Analysis of Rare Earth Elements in Rock and Mineral Samples by ICP-MS and LA-ICP-MS

Sven Sindern

doi:10.1515/psr-2016-0066

Article Publicly Available

Analysis of Rare Earth Elements in Rock and Mineral Samples by ICP-MS and LA-ICP-MS

Sven Sindern

Published/Copyright: February 17, 2017

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From the journal Physical Sciences Reviews Volume 2 Issue 2

Abstract

The group of the rare earth elements (REEs) serves as valuable indicator of numerous geological processes such as magma formation or fluid–rock interaction. The decay systems of the radioactive REE isotopes ¹³⁸La, ¹⁴⁷Sm and ¹⁷⁶Lu are used for geochronometric dating of a range of events, starting from first steps of planetary formation to younger steps of geodynamic development. Thus, the abundance of all REEs occurring in a large range of concentrations as well as precise isotope ratios must be analysed in different geomaterials.

The inductively coupled plasma (ICP) ion source and various types of mass spectrometers (MS) represent the basis to fulfil the analytical requirements of geoscientific studies. Today, ICP-quadrupole MS and ICP-sector field MS (SFMS) with a single detector or multiple ion collection (MC-ICP-MS) are standard instruments for REE analyses in the geosciences. Due to the need for in situ analysis, laser ablation (LA)-ICP-MS has become an important trace element microprobe technique, which is widely applied for determination of REE concentrations and isotope compositions in geoscientific laboratories.

The quality of concentration analysis or isotope ratio determination of REEs by ICP-MS and LA-ICP-MS is affected by many parameters. Most significant are interferences caused by polyatomic oxide and hydroxide ion species formed in the plasma as well as fractionation effects leading to non-stoichiometric behaviour during element determination or to biased isotope ratio measurements. Laser-induced fractionation and isobaric interferences have to be considered as additional effects for LA-ICP-MS. As analyte elements and matrix are unseparated, mineral standards matching the matrix of samples are a prerequisite for accurate and precise REE concentration and isotope ratio determination. Application of fs lasers instead of the more common ns lasers in LA-ICP-MS systems turns out to be a significant step to reduce laser-induced fractionation and to overcome effects of sample matrices.

Keywords: REE concentration; REE isotope ratio; ICP-quadrupole MS; ICP-sector field MS; laserablation ICP-MS

1 Introduction

The rare earth elements (REEs, i.e. lanthanides) are a ubiquitously distributed group of elements in nature and present in many earth materials, but at low concentration. In a range of element abundance in the earth’s crust ([1], noble gases not considered), they rank between positions 24 (Ce, 60 μg/g) and 61 (Tm, 0.3 μg/g). Their concentrations are similar to those of common metals such as Zn (65 μg/g), Ni (56 μg/g), Cu (25 μg/g) and Pb (14.8 μg/g), but significantly higher than the concentrations of precious metals such as Ag (0.07 μg/g), Au (0.0025 μg/g) and Pt (0.0004 μg/g) [1]. Only within a rare but complex group of minerals, the REEs reach major element levels.

The geochemical behaviour of REEs is controlled by their ionic radii, charge and complexation behaviour [2]. Owing to their electron structure and progressive filling of the 4f orbitals, REEs are characterized by the principal oxidation number of +3 and a steadily decreasing ionic radius with increasing atomic number (lanthanide contraction). Accordingly, within the group of REEs slightly different geochemical properties can be observed, leading to gradually varying smooth distribution patterns (e.g. [3, 4]) that are characteristic of different rock types (Figure 1). Exceptions to this behaviour can be observed for Ce and Eu, which may adopt the oxidation numbers +4 and +2, respectively, depending on the geochemical system. Furthermore, smoothly varying REE patterns can show significant perturbations as a consequence of fractionation caused by contrasting complexation behaviour of different REEs. This is a function of the electron configuration of different REEs as well as of the type of the complexing ligand and thus of the geochemical environment in which a mineral or rock was formed [2].

$Figure 1 Chondrite-normalized REE patterns for major domains of the silicate earth and different rock types are indicative of geological formation processes (chondrite values in ref. [5]). Diverging patterns of the average continental crust [1] and depleted mantle [6] relative to primitive mantle [5] reflect REE fractionation during formation of magmatic melts. Dunites (values of DTS-1 in ref. [7]) are residual rocks of the earth’s mantle formed after extraction of magmatic melts. Carbonatites [2] and alkaline granites [8] represent magmatic melts ascending in the crust. They can enrich REE and form deposits of these elements. Fe–Mn-oxide crusts are precipitates of the seawater [2] and reveal REE fractionation due to differences in marine particle reactivity. In particular, they show fractionation of Ce due to precipitation of CeO2.$

Figure 1

Chondrite-normalized REE patterns for major domains of the silicate earth and different rock types are indicative of geological formation processes (chondrite values in ref. [5]). Diverging patterns of the average continental crust [1] and depleted mantle [6] relative to primitive mantle [5] reflect REE fractionation during formation of magmatic melts. Dunites (values of DTS-1 in ref. [7]) are residual rocks of the earth’s mantle formed after extraction of magmatic melts. Carbonatites [2] and alkaline granites [8] represent magmatic melts ascending in the crust. They can enrich REE and form deposits of these elements. Fe–Mn-oxide crusts are precipitates of the seawater [2] and reveal REE fractionation due to differences in marine particle reactivity. In particular, they show fractionation of Ce due to precipitation of CeO₂.

