Analytical Chemistry of the Lanthanides Part 1. Atomic Absorption and Plasma Atomic Emission Spectroscopic Methods

ANALYTICAL CHEMISTRY OF THE LANTHANIDES PART 1. ATOMIC ABSORPTION AND PLASMA ATOMIC EMISSION SPECTROSCOPIC METHODS

Lauri H.J. Lajunen* and Gregory R. Choppin

Department of Chemistry Florida State University Tallahassee, Florida 32306-3006

CONTENTS

Page ABSTRACT 92 INTRODUCTION 92 ATOMIC ABSORPTION SPECTROMETRY 93 FLAME ATOMIC ABSORPTION SPECTROMETRY 93 GRAPHITE FURNACE ATOMIC ABSORPTION SPECTROMETRY 103 PLASMA ATOMIC EMISSION SPECTOMETRY 107 SUMMARY 125 REFERENCES 126

*On leave from the Department of Chemistry, University of Oulu, SF-90570 Oulu, Finland

91 ABSTRACT

Recent research on analytical atomic absorption and plasma atomic emission spectrometry of the lanthanides is reviewed and the conditions for use of these techniques for a variety of sample types discussed.

INTRODUCTION

Interest in sensitive methods of lanthanide element analysis has expanded greatly in the last 20 years as the scientific and technological value of the lanthanides has increased. The abundance of the lanthanide elements (the "rare earths") in ores, sediments and waters, has been studied extensively to obtain a better understanding of geological processes since their distribution in geological samples is affected by magmatism /1 / and by sedimentation /2/. The use of lanthanides in chemical and biochemical research continues to grow steadily. Industri- ally, the value of lanthanides as catalysts and phosphors, and in superconducting magnets has increased their use greatly. Many applications of lanthanides require high-purity materials particularly in those systems where even small impurities can markedly influence the properties of the lanthanide materials. Other applications require products of well known composition. The majority of the application of atomic spectrometric methods in the literature deal with determination of lanthanides in geological samples or in high-purity materials, reflecting the activity in these areas. Several general reviews of atomic absorption /3/ or atomic emission /4/ have recently been published, but these have not considered the lanthanide elements specifically. Review articles on the analytical chemistry of the lanthanides have appeared in Japanese /5/, Hungarian 16/, and Russian /7/. A review (in English) describes Chinese research on the analytical chemistry of the rare earths /8/. This paper reviews analyses of the lanthanides by absorption and plasma atomic emission spectrometry. We have not attempted a comprehensive review but have selected papers which seemed of broadest or most useful information. The literature search for this review ended in late 1985.

92 ATOMIC ABSORPTION SPECTROMETRY

Flame atomic absorption spectrometry

Flame atomic absorption spectrometry (FAAS) allows precise, and accurate determination of lanthanides. Unfortunately, FAAS methods are limited in application by insufficient sensitivity due to the formation of thermally stable lanthanide oxides and the complexity of the absorption spectra of many of the lanthanides. For example, more than 1000 absorption lines have been observed in the 250-650 nm region in fuel-rich, oxy-acetylene flames fed with ethanolic solutions of lanthanide Perchlorates /9/, 165 of which are due to samarium. Most of the lines do not involve excitation originating from the ground state and few of the lines are sufficiently intense for analytical use. However, despite the complexity of the absorption spectra, spectral interferences of different lanthanides are rare. Thus, only the neodymium line at 492.5 nm is unsuitable for neodymium determinations in the presence of praseodymium /10/. In the analyses of real samples, separation and preconcentration procedures are often needed prior to the absorption measurements. Unless the excitation occurs in a reducing atmosphere at very high temperature, lanthanides form stable oxides with a relatively low population of ground state atoms /ll/. Table I lists the ionization potentials of lanthanides and dissociation energies of lanthanide monoxides. The high thermal stability of these monoxides, LnO, makes the direct determination of lanthanum and cerium by FAAS insensitive even with hot nitrous oxide-acetylene or oxy-acetylene flames /12,13,24-26/. In these hot flames used in FAAS analyses, 35-80% of the lanthanide atoms are ionized /15/ (e.g., 75% for Eu at 100 mg/1 concentration /16/). The instrumental detection limits, the characteristic concentrations (1% absorption) and the recommended flame types for the most suitable absorption lines in aqueous solutions and in 80% methanol solutions buffered with sodium or potassium (2%) are given in Table II /10,18/. On the basis of the values for the characteristic concentration, sensitivities of the determination of lanthanides with the latest FAAS instruments are 3 to 5 times greater than 10-15 years ago /11,19/. The highest sensitivities and lowest detection limits are obtained for europium, dysprosium, erbium, thulium and ytterbium while lanthanum, cerium, praseodymium and gadolinium have the lowest sensitivities and

93 TABLE I

Ionization Potentials of Lanthanide Atoms and Dissociation Energies of Lanthanide Monoxides /17/

Ln/Lno m.p. b.p. Dissociation Ionization (K) (K) energy potential LnO Ln° Ln Ln+ + Fe" (eV/molecule) (eV)

La 1193 3742 5.61 LaO 8.2 Ce 1068 3741 6.54 CeO 8.03 Pr 1208 3400 5.76 PiO 7.9 Nd 1297 3300 6.31 NdO 7.4 Sm 1343 2173 6.6 SmO 6.1 Eu 1099 1712 5.67 EuO 5.8 Gd 1585 3300 6.16 GdO 7.4 Tb 1629 3073 6.74 TbO 7.4 Dy 1680 2873 6.82 DyO 6.5 Ho 1734 2873 6.9 HoO 6.5 Er 1770 3173 6.7 ErO 6.6 Tm 1818 2000 6.6 TmO 6.0 Yb 1097 1700 6.22 YbO 5.3 Lu 1925 3600 6.15 LuO 7.2

highest detection limits. This correlates well with the tendency of the lanthanides to form thermally stable monoxides and to ionize in hot flames (Table I). The sensitivity can be increased by addition of a material with a low ionization potential or by use of organic solvents. The addition of an

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95 element with a lower ionization potential (e.g., an alkali metal) decreases the ionization of the lanthanide due to the increased electron density in the flame. Thus, addition of potassium or sodium ions enhances the sensitivity because of the higher proportion of absorbing atoms present. The effect of added potassium on increased sensitivity is shown in Fig. 1 /2/.

~~Fig. 1: The effect of potassium concentration on the absorption of lanthanides /10 /.~~

Ishizuka and Sunhara /20/ investigated the effects of several metal ions on the absorption of terbium and dysprosium. That of terbium decreased in the presence of 30-300 mg/1 of Li, Na, K, Mg, Ca, Cr, Μη, Fe, Ni and Pb, but increased by 25-90% in the presence of 3-10 mg/1 of these elements. The presence of Sr and Ba in the 1-10 g/1 range showed similar effects of increasing the absorption. Copper had only a minor effect below 300 mg/1 but 110 g/1 increased the absorption by 40-50%.

