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Reply to the Comment on “Mass-Remainder Analysis (MARA): a New Data Mining Tool for Copolymer Characterization”: two names for one analysis fundamentally associated with mass defects Tibor Nagy, Ákos Kuki, Miklos Zsuga, and Sándor Kéki Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01886 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018
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Analytical Chemistry
Reply to the Comment on “Mass-Remainder Analysis (MARA): a New Data Mining Tool for Copolymer Characterization”: two names for one analysis fundamentally associated with mass defects Tibor Nagy, Ákos Kuki, Miklós Zsuga, Sándor Kéki Department of Applied Chemistry, University of Debrecen, Debrecen, Hungary.
This reply complements the comment of Thierry Fouquet on our recent work entitled “MassRemainder Analysis (MARA): a New Data Mining Tool for Copolymer Characterization”1. Indeed, it can be realized, that the formula for computing the “Remainders of Kendrick Mass” (RKM) in the paper entitled “First Gut Instincts Are Always Right: The Resolution Required for a Mass Defect Analysis of Polymer Ions Can Be as Low as Oligomeric” by Fouquet et al.2 is similar to the one proposed by us to calculate the “Mass-Remainder (MR) values”. The similarity of the two formulas as presented in the original papers1,2, however, is not straightforward. Fouquet et al. use the Kendric Mass (KM) to obtain RKM, and the nominal mass of the base unit can only be eliminated from the equation only after substituting the formula of KM. This yields a similar expression as MR divided by R (exact mass of base unit), as it is detailed in the comment. Of course, MR can be expressed using KM, but, probably due to the rounding in the calculation of the nominal mass of the base unit, the unified equation of Fouquet et al. (eq. 13 in the comment)
KMD(R) - KMD(R, n) =
MR(R, n) R
(1)
may result in negative values, as illustrated in Fig 1.
Figure 1a shows the MALDI-TOF mass spectrum of an ethylene oxide/propylene oxide/ ethylene oxide; EO/PO/EO triblock copolymer with approximately 10 wt% EO content (from VWR International, Leuven, Belgium). Figure 1b, c, and d plot the values of the terms (namely KMD(R), KMD(R, n), and MR(R, n)/R, respectively) in Eq. 1, where R is the exact mass of the PO unit and n=1. Consequently, the difference between KMD(R) and KMD(R, n) is negative for many m/z values (e.g. 0.05 – 0.31 for m/z 913.607), despite of the positive MR values.
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Figure 1. a. MALDI-TOF mass spectrum of an EOx–POy–EOx triblock copolymer recorded with a BIFLEX IIITM mass spectrometer (Bruker Daltonik, Bremen, Germany). b. Regular KMD plot (base unit: PO). c. Resolution-enhanced KMD plot (base unit: PO/59). d. MassRemainder vs. m/z plot”.
However, we independently used different approach. The mass defect (in mass spectrometry) is defined as the difference between the exact mass and the integer mass, but MARA calculates purely a remainder of a division, and does not involve any nominal mass in the calculation. In addition, MARA does not require any transformation to a new mass scale. Therefore, we accept one of our reviewers proposal, i.e., ”it would be incorrect to rename
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Analytical Chemistry
MARA with a unified name that involves the Kendrick mass because MARA is calculated without using this parameter.”
Acknowledgement We thank Fouquet and his colleagues for their interest in our paper. We are also grateful to our reviewers for their valuable comments.
References (1) Nagy, T.; Kuki, A.; Zsuga, M.; Kéki, S. Anal. Chem. 2018, 90, 3892−3897. (2) Fouquet, T.; Satoh, T.; Sato, H. Anal. Chem. 2018, 90, 2404-2408.
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