The group of REEs thus serves as highly valuable indicator of numerous geological processes like magma formation and differentiation [9] as well as interaction of hydrothermal fluids and rocks including the formation of ore deposits [2]. REEs can also reflect redox conditions in magmatic, hydrothermal or sedimentary systems (e.g. [10, 11]), and more and more they show the anthropogenic emission of REEs used in medical or technical applications to natural environments [12–14] (Figure 1).

Three of the REEs (i.e. La, Sm and Lu) have natural radioactive isotopes with half-lives in the order of magnitude of the earth’s age. Thus, considerable decay of the radioactive parent isotopes (¹³⁸La, ¹⁴⁷Sm and ¹⁷⁶Lu) and increase of the radiogenic daughter isotopes (¹³⁸Ce, ¹³⁸Ba, ¹⁴³Nd and ¹⁷⁶Hf) occur within the timescale of the earth’s history and can be used for geochronometric dating, starting from first steps of planetary formation taking place after onset of solar system condensation to crust formation or younger geodynamic events [15–24]. Figure 2 illustrates the concept of the Sm–Nd isochron dating of a metamorphic rock. Application of the ¹³⁸La–¹³⁸Ce/¹³⁸Ba-, ¹⁴⁷Sm–¹⁴³Nd- and ¹⁷⁶Lu–¹⁷⁶Hf-decay systems requires determination of isotope ratios, particularly in minerals that strongly fractionate the parent over the daughter isotope [25, 26]. To use the latter decay system, Hf isotopes must be analysed, which is addressed in this chapter too, although this element does not belong to REEs. For the application of the short-lived ¹⁴⁶Sm–¹⁴²Nd decay system for the study of early differentiation of the earth, see refs [27, 28].

$Figure 2 Samarium–neodymium isochron dating of rock formation. (a) Eclogite – a metamorphic rock formed during subduction of oceanic crust – composed of the minerals clinopyroxene (green), garnet (red) with minor amounts of quartz (white) and rutile (black). Sketch is drawn according to ref. [30]. (b) Isochron diagram showing the principles of dating using the 147Sm–143Nd system according to the decay equation displayed in the upper part of the diagram. The index “i” indicates the initial ratio of 143Nd/144Nd present at the time of crystallization of the rock, and λ denotes the decay constant of 147Sm (λ = 6.54 × 10−12/year, e.g. [19]). Rock-forming minerals, garnet (gt) and clinopyroxene (cpx), accessories (not considered) and accordingly also a whole rock sample (wr) initially are at isotopic equilibrium during formation of the eclogite (i.e. at metamorphic crystallization) and plot on a horizontal line. Due to radioactive decay of 147Sm to 143Nd, the isotopic signatures of mineral fractions and whole rock shift in an undisturbed system to a position on a line with positive slope – the isochron. Its slope (eλ t−1) indicates the time elapsed since rock formation and can be calculated after determination of 147Sm/144Nd and 143Nd/144Nd ratios [19, 25].$

Figure 2

Samarium–neodymium isochron dating of rock formation. (a) Eclogite – a metamorphic rock formed during subduction of oceanic crust – composed of the minerals clinopyroxene (green), garnet (red) with minor amounts of quartz (white) and rutile (black). Sketch is drawn according to ref. [30]. (b) Isochron diagram showing the principles of dating using the ¹⁴⁷Sm–¹⁴³Nd system according to the decay equation displayed in the upper part of the diagram. The index “i” indicates the initial ratio of ¹⁴³Nd/¹⁴⁴Nd present at the time of crystallization of the rock, and λ denotes the decay constant of ¹⁴⁷Sm (λ = 6.54 × 10⁻¹²/year, e.g. [19]). Rock-forming minerals, garnet (gt) and clinopyroxene (cpx), accessories (not considered) and accordingly also a whole rock sample (wr) initially are at isotopic equilibrium during formation of the eclogite (i.e. at metamorphic crystallization) and plot on a horizontal line. Due to radioactive decay of ¹⁴⁷Sm to ¹⁴³Nd, the isotopic signatures of mineral fractions and whole rock shift in an undisturbed system to a position on a line with positive slope – the isochron. Its slope (e^{λ t}−1) indicates the time elapsed since rock formation and can be calculated after determination of ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd ratios [19, 25].

Furthermore, stable isotopes may also show isotopic fractionation serving as indicator of a natural process. Slight anomalies of the ¹⁵³Eu/¹⁵¹Eu ratio in meteorite samples are interpreted to reflect early stages of planetary history, such as magnetic separation processes in the early ionized fraction of the solar nebula [29].