96 The signal increased with increasing aluminum concentration (35% at 300 mg/1), but this was reversed at higher concentrations. The absorption of dysprosium showed similar changes. The absorption of terbium and dysprosium also increased with increasing concentrations of other lanthanide elements (Fig. 2). Water soluble organic solvents increase the sensitivity as the nebulization is more efficient in the solutions of lower surface tension and decreased solvation /10,21/. Figure 3 shows the effect of added methanol on the absorption signal /10/. The sensitivity of atomic absorption of lanthanides was improved by a factor of 2-5 over an aqueous solution when an absolute ethanol solution of lanthanide Perchlorates was aspirated into a nitrous oxide-acetylene flame /22/. Lanthanum (1%) was used as a spectroscopic buffer to eliminate interferences and to enhance the sensitivity. This method was improved /23/ by using microsample injection which further increased the sensitivity by as much as 150%. Small sample volumes, typically, 200 μΐ

~~La, Pr, Nd,Sm,Eu Y, Er, Tm Gd, Tb, Ho,Yb, Lu Y, Oy, Ho, Er.Tm.Lu La, Pr, Nd, Gd~~

~~Sm, Eu,Yb~~

~~0.00 100 10000 10 1000 Added rare earth concentration (mg/l)~~

~~Fig. 2: The effects of rare earths on the absorption of terbium and dysprosium (Tb concentration 300 mg/l; concentration 100 mg/l) 1201.~~

~~97 METHANOL CONTENT (%)~~

~~Fig. 3: The effect of methanol on the absorption of lanthanides /10/.~~

injections of absolute ethanol solutions of the lanthanide Perchlorate buffered with 1% lanthanum, permit rapid FAAS determination of traces of lanthanides. Due to the low sensitivities of direct FAAS measurements of lanthanum and cerium, indirect methods have been developed /14,24-26/. Formation of the heteropolymolybdocerophosphoric acid (MCPA) allows indirect determination of cerium by measurement of the atomic absorption of molybdenum at 313.2 nm /14/. MCPA is formed by adding phosphate and molybdate to the test solution followed by adjustment to 0.4M with respect to nitric acid. The unreacted heteropolymolybdophosphoric is removed with a mixture of n-butanol and chloroform. After extraction of the MCPA into isobutyl acetate, the organic solution is aspirated into a nitrous oxide-acetylene flame and the atomic absorption of molybdenum measured. The

98 calibration graph was linear from 0.040 to 0.400 mg of cerium, corresponding to 1.6 to 16 mg/1 of cerium, in the initial aqueous solutions. The sensitivity (1% absorption) of cerium was 0.093 mg/1, which is about 300 times better than the corresponding value obtained by the direct method (Table I). The precision of the method is 4.7% (the percent relative standard deviation for ten samples, each containing 200 mg/1 cerium). Lanthanum has been determined by an indirect FAAS method using a wire loop atomizer /24/. Cadmium is preconcentrated by soaking the wire in CdCl2 solution, then immersing it in the lanthanide sample solution. Lanthanum(III) ions displace Cd(II) ions from the wire, resulting in a decrease of the atomic absorption signal of cadmium. Monovalent cations do not replace the Cd(II) so their presence has no affect on the cadmium signal (unless they are present in very high concentrations), but most other cations replace cadmium and interfere with the determination. If the sample contains sufficient concentrations of interfering ions, prior separation or masking is necessary. The absorbance change is linearly related to the soaking time of the wire in the lanthanide solution and measurements of ppb levels are achievable with increased soaking times. The average percent relative standard deviation was found to be 1.5-3%. As this technique is not selective for lanthanum, analyses of real samples require prior separation. Jackwerth et al 25/ developed an indirect atomic absorption determination of lanthanum with a sensitivity of 0.12 mg/1 based on the use of the copper(II) diethylenetriaminepentaacetate (CTPA) complex as an exchange reagent. In the presence of bathocuproine, which is selective for copper(II), lanthanum(III) displaces the copper. The amount of copper-bathocuproine complex is determined by FAAS after its extraction into hexanol. An indirect FAAS determination of cerium and lanthanum by flow injection analysis (FIA) using an air-acetylene flame has been reported /26/. An enhancement of the absorption signal of iron is produced by small amounts of cerium and lanthanum. This method allows the indirect determination of Ce and La separately in the concentration ranges of 0.07-0.70 mg/1 and 0.02-0.22 mgl, respectively. The procedure involves injection of 54 μΐ aliquots of cerium or lanthanum at pH 5.5 into a reagent carrier solution of 25 mg/1 iron(III) and 100 mg/1 EDTA or tartrate at pH 2. The enhancement in the iron signal at. 248.3 nm caused by cerium and lanthanum is used for determination of the latter elements. The FIA method is approximately 5000-fold more sensitive

99 for both La and Ce than the direct FAAS determination and offers advantages such as small sample volumes, sampling frequency as high as 150 per hour, increased selectivity and use with samples of high salt content. However, it is suitable only for relatively pure solutions of cerium and lanthanum unless a separation process is included. Several studies of determination of individual lanthanides in rare earth mixtures by FAAS have been reported /10,19,20,28/. The europium content of europium activated yttrium orthovanadate, yttrium oxide and yttrium oxysulfide phosphor powders used in the color television industry can be determined rapidly and simply by FAAS /29/. Other applications of FAAS deal with the determination of lanthanides in geological samples /2,11/14,15/19/. Lanthanides are separated from rock samples by precipitation with sodium hydroxide, ammonia, hydrofluoric acid and calcium oxalate /10,19,22,23/ and by ion-exchange /23/. Oghe and Verbeek /10/ determined lanthanides by FAAS in synthetic rare earth mixtures of known composition, in commercial lanthanide oxides of high purity, and in bastnasite and monazite. The ore samples were dissolved in hydrochloric acid (bastanasite and gadolinite) or perchloric • acid (monazite) to which a few drops of an oxidant (hydrogen peroxide or nitric acid) was added. The silicic acid present was dehydrated by heating. The residue was redissolved in hydrochloric acid and filtererd. The filtrate was reduced by evaporation and diluted with the alkali buffer solution and methanol to make the solutions 2% and 80% with respect to sodium (or potassium) and methanol, respectively. The standard addition method was applied to eliminate chemical, ionization and matrix interferences. In minerals and ores the less sensitive light lanthanides are generally present in larger amounts than the more sensitive heavier lanthanides which increases the value of FAAS for analysis of the lanthanides. The analyses were satisfactory with a precision of ca. 0.5%.

Sen Gupta /22,23,27/ has also studied FAAS methods for analysis of the lanthanides in a variety of geological samples using the procedure outlined in Figure 4. For geological samples containing small amounts of these elements, the procedure in Figure 5 was followed. The effectiveness of this method is shown in Table III where the results of analysis of two lanthanide minerals by FAAS and x-ray fluorescence show satisfactory agreement in general. FAAS allows determination of small amounts of Eu, Dy, Ho, Er, Tm and Yb in britholite. All three different sample preparation methods (direct determination after

~~100 Decompose the sample (2-10 g) with HF. Evaporate to dryness. Add 40- 50 ml HF and heat on the steam bath. Cool, filter and wash.~~

~~Reject filtrate. Decompose precipitate on filter paper by nitric and perchloric acids and evaporate. Dissolve residue 1n HNO, (101) and H,0, (5%) mixture. Filter Iny residue and wash.~~

Burn the filter, and fuse residue with Filtrate (soln. A). Precipi- K,S-07. Cool and dissolve residue in tate the oxalates. Filter and dflf H,S0. (soln. B). Precipitate wash. the oxalates. Filter and wash.

~~Rejec tfi1 träte. Combine oxalate precipitates Reject filtrate. and decompose by HNO,. Repeat oxalate precipitation. Filter and wash.~~

Reject filtrate. Decompose precipitate by nitric and perchloric acids and evaporate. Dissolve residue 1n HNO, (10Ϊ) + H.O, (5Ϊ), add Iron carrier (6 mg Fe,0,), hydroxylamlne hydrochloride (2 g) and precipitate R. E. hydroxides + Fe(OH), by anrionla. Filter and wash. 1 I ~

~~Reject filtrate. Decompose filter paper by HNO,. Add HC10., evaporate to a moist residue. Cool and dissolve In 5 or 10 ml of absolute etha'nol.~~

Determine La by flame-em1ss1on Transfer aliquot! to vol, flasks, add from an aliquot (no buffer) or lanthanum buffer (1% La 1n the final after diluting 1t with absolute vol.), and dilute to vol. with absolute ethanol. ethanol. Determine Y, Nd, Sm, Eu, Dy, Ho, Er, Tm and Yb by AAS.

