Analysis of Organic Sulphur Compounds in Coal Tar by Using Comprehensive Two-Dimensional Gas Chromatography-High Resolution Time-of-Flight Mass Spectrometry
Round 1
Reviewer 1 Report
Work taking into consideration 14 different classes of organic sulphur compounds in coal tar. The high sophisticated combination of gas chromatography and mass spectrometry was used for this purpose. The introduction introduces well into the subject of two-dimensional gas chromatography in relation to coal tar analysis. The methodology is described in detail and in accordance with all the requirements of the research work. The results presented in the tables and charts made the work clear and understandable. Thanks to the newly developed method of analysis, 60 sulphur compounds were detected.
I recommend the work for publication without changes.
Author Response
No request for modification has been made by the Reviewer.
Reviewer 2 Report
This manuscript presents a GCxGC-HRT method for coal tar and evaluates the method performance in regards to organic sulfur compounds. Overall, the paper is well written with a few minor grammatical errors. Scientifically, the authors could make some minor improvements to aid in the ease of data interpretation and repeatability of their study. The following changes are suggested:
Line 29 & 88-89 - define the ‘specific software function’
Line 44 - replace ‘and coauthor’ to Liu
Line 90 - move ‘it’ to after proving
Materials and Methods - please describe the data processing in greater detail. Include details about baseline corrections, mass calibration, etc. Also ‘mainlib’ is the NIST MS Library, please correct and specify version.
Figures 1 & 2 - add scale bars. Annotate Figure 1 to show the elation of different compound classes
Line 182 - replace ‘so called’ with software designated; delete ‘by the software’
Section 3.3 - I don’t agree with the way LOD and LOQ were calculated. Consider reevaluating LOD and LOQ according to the following reference: https://www.epa.gov/sites/production/files/2016-12/documents/mdl-procedure_rev2_12-13-2016.pdf
Author Response
Manuscript Separations-785582: Responses to Reviewer #2
Reviewer 2:
Point 1) Line 29 & 88-89 - define the ‘specific software function’
Response to Point 1) The name of software function has been included (see line 29 and 90).
Point 2) Line 44 - replace ‘and coauthor’ to Liu
Response to Point 2) The modification has been made (see line 45).
Point 3) Line 90 - move ‘it’ to after proving
Response to Point 3) The modification has been made (see line 92).
Point 4) Materials and Methods - please describe the data processing in greater detail. Include details about baseline corrections, mass calibration, etc. Also ‘mainlib’ is the NIST MS Library, please correct and specify version.
Response to Point 4) The information regarding data processing method has been added in a Table as supplementary material (Table S1). The version of the MS database has been specified (see line 134)
Point 5) Figures 1 & 2 - add scale bars. Annotate Figure 1 to show the elation of different compound classes
Response to Point 5) Scale bars have been added to both figures. The main compound classes of coal tar are now highlighted in Figure 1 and the figure legend has also been modified. Related modifications in the text can found on line 191 and on lines 198-199.
Point 6) Line 182 - replace ‘so called’ with software designated; delete ‘by the software’
Response to Point 6) The modification has been made (see line 200).
Point 7) Section 3.3 - I don’t agree with the way LOD and LOQ were calculated. Consider reevaluating LOD and LOQ according to the following reference: https://www.epa.gov/sites/production/files/2016-12/documents/mdl-procedure_rev2_12-13-2016.pdf
Response to Point 7) Different methods can be used for LOD and LOQ measurements. In the present research we applied the approach described in the following guideline document: ICH harmonised tripartite guideline: validation of analytical procedures: text and methodology Q2(R1); pp. 11-12.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
1, Line 163, (1D tR: ±2 min; 2D tR: ±0.3 s)
2min retention time for the first dimension and 0.3s retention time variation sound pretty big. The large variation only happened to the samples or both samples and standards? Why?
2, Line 119-121, The modulation period was set at 3.5 s: the hot jet was held for 0.3 s, and the cold jet for 1.45 s until 28 min; afterwards the duration of the jets was reversed in order to allow the release of high boiling point compounds.
