Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
Round 1
Reviewer 1 Report
Page 2 L65-78 The description of the XRF excitation process is wrong. Primary X-rays from the source cause the ionization of inner shell electrons of atoms of the target material (the sample). Subsequent de-excitation of the atom in which outer shell electrons fall down the potential energy gradient to replace the inner shell vacancy result in the emission of fluorescence X-rays characteristic of the atom in question. This mechanism is not clearly described in the text.
P2 L77 This sentence should explain that the X-rays produced are characteristic K and L line fluorescence X-rays of the target atoms as well as a lower intensity continuum. As well as the cited reviews, it would be sensible for the author to quote a classic XRF text for more information of the fundamentals of the technique.
L106 It would be preferable to use the term ‘aqua regia extractable’ rather than ‘pseudo-total’ as the latter term could be interpreted as pretending that this selective extraction is quantitative.
L152-155 This is not the formal definition of Limit of Detection. The ‘official’ Vocabulary of Metrology definition is ‘measured quantity value, obtained by a given measurement procedure, for which the probability of falsely claiming the absence of a component in a material is b, given a probability a of falsely claiming its presence’. I realise that this is difficult to interpret and may justify a more ‘friendly' definition. L156 What is σ the standard deviation of? Justify the factor of 3.3.
Eq 2 3.3 x 3.3 =10.89 not 10. Is this a problem as in 10σ=3.3LOD?
L164 ‘Method sensitivity’ is not the same as detection limit, as is implied by this sentence.
L189-190 I don’t think moisture is strictly an ‘interference’ effect in pXRF – it is a component that is removed by drying in other procedures.
L202-209 It would be preferable to put the search terms in an appendix.
General comment: since many of the pXRF review results are compared with aqua regia selective extractions, it would be useful for the review to include a summary of the expected capabilities of the latter extraction technique (which elements are likely to give near quantitative recoveries?
L329 ‘accurately determined’ implies a reliable calibration whereas the meaning here is that the LOQ for these elements is sufficiently low to allow them to be detected down to the regulatory limit.
L338-361 Accepting that different calibration procedures may have been used by difference investigators, the important point is that in each investigation, measurement results should have been validated by the analysis of a suitable reference material. Is this a problem in this area of study because of the unavailability of suitable RMs?
L362-377 Where discrepancies were observed between pXRF and aqua regia, presumable pXRF results should always be high. Was this the case?
L642 Discrepancies may occur due to different internal calibration procedures, but is not the problem then that the investigator has not properly validated the measurement results, possibly due to the lack of suitable matrix matched reference materials. Is this problem (lack of suitable RMs) also a problem in validating the comparative aqua regia data?
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
General Comments
This is a well-written review paper on an important application of portable x-ray fluorescence (p-XRF) technology: elemental concentration measurements of organic amendments required in the management and recycling of organic waste. The advantages of pXRF are undeniable and the author is correct in predicting room for growth and research in the future. The manuscript includes a comprehensive review of scientific literature in this field and describes well the state and future challenges of the employed quantitative methods.
The reviewer recommends the publication of this review paper pending the inclusion of some corrections and suggestions. The revisions are divided in major and minor categories as follows.
Major comments and revisions
(1) A general comment can be made on the statistical methods of agreement between pXRF-determined elemental concentrations and values determined by other reference analytical methods. While Pearson’s correlation coefficient is widely used in this review paper and several other references therein, there are pitfalls in its application as an agreement metric as described by Altman and Bland in their classic paper: D. G. Altman and J. M. Bland, Measurement in Medicine: The Analysis of Method Comparison Studies, Journal of the Royal Statistical Society. Series D (The Statistician) , vol. 32, no. 3, pp. 307-317, 1983. The reviewer does not necessarily suggest major revisions of this review paper regarding this technical issue, but the author should acknowledge this issue and include it in future research articles or literature reviews.
(2) Page 2, line 69. Description of the XRF process contains errors. The statement “When atoms decay from their excited states, electrons are emitted with less energy than the incident energy, which is therefore fluorescent energy” is misleading. XRF emissions occur in the aftermath of the photoelectric effect which refers to the absorption of an incident x-ray photon by an atom provided its energy is higher than the binding energy of the core shell electron. The incident photon absorption results in the emission of the core shell/subshell electron (a.k.a. photoelectron) which leaves a core shell/subshell vacancy (i.e., atom is in an excited state). There are two competitive atomic de-excitation processes: (1) characteristic or x-ray fluorescence (XRF) and (2) Auger electron emissions. Both processes imply an electron transition from an upper subshell to a lower one within the atom and are the basis of spectroscopic methods to identify the atom since both the XRF photons and the Auger electron have unique energies characterizing the electron transition in the given atom. XRF spectroscopy methods, however, are more practical than Auger electron spectroscopic methods due to high attenuation of electrons in matter. The author should revise the statements describing the XRF process accordingly.
(3) Page 2, line 75. The portable (or hand-held) XRF devices utilize miniature (coin-size) field-emission x-ray tubes and have a much simpler design than the large Coolidge-type x-ray tube used in industrial or diagnostic x-ray imaging applications.