~~Fig. 4: Sample preparation scheme for determination of lanthanides in rare earth minerals 1221.~~

phosphate separation, determination after separation by oxalate and hydroxide precipitation without using any carrier, and determination after separation by oxalate and hydroxide precipitation using iron as a carrier) gave essentially similar results for the minor lanthanides in britholite. The procedures in Figs. 4 and 5 are applicable to lanthanide minerals and to certain rock samples of relatively high lanthanide content, but it

~~101 Decompose the sample (0.1 - 0.2 g) with HNOj • HF and evaporate on steam bath.~~

I 1 Phosphate Minerals Non-Phosghate Minerals (B) Add 25 ml HF anä dige«t on Add HNO. and evaporate. the steam bath. Cool, Add HC104 (60Z) and filter and wash. evaporate to a moist residue. Cool in a desslca- tor. Dissolve In 25 ml abaolute alcohol.

~~Re.feet filtrate.~~

~~Decompose residues and filter paper with HNO^. Treat like non-phoaphat· mineral sample (B).~~

Determine La by flame- Determine the major Determine the minor emission from an rare earths by AAS rare earths by AAS aliquot (no buffer) or from a 1 mg/ml sample from a 3 mg/ml sample after diluting It with soln. (buffered soln. (buffered with absolute ethanol. with IZ la) In IZ La) in absolute absolute ethanol. e thanol.

~~Fig. 5: Sample preparation scheme for determination of lanthanides in rock samples containing small amounts of these elements 1221.~~

is not suitable for common rocks and minerals containing only traces of lanthanides. This is due to difficulties in the decomposition and subsequent handling of the large amount of sample (10-15 g) required for preconcentration of the lanthanides. Smaller samples (2-3 g) for FAAS determination of these elements using microsample injections have resulted in a significant saving of time and simplication of the sample dissolution and preconcentration in these materials /23/. Complete recovery of added lanthanides was obtained after their separation by double calcium oxalate and hydrous ferric oxide coprecipitation. The results obtained by the microsample injection technique were in reasonably good agreement with those of direct flame aspiration and GF-AAS methods. The microsampling method has the advantage of being easier to use than GF-AAS technique, suffering less interference from associated elements and being more rapid for routine work.

~~102 TABLE III~~

~~Determinations of Some Lanthanides in Two Rare Earth Minerals by FAAS and X-Ray Fluorescence~~

~~Oxide Britholite Cenosite FAASa X-raya FAASa X-rayc (%) (%) (%) (%)~~

~~La203 5.9 5.6 NdjOs 7.5 8.0 0.20 0.19~~

Sm203 1.3 1.5 0.22 0.27 EU2O3 0.29 d 0.13 d DyzOs 0.22 d 3.06 3.10 HO203 0.04 d 1.07 d ErjOs 0.03 d 2.65 3.53 ΤΠ12Ο3 0.006 d 0.45 d YbPa 0.012 d 2.33 2.71 a) ref. 22 b) ref. 31 c) ref. 32 d) not determined

In summary, relatively good sensitivities for Nd, Sm, Eu, Dy, Ho, Er, Tm and Yb have been obtained by aspirating methanolic or ethanolic solutions of their Perchlorates into a nitrous oxide-acetylene flame. They can be determined in a variety of geological samples /19,22,23/. However, for traces of La, Ce, Pr, Gd, Tb and Lu, the FAAS sensitivities are not adequate for determination in most rock samples.

~~Graphite furnace atomic absorption spectrometry~~

Recently, considerable attention has been paid to the determination of the lanthanide elements by graphite furnace atomic absorption spectrometry (GF-AAS), particularly for extremely low concentrations (Table IV). The tendency of lanthanides to form thermally stable carbides leads to reduced sensitivities in graphite furnaces. The determination of elements (including lanthanides) which react with graphite to form low volatility compounds were studies by GF-AAS using a tungsten furnace /43/. The sensitivities were higher in this furnace or in a graphite furnace lined with tantalum-foil /34/. Improvements in the sensitivities for the lanthanides by factors of

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105 2.5-10 were obtained with pyrolytically coated graphite tubes /38,39/. Under comparable experimental conditions, the sensitivities obtained with the tantalum-foil lined graphite tubes were 6 (Yb) to 40 (Ce) times better than with pyrolytically coated graphite tubes /36,37/. The lowest sensitivity and highest detection limit were obtained for cerium due to the complexity of the low intensity cerium spectra although more than 60% of the cerium atoms are ionized /13/. The determination of ytterbium is the most sensitive by GF-AAS with reported values for the detection limit and sensitivity (1% abs.) of 0.005 mg/1 and 0.0036 mg/1 with pyrolytically coated tubes, and 0.027 mg/1 and 0.005 mg/1 with uncoated graphite tubes /40/. With coated graphite tubes, the sensitivity of ytterbium by GF-AAS is about 800 times better than by FAAS (Table II). A study of sensitivity of lanthanides by GF-AAS using pyrolytically coated graphite tubes, tubes coated with carbides of tantalum, zirconium or lanthanum, and a tantalum-foiled tube gave best sensitivities with the last type of tube /41/. Increases of 1.4 to 8 fold in sensitivity were reported for tantalum coated tubes compared to those pyrolytically coated /42/. The atomization surface plays a major role in atom formation, analytical sensitivity, and reduction in memory effects associated with the formation of nonvolatile carbides /40/. A disadvantage of vaporization from the wall of the graphite tube is that temperature equilibrium may not be attained in the furnace while the lanthanide atoms are present so maximum population of excited energy levels may not occur until after the concentration of atomic vapor is already past its maximum /44/. Isothermal conditions can be obtained in the graphite furnace by the use of a platform (L'vov platform) /45,46/, which has received much attention in the literature recently as it promises an interference-free method. The platform temperature lags that of the graphite tube wall and the conditions closely approach isothermal /47/. The absorption intensities of ions are significantly improved in sensitivity with the platform and microboat although such ionic absorption is normally not used in AAS. The enhancement in sensitivity is more significant for nonresonance than for resonance lines. The detection limit (the amount giving a signal-to-noise ratio of 2) for the platform, microboat and tube wall vaporization were 3.2, 5.0 and 4.9 Mg/1, respectively. Peak height and peak area precision for ten replicate measurements of 50 μΐ aliquots of 7.5-25 pig/1 of 5.3% was obtained for the microboat. For yttrium, samarium and dysprosium, GF-AAS (using a tantalum

106 boat inserted into a graphite tube atomizer) gave better analytical sensitivities and negligible memory effects /48/. The sensitivities (1% abs.) for samarium and dysprosium with the tantalum boat were 0.86 and 0.17 ng, respectively. Ma et al 49/ investigated the effect of the heating rate on the sensitivity of the determination of rare earths by GF-AAS. GF-AAS methods have been applied to determination of lanthanides in geological samples /27,35-37/, in nuclear solutions containing uranium, thorium and fission products /33/, in misch metal /48/, and in animal tissue /49/. For determining the lanthanides in geological samples by GF-AAS, it is desirable to free these elements from the bulk of matrix elements. A convenient way to obtain the lanthanides as a group from such samples is to volatilize the silicon with hydrofluoric acid, co-precipitate the lanthanides as the oxalates with calcium and then separate the calcium by co-precipitation of the lanthanides as hydroxides with a small amount of added iron /35/. A lanthanide concentrate prepared in this way contains all the rare earth elements together with scandium, yttrium, thorium and added iron. The effect of iron on the absorbance of the individual lanthanides was found to be negligible up to the 200-ppm level. The analysis of four synthetic lanthanide solutions approximating the composition of some reference rocks revealed that interelement effects are negligible. For a number of standard reference rock samples, GF-AAS gave satisfactory agreement with values by other techniques.

