Investigating the Mineral Composition of Peat by Combining FTIR-ATR and Multivariate Analysis

Round 1

Reviewer 1 Report

The manuscript titled “Investigating the mineral composition of peat combining FTIR-ATR and multivariate analysis” is a paper that may be of interest to the readers of Minerals. The paper deals with an important topic in relation to understanding the chemical composition of soils/rocks. The authors discuss the development of FTIR spectroscopy for the detection of mineral components in the peat samples for the purpose of characterizing environmental changes and palaeoenvironmental processes. In my view, the article is of a satisfactory standard and the data was well presented. However, there is room for further improvements and below are some comments which I believe can be handled as a minor revision.

1. In the experimental section 2.2 there is no mention of how the samples were prepared for FTIR analysis and measured by the ATR. How was the ATR cleaned? Was a new and fresh background collected every time prior to sample measurement or was just a single background used for the entire study? The authors should provide more information on this in the experimental section.
2. Following on from the above comment was any effort made in improving the limit of detection of ATR-FTIR for the minerals? Why were 200 scans chosen and was it sufficient? There is no discussion of it in terms of an optimum value for the analysis of peat?
3. Why did the authors observe a C-H vibration in the 3000-2800 cm^-1 region on some ash samples and not on others when heated to 500° Could this be due to a carbonate overtone or something else?
4. It is not clear to me if the authors used the entire spectral region (i.e., 4000 to 400 cm^-1) for multivariate analysis or was the PCA performed on certain IR regions? It would be good if they can clarify this and add some comments about the spectral region for PCA.
5. Finally, why was the ATR method chosen to measure the peat samples rather than the DRIFT? Some comments about this in the introduction would be useful for the paper. The authors may want to consider the publication by Pejcic et al. Vibrational Spectroscopy, 2021, 112, 103186.

Author Response

Reviewer 1: The manuscript titled “Investigating the mineral composition of peat combining FTIR-ATR and multivariate analysis” is a paper that may be of interest to the readers of Minerals. The paper deals with an important topic in relation to understanding the chemical composition of soils/rocks. The authors discuss the development of FTIR spectroscopy for the detection of mineral components in the peat samples for the purpose of characterizing environmental changes and palaeoenvironmental processes. In my view, the article is of a satisfactory standard and the data was well presented. However, there is room for further improvements and below are some comments which I believe can be handled as a minor revision.

Authors response: we thank the reviewer for her/his positive evaluation of our manuscript and the suggestion provided. Below you will find our response to the comments.

1. Reviewer 1: In the experimental section 2.2 there is no mention of how the samples were prepared for FTIR analysis and measured by the ATR. How was the ATR cleaned? Was a new and fresh background collected every time prior to sample measurement or was just a single background used for the entire study? The authors should provide more information on this in the experimental section.

Authors response: a description of the preparation of the samples is already provided in section 2.1 of the manuscript (lines 95 to 107) together with a critical comment on possible modifications induced by the preparation method (for ash samples). Apart from drying and milling no other treatment was done. A background was collected before measuring each sample. This information has been added to section 2.2.

1. Reviewer 1: Following on from the above comment was any effort made in improving the limit of detection of ATR-FTIR for the minerals? Why were 200 scans chosen and was it sufficient? There is no discussion of it in terms of an optimum value for the analysis of peat?

Authors response: this was set based on literature and our previous studies on the application of FTIR-ATR to peat characterization. Most publications indicate a much lower number of scans (between 18 and 100), but in a previous research (Martínez Cortizas et al. 2021, Boreas https://doi.org/10.1111/bor.12527) we found that 200 scans provided high resolution and reproducibility and still a quite reasonably low time of spectrum acquisition – which is important when working with large number of samples. The consistency of our results for both peat and ash samples suggests that this number (200 scans) could even be reduced.

1. Reviewer 1: Why did the authors observe a C-H vibration in the 3000-2800 cm^-1 region on some ash samples and not on others when heated to 500° Could this be due to a carbonate overtone or something else?

