Fundamental Properties of Packing Materials for Liquid Chromatography
Round 1
Reviewer 1 Report
The title of the manuscript is too general; the review deals with several, rather randomly selected topics:
1. Molecular mechanics,
2. Definition of HILIC mechanism,
3. Ionic liquids as stationary phases,
4. Superficially porous column packing materials.
The topics 1, 2, 4 are dealt with superficially, contain mostly well-known information and in my opinion are not worth publishing. The parts on ionic liquids are most interesting and I recommend focusing on this topic and adding more details on newer applications. A new manuscript revised according these lines can be possibly considered for publication.
Specific comments:
1. References are missing to the statements on p. 2, lines 8, 43, p. 3, line 34.
2. P.3, lines 23 and 24, 42-51: Other authors more appropriately consider HILIC be the aqueous normal-phase LC (Pesek et al.). A standard distinguishing feature between RP and NP is that in reversed-phase LC the stationary phase is less polar than the mobile phase, whereas the case is opposite in the normal-phase LC. In practically all RP systems and in most NP systems (HILIC) both the polar and the solvophobic interactions may contribute to the retention, to the extent depending on the mobile phase. The author almost does not address the role of the mobile phase in LC, even though both the stationary and the mobile phase define the separation system!
3. P.6, line 8: The hydrophobicity of bonded alkyls increases with their length and is more significant than that of a siloxane oxygen.
4. P. 8, line 15: You can give more details on the “selective adsorption”, in addition to ref. [51].
5. P. 8, line 16: The numbers of figures (3H, 3I) are wrong.
6. P. 11, line 21: Please specify the “polymerizing organic silicones”.
Author Response
Thank you very much for your suggestion and pointed out my mistakes.
In this chapter, the editor (the publisher) did not demand to write a specific topic. I do not know about other chapters. I thought that I should not repeat to write about “quantitative in silico chromatography [34]. I tried to write a different way to explain properties of packing materials with up-to-date information. I also do not know about other chapters. Contribution of solvent (solvent effect) was quantitatively described [15, 16, 34]. Indeed, longer the alkyl-chain length, increasing the hydrophobicity. However, such tendency is not linearly related up-to C4 depending on analytes. The phenomenon was explained using calorimetric analysis, chromatographic results, and in silico analysis [34]. I tried to use explain the specificity of packing materials using electron localization. The method is used to teach organic synthesis chemistry and charge transfer complexes. The visualization should help beginners to understand chromatographic phenomena. No-one has demonstrated electron localization of packing materials, molecular interaction, and atomic partial charge to teach chromatographic molecular recognition.
The publisher did not demand to write a specific topic. Many reviews about ionic-liquid chromatography have been published; therefore, I should not write a typical subject. I cannot handle “Quantitative in silico chromatography” without technique and knowledge to synthesize bonded-phases; therefore, I had collected references about packing materials. Therefore, I wrote general information about packing materials suggested by the publisher. If a publisher asks me to write about ionic liquid phases, I need time. It took a long time to find a method for quantitative in silico analysis of reversed-phase ion-pair liquid chromatography; therefore, it will take times to find the solution about quantitative in silico analysis of ionic-liquid liquid-chromatography. Once I analyzed ionic-liquid phase GC, but analytes are polar in ionic liquid phase liquid chromatography; therefore, it won’t be simple to optimize the complex conformations, In addition, it seems that current ionic liquid phases are not end-capped; therefore, silanol effect is not avoidable.
Answers to specific comments:
1. I added references for p.2, line 8 [1, 15], 43 [16, 19, 20]. However, I did not add references for p.3, line 34. Manufactures did not provide the end-capping process and the guaranty of the chemical stability at high pH like C5, C8, PhHx, and C18. Water molecule can reach at polar group of bonded ligand, silanol and/or siloxane oxygen. This is the reason why we can use 100% aqueous eluent. Ions also reach at the same location. This is the reason why we can use 100% aqueous eluent while certain C18 columns are used.