~~PLASMA ATOMIC EMISSION SPECTROMETRY~~

Inductively coupled plasma (ICP) and direct current plasma (DCP) methods are used as atomization and excitation sources for simultaneous multielement trace analysis by atomic emission spectrometry (AES) /54-60/. Among the advantages of these techniques are high sensitivity and stability, large dynamic range of calibration curves, relative freedom from matrix effects, lack of chemical interferences, and ability to detect lanthanides at concentration levels usually not achieved by atomic absorption. However, with plasma AES the high number of emission lines which are excited can lead to interferences from other lanthanides and matrix elements /61,62/. Table V lists values reported in the literature for instrumental detection limits of ICP-AES and DCP-AES. The limits for lanthanides

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108 by plasma AES methods are about 1000 times lower than those for FAAS in Table II. In addition, the dynamic ranges of linear calibration curves for plasma AES are significantly greater than for FAAS. DCP-AES generally has somewhat lower detection limits than those of ICP-AES. Table VI lists the interfences in ICP-AES for the most sensitive emission line of each lanthanide /63/. The interference is indicated as a value proportional to the concentration of the analyte element (ng/ml of analyte element per 10 Mg/ml of interfering element). The spectral resolution of the conventional monochromator used in ref. 63 led to interference from emission lines within 0.02 nm of the analyte line. Even with a DCP instrument equipped with an Echelle monochromator (spectral resolution 5-10 times better than that of conventional monochromators) spectral interferences exist /64/. Readily ionized elements (such as alkali metals) in high concentrations often cause enhancement of the analyte line (especially in DCP). For example, Li, Na, K, Ca and Mg increase remarkably the sensitivity of lanthanum emission lines at 398.85 and 409.67 nm, when present in a 100:1 ratio (interferent: analyte) /65/. This effect can be used to improve the detection limits and to eliminate matrix effects by buffering the test and standard solutions with alkali metal ions (e.g., 2.5M L1NO3 /60/. With D.C. arc excitations, the bromide, chloride and fluoride salts of sodium increased the evaporation and decreased the temperature in the plasma by 200-300K /66/. A number of studies have been reported on the application of ICP-AES to the determination of lanthanides in geological samples /67-86/. In comparison with dissolution by a fusion procedure, acid digestion allows larger samples to be used with less contamination. However, acid digestion may not completely dissolve the sample. Since the lanthanides are usually in low concentrations with much higher concentrations of potentially interfering matrix elements, sample pretreatment by ion exchange separation is often employed /78,82/. The requirements of ion-exchange separation for the ICP-AES analysis of the lanthanides in geological samples are: (1) recovery of the lanthanides as a group, (2) dissolution of the sample and preconcentration of a small volume to allow maximum sensitivity, and (3) reduction of salt content of the final solution. In practice, compromises are necessary between resin size (larger resin size increases band spreading) and eluant flow rate which can lead to problems in either significant concentrations of matrix elements or reduced lanthanide recovery in

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110 the fraction needed for the spectrometric analysis /101/. Moreover, the relatively large eluant volumes increase separation time and require volume reduction. Many of these problems are avoided with high performance liquid chromatography (HPLC). The combination of HPLC with ICP or DCP detection is increasingly popular /63,100-105/. Fries and Lamathe /75/ used a sequential-scanning ICP spectrometer for the determination of the lanthanides in manganese nodules. The dissolution method consisted of digestion of the sample (0.5 g) in a mixture of hydrochloric, nitric and hydrofluoric acids (by volume, 7:4:3) in a sealed polycarbonate container immersed in boiling water for 30 minutes. After cooling, 1.5% boric acid solution was added and the resealed system heated in boiling water. The filtered solution was used for the emission measurements. Recoveries of lanthanides were better than 99%. The ICP emission spectra of the manganese nodules are so complex that interference-free lines for the lanthanides could not be found. Spectral interferences were eliminated with correction factors determined by aspirating 38 separate solutions into the plasma, each containing 1 mg/1 of an interfering element. Many interelement interferences were obtained which are not reported by Boumans /88/. The application of the ICP-AES method is demonstrated in Table VII which lists the results of analyses of two manganese nodule reference samples by ICP-AES, neutron activation analysis (NAA) and spark-source mass spectrometry (SSMS). The ICP-AES data are based on three repetitive determinations. The poor precision for Dy, Sm, Gd and Lu was attributed to large interference corrections which resulted in a relatively large uncertainty. The average relative standard deviation of 6-8% for the rest of the lanthanides is satisfactory. For NAA and SSMS, RSD-values of 1-5% and 2-5%, respectively, have been reported /89/. Using the standard addition method /75/, better results were obtained for these elements. With these exceptions, the data indicate that even without separation or preconcentration procedures, the results for most lanthanides by ICP-AES are in reasonable agreement with results obtained by mass spectrometric and neutron activation methods. Qing-Lie et al. /79/ studied the determination of lanthanides in geological samples by ICP-AES and x-ray fluorescence spectrometry (XRFS). The ICP-AES samples were prepared by fusion with sodium peroxide and sodium hydroxide followed by separation of the lanthanides by ion-exchange. The ICP-AES calibration involved a series

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112 of lanthanide standards prepared in 5% (v/v) nitric acid. The six lanthanides studies were determined at the following wavelenths (nm): La 398.9, Ce 394.3, Sm 442.4, Eu 382.0, Gd 376.8, and Yb 369.4. A series of ICP-AES determinations of lanthanides in the range 0.05-10 mg indicated 100% recovery after the ion-exchange separation. The procedure allowed the separation of lanthanides from the bulk of other constituents in the rock. The accuracy of the measurement was improved by reducing the spectral correction errors for ICP-AES determination. Matrix errors were found to be negligible in comparison with the possible errors associated with direct analysis of geological samples. The accuracy was shown to be satisfactory, by comparison with two independent analytical methods (ICP-AES and XRFS), as well as two independent types of standards (natural and synthetic). The results are in good agreement with those obtained by ICP-AES, XFRS, NAA and MS. The application of nitrogen-argon medium-power ICP and cation- exchange chromatography for the spectrometric determination of the lanthanides in geological samples has been reported /77/. Destruction of the matrix was accomplished by fusion with potassium hydrogen fluoride followed by separation of the lanthanides by cation-exchange chromatography. After evaporation of the eluent to a small volume, sodium nitrate and scandium were added so the final solution contained

2% (w/v) NaN03 and 50 mg/1 Sc (as did all calibration standards). It has been demonstrated earlier that the use of scandium as an internal standard compensates for variations in the nebulization process and results in a significant improvement in the precision /72,92,93/. The Sc content in the original sample was very low compared to the added Sc and did not affect the results. The emission lines selected for high sensitivity and minimum interference in the concentration range used are listed in Table VIII together with instrumental and practical detection limits. Five exposures of each sample were recorded on a photographic plate together with a synthetic calibration standard. Although calibration curves obtained by ICP-AES have generally been linear, these curves, extending over a range of 4 orders of magnitude, were slightly curved, due, possibly, to the emulsion calibration technique used. Consequently, most of the lanthanide calibration data were fitted to second order polynomials. The agreement for a large number of REE MINTEK reference material in a wide range of concentration of major elements with the available reference values is reasonable. Figure 6 illustrates the data for Gd, Eu,

