1. Introduction
Rare earth elements (REE) are a group of 17 metallic elements which appear in the periodic table. The group consists of the 17 lanthanide elements (which are: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu)) as well as yttrium (Y) and scandium (Sc). In recent years, REE demand has increased in electronics, catalysis, optical displays, high-performance magnets, batteries, aerospace manufacturing and diverse medical applications. While REEs are not uncommon in the earth’s crust, they are not often found in sufficient concentrations to make extraction commercially viable.
The determination of REEs can be performed by several analytical techniques such as neutron activation analysis, X-ray fluorescence spectrometry, ultraviolet-visible spectrometry, electrochemistry, atomic absorption spectrometry, inductively-coupled plasma-optical emission spectrometry (ICP-OES). And the various forms of inductively-coupled plasma-mass spectrometry (ICP-MS, i.e. inductively-coupled plasma quadrupole mass spectrometer (ICP-QMS), sector field inductively-coupled plasma mass spectrometer (SF-ICP-MS), time-of-flight inductively-coupled plasma mass spectrometer (TOF-ICP-MS)).
Most common methods to analyze REE in geological materials
However, two of the most common methods used to analyze REE in geologic materials are ICP-MS and ICP-OES. ICP-MS is often the technique of choice for such analyses because of the high sensitivity, selectivity, wide linear range and multi-element capability. ICP-OES is a suitable, low initial cost alternative for this kind of analyses. However, the accurate determination of rare earth elements remains complex mainly due to their low concentrations in natural samples associated with possible interferences. Indeed, one of the main challenges to be addressed during their analyses by ICP-MS is to handle polyatomic interferences (e.g. oxides and hydroxides) which are mainly related to the presence of Ba and light-REEs (LREEs) in the samples. Consequently, quantification of REE in geological materials by ICP-OES is one of the most challenging analytical routines.
On the other hand, the main difficulty related to the analysis of geological samples is attributed to sample preparation, due to the presence of refractory matrix components composed of silicate and oxide minerals. Since the RE minerals contain such a wide assortment of other elements, a variety of sample preparation methods exist, including fusions with Na2O2, Na2CO3, Li2CO3 and Li2B4O7, as well as digestions with strong mineral acids such as HF/perchloric/nitric/hydrochloric acid combinations. In this sense, microwave-assisted digestion reduces time and chemical reagent consumption and the possibility of contamination that may increase blank levels.
We present a sensitive, simple and robust method for REE determination in geological samples using OneNeb® and MassNeb® nebulizers with ICP-OES and ICP-MS. The optimal aerosol generated by the OneNeb® and MassNeb® nebulizers is also more efficiently desolvated and excited in plasma, helping to improve precision values, even at low sample flow rates, which also explains why it is much more sensitive than conventional nebulizers. Both nebulizers were tested in this study.
2. Experimental
Reagents and solutions
All reagents used were of the highest available purity. Water was purified with a Milli-Q Gradient system (Millipore, Watford, UK).
The aqueous levels of calibration standards were prepared by appropriate dilution of Monoelemental and multielement calibration standard contains 10 mg.L-1 of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc of 5% HNO3 (SPEX CertiPrep™). High purity He and H2 (> 99.999%) were used as collision/reaction gases in ICP-MS to study polyatomic interferences suppression.
Standard calibration covered the concentration range from 0.01 to 100 μg.L-1 for ICP-MS and the concentration range from 5 to 500 μg.L-1 for ICP-OES analysis. All aqueous solutions are acidified using 5% (v/v) purity nitric and hydrochloric acids (Trace Metal, Fisher Scientific). Two ores certified reference materials (OREAS, Australia), OREAS-110-112 Copper Sulphide Ore (Tritton Cu Project, NSW), were used to validate the ICP-OES and ICP-MS results. In addition, copper sulphide pulverized samples of regional mine from southwest of Spain, Iberian Pyrite Belt, were donated for this study.
Instrumentation
A microwave oven (CEM Matthews, C, USA, model MARS6) was used for the mineralization of certified reference materials OREAS (Copper Sulphide Ore ) used in this study. The microwave program is detailed in Table I.
Table I. Operational parameters used during microwave acid digestion.
All measurements were carried out using an Agilent 7900 ICP-MS (Agilent Technologies) and Agilent 5900 ICP-OES (Agilent Technologies) equipped with a SPS4 autosampler (Agilent Technologies). The instruments operating conditions are shown in Table II.
The OneNeb® and MassNeb® nebulizers use Flow Blurring nebulization technology instead of the traditional Venturi effect. This generates a fine droplet aerosol with narrow size distribution, improving efficiency across nebulization gas flows, especially 0.60-0.75 L min-1 at 100-250 kPa. Both nebulizers were tested in this study.
