Consequently, analysis of REE at concentrations < ~ 15/tg g-1 was not possible with direct injection of the rock solutions, and a separation procedure was developed to isolate the R E E from most of the major rock components. The method used, as described in Section 2.5, was a nitric acid elution procedure similar to t h a t described by Crock et al. (1984), but with an initial eluent containing oxalic acid to remove Fe. This combination of oxalic acid and 2 M nitric acid maintained small Kd-values for all the major components, and helped to pre-
189
0.6 Gd 0.4 sm 02
o , H,o,, 2~,No, 0.sMo×ALC , Aco,' , 20
Y
Ce
40
VOLUME
(m[)
iO
Fig. 2. Loss of REE due to early elution during group separation. Sample, 75 mg of Hawaiian basalt sample; column, 3-ml bed of 200-400 mesh AG 50Wx12®.
%
106
~
< ~o~ <
Ce
104
"k
~-~~,~, ~°~
4
~Gd
L~"
~87~o "K "-
\~
~ "" 12
16
20
24
28
'
32
\--~- -' 40' ~ 44' 35
VOLUME(m[) Fig. 3. Elution of REE from group separation column. Sample, REE standard containing 0.08-2 #g of the REE; column, 3-ml bed of 200-400 mesh AG 50Wx12®; arrows indicate sample loss if elution stopped at that point,
vent overloading of the small columns used for the group separation, The elution behavior of R E E and Y during removal of the major elements is shown in Fig. 2 for a basalt. H P L C analysis of the fractions showed that complete elution of partially retained elements, such as Ca and Mg, was achieved at ~ 30 ml, and that the loss of R E E was negligible at this point. Most R E E were at or below their detection limits for all of the fractions in Fig. 2. For the early eluting REE, Er, this corresponded to a sample loss of ~<0.5% for elution with ~<40 ml. This result was confirmed by the analysis of the 2 M nitric acid
eluent obtained just prior to the collection of the R E E during the analysis of the rock samples. The volumes of 6 and 8 M H N 0 3 eluent required for complete recovery of the R E E group were determined for a dilute "synthetic" standard (containing only R E E ) which had a similar R E E pattern and R E E weight as would be found in ~ 100 mg of a common basalt such as BCR-1. This aqueous standardwas used inplace of a rock solution to ensure the absence of overloading effects which would cause faster elution for R E E in rock samples. The results given in Fig. 3 show that the recovery of the latest eluting REE, La, should be ~>99.2 % if the volume of the eluent is >~28 ml. Analysis of fractions collected after the recovery of the R E E group from several types of rocks showed that slow elution of R E E caused a maximum sample loss of ~<0.1%. Sample blanks were at or below detection limits for all REE, and corresponded to ~<0.5% of that expected for a typical rock sample. For all samples the group separation gave a R E E fraction that was essentially free of major components and other interfering elements, and all R E E (see discussion for Dy on p. 191 ) were essentially well separated by the H P L C method (Fig. 4). For the komatiite, however, the H P L C separation of the R E E fraction from a second sample gave a large peak at the solvent front and some interfering peaks amongst the R E E peaks. These problems may have been related to the large concentration of MgO (29%) present in the sample as no interferences were observed during the analysis of two other komatiites (not reported here) having MgO concentrations of 23% and 20%. Another rock containing large amounts of Ca ( 47% CaO) has also been analyzed with no apparent problems, but the concentrations of the R E E in this rock were much higher than those present in komatiites.