~~113 "~~

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~~14 0.001 0.005 0.01 0.05 0.1 0.5 I QUOTED CONTENT (%)~~

~~Fig. 6: Comparison of measured and quoted content of Gd (·), Eu (*), Dy (•), and La (*) for standard reference materials using ICP-AES /77/.~~

~~Dy and La in Table VIII and lists the correlation coefficient, the slope and the intercept of the regression equation:~~

log y = a log χ + log b where y is the ICP-AES value, χ is the reference value, a is the slope and b is the intercept. The precision for each element varied from 3.5 to 11% (RSD). Crock et al. 78/ studied the group separation of the lanthanides from geological samples by cation-exchange chromatography. The samples were dissolved by digestion with a mixture of nitric, hydrochloric and perchloric acids. Any residue of the acid digestion was dissolved by lithium metaborate fusion followed by solution in dilute nitric acid. The lanthanides were separated with cation resin using in one case gradient nitric acid elution and in the other gradient hydrochloric acid elution. The elutions were followed by ICP-AES analysis.

115 In addition to the lanthanides, the cation-exchange separation of Fe, Μη, Th, Ti, U, V, and Zr (which have been reported to cause interference ) and Ca, Κ, Li, Mg and Na (whose removal is important in order to decrease the total salt content of the sample solution) were studied. Both separation procedures were successful in reducing the total salt content of the lanthanide fraction of the eluent and in allowing satisfactory preconcentration of the sample. The hydrochloric acid separation provided better separation from iron, but nitric acid eluted the rare earths in a smaller volume with good separation from the alkali earth elements. Since the nitric acid separation was preferred for the ICP—AES analysis, an anion-exchange separation with elution by 6M hydrochloric acid was used to remove the remaining iron prior to the ICP—AES analysis. The determination of the lanthanides in geological materials by ICP-AES interfaced with HPLC has been studied /63,100,101/. For the column separation of the lanthanides, Yoshida et dl. 63,100/ used ammonium lactate as the buffer solution for the mobile phase. The alkali solutions of α-hydroxyisobutyrate, EDTA and citrate, often used for lanthanide separation /106/, have too high a mineral content for ICP—AES as the salts can plug the ICP torch. The ammonium lactate made it possible to introduce solutions of concentrations as high as 1.5M into the ICP. Although the ammonium lactate allowed good separation of the rare earths, ammonium α-hydroxyisobutyrate would be equally suitable and provide better separation. The flow rate of the mobile phase is an important parameter for optimization of the HPLC/ICP-AES system. Maximum emission intensities were obtained for flow rates of 1.4-1.6 ml/min for sample introduction, but the separation of the rare earths was better with a flow rate of about 1 ml/min /63/. Chromatograms for the lanthanides from the HPLC/ICP-AES system are shown in Fig. 7. A concentration gradient elution technique was used and the detection wavelengths were the same as those given in Table VI. The HPLC separation of the lanthanides is satisfactory for reduction of the interelement interferences. Where adjacent elements in the elution order do not provide large spectral interferences (e.g., Lu, Yb, Tm, Ho, Sm), rapid chromatographic elution is possible. However, when adjacent elements have spectral interferences (e.g., Sm, Nd, Pr, Ca), elution must be performed slowly to obtain satisfactory separations. In this system mutual interferences were negligible when peak height measurements were used. Of the other elements, V, Fe, Ζ, Al, and Ti

~~116 Yb~~

~~>- Lu Dy CO Ζ UJ~~

~~ζ ο CO CO 2 ω~~

~~0 10 20 30 40 TIME (min)~~

Fig. 7: Chromatograms for rare earth elements obtained by the HPLC/ICP-AES system. Conditions: 10 μο of each element in the samle; mobile phase (linear concentration method), 0.4 Μ ammonium lactate, pH 4.22 (0-8 min). 0.6 Μ ammonium lactate, pH 4.22 (18 min), 1.0 Μ ammonium lactate, pH 4.22 (31-40 min) /63/.

showed no interferences since these elements eluted before the lanthanides. Cu, Zn, Ni, Na, Κ and Pb had overlap with some lanthanides, but the interferences were not significant. Mn, Mg, Ca and Cr did not cause interference. Table IX gives the detection limits and relative standard deviations for the lanthanides obtained with the HPLC/ICP-AES system. It also shows that the detection limits obtained by HPLC/ICP-AES are about 10 times higher than those obtained by the direct ICP-AES. The HPLC/ICP-AES system required only 0.1 ml of the sample solution compared to at least 1 ml of solution needed for the direct ICP-AES analysis. Furthermore, the HPLC separation minimizes erroneous analytical results from spectral interferences. The upper limit for the dynamic linear range of the calibration curves was about 500 mg/1. The HPLC/ICP-AES method was tested on some geological samples and on high purity lanthanide oxides /63/. The lanthanide concentrations in one USGS standard rock sample and three lanthanide ore samples

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118 were determined by the HPLC/ICP-AES system using two dissolution methods. A low-temperature acid digestion using nitric acid, hydrogen peroxide, hydrofluoric acid and perchloric acid was employed for digestion of a USGS standard sample. For the dissolution of ores the alkali fusion (Na2C03) followed by coprecipitation with cupferron was used (Fig. 8). With the coprecipitation the lanthanides can be quantitatively separated from the sodium salt of the alkali fusion. With extraction by CHC13 any lanthanides lost in the coprecipitation were recovered (although this procedure was not always necessary). The recoveries were relatively good, 95.3 to 103%, and 94 to 97.5% for the acid digestion and alkali fusion, respectively. The analytical results for Granodiorite standard rock sample showed good agreement with the reference values reported in the literature /107/. Analysis of ores may be more difficult than that of other geological samples because of the relatively large abundance of lanthanides which cause serious spectral interferences in the direct ICP-AES analysis. The chromatogram recorded for praseodymium showed that Er, Sm, Nd and Ce cause spectral interferences when the sample is directly measured by ICP-AES. With the HPLC/ICP-AES system, however, praseodymium can be purified sufficiently to be determined without interferences. Impurities of other lanthanides were determined successfully by the HPLC/ICP-AES system in high-purity lanthanide oxides. In some cases, the most sensitive line could not be used for the impurity analysis because of the spectral interference caused by the main component of the sample. For instance, Pr could not be detected using the emission line at 390.84 nm in cerium oxide (purity 99.9%). However, at 422.54 nm Pr could be measured despite the Ce. Similar experimental difficulties noted in the determinations of Nd in Pr203 and Sm in

Nd203 were overcome by measurement of Nd and Sm at 445.12 and 443.39 nm, respectively. Aulis et al. 101/ used the HPLC/ICP-AES system for the determination of yttrium and eight lanthanide elements in geological materials. Separation was effected without extensive precolumn sample modification. A 8PSCX 10 m Radial Pak Liquid Chromatography Cartridge in conjunction with a Z-module Radial Compression Separation

System was used with 0.1M NaN03 and 0.02M MgCl2 as eluants. The eluant was passed directly into the nebulizer of the multi-channel ICP spectrometer. Peak area analysis was employed to obtain quantitative determination of the lanthanides. Solutions containing suspensions of rock powders were prepared by

~~119 Sample (0.25 g)~~

~~Na2C03 - fusion (2 g) I~~

~~Acid digestion (HN03 and H.,0,,)~~

~~Dissolution with water (300 ml)~~

~~Addition of 10% cupferron (50 ml) Stand for 10 min. 1 I I Adjustment of pH at 9.0 with ΝΗ,ΟΗ~~

