Review Reports - Emerging Analytical Techniques for Rare Earth Element Study: Basic Principles and Cutting-Edge Developments

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Rare earth is the general name of 17 chemical elements. Because of its unique optical, electrical and magnetic properties, it is widely used in luminescence, permanent magnet, superconductivity and high-end electronics. Because of the similar properties, many rare earth elements often coexist in minerals, which makes the analysis methods and test accuracy of the content and phase composition of specific rare earth elements very important in the mineral mining and metallurgy. This paper systematically discusses the principles of ICP-OES, iCP-MS, INAA, XRF and their applications in rare earth analysis, which provides a certain basis for the improvement of rare earth analysis methods.

The following problems in this paper should be further improved.

This paper reviews the testing methods of rare earth elements, but the detection limits of various testing methods should be supplemented.
In which system or physical state, the rare earth can be tested more accurately using which testing method should be further discussed.
When commenting on hand-hoid test instruments, the test response time should be supplemented.
In the discussion of test methods such as ICP-OES, ICP-MS, XRF, ETC. we should illustrate the influence of the distribution and content of rare earth elements in the sample on the test results and the influence of sample pretreatment methods on the test accuracy.
In the final summary of this paper, it is suggested to list the suitable application scenarious, the state of samples to be tested, the detection accuracy and so on of each test method in the form of tables.

Comments for author File: Comments.pdf

Author Response

Response to the Reviewers’ Comments

Reviewer 1

The following problems in this paper should be further improved.

This paper reviews the testing methods of rare earth elements, but the detection limits of various testing methods should be supplemented.

Response: In some cases, we discussed the detection limit, if required for emphasis. However, not all cited references discuss such analytical performance, which indeed limits our discussion regarding this issue. According to the referee’s suggestion, in return, we attempted to summarize the analytical performance, including LOD, as presented in Table 4. (Page 31)

In which system or physical state, the rare earth can be tested more accurately using which testing method should be further discussed.

Response: We have covered this issue thoroughly in each section. Every technique demonstrates advantages and disadvantages for the particular state and type of samples, which we have incorporated into the text, including our perspective at the end of each section.

When commenting on hand-hoid test instruments, the test response time should be supplemented.

Response: Limited information in the cited literature prompted us not to present such data in this review.

In the discussion of test methods such as ICP-OES, ICP-MS, XRF, ETC. we should illustrate the influence of the distribution and content of rare earth elements in the sample on the test results and the influence of sample pretreatment methods on the test accuracy.

Response: This issue has been addressed in general under section 3 and specifically in each subsection. Please see the revised manuscript on page 8.

In the final summary of this paper, it is suggested to list the suitable application scenarious, the state of samples to be tested, the detection accuracy and so on of each test method in the form of tables.

Response: We added a table summarizing the analytical data as follows. (Page 31)

Table 4 presents representative data summarizing all the analytical information, including suitable application scenarios, analytical techniques, analytical performances, and sample information (type and handling).

Table 4: The analytical data for REE analysis employing emerging analytical techniques.