In nature, REE concentration patterns as well as isotope compositions can vary on a mineral grain scale. Complexly zoned minerals, such as zircon, monazite or garnet, can therefore serve as monitor for major geodynamic processes, for example crust formation or growth and collapse of mountain chains [31, 32], if they are studied in detail.

These brief explanations show that a large range of analytical capabilities is required for the study of REEs in geosciences. Usually all members of the group of REEs must be addressed for the determination of element concentrations in order to reveal the characteristic distribution patterns. Thus, the concentration range from several wt.% down to the sub-μg/g or sub-ng/L level has to be covered in different earth materials like minerals, bulk rock or soil samples as well as natural water. Furthermore, precise determination of isotope ratios is a prerequisite for application of the ¹³⁸La–¹³⁸Ce/¹³⁸Ba-, the ¹⁴⁷Sm–¹⁴³Nd- and ¹⁷⁶Lu–¹⁷⁶Hf-decay systems or for the investigation of fractionation of heavy stable isotopes (e.g. ¹⁵³Eu/¹⁵¹Eu). Both, element and isotope analyses are performed on bulk samples after chemical processing and also in situ using probe techniques, which allow analytical work with high spatial resolution. The inductively coupled plasma (ICP) ion source and various types of mass spectrometers (MS) represent the basis to fulfil the analytical requirements in the geosciences. This will be addressed in the following sections of this chapter. The important role of ICP-MS and laser ablation (LA) as efficient techniques for sample introduction and in situ analysis of solid matter to fulfil the analytical requirements in the geosciences are highlighted.

2 Technical development

Starting in the 1980s, ICP-quadrupole MS (ICP-QMS) has become the standard instrument for REE analyses in the geosciences. Within a few years, ICP-QMS-based analytical procedures replaced instrumental neutron activation analysis or procedures applying isotope dilution thermal ionization mass spectrometry (TIMS), which represented the standard methods since the 1960s. Most important arguments in favour of ICP-QMS were lower prime and operating cost, relatively low duration of analysis and large sample throughput as well as the option to cover all REEs in one step [33–38].

The combination of the ICP ion source with magnetic sector field (SF) mass filters represented a major step in the field of isotope geochemistry and cosmochemistry [39, 40]. Owing to the flat top peak signals and the ability of the plasma source to ionize most elements, ICP-SFMS systems allow precise isotope ratio measurements also for poorly ionizing elements such as Zr, Hf or W. In particular, geochronometry applying the Lu–Hf system was limited due to insufficient sensitivity of conventionally used TIMS for Hf [15]. The application of multiple ion collection (MC) in MC-ICP-SFMS instruments and simultaneous mass detection was a further step to enhance the precision of isotope ratio measurements [41, 42].

In contrast to QMS, double-focusing ICP-SFMS systems, commercially available today either with multiple or single ion collection, can be operated with significantly higher resolution at still reasonable sensitivity [43, 44]. Despite prices approximately >5 times higher than those of quadrupole instruments, (MC)-ICP-SFMS have become common equipment in most isotope geochemistry laboratories worldwide.

Due to the particular need for in situ trace element and isotope ratio determination with high spatial resolution, LA-ICP-MS has become an important technique in geosciences. After first steps in the late 1980s [36, 45–47], the potential of a technique for REE analysis avoiding the step of sample dissolution was intensively explored [48–50]. As an alternative to ICP-QMS and ICP-SFMS, time-of-flight MS (ICP-TOFMS) were used for LA-ICP-MS. However, due to limited ion counting rate and low sensitivity of such MSs, LA-ICP-TOFMS systems so far are not as effective as those applying QMS or SF-MS analysers [51]. In comparison to secondary ionization mass spectrometry, which has comparable analytical capabilities and which can also be considered an alternative method, LA-ICP-MS requires larger spot sizes but is significantly cheaper and faster, and does not require vacuum [52]. LA-ICP-MS thus turned out to be one of the “most affordable and most widely applicable trace element microprobe” techniques in geoscientific laboratories [53].

For LA-ICP-MS analysis, a pulsed high-energy laser beam is focused on the surface of a solid sample, which is placed in an air-tight sample chamber flushed with a carrier gas (Figure 3). Irradiation causes the release of aerosol particles from the sample surface, which are transported to the ICP ion source in which they are vaporized and finally ionized (e.g. [53, 54] and references therein). Helium allows better uptake of aerosol particles than Ar and usually serves as carrier gas [55, 56]. Small flows of other gases (e.g. N₂ and H₂) may be added to the carrier gas to suppress oxide formation and to enhance sensitivity [57, 58].

Figure 3

Schematic diagram of a common LA-ICP-MS system based on an ns-ArF excimer laser and an ICP-QMS.