The modulation changes at 28min. If the first dimension retention time can have ±2min variation, how do you do the calibration for the compounds at 28±2min?
Authors only reversed the modulation at 28min, did not increase modulation period?
If the modulation period is 3.5s and constant, how can you have compound’s 2nd dimension retention time to be over 3.5s (Table 1 & 2, Figure 1 & 2)?
3, In Figure 1, why there are two bands of solvent (one at about 1.6s and one at about 3.4s)?
4, The sample concentration was 100 mg/L. Authors used 5 to 1 split with 2 µL injection. If the conditions are changed to: 500 mg/L sample concentration, 1 µL injection, and 50 to 1 split, the results should look better, especially the Figure 1 will be much cleaner.
Author Response
Manuscript Separations-785582: Responses to Reviewer #3
Reviewer 3:
Point 1) Line 163, (1D tR: ±2 min; 2D tR: ±0.3 s)
2min retention time for the first dimension and 0.3s retention time variation sound pretty big. The large variation only happened to the samples or both samples and standards? Why?
Response to Point 1) The retention time windows in the target analyte finding data processing method were wide because for some chemical classes (e.g., benzothiophene derivatives) the number of positional isomers was high. If we consider that first-dimension peaks have a peak width at the base of 10-15 s, the 2 min is an appropriate time window for classes with many constituents. The following sentence has been added to the text (see line 175-176): “The 1D ± 2 min time window was selected to cover the chemical classes with a higher number of positional isomers.”.
Point 2) Line 119-121, The modulation period was set at 3.5 s: the hot jet was held for 0.3 s, and the cold jet for 1.45 s until 28 min; afterwards the duration of the jets was reversed in order to allow the release of high boiling point compounds.
The modulation changes at 28min. If the first dimension retention time can have ±2min variation, how do you do the calibration for the compounds at 28±2min?
Response to Point 2) The modulation period was the same even though the hot and cold jet times were inverted. As a consequence, first dimension retention times were not affected. The text has not been modified.
Point 3) Authors only reversed the modulation at 28min, did not increase modulation period?
Response to Point 3) As reported in the last point, the modulation period did not change. The duration of the jets was reversed, as reported in the text, to facilitate the release of high boiling point compounds. The text has not been modified.
Point 4) If the modulation period is 3.5s and constant, how can you have compound’s 2nd dimension retention time to be over 3.5s (Table 1 & 2, Figure 1 & 2)?
Response to Point 4) The modulation was set at 3.5 s for the entire run time. The compounds with a second dimension retention time higher than 3.5 s are compounds characterized by wrap-around. The second dimension retention times reported in Tables 1 and 2 are the “real” second dimension retention times, considering also the wrap-around time. With regards to Figures 1 and 2, these were automatically generated by the software, which takes wrap-around into account. The text has not been modified.
Point 5) In Figure 1, why there are two bands of solvent (one at about 1.6s and one at about 3.4s)?
Response to Point 5) There are two bands of solvent in Figure 1 presumably due to non-efficient entrapment of the solvent (dichloromethane – b.p. ≈ 40 °C ) during modulation. The following sentence has been added to the text (see line 191-193): “The presence of two solvent bands is evident in the chromatogram. This effect is probably due to non-efficient entrapment of dichloromethane (b.p. ≈ 40 °C) during modulation.”.
Point 6) The sample concentration was 100 mg/L. Authors used 5 to 1 split with 2 µL injection. If the conditions are changed to: 500 mg/L sample concentration, 1 µL injection, and 50 to 1 split, the results should look better, especially the Figure 1 will be much cleaner.
Response to Point 6) The injection of a ×5 more concentrated sample, even though at half the injection volume, will lead to a ×2.5 amount of coal tar reaching the liner. This will lead to a reduced liner lifetime due to higher contamination. The text has not been modified.
Author Response File: Author Response.pdf