Techniques	Samples	Analyzed REEs	Performance		Description	Ref.
Techniques	Samples	Analyzed REEs	Linearity	LOD	Description	Ref.
UV-Vis and multivariate analysis	Monazite	Sm, Eu, Gd, Tb, and Dy	-	1.38, 0.33, 42.12, 1.77, and 0.58 ppm, respectively, for Sm, Eu, Gd, Tb, and Dy	Sample digestion using concentrated nitric acid	66
XRF	Water samples	Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu	-	In the range of 0.4-4.7 ng mL^-1	Requiring solid phase extraction	71
NAA	Soil samples	La, Ce, Nd, Sm, Eu, Tb, Dy, Yb, and Lu	-	-	No requirement for sample dissolution	84
MP-AES	REE ore samples (OREAS-465, REE-1 and OKA-2)	La, Ce, Pr, Nd, Sm, Dy, and Yb	-	La (0.45 µg mL^-1), Ce (2.7 µg mL^-1), Pr (2.3 µg mL^-1), Nd (0.7 µg mL^-1), Sm (0.76 µg mL^-1), and Yb (0.31 µg mL^-1)	Sample treatment using the fusion digestion method	49
LIBS	Monazite, xenotime, cerite, and white fluorapatite	Eu, Sm, Gd, Tb, Ce, Ho, Er, Tm, and Yb	-	-	-	98
ICP-OES	Rock, soil, and core samples	-	-	-	Solvent extraction separation using a bi-dentate ligand, 2,3-dihydroxynaphthalene (2,3-H₂ND) done prior to measurements	112
ICP-MS	Lignites and volcanic fumarole sediment	13 lanthanides, La, Sc, and Y	-	In the range of 0.04-10.9 ng L^-1	Preconcentration conducted with biosilica layer-by-layer modified with polyamines and carboxyarsenazo	124
SHRIMP-RG ion microprobe	Coal combustion fly ash	¹³⁹La⁺, ¹⁴⁰Ce⁺, ¹⁴⁶Nd⁺, ¹⁴⁷Sm⁺, ¹⁵¹Eu⁺, ¹⁵⁸Gd¹⁶O⁺, ¹⁵⁹Tb¹⁶O⁺, ¹⁵⁹Dy¹⁶O⁺, ¹⁶⁶Er¹⁶O⁺, ¹⁷²Yb¹⁶O⁺, ¹⁷⁵Lu¹⁶O⁺, Pr, ⁸⁹Y⁺, and ⁹³Nb⁺	-	mg kg^-1 to sub-mg kg^-1 level	-	177
µXRF and advanced minerals identification and characterization system (AMICS) software	Britholite, aeschynite, cerite, monazite, thorite, andthorutite, pyrite, actinolite, apatite, ilmenite, hematite, zircon, and ankerite	All REEs in the analyzed minerals	-	-	-	210
Handheld LIBS device	Geological solid and concentrated liquid samples	-	-	-	Device development	214
Raman micro-spectroscopy	Carbonate, phosphate minerals, and cyanobacteria in rock	Sm³⁺, Eu3+, Pr³⁺, Ho³⁺, and Er³⁺	-	-	-	217
Hyperspectral remote sensing	Carbonatite samples	Nd and associated REEs, e.g., Sm, Pr, and Eu	-	-	-	221
Fluorescence sensor	Lanthanide metals (Dy, Eu, Tb, and Nd)	Industrial stream	Up to 8 mM	0.227 mM	-	237

Reviewer 2

Considering the breadth this review covers, it is surprisingly concise. The authors do a great job of hitting key points and not getting bogged down in details.

I can imagine this review being used by advanced undergraduate & graduate students as well as more experienced researchers looking for a guide into REE analysis.

The literature cited covers key examples. It would not be practical to try to cite all the publications in this subject area, so the authors do a good job of citing representative studies.

The authors use these studies and their own insights to provide a critical review that will be helpful for those needing to understand the advantages and limitations of each method.

This reviewer particularly likes the figures provided. Combined with the text, the review goes a long way to explaining how each technique works. This is what will make the review valuable as a teaching tool.

This reviewer would like to see similar figures and/or photos for some of the portable analysis techniques. This would go well with the early section on REE provenance because the portable analytical tools are used heavily in mineral exploration.

Response: We added two additional figures as the referee’s suggestion.

A real photograph of a portable SWIR (400-2500 nm) used in geological studies is shown in Figure 11. (Page 28)

References

[211] van der Ent, A., et al., X-Ray Fluorescence Ionomics of Herbarium Collections. Scientific Reports, 2019. 9(1).

[227] Balaram, V., Advances in Analytical Techniques and Applications in Exploration, Mining, Extraction, and Metallurgical Studies of Rare Earth Elements. Minerals, 2023. 13(8).

Reviewer 3

In this review paper a thorough description is made about the function as well as the advantages and disadvantages of all analytical techniques that can be used for detecting REEs. The authors also present nice examples from the literature concerning their applications.