After intensive technical development in the last 20 years, different lasers providing pulsed laser light are used in LA-ICP-MS systems. Lasers differ with respect to the pulse length, which can be in the range of 3–20 ns (ns lasers) or in the sub-picosecond range (often 60–150 fs, generalized as fs lasers [58–60]). Furthermore, lasers differ with respect to the type. In commercial instruments, ns laserlight is either produced in solid-state Nd:YAG or ArF excimer lasers. The latter laser type, which is more expensive but delivers higher photon energy, has a fundamental wavelength of 193 nm, whereas in a Nd:YAG laser such light is produced from the fundamental wavelength of 1,064 nm using an optical parameter oscillator ([51] and references therein). Solid-state Nd:YAG ns lasers emitting light with 266 or 213 nm, which have been most common in earlier commercial systems, become less attractive for application in geoscientific laboratories.

Femtosecond LA systems, which are based on solid-state lasers (Ti:sapphire), are significantly more complex than ns instruments and have become commercially available only in the last years [61].

3 Physical and chemical effects on concentration and isotope ratio determination

The quality of concentration or isotope ratio determination by ICP-MS and LA-ICP-MS is affected by many factors. In addition to general aspects related to any analytical activity (e.g. memory effects and contamination) or general technical conditions (e.g. signal drift, dead time of ion detector and plasma stability), there are further factors, which are specifically associated with physical and chemical processes in the ICP source leading to interference and fractionation (e.g. [62]).

The occurrence of polyatomic ion species due to oxide and hydroxide formation of REEs and Ba as well as of BaCl in the plasma is a well-known phenomenon causing serious interference mainly on Eu, Gd, Tb, Yb and Lu [34, 35]. Oxide production is influenced by the availability of oxygen, which is significantly higher in solution-ICP-MS than at “dry plasma” conditions in LA-ICP-MS [36, 63]. Different to sample introduction by solution LA-ICP-MS allows routine analytical procedures during which oxide contributions to most REE isotopes chosen for analysis can be neglected [64].

In addition to interferences, fractionating effects have to be considered. Plasma temperature is important for the degree of ionization and ion energies, which have an effect on mass-dependent vaporization, diffusion or ion trajectories in the plasma ([65] and references therein). In addition, space charge effects causing mutual repulsion of positively charged ions lead to enhanced deflection of the lighter masses in the ion beam [66, 67]. In combination, these physical processes, which occur in the plasma, between sampler and skimmer cones as well as immediately behind the skimmer cone, are the reason for mass-dependent fractionation [65, 68]. Any instrumental parameter affecting these physical processes in plasma and interface region, in turn, has an influence on fractionation that is reflected by non-stoichiometric behaviour during element determination or by biased isotope ratio measurement. Such instrumental parameters are nebulizer gas flow, extraction lens voltage, torch position, rf power and cone design [65, 67]. Furthermore, as the total ion current also affects physical conditions in the plasma sample, related parameters like concentration or sample matrix influence mass-dependent fractionation, too.

Other instrumental factors, such as focusing and shape of the ion beam, collision or scatter of ions in the flight tube or in the ion collection system, inefficient suppression of secondary electrons or nonlinear response of resistors, lead to mass-independent fractionation effects [41]. These are most obvious if the beam is split into various parts in MC-ICP-MS.

All of these factors leading to elemental and isotopic fractionation are independent of the method of sample introduction (solution or LA). Such instrumental mass bias must be distinguished from laser-induced fractionation processes, which are specifically associated with sample uptake by LA (Figure 4). Elemental and isotopic fractionation may occur immediately in or around the ablation pit during laser sampling due to non-congruent evaporation and redistribution of elements on the basis of different volatilities between subsolidus phases (vapour and melt) forming in the ablation pit or condensing around it [59, 63 and references therein, 69, 70].

$Figure 4 Schematic overview of processes causing laser-induced fractionation and interference in (ns-)LA-ICP-MS system on the way of an analyte element from the ablation pit in a solid sample with unseparated matrix via transfer tubing and ICP interface to the detector system [59, 63, 69, 70, 72, 74, 75].$

Figure 4

Schematic overview of processes causing laser-induced fractionation and interference in (ns-)LA-ICP-MS system on the way of an analyte element from the ablation pit in a solid sample with unseparated matrix via transfer tubing and ICP interface to the detector system [59, 63, 69, 70, 72, 74, 75].

Differential transport properties of ablation products (i.e. melt droplets, vapour phase condensates, solid fragments and agglomerates) were invoked to cause fractionation during passage through the transfer tubing [71 in 63]. Most studies on laser-induced fractionation point to the crucial role of melt formation at the ablation site (Figure 4).

While incomplete vaporization and ionization of larger particles in the plasma are shown to lead to preferential transmission of more volatile elements and lighter isotopes, enhanced loading of the plasma with aerosol particles causes element-dependent suppression of signal intensity [63, 65, 72, 73]. Although the latter effects occur in the plasma they are controlled by the properties of laser-derived aerosols and thus belong to laser-induced fractionation.

Other studies explore the role of the position of a sample in a large-volume open design ablation cell on variation of Sm–Nd inter-element and Nd isotope fractionation [74]. Two-volume ablation cells help to establish constant conditions of fractionation [59].

