Analysis of Rare Earth Elements

The complexity of modern technology has rapidly increased over the last 30 years.

This has been driven by demand for more sophisticated and efficient electronic devices, ranging from everyday consumer products like cameras, smartphones, computers and LED lights to improvements in medical and material science and advanced breakthroughs in renewable energy and battery storage.

This constantly increasing demand for complex technologies has stemmed from the use and applications of the unique features of rare earth elements (REEs), for example, catalytic, magnetic and phosphorescent properties.

Rare earth elements (REEs) are made up of seventeen chemical elements in the periodic table, including the lanthanides, scandium and yttrium. While these elements may be found in nature, they are not naturally found in a pure metal form due to their reactivity.

Rather, they are found together in a range of ores and minerals, requiring further extraction and separation processes before they can be used.

Isolated REEs can be employed in a diverse array of applications ranging from manufacturing processes (autocatalytic converters, ceramics and industrial catalysts) to improvements in energy consumption reductions, enhanced efficiency, better durability, miniaturization and improved thermal stability for electronic devices.

As high tech applications continue to develop, REEs are deemed increasingly vital elements for current and future industries.

There is mounting pressure around the world for analytical solutions able to accurately characterize and quantify REE content in a high throughput manner in order to meet the application needs of expanding industries.

These applications must be able to offer reliable results, even when working with a highly diverse selection of mineral samples.

This article outlines the use of PerkinElmer’s NexION® 2000 ICP-MS for in situ analysis of certified reference materials (CRMs) in selecting five mineral matrices.

The NexION® 2000 ICP-MS is coupled to a laser ablation system in order to demonstrate the potential of laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) for the quantification of rare earth elements.

The article outlines instrument setup, sample preparation, calibration and an appropriate analytical approach for verifying the accuracy, precision, percent recovery, calculated uncertainty and detection limits for the samples and matrices utilized in this experiment.

Experimental

Samples and Preparation

Certified reference materials (CRMs) were procured with various elemental concentrations. These were prepared for five different sample matrices, including sediment, diorite, andesite, basalt and obsidian.

A lithium metaborate fusion procedure was used to prepare each certified reference material (CRM). In each instance, 0.6 g of CRM was mixed with 6 g of fusion flux according to the content percentages shown in Table 1.

Table 1. Composition of Fusion Flux. Source: PerkinElmer 

Material Content
Lithium Tetraborate, Li2B4O7 49.75%
Lithium Metaborate, LiBO2 49.75%
Lithium Bromide, LiBr 0.5%

 

Fusion beads prepared from five different matrices of CRMs labeled as per Table 2 were utilized for calibration.

Table 2. Elemental Concentration (mg kg-1) in CRMs. Source: PerkinElmer 

  STD 1 STD 2 STD 3 STD 4 STD 5
Matrix Diorite Sediment Andesite Obsidian Basalt
CRM ID SY4 STSD-3 JA-1 133 134
Sc 1.1 13 28.5 38.1 5.1
Y 119 36 30.6    
La 58 39 5.24    
Ce 122 63 13.3 13.3 62.2
Pr 15   1.71    
Nd 57 33 10.9    
Sm 12.7 7 3.52 2.79 5.7
Eu 2 1.3 1.2 1.07 0.84
Gd 14   4.36   5.3
Tb 2.6 1.1 0.75 0.448 1.00
Dy 18.2 5.4 4.55    
Ho 4.3   0.95    
Er 14.2   3.04    
Tm 2.3   0.47    
Yb 14.8 3.4 3.03 2.09 4.50
Lu 2.1 0.8 0.47 0.34 0.73
Th 1.4 8.5 0.82   12.4
U 0.8 10.5 0.34 0.37 4.58

Note: Some CRMs do not contain concentration information for certain elements, hence, they were left blank in the table.

Instrumentation

Analyses were performed using an LSX-213 G2+ laser ablation system that had been fitted with a HelEx II ablation cell (Teledyne Cetac Technologies, Omaha, Nebraska, USA).

This was coupled to a NexION 2000 quadrupole ICP-MS system (PerkinElmer Inc., Shelton, Connecticut, USA) using the parameters displayed in Tables 3 and 4.

