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 applica- tions 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 determina- tion 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 94 oqhooqoowrtrtHO Ν a ΙΟΗΟΟΟΜΝ'ΟΟΟ'ΟΟ r- ο τι· η ^ ο\ ^ \0 Ν Η Μ «Ο ^ Ν Ο § s ri τ}·' cn ν ο σ\ νο ο ö ο ο ο ΐΛ!0»0<ΛΐΛΙ0·0ΐ0·0ΐη·Λ<Λ e .2 « ~ r- r- r-^ r^ r- Γ-' Γ- r- r- r^ ο η η Μ w Ο t "ο "t "Λ ^ m ο ·> '5 «uz νονονονονονονονονονονονο Ά -2 3a ομ 3 ö § •υ5 z—s ο S ^ ec c tfc C- .2 ο b^ OOcoOOOOOO u»* 11 ° ΪΪS e 2 χι £ ο "g λ » 00 Μ S M ω (Ν 00 ί ε Ο Ο Ο ο ο g8». aO e3s h S« 1 I < 1 ·5 §C <« Ä Λ - Α J3 .a .2 ο I •5 υ υ $ -δ J. Μ Μ "C 11 "C "G 'Π Ν Π (S CS Μ <S Μ Ρ» CS (Ν c' c. ^ C •O 9S5.3?.:S. =5. Χ(Ν Εd =5. Κr . ΤΧ. a; Οε ν ν ν ν V ν ν V ν V V V V 2 9.9.9.9, 9 99 99 9 99 ζ ζ ζ ζ ζ ζ ζ ζ ζ ζ ζ ζ ώ V ^ rtOH^ift^^ft^rj^eoDOoq >λ CΜ ) CC oovimtNaw^N^ödrt« J U £ Ζ MWOHOKäH^ 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/.