Table II. Operational conditions for 7900 ICP-MS and 5900 ICP-OES using OneNeb® and MassNeb® nebulizers
Samples preparation
First, 0.500 g of each sample was weighed into EasyPrep iWave vessels (CEM) and 3 mL of HNO3 and 9 mL of concentrated HCl were then added. The mixture was left 30 min for pre-mineralization at room temperature. The vessels were capped and placed in the CEM MARS 6 iWave microwave for digestion. After completion of the digestion process, digested solutions were left to cool to room temperature, then transferred to clean PFA vials (Savillex Corporation, Eden Prairie, MN, USA) and diluted with ultrapure water to a final volume of 25 g for ICP-OES. On the other hand, 5 g of the resulting solution were transferred to another PFA vials and diluted 10 times, to a final volume of 50g for ICP-MS analysis. An overview of the total sample preparation procedure is presented in Figure 1.
Figure 1. Workflow used for sample preparation.
Samples concentrations were calculated after applying the corrections with the calibration blank and analytical blank. First, the calibration blank signal was subtracted underwent ensure that the ICP-OES or ICP-MS measurement provides a zero signal when no analyte is present. Finally, an “analytical blank” prepared using the whole set of reagents (HNO3 and HCl) were mineralized and underwent the same preparation procedure as the samples under the same conditions. This blank reflects the potential contamination that occurred during the whole sample preparation workflow, and it was subtracted from the samples’ elemental concentrations in order to calculate the “true” concentration of each element. All labware was washed in 20% (v/v) HNO3 and rinsed with ultrapure water before use.
Prior quantification of REEs by ICP-OES and ICP-MS, diluted solutions were filtered using PTFE syringe filter (0.45 μm). All samples were prepared by triplicated.
3. Results and discussion
Selection of the isotopes and wavelengths monitored for analysis
As is well known, in ICP the formation of polyatomic species like MO+ and MOH+ is greatly influenced by the chemical nature of the respective element. Since oxide and hydroxide formation will follow particular stoichiometric reactions, their contribution to an analyte signal can be corrected by a fixed numerical coefficient (correction equation), determined for the specific analysis conditions. Cerium for example has a strong affinity for oxygen with oxide levels of less than 1% being typical under optimized instrument conditions. In this sense, ICP-MS has become one of the most powerful technique for the determination of REEs due to its high sensitivity, selectivity, wide linear range and multi-element capability. However, the accurate determination of rare earth elements remains complex mainly due to their low concentrations in natural samples associated with possible interferences. Indeed, one of the main challenges to be addressed during their analyses by ICP-MS is to handle polyatomic interferences (e.g. oxides and hydroxides) which are mainly related to the presence of Ba, and other common elements and light-REEs (LREEs) in the samples. Several approaches have been considered to overcome such spectral interferences and to improve the detection limits.
As a previously mentioned, two copper sulphide ore as a certified reference materials (OREAS-110 and OREAS-112) were selected to validate the analytical methodology. For selection of isotopes and wavelengths for ICP-MS and ICP-OES, respectively, the typical multielemental composition of copper sulphide ores (Table III) was considered in the formation of polyatomic and isobaric interferences based on the literature. Isotopes and wavelengths selected in this study are shown in Table II. Major elements presents in these samples, such as Cu, Fe, S, between others, were considered in the selection.
Table III. Multielemental composition in certified copper sulphide ores OREAS-110-112.
Optimization of Operational Instruments Conditions
The operational instruments conditions were optimized to achieve the high sensitivity, but low background, and polyatomic, oxide and double charge interferences prior analysis.
On the one hand, higher power generates high energy plasma conditions that lead to increases in intensity for atomization and ionization processes. The plasma power can have a major effect on the formation of oxides of REEs as well as in signal intensity and plasma robustness. For this purpose, power RF values between 1400-1600 W were checked in this study using MassNeb® and OneNeb® nebulizers and both ICP detectors and the best results were obtained using 1500 W for all cases (Table II). Robust conditions of the plasma have been associated to high applied power by radiofrequency generator (RF). The standardized intensity rises with increased power levels due to the high energy of the plasma at higher power levels. The high plasma power also increases the background levels and sometimes at a rate higher than what the analyte increases. This rise indicates that the monitoring of background signal is important.
On the other hand, additional parameters closely related with the RF power effects are the pump speed rate or uptake sample flow and nebulization gas flow employed. In this sense, each nebulizer has got its own optimum gas flow that directly controls the sample uptake en route the plasma. The longer the sample interacts in the plasma, the more optical transitions of the elements are possible by it acquiring the energy for high energy interactions. High nebulizer gas flow rate gives rise to reduced plasma temperature and lowers the atomization and ionization processes. The pump speed affects the uptake of sample and the efficiency of nebulization that is very critical to sensitivity. The nebulizer gas pressure and the speed of the peristaltic pump determine the volume uptake of sample, and both influence the sample transit to the plasma. Large sample volumes increase the background level due to poor aerosol formation in the spray chamber. Related to this, the dimensions of pump tubing, PFA capillaries and sample probe have effect on the pumping speed; hence, the ideal tubing must be sought.
In this work, different dimensions of pump tubings, PFA capillaries, sample probe, pump speed rates and nebulization gas flow were tested, and the best conditions are show in Table II.