~~Filtration~~

~~Dilution to 50 ml~~

~~Fig. 8: The dissolution of rare earth ores by the alkali fusion 1631.~~

120 Bolton et al. 82/. After the sample was subjected to lithium metaborate fusion, the melt was dissolved and diluted to the desired volume with 2M nitric acid. For the preparation of the calibration solutions, the appropriate volumes of the lanthanide spectroscopic standards were added before final dilution. A schematic representation of the chromatograms obtained for a sample is illustrated in Fig. 9. Each chromatographic band is represented as a triangle, the length of the base representing the base-line band width and the apex position indicating the band maximum. The lanthanides and yttrium were well separated as a group from other elements except aluminum. In all the chromatograms, the lanthanide bands were broad and asymmetric, features ascribed to the inefficiency of the sodium counter ion in moving the more highly charged analyte ions along the column. Mg2+ which has a greater affinity for the column ion exchange sites than Na+, was used as the counter ion. Additional factors favoring the use of Mg

~~Ce_ Nd — Eu — Lo- Y- Sm ^ Dy- Yb" •Ag -Cu Be Al Al »-Fe h-Mn • Sc Ni ^Co ^ Pb Gd- ^ Zn~~

~~10 15 20 TIME (min)~~

~~Fig. 9: Schematic representation of combined chromatograms obtains by the HPLC/-ICP-AES system for the separation of a mixed element standard solution (all at concentration of 100 mg/l) /101/.~~

121 were the availability of the Mg channel on the ICP spectrometer for monitoring concentration changes and/or gradients, and the absence of spectral interference by Mg. The Mg2* ions showed no significant change in the column behavior of manganese, but the retention time and band width of yttrium(III) was decreased dramatically. A double gradient procedure was introduced to effect the required separation. Major and principal trace elements were removed by applying a small pulse of magnesium (gradient increase from 0 to 25 % in one minute, followed by a return to 0% in the second minute). A second gradient increase (up to 95%) was used to remove yttrium and lanthanides after passing water through the column for a short time. Although the HPLC separation of the lanthanide elements reduced the amount of interference due to the matrix elements, mutual spectral interference of lanthanides occurred. These spectral interferences were eliminated with correction coefficients in the calibration. Calibration plots for lanthanides in synthetic geological samples (with a matrix matched to typical silicate rocks) showed linear relationship between the peak areas and concentrations. The following practical detection limits in geological samples were reported: <1 jug/g for Eu and Yb; 1-2 μg/g for La, Dy, Gd and Y; 5 ßg/g for Sm; and 10 μg/g for Ce and Nd. Yttrium and eight lanthanides were determined in the following standard reference materials: SY-2, SY-3 and NIM-G (National Institute for Metallurgy, South Africa Bureau of Standards, Pretoria, South Africa), and MRB-14 (Geological Survey, Ontario, Canada). The agreements were satisfactory. Plasma source mass spectrometry is a new and very promising technique for trace elemental analysis. The technique was first demonstrated in 1976 by Gray /108/ using a capillary arc (d.c.) plasma. The technique allows rapid determination of trace elements and an estimate of the isotope ratio of each element in the sample. The DCP source was found to suffer from severe interelement and matrix effects, caused by its low temperature and insufficient sample introduction /109/. Date and Gray /II0/ compared the performance of the ICP source under two different modes of operation, boundary layer sampling and bulk plasma (or continuum) sampling. Douglas et al. 111/ investigated the performance of a microwave induced plasma (MIP), comparing it with data obtained by the ICP-AES. ICP is favored over MIP and continuous sampling from ICP is the usual technique for plasma source mass spectrometry /112/. Date and Gray /112/ recently reported on the application of ICP-MS

122 to the determination of 27 trace elements, including lanthanides, in geological samples. The mass spectrometer used in that study has been described in detail /113,114/. For simultaneous multi-element work, the mass spectrometer covered a wide mass range from 7 (lithium) to 238 (uranium). Each scan across the mass range was triggered by a multi-channel scaling data system (MCS) used for the data acquisition. In order to assign a reasonable number of MCS channels to each isotope a data acquisition memory group of 2048 channels was used. With a dwell time per channel of 500 μ& and 120 separate sweeps, a complete spectrum was accumulated in about 2 inin. For the determination of elements in a more restricted mass range, the Canberra Series 80 MCS data acquisition memory group was limited to 1024 channels with a dwell time per channel of 1 ms. A complete spectrum of 60 separate sweeps was accumulated in about 1 min. Performance in this mode of operation was demonstrated by determination of the lanthanides plus yttrium in 3 standard reference silicate rock samples. The power of detection of ICP-MS is excellent over a wide range of elements /II2/. For the lanthanides, the detection limits of La and Ce are given as 0.05 μφ by ICP-MS, which is about 10 times better than obtained by conventional ICP-AES (Table V). The spectra for lanthanides from a Canadian certified reference rock sample and a lanthanide reference standard solution are shown in Fig. 10. The signal for the heavy lanthanides is significantly lower than that for lanthanum and cerium (Fig. 10A). Although several peaks are overlapping, every lanthanide has at least one isotope free from interference. The less abundant heavy lanthanide elements are mono- isotopic (159Tb, 16SHO and 169Tm) or nearly so (17SLu: 97.4%), and have higher ionization energies than the lighter ones. Signal levels for the singly ionized species are high, and the detection limits for the whole group lie within a narrow range. The geological samples were dissolved by acid digestion with hydrofluoric, nitric and perchloric acids. Any residue after the digestion was dissolved in a PTFE bomb using nitric acid. The final solution obtained by this sample preparation method contains all major elements of the sample except silicon (lost as SiF4 during the digestion). No sample pre-treatment was necessary. Calibration was achieved with a single reference standard solution containing each lanthanide at 0.1 mg/1 in 1% (v/v) HN03. Data for the reference rocks analyzed in Table X show good agreement with previously published values / 115/. ICP-MS has several advantages in the determination of lanthanides

123 l40Ce*

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175lu *

~~I59TK* Ho* Pr * 153Eu ' JIAIIA^VA-^wv^AAAWWI~~

Fig. 10: Rare earth element spectra for a Canadian certified reference material (Project SRM SY-3; 1.0 g in 100 ml) (A), and for a reference standard solution containing each lanthanide at 110 mg/l in 1% (ν/ν) HNO3 (B) (1024 channels, 1 ms dwell time per channel, 60 sweeps = 1 min).

over other spectroscopic methods of analysis; it is very rapid, detection limits are extremely low, there is little or no chemical and spectral interferences, only one calibration solution is needed and sample preparation is easy. In addition to geological and high-purity lanthanide samples, lanthanides have been determined by ICP-AES in metallurgical samples /61/, in yttrium oxide /94/, aluminum /62/, in concentrated brines /95/, in boron and in boron compounds /96/, in animal tissue 1911, and in spellings, ark shell and coral /98/. DCP-AES has been used for determination of neodymium in iron-neodymium-boron alloys /99/.

~~124 TABLE Χ~~

~~Determination of Lanthanides in a USGS GSP-1 Made Sample by the HPLC/ICP-AES System /63/~~

~~Ln HPLC/ICP-AES USGS (certified value) (Wig) OUg/g)~~

La 180 ±10 191 Ce 390 ± 10 394 Pr 57 ± 4 50 Nd 180 ± 10 188 Sm 31 ± 3 27.1 Eu 2.8+ 0.3 2.4 Gd 13 ± 3 15 Tb - 1.3 Dy 5.1 ± 0.5 5.4 Ho 2 ± 1 <5 Er 4.7 ± 1.4 3.0 Tm 0.3 ± 0.1 _ Yb 2.0 ± 0.4 1.8 Lu 0.05 0.23

~~SUMMARY~~

In this brief review, we have attempted to show the wide versatility of atomic absorption and plasma atomic emission techniques in the analysis of lanthanide elements. Further, earlier review of such methods can be found in ref. 116. A future article will review other methods of lanthanide analysis. Prepared of this article was assisted by a contract with the U.S.D.O.E. Division of Chemical Sciences.