2. You are right. However, I would like keep normal-phase (previously “adsorption”), and ask people carefully use a word “HILIC”. Many people buy so-called HILIC columns, and performed HILIC separation without consideration of the interaction mechanisms quantitatively. As mentioned above, I avoided demonstrating desorption mechanisms “solvent effect”. References for solvent effect: [15, 16, 34]; additional references, Quantitative In silico analysis of retention of nitrobenzofurazan-amino acids in reversed-phase ion-pair liquid chromatography. Toshihiko Hanai, Masae Sekine, Hiroshi Homma, Journal Chromatographic Science, 54 (2016) 1723-1726. Quantitative In silico analysis of retention of phenylthiohydantoin-amino acids in reversed-phase ion-pair liquid chromatography. Toshihiko Hanai, Journal Chromatographic Science, 54 (2016) 604-608.
3. Yes, the hydrophobicity of alkyl-ligands increases as the numbers of methylene unit; however, the relation is not linear by C4 depending on analytes. The non-linearity is affected by oxygen [17].
4. The published chromatograms demonstrated the selectivity. However, I have not done the quantitative in silico analysis. It was simple to analyze log k values measured using ionic-liquid GC, but it is difficult for the quantitative analysis in liquid chromatography. The mechanisms are like reversed-phase ion-pair liquid chromatography and enantiomer separation.
5. p 8. Line 16 Thank you very much for finding mistypes. The publisher permits to use color figures; then I separate Figure 3, but I forgot the correction.
6. p. 11, line21. I added the example of the name of organic silicone. “tetraethoxysilane, tetramethoxysilane”
In addition, I added a short summary by requested by a reviewer in Introduction.
Reviewer 2 Report
The revision is very complete and appropriate. However, the introduction is somewhat confusing and the article will clearly improve if it was divided in two sections. A small introduction for showing the objectives of the article and another section for the rest of the current introduction on the study of the quantification of the interactions analyte-stationary phase.
The meaning of unstable in line 8 and 35 should be clarified.
Section 3, "New Developments in Bonded -Phases" is dedicated almost exclusively to ionic liquids that are used mostly as modifiers, not as bonded-phases. This should be explained.
Author Response
Thank you very much for your suggestion and corrections.
The requested subject is very broad; therefore, introduction is also broad. I added short summary in introduction.
Line 8 & 35: I added “chemically” unstable.
Yes, there are many ionic liquids that have been used for extraction and additives. I described briefly, but I focused for the bonded-ionic liquid phases based on the publisher’s request.
This requested chapter is written to visualize the selective molecular interaction of bonded-phases using electron localization, but not quantitative in silico analysis of log k values; therefore, solvent effect (desorption mechanisms) were not described. Hoping beginners will understand the retention mechanisms like contact charge-transfer or mechanisms of organic synthesis.
Reviewer 3 Report
The author thoroughly reviewed the packing materials for chromatography with deep knowledge and lots of references. Some comments are the following:
Please check the required style of the paper carefully, such as line space and space between words.
some letters and special characters might be missed, such as page 11, line 15. Please check carefully.
The headlines of the paper is confusing, For example, Part 3 was followed by sub-section 3.1, but par 4 was followed by 3.2, 3.3...Please check carefully and rearrange the text.
Author Response
Thank you very much for your suggestion and corrections.
The word space and line space of the title was arranged for the easy reading, but I corrected to the original.
I could not find this mistake in my original text, and other micron is correct. I wonder what was happened. I corrected.
Thank you very much for your finding. At final stage, I rearranged but forgot correcting numbers. I corrected.
In addition, I added a short summary by requested by a reviewer in Introduction
Round 2
Reviewer 1 Report
The author explained the purpose of this manuscript. This work may be suitable as a chapter discussing some problems with tne silica gel based stationary phases.