3.2. H P L C separation A gradient elution was used to effect the elution of all R E E in a reasonable time. A typical
190 T A B L E II ,
u~
Analysis of s t a n d a r d R E E solution 7 hr. after calibration 3e La
Tb
v J-o
Element
Nanograms injected
Nanograms found
% deviation
La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu
60.4 146.0 22.3 108.9 34.4 13.9 46.9 8.64 57.7 195.8 12.1 37.7 6.16 39.2 5.71
61.0 146.8 22.5 109.1 34.7 14.1 46.9 8.72 58.6 193.1 12.1 37.7 6.24 39.9 5.69
1.0 0.5 0.9 0.1 0.9 1.4 0.0 1.0 1.5 1.0 0.8 0.0 1.3 1.8 0.3
;m
KL2 5.6 mg
s
P'
lo (rain)
Fig. 4. Separation of R E E after group separation from Hawaiian basalt. Conditions: gradient elution from 0.05 M H I B A to 0.5 M H I B A over 15 min. at 1.5 ml min. -1 and p H 3.8 with 0.01 M OS; sample, 100/~l of a 1-ml solution from R E E group separation from 50 mg of Hawaii basalt,
chromatogram obtained for a rock sample after a group REE separation is shown in Fig. 4. The amounts of the REE in this injected sample varied from 1.8 ng for Lu to 183 ng for Ce; this represented approximately one-tenth of the REE present in ~ 50 mg of a rock sample having REE concentrations from a minimum of 0.3 /lg g - 1 for Lu to a maximum of 34/lg g- 1 for Ce. Detection sensitivity decreased ~5-fold from the light REE (LREE) to heavy REE ( HREE ). For the REE with the poorest sensitivity, Lu, the % r.s.d, was 14 for injections of 1.2 ng, but for 2.5-ng samples of Ho the % r.s.d, was 2.6. When the sample sizes were >t 10 times the detection limits ( 0.1-0.5 ng for 2 × baseline noise ), the % r.s.d, values were in the range of 0.5-2.0 (50-/11 injections ). With proper attention to eluent purity, the stability of the HPLC system was good, as illustrated by the data in Table II, which show the analysis results for a standard REE mixture injected 7 hr. after the HPLC had been calibrated and used for the analyses of a number of rock samples. Previous studies (Cassidy et al., 1986) have shown that similar column systems
can be used for several months without serious changes in column behaviour. During the initial part of these studies, however, a gradual and irreversible reduction in retention times was observed, and this appeared to be associated with the presence of bacteria (2-104 living cells/l) in the water obtained from an ion-exchange purification system. Although the system was equipped with a bacterial filter (sorption type), it could not remove the large numbers of bacteria (~2.1061-1) present in the feed water (tap water purified by mixedbed ion exchange). The addition of bacterial inhibitors to the eluent was not considered, because this action would still leave the dead bacteria in the eluent and the chemicals could also affect the separation. However, the use of distilled water and filtration of all eluents through 0.2-/tm filters did eliminate this problem. At one point in this investigation, a rapid loss of column resolution was observed for metal ion separations, but not for the separation of neutral organic molecules. This loss of column resolution was irreversible, and was traced to an impurity (possibly a large anionic organic compound) in the HIBA. This impurity could not be removed from the eluent
191 by ion-exchange purification procedures (Cassidy et al., 1986), but was removed by vacuum distillation of the HIBA. Reliable operation of a postcolumn-reaction system is required for proper baseline stability and reproducible peak areas. The dual regulator system (Fig. 1 ) used in these studies to control the He pressure on the Arsenazo III solution maintained flow rates to _+0.008 ml min.-1. The He pressure over the Arsenazo III solution was adjusted to give a predetermined absorbance (normally 0.085 AU for an Arsenazo III flow rate of 0.7 ml m i n . - 1with an eluent flow rate of 1.5 ml m i n . - 1), and this absorbance value remained constant ( + 0.002 AU) throughout a normal working day. The peak-to-peak baseline noise obtained with this postcolumn reaction system was ~ 3" 10 -5 AU. Because total sample volumes were limited, samples were injected directly into a large sampie loop. A study of the effect of sample volume on peak areas showed that, for a constant amount of REE, peak areas were constant to within + 1% for sample volumes from 8 to 300 /~l. This simplified the preparation of standard curves and the determination of REE in rock samples where the variation in REE concentration was larger t h a n the calibration range,