~~125 REFERENCES~~

1. HANSON, G.N.,/4«/!. Rev. Earth Planet Sei., 8, 371 (1980). 2. McLENNON, S.M., NANCE, W.B., and TAYLOR, S.R., Geochim. Cosmo- chim. Acta, 44,1833 (1980). 3. HORLICH, G.,Anal. Chem., 56, 278R (1984) and references therein. 4. KELIHER, P.N., BOYKO, W.J., PATTERSON III, J.M., and HERSHEY, iW.,Anal. Chem., 56,133R (1984). 5. SATOH, M., bundeki Kakagu, 3,190 (1984). 6. NAGY, Β.,Magy. Lapja, 39, 254 (1984). 7. SMIRNOVA, E.V., LONTSIKH, S.V., and SIDOROVSKI, A.I., Zh. Anal. Khim., 39,1804 (1984). 8. CHENG, J.,Inorg. Chim. Acta, 94,249 (1984). 9. MASOTTI, V.G., and FASSEL, V.A., Spectrochim. Acta, 20,1117 (1964). 10. DOGHE, W„ and VERBEEK, F., Anal. Chim. Acta, 73, 87 (1974). 11. JAWOROWSKI, R.J., WEBERLING, R.P., and BRACCO, D.J., Anal. Chim. Acts, 37,284 (1967). 12. SHIFRIN, S„ HELL, Α., and RAMIREZ-MUNOX, i.,Appl. Spoectrosc., 23, 365 (1969). 13. THOMAS, P.E., Resonance Lines, 1, 519 (1970). 14. JOHNSON, N., KIRKBRIGHT, G.F., and WHITEHOUSE, R.J., Anal. Chem., 45,1603 (1973). 15. MANNING, D.C.,At. Abs. Newsl., 5,127 (1966). 16. MANNING, OJC.,At. Abs. Newsl., 5, 63 (1966). 17. LINDSJO, O., and RIEKKOLA, M., "Atomiabsorptiospektrometria", Teknillisten Tieteiden Akatemia, Vammalan Kirjapaino Oy, 1976. 18. PRICE, W.J., "Spectrochemical Analysis by Atomic Absorption", Heyden, London,1979. 19. VANLOON, J.C., GALBRAITH, J.H., and AARDEN, H.M., Analyst, 96, 47 (1971). 20. ISHIZUKA, T., and SUNHARA, Y\.,Anal. Chim. Acta, 66,343 (1973). 21. CHRISTIAN, C.D., and FELDMAN, F.J., "Atomic Absorption Spectro- scopy", Wiley-Interscience, New York, 1970. 22. SEN GUPTA, J.G., Talanta, 23, 343 (1976). 23. SEN GUPTA, J.G., Talanta, 31,1045 (1984). 24. NEWTON, Μ .P., and DAVIS, D.G.,,4na/. Lett., 6,923 (1973). 25. JACKWERTH, Ε., and GRAFFMAN, G.,Z. Anal. Chem., 257,265 (1971). 26. MARTINEZ-JIMENEZ, P., GALLOGO, M., and VALCARCEL, M., At. Spectrosc., 6,137 (1985). 27. SEN GUPTA, J.G., Geostand. Newsl., 6,241 (1982). 28. SZAPLONCZAY, A.M., Analyst, 97, 29 (1972). 29. SCOTT, R.L.,Xi. Abs. Newsl, 9,46 (1970). 30. CARRON, M.K., SKINNER, D.L., and STEVENS, R.E., Anal. Chem., 27, 1058 (1955). 31. HUGHSON, M.R., and SEN GUPTA, J.G., Am. Minerologist, 49. 937 (1964).

126 32. POULIOT, G., MAXWELL, J .Α., and ROBINSON, S.C., Can. Minerologist, 8,1 (1964). 33. GERARDI, M. and PELLICCIA, G.A.,At. Spectrosc., 4,193(1983). 34. LVOV, B.V., and PELIEVA, L.A., Can. J. Spectrosc., 23,1 (1978). 35. SEN GUPTA, J.G., Talanta, 28, 31 (1981). 36. SEN GUPTA, J.G., Talanta, 32,1 (1985). 37. SEN GUPTA, J.G., Talanta, 31,1053 (1984). 38. KUGA, K., and TUJII, K.,BunsekiKagaku, 27,441 (1978). 39. GROBENSKI, Ζ.,ΑηαΙ. Chem., 289, 337 (1978). 40. SNEDDON, J., and FUAVAO, V.A.,/4f. Spectrosc., 3,51 (1982). 41. ZHAO, G., WANG, S„ ZHU.M., and SARAI, K., Bunseki Kagaku, 32,164 (1983). 42. ZHEN, R„ and YANG, Β., Xiyou Jinshu, 2, 178 (1983); Chem. Abstr., 100:167356r. 43. SYCHRA, V., KOLIHOVA, D., HLAVAC, R., DOLEZAL, J., PUSCHEL, P., and FORMANEK, Z., Analytiktreffen: Atomspedtrosk., Forschr. Anal. Anwend., Hauptvortr. 1982 (Publ. 1983), 154; Chem. Abstr., 102:89240y. 44. FAZAKAS, J.,Anal. Lett., 15A, 245 (1982). 45. FERNANDEZ,FJ., BEATY, M.M., and BARNETT, W.B„At. spectrosc., 2,16 (1981). 46. SLAVIN, W., MANNING, D.C., and CARNRICK, G.R., At. spectrosc., 2, 137 (1981). 47. FUAVAO, V.A., and SNEDDON, J., At. Spectrosc., 4, 179 (1983). 48. DAIDOJI, H„ and TAMURA, S„Bull. Chem. Soc. Japan, 55, 3510 (1982). 49. MA, Y„ LI, S., ZHANG, Z., WU, Z., FENG, X., SU, W„ and SUN, D., Huaxe Tongbao, 8,18 (1984); Chem. Abstr., 102:89158c). 50. HORSKY, S>^„At. Spectrosc., 1,129 (1980). 51. ABBEY, S., Canada Centre for Mineral and Energy Technology Report, 79-35. 52. VAN DER SLOOT, H.A., and ZONDERHUIS, J., Geostandards Newsl., 3, 185 (1979). 53. ABBEY, S., Geol. Surv. Can. Paper, 77-34. 54. FASSEL, V.A., and KNISELEY, R.N., Anal. Chem., 46, 1110A and 1115A (1974). 557^BOUMANS, P.W.JΜ.,Ζ. Anal. Chem., 299, 337 (1979). 56. BARNES, R.M., CRC Crit. Rev. Anal. Chem., 7, 203 (1978). 57. GREENFIELD, S., McGRECHIN, H. McD., and SMITH, P.B., Talanta, 23, 1 (1976). 58. BURMAN, J.O., JOHANSSON, B., MOREFALT, B., and NARFELDT, ΚΜ.,ΑηαΙ. Chim.Acta, 133,379(1981). 59. KELIHER, P.N., BOYKO, W J., PATTERNSON III, J.M., and HERSHEY, ]W„Anal. Chem., 56,133R (1984). 60. FRANK, Α., and PETERSSON, L.R., Spectrochim. Acta, 38B, 207 (1982). 61. FLISHER, P.T., and ELLGREEN, A.J., Spectrochim. Acta, 38B, 309 (1983). 62. MAHANITI, H.S., and BARNES, R.M.,/4pp/. Spectrosc., 37, 261 (1983).

