Balancing Resolution with Analysis Time for Biodiesel–Diesel Fuel Separations Using GC, PCA, and the Mahalanobis Distance

Round  1

Reviewer 1 Report

This paper describes the use of PCA and Mahalanobis distance method for the study and separation of fatty acid methyl esters in different biodiesel fuels using two types of columns. The article is very complete, it is of interest to the scientific community, the methods used are appropriate and the statistics used are correct. The work is very well written and the experiments exposed are very well developed and presented. The work is interesting and deepens in the knowledge of the use of PCA and Mahalanobis distance method in the gas chromatography separation of organic compounds, in this case, fatty acid methyl esters in different biodiesel fuels. The authors have a great knowledge of the subject as it is observed in the bibliography, and this work deepens even more in this field.

I consider that the article is appropriate to be published in Separations journal once the authors have made slight modifications to it.

Observation:

Lines 123, 126, 129, 136………: Put a separation between the number and ºC. Apply to the entire document.

Lines 124, 129, 131, 132………: Put a separation between the number and mL. Apply to the entire document.

Lines 125, 131, 132, 143, 146, ……..: Include the city, state and country of the corresponding manufacturing companies. Apply to the entire document.

Lines 131 and 132: Fisher Scientific and Fisher Chemical are the same?. Check.

Lines 144, 147, 148, ….: Delete the separation between the number and “/”. Unify and apply to the entire document.

Line 149: Put a separation between the number and µL. Apply to the entire document.

Line 152: Put m/z in italics.

Line 182: Use Figure instead of Fig. according to the style of MDPI.

Lines 191, 206, 228, 240: Capitalize each word according to the style of MDPI.

Line 240: ¿Subsection 4.1?.

References: Do not put a full stop after the name journal if the last word of the journal is not abbreviated, for example in lines 286, 289, 292, 311, …… for the journals “Talanta”, “Anal. Chim. Acta”, “Fuel”, “J. Chrom. A”.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Journal: Separations,

Manuscript ID: separations-485926

Title: Balancing resolution and run time for biodiesel-diesel fuel separations: A study of GC column conditions using PCA and the Mahalanobis distance

Authors: Edward J. Soares, Alexandra J. Clifford, Carolyn D. Brown, Ryan R. Dean, and Amber M. Hupp

Characterization of chromatographic separation (and resolution) is of paramount importance. The binary classification of biodiesel-diesel separation is also a serious problem and the authors have solved it properly. Either they reinvented the wheel (i.e. the linear discriminant analysis) or they elaborated a similar technique based on Mahalanobis distance.

In any case the corresponding literature has not been cited: e.g.

Suggestion for using Euclidean distance:

Witold Nowik, Sylvie Héron, Myriam Bonose and Alain Tchapla. Separation system suitability (3S): a new criterion of chromatogram classification in HPLC based on cross evaluation of separation capacity/peak symmetry and its application to complex mixtures of anthraquinones Analyst, 2013, 138, 5801.

Suggestion for using Manhattan distance:

Filip Andric, Károly Héberger. How to compare separation selectivity of high-performance liquid chromatographic columns properly?  Journal of Chromatography A, 1488 (2017) 45–56.

Connection to Mahalanobis distance (MD) and linear discriminant analysis (LDA):

Richard G. Brereton, and Gavin R. Lloyd. Re-evaluating the role of the Mahalanobis distance measure J. Chemometrics 2016; 30: 134–143.

See also:

https://stats.stackexchange.com/questions/26193/mahalanobis-distance-in-a-lda-classifier

The statement “square of the MD … follows an F-distribution” needs some proof or at least a reference. It can only be true if multivariate normal distribution is assumed.

Minor errors

Title

It contains unambiguous and superfluous terms and rethinking is advisable.

e.g. What does balancing run time means? The words “A study of” is totally superfluous. The definite article should not be used in titles.

Table 1 The MD values are too small close to the accuracy of number precision, presumably an artefact. Details about the programs (computer codes) should be given.