3.3. HPLC analysis of rock samples The H P L C results obtained for the rock samplesldescribed in Table I are compared to reference values in Table III; the sources of the reference data are described in Section 2.3. These data are also displayed as plots of chondrite-normalized concentrations in Figs. 5-8; the chondritic values used were those determined by N a k u m u r a (1974). Fig. 8 also contains data for a NBS 4350 Columbia River sediment, but no reference data are available for REE. Because of the appreciable overlap between the Dy and Y peaks (Fig. 4), it was not possible to obtain reliable peak areas for these elements; however, some studies with more efficient columns have shown that it should be
possible to determine these two elements with an accuracy of _+3-5%. These data show that there was good agreement between HPLC and reference results, especially for the L R E E from Tb to La. Although a larger scatter in the data for the H R E E is clearly shown in the chondritenormalized plots (Figs. 5-8), this scatter appears to be as great for the reference values as it is for the HPLC results. For the BR, DR-N and MF217 samples the estimated errors given for several REE reference values were >/_+ 10% and these REE have been indicated in Table III. For some REE the HPLC results were obtained from only one or two injections and the H R E E were not always analyzed at a concentration that was >f 10 X the detection limit. In some samples (DR-N, HUD, BR, MF217, and the sediment) an apparently high value was obtained for Er (Figs. 5 and 7). W h e n second samples of HUD and MF217 were analyzed, the result for Er lay on a smooth curve with the rest of the data (Figs. 5 and 7). Recent results obtained with additional rock samples have suggested a correlation between high Th concentrations and this interference, which appeared as a separated peak between Er and Ho when the column capacity was increased. This is the retention zone expected for Th, and the identity of this peak will be determined during future applications of this method. The HPLC results for all the rock samples are summarized in Table IV. Reference data that were suspected to be in error were not included in Table IV; a data point was considered unacceptable if it had a large r.s.d. (some reference data) and/or showed a large deviation from an otherwise smooth plot of the chondrite-normalized values. The four HPLC values for Er that were known to be high were also discarded. To produce the summary in Table IV, each HPLC REE result was ratioed to the reference value, and for each REE, a mean and r.s.d, value was calculated for these ratios. T h e deviation of the mean value of each ratio, )~r, from 1.0 can be considered to be a measure of the bias (relative to reference value) for a given
192 T A B L E III H P L C analysis {in #g g - 1) of rock samples Element
BCR-I
W2
HPLC
reference
La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu
23.7 52.4 6.7 28.6 6.5 1.93 6.70 1.04 1.31 3.80 0.53 3.50 0.48
25.0 53.7 6.9 28.7 6.6 1.96 6.68 1.05 1.25 3.61 0.59 3.39 0.51
Element
HUD
reference
10.4 23.1 3.0 13.1 3.25 1.07 3.70 0.58 0.76 2.23 0.30 1.98 0.34
10.4 23.4 3.18 13.4 3.3 1.1 4.15 0.63 0.83 2.35 0.4 2.1 0.33
D-IO
HPLC
reference
La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu
28.3 56.6 6.74 25.5 4.17 1.09 2.71 0.31 0.26 0.66 0.075 0.54 0.08
27.6 55.8
Element
DR-N
La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu
HPLC
KL2
25.3 4.2 1.11 2.77 0.68 0.59 0.09
HPLC
0.97 2.65 2.12 0.30
reference
6.29 17.4 2.7
82.0 152 17.3 66.0 12.0 3.5 9.75 1.25 1.05 3.4 0.32 1.91 0.29
83.0 148 5* 65.0* 12.1 3.5 9.67*
4.38 1.54 5,7 0.91 1.33 4.1 3.85 0.56
2.4* 1.75" 0.23*
M664
HPLC
reference
HPLC
reference
21.2 45.6 5.7 23.4 5.3 1.45 5.1 0.78 0.93 3.2 0.39 2.5 0.37
20.7 46* 6.0* 23.4* 5.3 1.42" 4.3*
281 459 44 139 19.6 5.7 14.4 1.93 1.95 5.4 0.67 4.55 0.58
299 492 53* 168 20.8 5.6 16" 2.11 2.5* 0.85* 4.77 0.63
*Estimated errors for referenced value > + 10To.
13.7 33.7 4.71 22.3 5.78 1.98 6.16
HPLC
MF217
2.7* 0.39
13.2 32.7 4.64 21.6 5.63 1.91 5.86 0.87 0.99 2.56 0.33 2.03 0.30
reference
4.26 1.46 5.54 0.93 1.29 3.8 0.56 3.52 0.60
reference
BR
7.36 18.7 2.81 -
HPLC
HPLC 0.54 1.43 0.26 1.37 0.54 0.21 0.84 0.145 0.22 0.72 0.1 0.69 0.1
reference 0.54 1.78 0.28 1.49 0.61 0.23 0.84 0.15 0.24 0.69 0.66 0.097
193 100
50
4--4,,
oZ ~
10
Ld J o_~ < cn
5
1
I
l]
KL2
~,
•
o
KL2R HUD
•
HUDR
I
I
~ "o. ~
I
~
I
La Ce Pr Nd
I
I
I
I
I
I
I
I
I
~= ~ 10
A •
~ z 7, ~) ~ ~
00-10 • D-10R
J
5
!
•
M664 M664R
I
I
o
1
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
W2 W2R
L
"~-- ~'~",,~...~ " "~-
I
Lo Ce Pr Nd
Fig. 5. Chonclrite-normalized concentrations for H P L C and reference data. Open points are H P L C data, solidpoints are reference data, and starredpoints are for closely overlapping H P L C and reference data and cross is initial Er value for H U D . ).
I
I
~
I
I
I
L
I
I
1
I
I
t
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 7. Chondriteinormalized concentrations for H P L C and referencedata. Open points are H P L C data, solid points are reference data, and starred points are for closely overlapping H P L C and reference data. 120
1000
500 ~-
1 O0
\.