Finally, in ICP-MS additional instrument operational conditions, such as kinetic energy discrimination (KED) voltages and He gas flow employed in collision-reaction cell (CRC), and ion lens voltages were optimized with order to obtain the optimal results for sensitivity, low background, and polyatomic, oxide and double charge interferences prior analysis. In this sense, the preliminary step of this study using ICP-MS analytical technique was to select the optimal gas and flow rate in CRC and the KED voltage to handle spectral interferences in CRC during the analysis of REEs. In the light of the information found in the literature and the results obtained, helium appeared to be the most promising gas for suppressing polyatomic spectral interferences. In this study, for the 32 isotopes initially tested, the use of He always allowed reduction of the interference in comparison with the no gas mode. For this purpose, different He flow rate employed in CRC and KED voltage were evaluated in this work and the optimal values are shown in Table II.
Selection of the nebulizer for REEs determination using ICP-MS and ICP-OES
In addition to the results obtained during the optimization of the instrument operational parameters previously discussed in the previous section using MassNeb® and OneNeb® nebulizers and ICP-MS/ICP-OES detectors, with order to evaluate the optimal nebulizer for REEs analysis, the sensitivity and accuracy of the analytical methodologies using two copper sulphide ores CRMs (OREAS-110-112), were tested, and the result are shown in Table IV and Table V, respectively.
For this purpose, the instrument detection limits (IDL) was calculated by three times the standard deviation of a ten-replicate analysis of the calibration blank. On the other hand, accuracy of an analytical method may be defined as the closeness of agreement between test results and the accepted reference value after sample preparation and operational instruments conditions optimized (Table II). In both cases, we can observe lower levels of ILDs (Table IV) and higher accuracy using MassNeb® in comparison with OneNeb® nebulizer (Table V).
Table IV. Instrumental limit of detections (ILDs) using MassNeb® and OneNeb® nebulizers and ICP-MS/ICP-OES analysis
Table V. Instrumental limit of detections (ILDs) using MassNeb® and OneNeb® nebulizers and ICP-MS/ICP-OES analysis.
Application of optimized methodology to REEs determination on copper sulphide ore from Iberian Pyrite Belt, SW Spain
Based on the previously discussed results, MassNeb® nebulizer turns out to be the optimal nebulizer for the quantification of REEs using ICP-MS and ICP-OES, in terms of higher sensitivity and accuracy, but lower background, and polyatomic, oxide and double charge interferences.
It is well known, in both ICP-MS and ICP-OES, an internal standard is used to improve the accuracy and precision of measurements by correcting for variations in matrix effects, and instrument drift. For this purpose, a REEs was normally employed. For this reason, in this study, these effects were corrected by reslope of calibration curves in both analytical techniques.
This change in slope can affect the accuracy of analyte concentration determination if not properly addressed. Essentially, the calibration curve is re-calculated with a different slope to reflect the new relationship between analyte concentration and instrument response.
For this purpose, a quality control samples with known concentrations throughout the analysis to monitor for drift and adjust the calibration curve if necessary. In this study, a quality verification control sample of 25 ng.g-1 and 100 ng.g-1 for ICP-MS and ICP-OES analysis, respectively, were employed and analyzed for each 5 samples during the analytical sequence. The results obtained are shown in Figure 2.
Figure 2. Recovery results on slope calibration curves obtained in this study using ICP-MS and ICP-OES with MassNeb® nebulizer in a sequence of 50 samples in the same analytical batch.
In all cases, the recovery results on slope calibration curves must fall within the limits of 95-105 % and 94-104 % using ICP-MS and ICP-OES, respectively (Figure 2).
On the other hand, the precision of the analytical methodology optimized was evaluated based on recovery assays, spiked twice time the concentration of each analyte in copper sulphide ore sample employed in this study. The results are shown in Table VI.
Additionally, the reproducibility was evaluated analyzing 5 replicates of each sample, and the results obtained, expressed as relative standard destination (RSD, %) are shown in Table VI.
Results show remarkable precision and reproducibility of the proposed methodology for multielemental quantification of REEs in copper sulphide ore samples. In Table VI we can observe higher precision and reproducibility using MassNeb® nebulizer in ICP-OES analysis.
Table VI. Experimental values for REEs concentrations means, relative standard deviation (RSD, %) for 5 replicates, and recoveries results each analyte using MassNeb® nebulizer employed in this work by ICP-MS and ICP-OES detection in copper sulphide ores samples from Iberian Pyrite Belt, SW Spain.
4. Conclusions
The results obtained in this study, MassNeb® nebulizer provides better sensitivity, reproducibility, accuracy and precision to lower speed pump for sample introduction or sample flow rate to the ICP-MS and ICP-OES instruments in comparison with the results when OneNeb® nebulizer is employed. This fact makes MassNeb® nebulizer more appropriate for REEs determination in digested samples with characteristic high contents of total dissolved solids (TDS) and salts in acid digested geological samples, as is the case with the samples analyzed in this study.
Related to this, when MassNeb® nebulizer and optimized operational instrument conditions were employed, lower backgrounds, polyatomic and oxide and double charge interferences were demonstrated in this work.
In summary, combining ICP-MS and ICP-OES with MassNeb® nebulizer is the best option for profiling REE contents, quantitative analysis and result validation.