(4) Page 2, lines 79 to 88. The development of hand-held XRF devices is mostly due to Dr. Lee Grodzins, Professor of Physics at MIT who founded the Niton Corporation in 1986 and directed its R&D efforts after his retirement in 1998. Among many awards, Lee Grodzins received the 2021 Birks Award for his contributions to the development of portable XRF devices. Among the other references included here, the reviewer suggests recognition of Dr. Grodzins’ pioneering work.
(5) Page 3, lines 120 to 123. Infrared spectroscopies identify molecules via quantized absorption of infrared light by molecules in their transitions between various vibration/rotational quantum states. The author refers to this later in the manuscript, but it is good to mention this feature here setting it apart from pXRF which is an elemental detection and quantitation technique.
(6) Page 34, line 156. The author does not specify to which quantity the standard deviation (i.e., sigma) in equation (1) belongs to. From the context, one deduces easily that sigma is the standard deviation of the determined concentration, and it is computed based on estimates from several independent measurements. Apart from this minor comment, there is a larger issue related to LOQ definition as 10 sigma. From a standard Gaussian statistics point of view, the LOQ definition as 10 sigma is puzzling as scientific discoveries only require a 5 sigma confidence interval for experimental validation. The reviewer acknowledges this quantity as standard metric in the analytical chemistry field, but its precise meaning must extend beyond statistics and perhaps includes other measurement-related quantities such as unknown or presumed systematic uncertainties. Since this is a review paper, the author could perhaps clarify the meaning of LOQ and provide a rough value of the “acceptable precision” mentioned in this paragraph.
Minor comments/revisions
(1) Page 1, Abstract, line 24. Acronyms NIRS and MIR should be spelled out.
(2) Page 2, line 83. In the sentence “… who reported the 2012 James L. Waters Symposium” there is a missing preposition such as “to the 2012 … “ or “within…” . Please rephrase or correct.
(3) Page 3, line 121. Fourier Transform is the correct term in FTIRS acronym.
(4) Page 4, line 127. Rewrite the sentence as: “There are two standard XRF techniques that can be applied to residues.”, assuming the author refers to the two EDXRF and WDXRF spectrometric techniques employed in XRF analysis.
(5) Page 5, table 1. Left text alignment would be better for an easier reading of the third column.
(6) Page 6, line 184. Missing “c” letter in subsection 1.4 title.
(7) Page 7, line 234. Replace “1” with “one”.
(8) Page 7, line 270 & page 8, line 274. “Aqua regia” should be written in italic font.
(9) Page 8, lines 297 and 298. The reference seem a bit isolated, not much is mentioned regrading the study of Thomas et al. The author should consider removing this reference or substantiate its addition with more details and reasons for its inclusion in the paper.
(10)Page 8, line 307. “Correlations” is perhaps a better term than “relationships”.
(11)Pages 10 & 11. Numbering of pages seems to have been interrupted. Table 2 divided on these two pages appears to have bold fonts. Are these indicating something, or are just editing errors?
(12)Page 12, line 412. Replace “fluorescence energy values” with the more precise “XRF photon energy values”.
(13)Page 13, line 432. Used past tense for “compare”.
(14)Page 13, line 448. Replace “researches” with “research studies”.
(15)Page 13, line 458. What does “photon data” mean?
(16)Page 14, line 508. Replace “grow” with “growth”.
(17)Page 14, line 517. Replace “… does not have to be homogeneous” with a stronger statement: “is not homogeneous”.
(18)Page 15, lines 546 to 548. Please rephrase, the message is not clear.
(19)Page 15, line 561. What does the author mean by “… critical dimension values may be unusually high”. I believe this referrers to XRF signal saturation occurring past a certain sample thickness assuming homogeneous elemental distribution, but it should be more clearly written.
(20)Page 16, line 598. Replace “propose” with “proposed”.
(21)Page 16, line 606. Replace “researches” with “research studies”.
(22)Page 19, line 724. Missing journal title from reference #19.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
This very interesting review includes the studies conducted to date on amendments and organic residues with pXRF instrumentation, although it is rarely used for the characterisation of this type of material.
After a brief explanation of the XRF technique, it is explained how a careful literature study was conducted there and the main results obtained from these studies are summarised. In addition, previous studies conducted on other matrices, such as soils and sediments, are used, due to the scarce number of studies conducted on organic matrices. These studies were commented on, showing analysis methodologies, results, and conclusions, highlighting the advantages and disadvantages of each study with special mention of the disadvantages that make the results not entirely accurate for certain samples or elements analysed. Interference effects on measurements due to sample moisture, organic matter content and other measurement conditions were illustrated. The author described his research investigation in detail.
I think that after minor revision it can be accepted.
There are some punctuation errors: e.g. line 97, 313, 362
Line 156, I would specify what LOQ refers to.
In table 2 (line 379), it is unclear which mode of analysis is used, it would be good to specify this better.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