The physical processes of aerosol formation and interaction occurring during ablation are controlled by absorption of the laser energy by the solid sample. Thus, in addition to the sample matrix, instrumental parameters that influence absorption also have an effect on laser-induced fractionation. Numerous studies conducted in this field have shown that laser wavelength and power density as well as laser pulse length are most important (see [63] for further parameters). Different to ns pulses, thermal diffusion, which is the reason for melt production in an ablation pit, does not occur during ablation with fs pulses [58, 63]. Taking the crucial role of melts for laser-induced fractionation into consideration fs-LA-ICP-MS can be regarded as a major step to reduce laser-induced fractionation and to overcome effects of sample matrices on fractionation [58, 76, 77].

For ns LA-ICP-MS, the application of deep UV wavelengths (in particular 193 nm) turned out to be absorbed best by transparent matrices [53, 72, 78]. This allows efficient ablation of silicates (e.g. zircon and feldspar) or oxides (e.g. quartz) that constitute a dominant part among mineral and rock samples.

4 Determination of REE concentrations

4.1 Sample preparation

The standard procedure of operation requires sample supply to the ICP ion source as liquid aerosol droplets via a nebulizer and a spray chamber. Therefore, dissolution is the first sample preparation step of earth materials after homogenization and separation of a representative aliquot.

As a characteristic feature of the REE deportment in earth materials, their abundance is often controlled by accessory minerals such as allanite, apatite, monazite, xenotime or zircon [79]. Some of these accessories are of low solubility, which has to be considered for the choice of dissolution procedures. Acid dissolution is usually performed with mixtures of either HNO₃ or HClO₄ and HF necessary to attack silicates and quartz that are dominant in many rocks. Evaporation to dryness leads to disappearance of Si. Residues are taken up with either HCl or HNO₃. Pressure containers are preferred because they allow digestion of accessory minerals of low solubility. Procedures employing microwave digestion systems are also successfully applied for analysis of REEs in silicate rocks [80].

Furthermore, effective digestion of solid earth materials can be achieved by fusion dissolution with lithium borate. Glasses formed in this step can further easily be digested in HCl or HNO₃ [43].

Bulk rock REE analyses are also performed on homogenized solid samples applying LA as sampling tool. For this purpose, pressed powder pellets have been applied [48, 81]. Limited precision due to heterogeneity on the scale of a single laser spot is compensated by averaging results from a raster of laser spots [36]. Better homogenization of rock samples is achieved in samples, which are fused either directly or using a flux. Application of the relatively expensive method of direct fusion for REE analysis in rock samples is less common [82, 83] but sometimes rock glasses suitable for LA-ICP-MS are provided by natural volcanic processes [84]. Flux-based methods, using mixtures of Li-borates added to the sample at ratios ranging between 7:1 and 5:1, are applied most often [36, 43, 49, 85, 86].

4.2 Quantification

Quantification of REE concentrations by solution-ICP-MS is mainly based on external calibration using dilutions of stock solutions [33, 34]. The methods of standard additions and isotope dilution are alternative ways of calibration, which are less vulnerable to complex matrices in earth materials and thus offer better accuracy and precision of REE analysis [87]. However, both methods require labour-intensive and time-consuming steps of sample handling, which also leads to higher costs of analyses. Therefore, they are usually not applied as routine methods in geoscientific laboratories.

Drift of signal intensity is corrected by internal standards for which elements that are naturally not contained in the sample are added at defined concentrations. For analysis of REEs in earth materials, Ru or Rh and Re are appropriate as their masses bracket the range of masses of REEs [88].

While one or more isotopes free of isobaric interferences exist for each REE, the interferences caused by polyatomic oxide and hydroxide species can be significant for the group of REEs and can require correction applying experimentally derived factors accounting for yields of formation of interfering ions in the plasma [34, 42] unless high mass resolution is available using double-focusing ICP-SFMS. Increasing the resolution (m/Δm) to values in the range of 8,000–10,000 REE-oxide ions can be separated from REE-atom ions, which, however, leads to a significant decrease in sensitivity [43, 89]. Several authors explore the effects of high resolution, collision cell technology and doubly charged ions to minimize interferences [37, 90, 91].

Detection limits and precision are not entirely dependent on instrumental parameters as they are also affected by factors such as laboratory blank and numerous processes during the entire analytical procedure starting from the sample preparation and ending with data reduction and concentration calculation. Using solution-ICP-MS for determination of REE concentrations, even with quadrupole mass filters, detection limits ranging around 1 ng/g (in solid samples) and run-to-run (external) precision <5 % for REE concentrations ≥0.1 μg/g can be reached. Accuracy of REE determinations based on repeated dissolution and analysis of certified rock standards varies between 5 % and 10 % [34, 88].