" 0 , ,
a z
o:=
\ .
z
5o
MF217 MF217R
~
u
BR
I
• ~
"*.•
o •
~
DR-NR
•
:
z
:
# a_
=: I
I
I
I
La Ce Pr Nd
I
I
1
I
-"'~'t~
[3
SED
0
BCR-1
°
"~ d
5¢
50
X x t l
I
I
I
I
J
I
I
S m Eu Gd Tb Dy Ho Er Tm Yb Lu
__ 10
s
I I I I I I I I I I I I I I I Lo Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig.8. Chondrite-normalized concentrations for H P L C
and
Fig. 6. Chondrite-normalized concentrations for H P L C and reference data. Open points are H P L C data, solidpoints are reference data, starred points are for closely overlapping H P L C and reference data, and cross is initialEr value
referencedata. Open-points are H P L C data, solidpoints are reference data, and starred points are for closely overlapping H P L C and reference data.
forMF21?.
for all R E E was 4.1% which corresponds to a
R E E determination, and the r.s.d,of each -~r-
r.s.d,value of 2.9% for the H P L C assumes that:
value gives a m a x i m u m r.s.d, value for the H P L C determination of the REE; this r.s.d, value is also affected by the r.s.d,value of the analytical technique used to produce the reference value by the relationship: (r.s.d.2)z,= (r.s.d.2)HPLC + (r.s.d."~)re~,~ The r.s.d,data in Table IV show that the best agreement with the reference data was obtained from La to Tb; the average r.s.d,for this range of R E E was 3.4%. The mean r.s.d,value
results if one
r.s.d.HPLC-~r.s.d.~fe ..... The H P L C results obtained for some of the individual rock samples indicated a potential negative bias for the H P L C method. This was confirmed by the pooled data in Table IV where the mean value, .~(z,) of all ratios differed from the expected value of 1.0 b y - 1 . 7 % . With an r.s.d,value of 1.05% for X~z,), this bias was significantat the 95% confidence level.This small
194
TABLEIV
group separation) is fast relative to many other
SummaryofHPLCresultsfor REE
techniques, and sample consumption (1-5 mg rock per analysis) is low. Consequently small columns can be used for the group separation
Element Number Meanratio,)~r % r.s.d. Concentration of (HPLC/reference) range
samples
(gg-1)
La
8
0.981
3.9
0.5 -300
Sm Eu Gd Tb Ho Er Tm Yb
8 0.982 8 0.982 7 0.970 5 0.972 5 0.974 5 0.987 no referencedata available 7 0.992
2.4 2.2 3.7 3.8 5.7 5.4 6.5
0.5 - 20 0.2 - 6 0.8- 16 0.1 - 2 0.2 - 2 0.7- 1 0.8- 0.7 0.6 - 5
5.2
0.0S- 0.6
Ce Pr Nd
au
8 6 6
7
1.005 0.973 0.982
0.994
3.5 3.9 3.5
1 -50o O.3 - 50 1 -150
-~(xr) =0.983, % r.s.d.= 1.05;X (% r.s.d.)=4.1,
bias may be associated with incomplete sample dissolution, or precipitations of insoluble components, a n d / o r an error in the concentration of standards used for calibration; these factors will be examined in more detail as this method is applied to REE determinations on a routine basis. Sample losses during the group separation may also be a contributing factor, but significant losses at this stage should result in larger biases for the LREE, which is not evident from the data in Table IV. For the routine application o f t h i s m e t h o d t o g e o c h e m i c a l s t u d ies the relative concentrations of the REE is most important, and a comparison of the relative concentrations for a series of R E E pairs in the samples and in the reference data showed t h a t the relative concentrations were consist e n t to _+1.6%. 4. C o n c l u s i o n s
and only 0.3 mol HNO3 are required for each sample. The method gives consistent relative concentrations for all REE except Dy, and the small bias of - 1.7% for the H P L C results relative to reference data can possibly be eliminated through minor changes i n t h e analytical procedures. Acknowledgements I would like to t h a n k A1 H o f m a n n for his interest and support throughout these studies. I would also like to t h a n k Peter Pfeiffer for his analysis of my eluents for bacteria, Nick Arndt and Catherine Chauvel for their m a n y suggestions, Steve Elchuk for his technical input, and Chris Roddick for his comments concerning oxalic acid. References Aulis, R., Bolton, A., Doherty, W., VanderVoet, A. and
