Analyzing and Controlling Rare Earth Elements (REEs) in High-Purity REE Oxide

Table of Contents

Introduction
Agilent ICP-MS Technology
Experimental Method
     Instrumentation
     Analysis in Cell Gas Mode
     Mass-Shift Mode with Oxygen as Carrier Cell Gas
     NH3-Based On-Mass Measurement
Experimental Results
Conclusion
About Agilent Technologies Inc.

Introduction

Rare earth elements (REE) have many important applications in modern technology. For instance, elements such as dysprosium (Dy), samarium (Sm), neodymium (Nd) and praseodymium (Pr) are used in high-power permanent magnets and holmium (Ho), erbium (Er) and ytterbium (Yb) are used in lasers. Fluorescent and luminescent phosphor components such as europium (Eu), terbium (Tb), gadolinium (Gd) and lanthanum (La) are used in plasma displays, radar screens and fluorescent lamps. Other applications of REEs include high- technology glasses and auto exhaust absorption catalysts.

It is evident from the above examples that REEs are the crucial constituents of materials employed in high-technology industrial applications. However, REEs exist as impurities in a pure single-element material, thus affecting the efficiency of the final product. Therefore, it is necessary to control the contaminant level in REE oxide raw materials.

Agilent ICP-MS Technology

ICP-MS is the one of the common atomic spectroscopic methods used for measuring the traces of REEs owing to its simple REE spectra over other emission techniques. However, it is very difficult to measure traces of REEs with higher mass on a low-mass REE matrix. This is due to the highest metal-oxide bond strengths of REEs and interference of oxide ions of low mass REEs with the high-mass and mid-mass REEs.

This experiment used an Agilent 8800 Triple Quadrupole ICP-MS, which is designed for measuring the trace REE contaminants of high- purity REE materials. The device is equipped with an additional quadrupole mass filter (Q1) placed before the Octopole Reaction System (ORS) cell, and quadrupole mass filter (Q2). In this configuration, Q1 acts as a 1amu mass filter, facilitating the entry of target analyte mass into the cell while neglecting all other masses. Therefore, elimination of matrix and plasma ions by Q1 filter ensures precise control of ORS reaction processes, providing accurate results for complex, high-matrix samples.

Experimental Method

Instrumentation

The Agilent 8800 Triple Quadrupole ICP-MS consists of a 2.5-mm injector integrated quartz torch, a Peltier-cooled quartz double-pass Scott-type spray chamber, standard ion lens and a sample introduction system with a MicroMist glass concentric nebulizer and Ni interface cones.

Analysis in Cell Gas Mode

The 8800 includes two operation modes in MS/MS configuration - direct/on-mass mode and indirect/mass-shift mode. The device is operated in on-mass mode in absence of analyte reaction and when the chosen cell gas reaction eliminates any interference.

In this mode, the analyte is directly measured. In mass-shift mode, the analyte is made to react with the cell gas and it acts as a reaction product ion. REEs can be determined in both the operating modes, and fast sequential analyte measurement can be facilitated by a single pre-defined method in either or both modes.

In tandem MS/MS mode, Q1 rejects the matrix ions or co-existing analyte that overlap the new analyte product ion mass or the original analyte mass. Therefore, the MS/MS product ion spectra are simpler when compared to conventional ICP-QMS. This proves that accurate results can be obtained in the mass-shift mode, and there is no or less need for method development.

The efficiency of the 8800 operating in mass-shift mode with oxygen cell gas carrier was tested, in addition to on-mass measurement of REE impurities in single element REE oxides with ammonia cell gas. The 8800 was operated under built-in 'general purpose' preset plasma conditions for all cell modes, providing CeO+/Ce+ <0.8% in no gas mode. The table below provides the cell-related tuning parameters.

Table 1. Agilent 8800 Triple Quadrupole ICP-MS ORS3 cell-related tuning parameters

Cell mode No gas O2 NH3
Scan mode MS/MS MS/MS MS/MS
Octopole bias (V) -8 -5 -18
Octopole RF (V) 180 180 180
KED (V) 5 -8 -8
Cell gas N/A O2 NH3/He
Cell gas flow rate (mL/min) N/A 0.35 9.0
Cell entrance (V) -80 -90 -110
Cell exit (V) -80 -90 -110
Deflect (V) 20 10 -3
Plate bias (V) -80 -90 -110

Mass-Shift Mode with Oxygen as Carrier Cell Gas

The evaluation results of reaction efficiency of REE using O2 as carrier cell gas are shown in Figure 1.