Quantification of REE concentration in homogenized rock or mineral samples by LA-ICP-MS is mainly performed after external calibration using solid reference materials. As reference materials, either synthetic glasses or rock glasses produced by melting of natural rocks or minerals as well as synthetic crystalline material or natural minerals are applied for REE analysis [92, 93]. Synthetic polyethylene-based REE standards have been tested but are not common today [94]. The glasses 610 and 612 synthesized by the National Institute of Standards and Technology (NIST) are certainly most widely applied for LA-ICP-MS mineral and rock analysis; see GeoReM database in refs [95, 96]. Mineral standards, for which REE concentrations are known, are zircon 91500 [97], zircon Plesovice [98] or titanite TIT-200 [99]. An alternative technique based on simultaneous ablation of a Li-borate blank disc and nebulization of a standard solution was tested by Pickhardt et al. [100].

It has to be considered for external LA-ICP-MS calibration that the absolute mass of material ablated per time unit (ablation yield) varies between different materials [53, 63, 70], which leads to systematic errors if standard materials are different to samples. Therefore, at least one element with independently determined concentrations in standard and sample serving as internal reference standard is required for quantification of concentrations using the equation:

CauCru=CastCrst⋅Iau⋅IrstIru⋅Iast⋅Sau⋅SrstSru⋅Sast

[53, 101]. Here, C and I denote concentrations and signal intensities, respectively, in an unknown sample (upper index u) and a standard (upper index st). The lower index a indicates the analyte element, whereas r marks the internal reference element. Usually external calibration standards and samples are analysed in a bracketing sequence, which also helps to correct for drift effects.

As physical processes responsible for laser-induced fractionation are controlled by material-dependent absorption of laser energy in the solid (see 3), signal intensities obtained in a standard are not representative for an individual sample unless both matrices are identical. Such differences in sensitivity can be corrected with element-specific factors (S) accounting for contrast of sensitivity between internal reference element and the analyte element [63]. Owing to their overall similarity, sensitivities within the group of the REE as well as those of Y and Sc are almost identical. Furthermore, sensitivities of REEs are not significantly different from those of the alkaline earth elements Mg, Ca, Sr and Ba [70]. Therefore Ca, which is abundant in many rock types and minerals as well as in the NIST 600 series standards, can be recommended as suitable internal standard element for REE analysis. In such case, errors caused by insufficiently constrained sensitivity correction factors are minimized. When using a LA-ICP-MS system affected by mass- and element-dependent fractionation (particularly for ns LA-ICP-MS) these errors can only be neglected if matrix-matched standards are applied.

This has an effect on accuracy of LA-ICP-MS, which is known to be <10 % when analysing silicate minerals without matrix matching but may be reduced to <5 % if matrix matched standards are used [33, 63]. External precision has improved since the early days of REE analysis by LA-ICP-MS (e.g. [94]) and now is similar to precision of solution-ICP-MS. Jochum et al. [102] report precision of 0.8–3.5 % (RSD) for REE analysis by 193 nm ns LA-ICP-SFMS in the concentration range of 38 and 51 μg/g in natural glass standards after repeated analysis. The potential to improve precision as well as to overcome matrix dependency using fs LA-ICP-MS is shown in Refs [103, 104]. Detection limits for LA-ICP-MS significantly depend on the size of the laser pit and on the isotope used for detection next to the sensitivity of the instrument. For a beam diameter of 100 μm sensitive LA-ICP-SFMS systems can reach detection limits for REEs as low as 0.001 μg/g [86, 102].

In addition to concentration analysis, LA can also be used as a tool for chemical imaging if operated in a raster mode. Cook et al. [105] create REE distribution maps of mineral grains based on LA-ICP-MS signal intensities with a spatial resolution of 7–12 μm.

5 Determination of isotope ratios by multi-collector (MC)-ICP-MS

The application of ¹³⁸La, ¹⁴⁷Sm, ¹⁴⁶Sm or ¹⁷⁶Lu decay for geo- or cosmochronological studies or the use of isotope ratios as geochemical tracers requires the precise determination of isotope ratios (Figure 2). These include ratios of the radiogenic daughter isotopes and a non-radiogenic stable reference isotope of the same element in the denominator (i.e. ¹³⁸Ce/¹⁴²Ce, ¹⁴³Nd/¹⁴⁴Nd, ¹⁴²Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf) as well as ratios of the radioactive parent isotope and the reference (i.e. ¹³⁸La/¹⁴²Ce, ¹⁴⁷Sm/¹⁴⁴Nd, ¹⁴⁶Sm/¹⁴⁴Nd, ¹⁷⁶Lu/¹⁷⁷Hf).

For this analytical task, MC-ICP-MS offers best ionization of REEs and Hf in the ICP source as well as most precise isotope ratio determination due to MC, which allows simultaneous mass detection. This enhances precision because effects due to fluctuations in signal intensity cancel out [41, 42]. Isotope ratio determination is either performed in static mode with stable magnetic field or in dynamic mode, in which the field is changed to detect individual masses in different collectors. While the static mode allows faster analysis, the latter helps to circumvent errors caused by different efficiencies within the array of ion collectors [41].