“correlation optimized warping” also needs proper referencing, e.g.:

G. Tomasi, F. van den Berg, C. Andersson. Correlation optimized warping and dynamic time warping as preprocessing methods for chromatographic data Journal of Chemometrics, 18 (2004) 231–241.

Checking the English would certainly be useful.

In summary, I warmly recommend publication provided the discussion part will be extended with the literature predecessors and with an outline to the connections to LDA.

April 27 / 2019                        referee:

Author Response

see attached pdf

Author Response File: Author Response.pdf

Reviewer 3 Report

The topic of the research presented into the manuscript entitled: “Balancing resolution and run time for biodiesel-diesel fuel separations: A study of GC column conditions using PCA and the Mahalanobis distance” is reasonable, and its relevancy in the literature is well described and contextualized. Nevertheless, in my opinion, few modifications are needed, in order to clarify the procedures, with the aim of increasing the general readability of the paper.

General comments

How are calculations made? Which software has been used?

Introduction

General comment:

The possibility of reducing the time of analysis in chromatography is a hot topic. As the authors properly describe at the beginning of the introduction, it is possible to change experimental conditions in order to reduce it. Nevertheless, also chemometric-based approaches have been proposed on this regard. 

A quite widely applied solution is to use a rapid gradient and then to exploit a chemometric curve deconvolution method such as Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS).

I personally think it would be nice if the authors would also mention the possibility of reducing the time of the analysis by chemometric approaches, reporting chemometric-based solutions.

An example can be found in:

De Luca, S; Ciotoli, E; Biancolillo, Magrì, A.D.; Marini, F. Simultaneous quantification of caffeine and chlorogenic acid in coffee green beans and varietal classification of the samples by HPLC-DAD coupled with chemometrics, Environ. Sci. Pollut. Res. 25, 2018, 28748–28759. 

I think it would be appropriate if you could include also some others chemometric-based solutions.

For what concerns the rationale behind MCR, please refer to:

Maeder, M. Multivariate curve resolution applied to second order data. Anal. Chem. 59, 1987, 527–530. https://doi.org/10.1021/

Tauler, R. Multivariate curve resolution applied to second order data. Chemometr. Intell. Lab. Syst. 30, 1995, 133–146. https://doi.org/10.1016/0169-7439(95)00047-X

Tauler, R. Smilde, A.K.; Kowalski, B.R. Selectivity, local rank, threeway data analysis and ambiguity in multivariate curve resolution. J. Chemom. 9, 1995, 31–58. https://doi.org/10.1002/cem.1180090105

Line 176: How did the authors build the PCA model? How was the optimal number of PC chosen? Since the definition of the optimal complexity is a relevant point in chemometric, please, specify. 

Line 197: In the manuscript is written: “It should be noted that for a sample of size 6, there are only 5 non-zero PCs and so the maximum L can be is 5”.

Which makes completely sense; nevertheless, what is not completely clear to me, is what is the underlined meaning of this; what is the message you want to communicate adding this sentence? Are you somehow explaining why you took 3 PCs?

Line 201: Why do you repeat the analysis using 5 PCs? Is the number of PCs not optimized?

References

The topic of the paper is well described and the rationale behind the study is supported by several references. Nevertheless, despite the references provided are for sure appropriate, mentioning a method, it would be nice to give reference also to the first author(s) who discussed it. Consequently, I would suggest you to add at least the following references to PCA:

-Parson, K. On lines and plans of closes fit to systems of points in space Philos. Mag. 1901, 2, 559–572.

-Wold, S; Esbensen, K; Geladi, P. Principal component analysis. Chemometr. Intell. Lab. Syst. 1987, 2, 37–52.

-Jolliffe, I.T. Principal component analysis; Springer, New York, NY, 2002.

Author Response

see attached pdf

Author Response File: Author Response.pdf

Round  2

Reviewer 2 Report

The authors have accepted a minimu change approach, but, indeed, the MS becaem much better now.

I suggest acceptance.