127 63. YOSHIDA, Κ., and HARAGUCHI, ϋ.,ΑηαΙ. Chem., 56, 2580 (1984). 64. ANTTILA, R„ and LAJUNEN, L.H.J., unpublished results. 65. ANTTILA, R., and LAJUNEN, L.H.J., Symposium on Inorganic and Analytical Chemistry, JyvaskyJa May 16,1984, Abstracts, 38. 66. TONG, Y., Chem. Abstr., 100:16355q. 67. CHEN, F., and WANG, Z., Gaodeng Xuexiao Huaxe Xuebao, 5, 181 (1984);Chem. Abstr., 100:184990k. 68. BROEKAERT, J.A.C., FELIS, F., and LAQUA, K., Spectrochim. Acta, 34B, 73 (1979). 69. FLOYD, M.A., FASSEL, V.A., and D'SILVA, A.P., Anal. Chem., 52, 2168 (1980). 70. CHURCH, S.E., Geostand. Newsl., 5,133 (1981). 71. WALSH, J.N., BUCKLEY, F., and BARKER, }., Chem. Geol., 33, 141 (1981). 72. BRENNER, I.B., WATSON, A.E., STEEL, T.W., JONES, E.A., and GONCALVES, HI., Spectrochim. Acta, 36B, 785 (1981). 73. CROCK, J.G., and LICHTE, Υ.Έ,.,ΑηαΙ. Chem., 54, 1329 (1982). 74. CASETTA, B., GIARETTA, Α., and RAMPAZZO, G., At. Spectrosc., 4, 152 (1983). 75. FRIES, T„ LAMOTHE, P.J., and PESEK, J.J „Anal. Chim. Acta, 159, 329 (1984). 76. SESHAGIRI, T.K., BABU, Y„ JAYANTH KUMAR, M.L., DALVI, A.G.I., SASTRY, M.G., and JOSHI, B.D., Talanta, 31, 773 (1984). 77. BRENNER, I.B., JONES, E.A., WATSON, A.E., and STEELE, T.W., Chem. Geol., 45,135 (1984). 78. CROCK, J.G., LICHTE, F.E., and WILDEMAN, T.R., Chem. Geol., 45, 149 (1984). 79. QING-LIE, H„ HUGHES, T.C., HAUKKA, M„ and HANNAKER, P., Talanta, 32,495 (1985). 80. CROCK, J.G., LICHTE, F.E., and BRIGGS, P.H., Geostand. Newsl., 7, 335 (1983). 81. YUAN, X., QUE, S„ WU, X., and YIN, N„ Yankunang Ceshi, 2, 127 (1983); Chem. Abstr., 100:95678x. 82. BOLTON, Α., HWANG, J., and VANDER VOET, Α., Spectrochim. Acta, 38B, 165 (1983). 83. LI, Y., and LIN, M„ Kuangye Gongcheng, 3, 63 (1983); Chem. Abstr., 99:47159t. 84. VANNUCCI, R„Rend. Soc. Ital. Miner. Petrol., 38, 781 (1983). 85. ZOU, J., Ζ LAI, M., WANG, H., and CHEN, B., Yashi Kuangwu Ji Ceshi, 3, 149 (1984); Chem. Abstr., 102:89201m. 86. ERZINGER, J., HEINSCHILD, H.J., and STROH, Α., Fortschr. Atom- spektrom. Spurenanal., 1, 251 (1984); Chem. Abstr., 102:197156j. 87. WINGE, R.K., PETERSON, V.J., and FASSEL, V.A., Apl. Spectrosc., 33, 206 (1979). 88. BOUMANS, P.W.J.M., Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry, Pergamon Press, New York, 1980.

128 89. REEVES, R.D., and BROOKS, R.R., Trace Element Analysis of Geological Materials, Wiley, New York, 1978. 90. FLANAGAN, F.J., and GOTTFRIED, D., USGS Professional Paper 1155, 1980. 91. RANKIN, P.C., and GLASBY, G.P., in J.L. Bischoff and D.Z. Piper (eds), Marine Geology and Oceanography of the Pacific Manganese Nodule Province, Plenum Press, New York, 1979. 92. WATSON, A.E., and RUSSELL, GM.JCPInfo. Newsl., 4, 441 (1979). 93. BRENNER, I.B., WATSON, A.E., RUSSELL, G.M., and GONCALVES, M., Chem. Geol, 28, 321 (1980). 94. NI-CHUNG.R., WU-MIN.C., ZE-CHENG, J., and YUN-E, Z., Spectrochim. Acta, 38B, 175 (1983). 95. BUCHMAN, A.S., and HANNAKER, P., Anal. Chem., 56, 1379 (1984). 96. HUMSTON, P.,Anal. Chim. Acta, 155, 247 (1983). 97. JIANG, Z., LI AO, Z., and LE, X., Wuhan Daxue Xuebao, Ziran Kexueban, 2,123 (1984); Chem. Abstr., 101:207040m. 98. DAIDOJI, H., and MATSUBARA, M.,Bunseki Kagaku, 34,T123 (1985). 99. POTTER, N.M., and VERGOSEN III, H.E., Talanta, 32, 545 (1985). 100. YOSHIDA, K., FUWA, K., and HARAGUCHI, H., Chem. Lett., 1879, (1983). 101. AULIS, R., BOLTON, Α., DOHERTY, W„ VANDER VOET, Α., and WONG, P., Spectrochim. Acta, 40B, 377 (1985). 102. GAST, C.H., KRAAK, J.C., POPPE, Η., and MAESSEN, F .J.M.J., J. Chrom., 185,549 (1979). 103. FRALEY, DM., YATES, D.A., and MANAHAN, S.E., Anal. Chem., 51, 2225 (1979). 104. MORITA, M„ UEHIRO, T., and FUWA, F., Anal. Chem., 52, 349 (1980). 105. WALEY, B.S., SNABLE, K.R., and BROWNER, R.F., Anal. Chem., 54, 162 (1982). 106. TOPP, N.E., "The Chemistry of Rare Earth Elements", Elsevier, New York, 1965. 107. FLANAGAN, F., J. Geochim. Cosmochim. Acta, 37,1189 (1972). 108. GRAY, A.LAnalyst, 100, 289 (1975). 109. GRAY, A.L., "Dynamic Mass Spectrometry", D. Price and J.F.J. Todd (eds), vol. 5, Chap. 8, p. 106, Heyden, London, 1978. 110. DATE, A.R., and GRAY, A.L., Spectrochim. Acta, 38B, 29 (1983). 111. DOUGLAS, D.J., QUAN, E.S.K., and SMITH, R.G., Spectrochim. Acta, 38B, 39 (1983). 112. DATE, A.R., and GRAY, A.L., Spectrochim. Acta, 40B, 115 (1985). 113. GRAY, A.L., and DATE, A.R., Analyst, 108,1033 (1983). 114. DATE, A.R., and GRAY, A.L., Analyst, 106, 1255 (1981). 115. ABBEY, S., Geol. Surv. Canada, paper 83-15 (1983). 116. DeKALB, E.L., and FASSEL, V.A., Handbook on the Physics and Chemistry of Rare Earths, eds., K.A. Gschneidner and L. Eyring, Chapter 37D, Vol. 4, North-Holland Publ., New York, 1979.

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