The following parts, therefore, focus on MC-ICP-MS. For isotope ratio determination by MC-ICP-MS, sample uptake is done by solution as well as by LA.

5.1 Solution-MC-ICP-MS

For solution-MC-ICP-MS, powdered bulk rock samples or mineral grains must be dissolved. Due to blank restrictions and in order to keep the sample matrix simple, only acid dissolution, mainly with mixtures of HF and HClO₄ or HNO₃, is performed using Teflon® vessels (with steel jacket if necessary) in clean air laboratory environment.

While ratios of two isotopes of the same element can be determined in one step, isotope ratios involving two elements (e.g. ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁷⁶Lu/¹⁷⁷Hf) are determined in two separate steps of concentration analysis using isotope dilution [25]. Therefore, mixed spike solutions usually enriched with the isotopes ¹⁴⁹Sm–¹⁵⁰Nd, ¹⁷⁶Lu–¹⁷⁸Hf and ¹⁷⁶Lu–¹⁸⁰Hf are added to the sample solution.

In order to keep the matrix of the elements as simple as possible, which helps to optimize signal stability and to minimize mass discriminating effects (see 3) and in order to avoid isobaric interferences of atomic ions (e.g. ¹⁴⁴Sm and ¹⁴⁴Nd) as well as of molecular species forming with elements of lower masses and Ar, O, N or C, element fractions are isolated from the spiked sample solutions [41]. Several methods of extracting REE and Hf from the bulk rock or mineral matrix and subsequent purification of Nd, Sm and Lu employing different chromatographic materials (e.g. Biorad® AG50WX8 cationic resin, Eichrom® TRU resin, resins conditioned with α-hydroxyisobutyric acid/α-HIBA, Teflon®-based resins conditioned with di(2-ethylhexyl)orthophosphoric acid/HDEHP) but also liquid–liquid extraction have been developed [106–118].

Despite chemical isolation, trace impurities can occur in the fractions of analyte elements, which impose the problem of isobaric interferences (e.g. ¹⁷⁶Yb–¹⁷⁶Lu and ¹⁴⁴Sm–¹⁴⁴Nd). Correction of interference-related signal intensities can be performed using interference-free masses, such as ¹⁷³Yb, ¹⁷⁵Lu, ¹⁴⁶Nd or ¹⁴⁵Nd and ¹⁴⁹Sm, as monitors for the presence of the interfering masses [113].

Correction of mass bias due to fractionation effects in the ICP ion source or detection system can be performed by bracketing of the sample analysis with two standard analyses. This requires identical fractionation properties of sample and standard solutions [41]. For independent calculation of bias caused by mass-dependent factors, different mathematical approaches are known among which an exponential law is applied most [41, 42]. For the latter, a stable reference isotope ratio unaffected by interferences is required for normalization. While ¹⁴⁶Nd/¹⁴⁴Nd, ¹⁴⁷Sm/¹⁵³Sm and ¹⁷⁹Hf/¹⁷⁷Hf ratios can be used for correction of unknown isotope ratios of Nd, Sm and Hf, respectively, Lu only has the two isotopes ¹⁷⁶Lu and ¹⁷⁵Lu. Mass bias of the ¹⁷⁶Lu/¹⁷⁵Lu ratio can be evaluated using an interference-free isotope ratio of another REE after doping of the solution (e.g. ¹⁶⁷Er/¹⁶⁶Er [119]) or using Yb occurring in the Lu fraction due to incomplete chromatographic separation. To avoid the incorrect assumption of identical fractionation properties of Lu and Yb, the difference in mass discrimination is externally evaluated by repeated systematic analysis of mixed Yb and Lu standard solutions – a method also allowing assessment of isobaric interference by ¹⁷⁶Yb on ¹⁷⁶Lu [15, 41]. Alternatively, normalization to the ¹⁸⁷Re/¹⁸⁵Re ratio of an admixed standard solution is used for correction [120].

Any mass-independent fractionation must be corrected using factors derived from the analysis of standard solutions with known isotopic composition. Thus, often a final normalization to a standard solution run during analytical sessions is applied to yield accurate and reproducible isotope ratio measurements [41, 119].

For normalization or general evaluation of accuracy and correction procedures for determination of the radiogenic isotope ratios, standard solutions prepared with pure metals or oxides such as La Jolla, Ames-Nd or JNdi-1 for Nd, JMC 475 for Hf or JMC 304, Ames-Ce for Ce are common [95, 121–123].

Precision of isotope ratio determination by MC-ICP-MS depends on instrumental properties and correction procedures as well as on concentration of the analyte element or analysis time. For common analytical conditions applied for rock and mineral analysis, external precision (2σ) obtained from repeated analysis of standard solutions is often reported to be ≤ 20 ppm for ¹⁷⁶Hf/¹⁷⁷Hf or ¹⁴³Nd/¹⁴⁴Nd [68, 119, 124]. For the latter element, precision of MC-ICP-MS is comparable to values determined by TIMS [125].