Authors response: as indicated in the manuscript (lines 317-319) C-H vibrations were detected in the peat section showing the largest OM content. The specific vibrations (2920 and 2850 cm-1) are known to correspond to aliphatic absorbances. On the other hand, these vibrations did not covary with any other that could be assigned to carbonates (as the 1400 cm-1 region, for example; see upper panel of Figure 4).

1. Reviewer 1: It is not clear to me if the authors used the entire spectral region (i.e., 4000 to 400 cm^-1) for multivariate analysis or was the PCA performed on certain IR regions? It would be good if they can clarify this and add some comments about the spectral region for PCA.

Authors response: yes, we used the whole spectrum for each sample. We are sorry the description in section 2.3 was not clear enough, even we wrote that “the spectrum of each sample is decomposed into several principal components …” which are shown in Figure 4. We have added that the whole spectrum was used (line 127).

1. Reviewer 1: Finally, why was the ATR method chosen to measure the peat samples rather than the DRIFT? Some comments about this in the introduction would be useful for the paper. The authors may want to consider the publication by Pejcic et al. Vibrational Spectroscopy, 2021, 112, 103186.

Authors response: We have chosen FTIR-ATR because it does not require other sample pretreatment than drying and fine milling. We are aware of other spectroscopy methods and used them sometimes (we are currently involved in the study of lake sediments using DRIFT). We thank the reviewer for providing the Pejcic et al reference. Although the aim of the present manuscript is not the comparison between spectroscopy methods, we feel this issue can be developed further in future studies. We now mention this in the last paragraph of the conclusions as a future development (line 382) and have included the recommended reference.

Reviewer 2 Report

After reading the manuscript carefully, I feel I am not able to review the paper. I am not sufficiently expert in several key aspects of the manuscript. Here are a few minor edits:

Lines 35, 66: Is that volume %, weight %, or something else? Wet or dry? It’s hard for me to get my mind around comparing percentages of organics and minerals. And how are such estimates made if it’s virtually impossible to separate the two.

Line 249: Change “in this mineral” to “of this mineral”

Lines 369-370: I think the phrase “phases of change in the mineralogy” is confusing here, because for many mineralogists “phase” denotes a thermodynamically distinct individual mineral. Is there a better word, perahaps “assemblage”?

Author Response

Reviewer 2: After reading the manuscript carefully, I feel I am not able to review the paper. I am not sufficiently expert in several key aspects of the manuscript. Here are a few minor edits:

Authors response: we thank the reviewer for taking the time to read our manuscript and the suggestions she/he provides.

Reviewer 2: Lines 35, 66: Is that volume %, weight %, or something else? Wet or dry? It’s hard for me to get my mind around comparing percentages of organics and minerals. And how are such estimates made if it’s virtually impossible to separate the two.

Authors response: the text reads “… 99% organic matter (dry mass) …”, so it may be understood that it refers to dry mass not volume. An approximation to the mineral content of the peat can be obtained by ashing, as this eliminates almost all the organic matter. So, it is not that they cannot be separated but that the large amount of OM implies that a large mass (or volume) of peat has to be sampled to obtain reasonable amounts of the mineral fraction, and the procedure used for that may introduce unwanted modifications to the minerals -as we discussed.

Reviewer 2: Line 249: Change “in this mineral” to “of this mineral”

Authors response: changed accordingly

Reviewer 2: Lines 369-370: I think the phrase “phases of change in the mineralogy” is confusing here, because for many mineralogists “phase” denotes a thermodynamically distinct individual mineral. Is there a better word, perahaps “assemblage”?

Authors response: we understand the reviewer concern since, as she/he indicates, the term “phase” is also used for minerals. The term “assemblage” is not adequate in this case as we are describing periods of time and not mineralogical assemblages. We have modified the text a bit avoiding the use of “Phase” and substituting it for “period” (lines 339 to 345).