Voltammetric MDMA Analysis of Seized Ecstasy Samples

General Comments - “This manuscript clearly describes a potentially useful technique for measuring  MDMA in ecstasy or other mixed samples The rationale for the development of the technique is rationally argued and the methods for the development and evaluation are clearly described. The results describe both the strengths and limitations of the technique which are extensively discussed in the discussion.  A general point that the authors may want to discuss is that for the only drug that they observed simultaneous detection, procaine, they only tested one concentration of each drug and they may wish to comment on what would be the effect of different relative concentrations.  Additionally with reference to figure 7b they say there is no variation for procaine when the pre-concentration time is varied while there is an apparent change. This is referred to in the discussion (line 472) but it would be useful to mention this in the results section.

Overall, this study represent a useful addition to the literature and could be describing a potentially practically useful technique. (A reference to cost benefit analysis could also be useful)”

Answer General Comments: The authors acknowledge the comments of the Reviewer. We added an information in the item “4.1.4. Selectivity of the developed CME” about the possibility to evaluate other concentrations for the selectivity test. And, we added more information on item “3.1.4. Selectivity of the developed CME”.

Additionaly, information about cost-benefit ratio has been added in the Conclusions topic. 

Comment 1- “Line 39: the expressions ‘from mescaline’ and ‘from amphetamine’ are not clear.”

Answer 1: We rewrote that text and added a reference.

Comment 2- “Line 48: The value of %usage  does not seem to match the report (ref 11) ,. Page 43 of this report says that the average use of MDMA is 2.3%”

Answer 2: We appreciate your observation. The numbers have been corrected in the text.

Comment 3- “Line 270: Description of A and B seems to be inverted”

Answer 3: Thanks for your observation, we correct the description in the text.

Comment 4- “Line 308: In figure 4 A: line 2 represent a pre-concentration measurement , why does it look different (considering the scale of the graph) from all the traces in B which look similar to the traces of 3 in figure A where there was no pre-concentration?”

Answer 4: In this case, the first cycle will always be the most intense (pre-concentration time of 60 s), consecutive cycles will be less intense (no pre-concentration time). On the layer diffuse, the electroactive species in bulk are attracted to the electrode surface, thus we can believe that the second and third cycles of this sequence still contain more traces of the MDMA molecule near the electrode surface, which results in a signal slightly greater than the next cycles. On next cycles, the bulk contains more MDMA and the time to reach the electrode surface is constant, thus there is a significant reduction in the signal, that turns constant (which corresponds to the population of molecules that is oxidized).

General Comments – “This contribution will be of significant interest to a wide audience; it contains good science and should be published. However, the present format is convoluted and much too long. Particularly, sections 2,3 and 4 read as if taken largely from a thesis or from a very detailed technical report. It should be rewritten in a concise, scientific style so as to emphasise the advances found. Some further, more robust conclusions would be an advantage”

Answer General Comments:

The authors acknowledge the comments of the Reviewer. We believe that if we had to make the manuscript more concise, it would be necessary to expand the “supplementary material” section. Especially for guiding the electrode development and setting parameters for its optimization.  

General Comments - “The authors propose a newly developed voltammetric sensor for MDMA identification in ecstasy samples. The paper is certainly informative and well written. The conclusions are supported by the data and the paper is clearly worth publishing. I suggest a minor revision.”

Answer General Comments: The authors acknowledge the comments of the Reviewer.

Comment 1- “Page 2, line 93: “The electrochemical system consisted of an electrochemical cell”. This statement is obvious, it should be reframed.”

Answer 1: The comment was attended.

Comment 2- “Page 4, line 152/153: “active area” is repeated in the same sentence.”

Answer 2: The comment was attended.

Comment 3- “Page 6, line 265. I suggest to use “working potential range” or just “potential range” instead of “working range”. I also do not understand why the current decreases at potential values larger than 1.20 V, I expected an increase (potential range delimited by the solvent).”

Answer 3: We appreciate your observation about the current. The comments were attended.

Comment 4- “Did the Authors try more sensitive techniques such as differential pulse voltammetry?”

Answer 4: Yes, we studied this working electrode in differential pulse voltammetry, the results are being compiled for another article with a different purpose.

Comment 5- “Page 13, the sentence starting at line 463 is too long, please reframe”

Answer 5: The authors provided corrections in this section and added complementary information about electrochemical behaviour.

Comment 6- “Conclusions: the first sentences are too generic and belong more to an introduction. I suggest to limit the conclusion to the results relevant to the presented sensor.”

Answer 6: A part of the conclusion was moved to the introduction.