Table 3. Instrument Parameters. Source: PerkinElmer 

Laser Ablation System
Instrument Teledyne Cetac LSX-213 G2+ with HelEx II Ablation Cell
Laser Fluence 6.69 J/cm2
Laser Rep Rate 20 Hz
Spot Size 50 µm
Scan Speed 10 µm/s
Scan Time 58 s
He Gas Flow 0.65 and 0.35 L/min
ICP-MS System
Instrument PerkinElmer NexION 2000 ICP-MS
RF Power 1500 W
Nebulizer Gas Flow 1.04 L/min
Auxiliary Gas Flow 1.2 L/min
Plasma Flow 15 L/min
Measurement Mode 20 Replicates
Monitored Isotopes 18

 

Table 4. Analytes, Their Masses and Used Dwell Time. Source: PerkinElmer 

Element Mass (amu) Dwell Time (ms)
Sc 45.0 20
Y 88.9 20
La 138.9 20
Ce 139.9 20
Pr 140.9 10
Nd 145.9 10
Sm 146.9 10
Eu 152.9 10
Gd 156.9 20
Tb 158.9 20
Dy 162.9 20
Ho 164.9 10
Er 165.9 20
Tm 168.9 10
Yb 172.9 20
Lu 174.9 10
Th 232.0 20
U 238.1 20

 

Laser sampling was performed in a straight line from one point to another. A defined scan rate of 10 µm/s was used along with a spot size of 50 µm, allowing measurement of 18 elements to be completed in 58 seconds.

A HelEx II cell was employed in this application. The instrument’s two separate cell flow controls (inner and outer) meant that the signal intensity and washout could be optimized to the NexION 2000 ICP-MS. This approach offers an ideal solution for high throughput laboratories undertaking low level analyses in rare earth elements.

Results and Discussion

Calibration

A calibration curve was constructed using five different matrices with a range of concentrations (Table 5). Good correlation coefficients of calibration curves (Table 6) signal that analyte responses remained similar irrespective of the matrix.

Table 5. Calibration Standards Used for Five Calibration Points. Source: PerkinElmer 

  STD 1 STD 2 STD 3 STD 4 STD 5
Matrix Diorite Sediment Andesite Obsidian Basalt
CRM ID SY4 STSD-3 JA-1 133 134

 

Table 6. Summary of Calibration Curve Results. Source: PerkinElmer

Analyte Mass
(amu)
Curve
Type
Slope Intercept Corr.
Coeff.
Sc 45.0 Simple Linear 110.61 149.47 0.996927
Y 88.9 Simple Linear 91.08 -118.81 0.999999
La 138.9 Simple Linear 206.24 -175.75 0.995163
Ce 139.9 Simple Linear 203.12 -70.72 0.999612
Pr 140.9 Simple Linear 213.31 115.75 1.000000
Nd 145.9 Simple Linear 33.50 21.80 0.999814
Sm 146.9 Simple Linear 30.22 -7.78 0.997846
Eu 152.9 Simple Linear 110.95 -8.11 0.995199
Gd 156.9 Simple Linear 28.66 -13.44 1.000000
Tb 158.9 Simple Linear 179.48 -2.67 0.996987
Dy 162.9 Simple Linear 44.49 20.44 0.999890
Ho 164.9 Simple Linear 170.75 39.79 1.000000
Er 165.9 Simple Linear 59.88 6.96 1.000000
Tm 168.9 Simple Linear 185.25 -6.57 1.000000
Yb 172.9 Simple Linear 31.99 -7.13 0.998124
Lu 174.9 Simple Linear 179.88 4.68 0.999912
Th 232.0 Simple Linear 227.36 -22.37 0.998836
U 238.1 Simple Linear 253.84 56.48 0.997054

Note: Calibration curve obtained with different mineral matrices.

This also confirms that samples prepared as fusion beads with different matrix compositions may be analyzed against calibration curves obtained in the same fashion.

CRM Recovery and Establishing Measurement Uncertainty

Measurement uncertainty is calculated based on processes outlined in the EURACHEM/CITAC Guide - Quantifying Uncertainty in Analytical Measurement.

A CRM SY4 was analyzed over a period of 10 days in order to obtain the uncertainty, with recoveries obtained and uncertainty calculated (Table 7) from these measurements.

Table 7. QC Recovery and Uncertainty Data. Source: PerkinElmer 

Element % Range of
QC Check
Measurement Uncertainty
 X ± mg/kg
Sc 83.8-109.8 0.12
Y 98.0-106.9 0.03
La 98.0-106.3 0.03
Ce 97.7-107.4 0.03
Pr 97.9-108.3 0.03
Nd 95.1-106.6 0.03
Sm 91.9-106.7 0.05
Eu 91.2-108.3 0.04
Gd 97.5-108.0 0.04
Tb 94.3-103.5 0.05
Dy 95.2-104.0 0.04
Ho 96.9-105.9 0.03
Er 93.3-104.9 0.05
Tm 91.8-107.0 0.05
Yb 92.7-105.4 0.04
Lu 93.5-114.0 0.06
Th 93.4-111.0 0.13
U 94.0-117.0 0.15

 

The majority of elements in CRM SY4 displayed good recoveries within 90-110%. The only exceptions were Lu, Th and U that fell within 90-120% due to their concentration being on the low side of 0.8-2.1 mg/kg.