The manuscript is very-well structured and the quality of English language is very high.

Very nice and well-explained figures and tables are presented by the authors.

Below are my comments:

1) I haven't read anywhere before that LREEs are called 'the cerium group' and HREEs are called 'the yttrium group'. Are these terms right? If not, the authors should remove them from the text.

Response: These terms are introduced in several literature, a few of which are listed below. (Please also see reference 5 in the reference section of the revised manuscript)

[1] https://doi.org/10.14434/ijes.v4i1.33628

[2] Greenwood, N.N. and Earnshaw, A., 2012. Chemistry of the Elements. Elsevier.

[3] Daane, A. H. (1968). "Yttrium". In Hampel, Clifford A. (ed.). The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corporation. pp. 810–821. LCCN 68029938. OCLC 449569

2) You talk more extensively about the collision cell only for ICP-MS/MS. The collision cell is also included in all ICP-MS instruments. In ICP-MS paragraph, you say that the collision cell could be applied, giving the impression to the reader that it may not be already included.

Response: A collision cell is not found in all ICP-MS instruments, as high-resolution ICP-SFMS systems can eliminate interferences through spectral resolution instead of a collision cell.

In addition, you don't talk about the ion lenses which is a very important step in determining accurately REE levels in a sample. The implementation of collision cell and ion lenses in an ICP-MS system has made the interferences very minimal when analyzing REEs.

Response: To address this issue, we added the following lines.

…Moreover, the optimization of ion lenses plays a decisive role in ensuring accurate determination of REEs, as it directly influences ion transmission efficiency and signal stability in ICP-MS analysis. When coupled with a collision cell, ion lens tuning enables the significant reduction of interferences, thereby enhancing the reliability and precision of REE measurements, even in complex sample matrices [121]. (Page 17)

3) I propose that you include the following article in your manuscript that compares ICP-MS and TXRF when analyzing REEs in various environmental samples: Levels of rare earth elements on three abandoned mining sites of bauxite in southern Italy: A comparison between TXRF and ICP-MS – ScienceDirect

Response: We included this study under Section 4 as follows.

Brouziotis et al. [254] carried out a comparative study on the analysis of REE contents in abandoned mining sites of bauxite using ICP-MS and TXRF. The significant amounts of REEs were observed through the corresponding techniques, where among the REEs, Ce is the most abundant. Conclusively, ICP-MS performed better than TXRF in this study. (Page 31)

Reference (Page 47)

[254] Brouziotis, A.A., et al., Levels of rare earth elements on three abandoned mining sites of bauxite in southern Italy: A comparison between TXRF and ICP-MS. Talanta, 2024. 275.

4) Make the following grammar corrections:

Abstract: which have --> them having

Response: We changed it to be “…have taken place, with some of them having shrunk in size,…”. (Page 1)

Introduction: Otherwise --> However

Response: We revised it to be “…lanthanide series. However, these elements…”. (Page 4)

Geological Occurrence and Distribution of REEs:

Of the REE minerals --> Among the REE minerals

Xenotime is a yttrium phosphate mineral --> Xenotime is an yttrium phosphate mineral

Response: We revised them to be

“…minerals [35, 36]. Among the REE minerals, bastnäsite…”. (Page 7)

“…Xenotime is an yttrium phosphate mineral…”. (Page 7)

3.12. Chemical Sensors: Chemical sensors are an analytical technique --> Chemical sensors consist an analytical technique

Response: We revised it to be “…Chemical sensors consist of an analytical technique that works on…”. (Page 28)

Conclusions and challenges:

compositions. minimal --> compositions, minimal

Response: We changed it to be “…and mineralogical compositions, as well as minimal interference, multi-element and isotopic capabilities,…”. (Page 31)

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Review attached

Comments for author File: Comments.pdf

Author Response

Considering the breadth this review covers, it is surprisingly concise. The authors do a great job of hitting key points and not getting bogged down in details.

I can imagine this review being used by advanced undergraduate & graduate students as well as more experienced researchers looking for a guide into REE analysis.