5.2 LA-MC-ICP-MS

Determination of Sm–Nd isotopes with LA-MC-ICP-MS has increasingly been applied to minerals that enrich the light REE in the last years. Minerals such as monazite, apatite, titanite or allanite in which Nd can reach concentrations of several wt.% are highly suitable for this technique because precise isotope ratio analysis can be performed also in small laser spots [126]. Isotopes of the Lu–Hf system are mainly analysed in zircon. Such measurements in combination with other in situ trace element and isotope (U–Pb, O) data obtained from the same single crystal make zircon one of the most important tracers of the earth’s crustal evolution [57, 127].

LA-MC-ICP-MS thus combines the advantage of in situ analysis at high spatial resolution with the convenience of a method that does not require time-consuming dissolution and chromatographic sample preparation. However, leaving analyte elements and matrix unseparated leads to analytical tasks that require further consideration.

Isobaric interferences are more crucial for in situ analysis than for solution-ICP-MS. The signal of ¹⁴⁴Nd can be affected by a contribution of ¹⁴⁴Sm as high as 4 % in monazite because this mineral, like other light REE-rich minerals, contains significant amounts of Sm next to Nd [75]. Likewise, the REE isotopes ¹⁷⁶Lu and ¹⁷⁶Yb interfere to higher degrees with ¹⁷⁶Hf in in situ mineral analyses than in elemental fractions after chemical separation. Correction of an interfering isotope (e.g. ¹⁴⁴Sm) can be performed using its ratio with an interference-free isotope (e.g. ¹⁴⁹Sm) and additional correction of mass bias on the ratio. However, stable non-radiogenic ratios used for normalization in mass bias correction procedures may also be affected by interfering matrix elements (e.g. ¹⁴²Ce on ¹⁴²Nd) that can have high concentrations in unseparated solid mineral matter. Different procedures for such correction are given in refs [74, 75, 126].

In general, due to laser-induced fractionation (see 3), which occurs in addition to fractionation caused in plasma and interface, mass bias is a more pronounced effect of in situ LA-MC-ICP-MS isotope ratio determination. Furthermore, inter-element fractionation affecting analyses of parent–reference ratios (e.g. ¹⁴⁷Sm/¹⁴⁴Nd, ¹⁷⁶Lu/¹⁷⁷Hf) cannot always adequately be corrected using stable isotope ratios of only one element.

In many laboratories, both insufficiently corrected mass bias of daughter–reference ratios and fractionation of parent–reference ratios are corrected using mineral standards often analysed in a standard-sample bracketing mode [74]. Thus, well-characterized and homogeneous mineral standards matching the matrix of REE-enriched minerals including zircon are a prerequisite for accurate and precise isotope ratio measurement in particular for ns LA-MS-ICP-MS, which is still more often applied than fs LA-MS-ICP-MS. Instead of mineral standards, such as zircon 91500 [97, 128, 129], synthetic glass standards have also successfully been applied [74, 75].

Precision of LA-MC-ICP-MS isotope ratio determination depends on signal intensity, and thus on the concentration of the analyte element as well as on the pit diameter. Furthermore, integration time and correction procedures also control precision [65; 129]. McFarlane and McCulloch [126] report internal precision of the ¹⁴³Nd/¹⁴⁴Nd ratio as good as in TIMS instruments (<10 ppm, 2σ) for monazite with Nd at 9 wt. % and a crater diameter of 50 μm. External precision reaches best values between 60 and 95 ppm (2σ) for analytical conditions applicable to natural minerals (e.g. zircon and ¹⁷⁶Hf/¹⁷⁷Hf [57, 129]).

6 Concluding remarks

In the last 30 years, ICP-MS has been a rapidly developing technique. The robust and versatile ICP source allows efficient ionization and can be combined with different systems of sample input as well as with different MSs (quadrupole, magnetic SF and time of flight) at various detector configurations (single collector and multi-collector). Recent advance, for example in the field of fs LA-ICP-MS and MC-ICP-MS, shows that the technical and methodological evolution is still ongoing – certainly also driven by the analytical needs of the geosciences.

The introduction of ICP-MS as a standard instrument in geochemical laboratories significantly enhanced the availability of REE analyses of rocks and minerals, and due to the excellent ionization capability of the source ICP-MS also marked a major step in isotope geochemistry and cosmochemistry. Taking the role of REEs as indicator elements for geological processes and as geochronometers based on the decay of La, Sm and Lu isotopes into consideration, the advent of ICP-MS can be considered an important step in geosciences.

Acknowledgment

This article is also available in: Golloch, Handbook of Rare Earth Elements. De Gruyter (2016), isbn 978–3–11–036523–8.

The author thanks Mrs. G. Günther and M.-A. Voß for assistance with bibliographical data.

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Published Online: 2017-2-17

Published in Print: 2017-2-28

Articles in the same Issue

https://doi.org/10.1515/psr-2016-0066

Keywords for this article

REE concentration; REE isotope ratio; ICP-quadrupole MS; ICP-sector field MS; laserablation ICP-MS