Uncertainties for every element obtained have been based on a coverage factor of two with a confidence level of 95%, resulting in a comparatively low range of possible values where the exact value of the measurement may be found.

Detection Limits

Seven individual analyses of a CRM SY4 were performed, with the lowest weight (~0.1 to 6 g fusion flux). The standard deviation of these seven readings enabled the limits of detection and quantitation to be tabulated (Table 8).

Table 8. Limit of Detection (LOD) and Limit of Quantitation (LOQ) of Elements with LA-ICP-MS. Source: PerkinElmer 

Element LOD (3.14 x SD) mg/kg LOQ (10 x SD) mg/kg
Sc <0.5 <2
Y <2 <10
La <1.5 <5
Ce <3.5 <10
Pr <0.5 <2
Nd <2.0 <5
Sm <1.0 <5
Eu <0.2 <0.5
Gd <1.5 <5
Tb <0.2 <0.5
Dy <1.0 <5
Ho <0.2 <1
Er <0.7 <5
Tm <0.2 <1
Yb <1 <5
Lu <0.2 <0.5
Th <0.2 <0.5
U <0.2 <0.5

 

Accuracy (Bias) and Precision Results

In order to determine accuracy and precision, two sets of experiments were performed (Table 9):

  1. Set of experiments prepared by the first operator on CRM SY4 on the first day
  2. Set of experiments prepared by the second operator on CRM SY4 on the second day

Experiments were all set up with ten individual preparations of fusion beads that had been verified from sample preparation. These were analyzed as samples after calibration in order to confirm the method’s repeatability, reproducibility and inter-laboratory precision.

Table 9. Accuracy (Bias) and Precision Study on CRM SY4. Source: PerkinElmer 

Element Certificate Accuracy (Bias) and Precision
Concentration
mg/kg
(A)
Bias %
(B)
Bias %
(A)
Precision %
(B)
Precision %
Sc 1.1 13.82 -12.78 8.29 9.73
Y 119 7.10 -4.47 3.47 1.73
La 58 3.55 -8.12 2.98 1.86
Ce 122 1.18 -12.26 3.38 2.05
Pr 15 3.82 -12.78 3.29 1.73
Nd 57 3.17 -13.11 2.77 1.91
Sm 12.7 -0.85 -10.30 4.75 4.27
Eu 2 -2.40 -11.09 2.25 4.13
Gd 14 -3.54 -8.38 3.48 3.19
Tb 2.6 -0.74 -8.56 2.49 2.01
Dy 18.2 0.21 -7.78 3.53 2.00
Ho 4.3 -0.24 -10.38 2.81 1.99
Er 14.2 -1.06 -8.66 3.98 2.34
Tm 2.3 -0.60 -10.45 4.86 4.19
Yb 14.8 -2.94 -8.54 4.61 2.22
Lu 2.1 -0.09 -7.00 4.56 3.86
Th 1.4 2.86 -1.87 3.80 3.36
U 0.8 15.07 -7.58 6.30 6.24

 

Conclusion

The method developed and outlined within this article is rapid and easy to use, offering analysis times as short as 58 seconds for 18 elements. It also offers exceptional accuracy and precision.

The XRF technique may be utilized alongside LA-ICP-MS – a sample analyzed using LA-ICP-MS for REE determination may also be analyzed via XRF for main elements in minerals; for example, Al, Si, Na, Mg, P, K, Ca, Ti, Mn and Fe.

The method’s validity was confirmed through the incorporation of robustness and ruggedness tests on energy output, additional sample preparation weights and different mineral matrices in the fusion bead.

QC, CRM recovery and spike recovery checks performed during the analysis revealed good recoveries that were comfortably within acceptable limits.

These checks provided additional confirmation that the NexION 2000 ICP-MS coupled to a laser ablation system is able to achieve exceptional detection limits while offering good accuracy (bias %) and precision (repeatability %) that are well within acceptable limits.

Consumables Used

Table 9. Source: PerkinElmer 

Component Part Number
Baffled Cyclonic Spray Chamber N8152389
MEINHARD® Nebulizer N8152373
One-piece Quartz Torch with 2.5 mm Injector,
PVDF O-ring Freeholder (Blue Mark) Torch
N8152473

 

This information has been sourced, reviewed and adapted from materials provided by PerkinElmer.

For more information on this source, please visit PerkinElmer.

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