Wong, P., 1985. Determination of yttrium and selected rare earth elements in geological materials using high performance liquid chromatographic separation and ICP spectrometric detection. Spectrochim. Acta, 40B: 377-387. Barkley, D.J., Blanchette, M. and Cassidy, R.M., 1986. Dynamic chromatographic systems for the determination of rare earth and thorium in samples from uranium ore refining processes. Anal. Chem., 58: 2222-2226. Cassidy, R.M., Elchuk, S., Elliot, N.L., Green, L.W., Knight, C.H. and Recoskie, B.M., 1986. Dynamic ion exchange chromatography for the determination of number of fissions in uranium dioxide fuels. Anal. Chem., 58:
1181-1186. Cassidy, R.M., Elchuk, S. and Dasgupta, P.K., 1987. Per-
The combination of a simplified group REE separation and H P L C analysis is an attractive method for the determination of REE in a wide variety of rocks. The average % r.s.d, of ~ 2.9 is comparable to other techniques and can he iraproved by repetitive sample analysis. The 10-20-min. time required for one analysis (after
formance of annular membrane and screen-tee reactors for postcolumn-reaction detection of metal ions separated by liquid chromatography. Anal. Chem., 59: 85-90. Crock, J.G., Lichte, F.E. and Wildeman, T.R., 1984. The group separation ofthe rare-earth elementsand yttrium from geologic materials by cation-exchange chromatography. Chem. Geol., 45: 149-163. Date, A.R. and Gray, A.L., 1985. Determination of trace elements in geological samples by inductively-coupled-
195 plasma source mass spectrometry. Spectrochim. Acta, 40B: 115-122. Doherty, W. and VanderVoet, A., 1985. The application of inductively-coupled-plasma mass spectrometry to the determination of rare earth elements in geological materials. Can. J. Spectrosc., 30: 135-141. Feigenson, M.D. and Carr, M.J., 1985. Determination of major, trace, and rare-earth elements in rocks by DCP-AES. J. Chem. Geol., 51: 19-27. Govindaraju, K., 1984. Compilation of working values and sample description for 170 international reference samples of mainly silicate rocks and minerals. Geostand. Newsl., 8: 3-16. Hofmann, A.W., Jochum, K.P., Seufert, M. and White, W.M., 1986. Nb and Pb in oceanic basalts: new constraints on mantle evolution. Earth Planet. Sci. Lett., 79: 33-45. Jochum, K.P., Seufert, M. and Knab, H.-J., 1981. Quantitative multielement analysis of geochemical and cosmochemical samples using spark source mass spectrometry. Fresenius Z. Anal. Chem., 309" 285-290. Joron, J.L. and Ottonello, 1985. Radiochemical neutron activation analysis of rare-earth elements in peridotitic rocks. J. Radioanal. Chem., 88: 259-272. Mattinson, J.S., 1972. Preparation of hydrofluoric, hydrochloric, and nitric acids at ultralow lead levels. Anal. Chem., 44: 1715-1716. Mazzucotelli, A., Dadone, A., Frache, R. and Baffi, F., 1985. Determination of trace amounts oflanthanides in rocks and minerals by high-performance liquid chromatography. J. Chromatogr., 349: 137-142. Moody, J.R. and Beary, E.S., 1982. Purified reagents for trace metal analysis. Talanta, 29: 1003-1010.
Nakumura, N., 1974. Determination of rare earth elements, Ba, Fe, Mg, Na, and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta, 38" 757-775. Newsom, H.E., White, W.M., Jochum, K.P. and Hofmann, A.W., 1986. Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core. Earth Planet. Sci. Lett., 80: 299-313. Robinson, P., Higgins, N.C. and Jenner, G.A., 1986. Determination of rare-earth elements, yttrium and scandium in rocks by an ion exchange-X-ray fluorescence technique. Chem. Geol., 55: 121-137. Roelandts, I. and Michel, G., 1986. Sequentially inductively coupled plasma determination of some rare-earth elements in five French geostandards. Geostand. Newsl., 10: 111-120. Strelow, F.W.E. and Jackson, P.F.S., 1974. Determination of trace and ultratrace quantities ofrare-earthelements by ion-exchange chromatography and mass-spectrometry. Anal. Chem., 46: 1481-1486. Taylor, S.R. and Gorton, M.P., 1977. Geochemical applications of spark-source mass spectrography, III. Element sensitivity, precision and accuracy. Geochim. Cosmochim. Acta, 41: 1375-1380. Wanke, H., Kruse, H., Palme, H. and Spettel, B., 1977. Instrumental neutron activation analysis of lunar samples and the identification of primary matter in lunar highlands. J. Radioanal. Chem., 38: 363-378. Yoshida, K. and Haraguchi, H., 1984. Determination ofrareearth elements by liquid chromatography/inductively coupled plasma atomic-emission spectrometry. Anal. Chem., 56: 2580-2585.