The literature cited covers key examples. It would not be practical to try to cite all the publications in this subject area, so the authors do a good job of citing representative studies.

The authors use these studies and their own insights to provide a critical review that will be helpful for those needing to understand the advantages and limitations of each method.

Response: We added two additional figures as the referee’s suggestion.

A real photograph of a portable SWIR (400-2500 nm) used in geological studies is shown in Figure 11. (Page 28)

References

[211] van der Ent, A., et al., X-Ray Fluorescence Ionomics of Herbarium Collections. Scientific Reports, 2019. 9(1).

[227] Balaram, V., Advances in Analytical Techniques and Applications in Exploration, Mining, Extraction, and Metallurgical Studies of Rare Earth Elements. Minerals, 2023. 13(8).

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript is very-well structured and the quality of English language is very high.

Very nice and well-explained figures and tables are presented by the authors.

Below are my comments:

1) I haven't read anywhere before that LREEs are called 'the cerium group' and HREEs are called 'the yttrium group'. Are these terms right? If not, the authors should remove them from the text.

4) Make the following grammar corrections:

Abstract: which have --> them having

Introduction: Otherwise --> However

Geological Occurrence and Distribution of REEs:

Of the REE minerals --> Among the REE minerals

Xenotime is a yttrium phosphate mineral --> Xenotime is an yttrium phosphate mineral

3.12. Chemical Sensors:
Chemical sensors are an analytical technique --> Chemical sensors consist an analytical technique

5. Conclusions and challenges:

compositions. minimal --> compositions, minimal

Author Response

The manuscript is very-well structured and the quality of English language is very high.

Very nice and well-explained figures and tables are presented by the authors.

Below are my comments:

1) I haven't read anywhere before that LREEs are called 'the cerium group' and HREEs are called 'the yttrium group'. Are these terms right? If not, the authors should remove them from the text.

Response: These terms are introduced in several literature, a few of which are listed below. (Please also see reference 5 in the reference section of the revised manuscript)

[1] https://doi.org/10.14434/ijes.v4i1.33628

[2] Greenwood, N.N. and Earnshaw, A., 2012. Chemistry of the Elements. Elsevier.

[3] Daane, A. H. (1968). "Yttrium". In Hampel, Clifford A. (ed.). The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corporation. pp. 810–821. LCCN 68029938. OCLC 449569

Response: A collision cell is not found in all ICP-MS instruments, as high-resolution ICP-SFMS systems can eliminate interferences through spectral resolution instead of a collision cell.

Response: To address this issue, we added the following lines.

Response: We included this study under Section 4 as follows.

Reference (Page 47)

[254] Brouziotis, A.A., et al., Levels of rare earth elements on three abandoned mining sites of bauxite in southern Italy: A comparison between TXRF and ICP-MS. Talanta, 2024. 275.

4) Make the following grammar corrections:

Abstract: which have --> them having

Response: We changed it to be “…have taken place, with some of them having shrunk in size,…”. (Page 1)

Introduction: Otherwise --> However

Response: We revised it to be “…lanthanide series. However, these elements…”. (Page 4)

Geological Occurrence and Distribution of REEs:

Of the REE minerals --> Among the REE minerals

Xenotime is a yttrium phosphate mineral --> Xenotime is an yttrium phosphate mineral

Response: We revised them to be

“…minerals [35, 36]. Among the REE minerals, bastnäsite…”. (Page 7)

“…Xenotime is an yttrium phosphate mineral…”. (Page 7)

3.12. Chemical Sensors: Chemical sensors are an analytical technique --> Chemical sensors consist an analytical technique

Response: We revised it to be “…Chemical sensors consist of an analytical technique that works on…”. (Page 28)

Conclusions and challenges:

compositions. minimal --> compositions, minimal

Response: We changed it to be “…and mineralogical compositions, as well as minimal interference, multi-element and isotopic capabilities,…”. (Page 31)

Author Response File: Author Response.pdf