Figure 1. Measured REE oxide formation ratio and enthalpy of oxide formation reaction. MO+ formation ratio is calculated from MO+/(M++ MO+), for each analyte mass number M.

Figure 1 reveals that all REEs other than Yb and Eu are capable of forming oxide ions, while the reactions with Eu and Yb elements are endothermic and form low efficiency oxide ions. However, it is possible to detect Eu and Yb as MO+ with <1 ppt detection limits. Considering the reaction of REEs with O2 gas, this mode is suitable for eliminating the interference of original oxides.

NH3-Based On-Mass Measurement

Due to its highly reactive nature, NH3 gas may likely react with the target REE analyte ion or corresponding REE-oxide interference at various reaction rates. REE-oxide overlap can be eliminated to detect the target REE analyte ion, if the REE-oxide reacts with NH3 gas.

Preliminary tests showed that REEs can be classified into two groups. The first group consists of REE elements that react with NH3 gas such as Yb, Tm, Ho and Eu. High-sensitivity measurements of these REEs can be performed at their original masses and in NH3 cell gas mode.

The second group consists of REEs such as Lu, Tb, Gd, Sm, Nd, Ce, and La that reacted with NH3 gas under test conditions (Table 1). These REEs, upon reacting with NH3 gas, reduce the sensitivity of analyte ion measurement in cell gas mode. The sensitivity of each element in the three cell modes is represented in Figure 2.

This mode can reduce the oxide interference if the oxide ions of REEs react with NH3. The signal response of 156GdO+ was evaluated corresponding to varying NH3/He cell gas flow. The response of 172Yb+ isotope overlapped by 156GdO + also was investigated simultaneously.

Figure 2. Sensitivity of second group elements in three cell modes

The evaluated background equivalent concentration (BEC) of Yb in 1ppm Gd solution at the optimum cell gas flow rate of 9.0mL/min is shown in Figure 3.

Figure 3. Sensitivity of GdO and Yb at m/z 172, plus Yb BEC as a function of NH3/He flow rate

BEC of individual REE impurity present in 1ppm Gd solution is given in Figure 4.

Figure 4. BECs of each REE impurity in the 1 ppm Gd solution

It is evident from the above figure that REEs such as 175Lu, 172Yb and 159Tb have increased BEC owing to interferences of Gd polyatomic ions. O2 mass-shift mode is preferred for measuring Tb and Lu elements. However, the O2 mass-shift method may not be suitable for measuring Yb+. The BEC of Yb is significantly improved in the NH3 on-mass mode to < 1ppt.

Experimental Results

The experiment involves dissolution of two pure REE oxide materials in semiconductor grade HNO3 solution and diluted at a concentration of 1ppm. Measurement of other REEs is carried out in three cell gas modes. Comparison of matrix element oxide interferences on other REEs is performed based on the apparent concentration of each trace REE in each matrix. The following figure shows the BEC of each REE impurity in the 1 ppm Sm solution.

Figure 5. BEC of each REE impurity in the 1 ppm Sm solution

172Yb, 169Tm, 166Er, 165Ho and 163Dy have higher BEC in no gas mode owing to SmO+, SmOH+ and SmOH2+ polyatomic ions interferences.

Conclusion

The Agilent 8800 Triple Quadrupole ICP-MS employs a single acquisition method and runs on two operational modes: NH3 on-mass mode and O2 mass-shift mode for the measurement of REE impurities in REE materials of high purity. Hydroxide polyatomic interferences, oxide interferences and REE matrix-based hydride can be effectively eliminated using these cell gas modes.

Measurement of trace REE impurities can be carried out at < 1ppt levels in a 1ppm solution by combining both the modes. The optimized routine method resulting from the combination of gas modes provides accurate results on trace REEs of natural samples as well.

About Agilent Technologies Inc.

Agilent is the proven leader in elemental analysis for geochemistry, mining, and metals industries. We offer the broadest atomic spectroscopy portfolio on earth, with AA, MP-AES, ICP-OES and ICP-MS solutions that solve your toughest challenges.

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This information has been sourced, reviewed and adapted from materials provided by Agilent Technologies Inc.

For more information on this source, please visit Agilent Technologies Inc.

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