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First gut instincts are always right: the resolution required for a mass defect analysis of polymer ions can be as low as oligomeric Thierry Nicolas Jean Fouquet, Takaya Satoh, and Hiroaki Sato Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04518 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018
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Analytical Chemistry
First gut instincts are always right: the resolution required for a mass defect analysis of polymer ions can be as low as oligomeric Thierry Fouquet (
[email protected]),1* Takaya Satoh2 and Hiroaki Sato1 1
National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565,
Japan. 2
JEOL Ltd., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan.
ABSTRACT Its recent adaptation to low-resolution mass spectra of polymers using fractional base units raises the question of the minimal resolution needed for a Kendrick mass defect (KMD) analysis. Intuiting an oligomeric resolution since the mass of a repeat unit is the sole value to be known, it is challenged by the relative failure of the KMD plots computed from an isotopically resolved MALDI-TOF mass spectrum to display clear alignments in the high mass range. Another procedure based on the remainders of Kendrick mass (RKMs) overcomes this pitfall with oligomers perfectly aligned in a new RKM plot. Despite a concomitant degradation of the resolving power and accuracy with the example of MALDI-TOF/TOF mass spectra of a variety of homo- and copolymer ions, the RKM procedure still allows a rapid enumeration, assignment and any further manipulation of all the product ion series in visual RKM plots. Successfully extended to the critical case of a MALDI mass spectrum recorded with a linear TOF analyzer allowing a bare oligomeric resolution, the RKM plot turns the distributions differing by their end-groups or adducted ion into clear horizontal lines. It eventually gives intuition its due by answering the original question: the minimal resolution required for a mass defect analysis can be as low as oligomeric with the appropriate formulas.
The extension of the Kendrick mass defect (KMD) analysis1-4) to polymer chemistry has been proposed recently for a convenient visualization of mass spectra of blended homopolymers and copolymers from single stage, tandem and multiple stage experiments.5-8) A new mass scale is based on the nominal mass of the repeating unit R of a polymer backbone (e.g. ε-caprolactone CL, C6H10O2 at
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114.0681 in the IUPAC scale, set at 114.000) and mass-to-charge ratios (m/z) are re-calculated accordingly, becoming Kendrick masses (KMs, glossary in the Supporting Information).1,5) KMDs are calculated by subtracting the exact KMs to their nominal counterparts (NKM). Homolog ions whose composition varies by a number of base units only (e.g. +/-(C6H10O2)n for PCL) have the same KMD while ions with elemental compositions differing by other elements (e.g. different end-groups or adducted ions) have different KMDs. Considering the variation of NKMs with the degree of polymerization, data are displayed as horizontal lines, oblique lines or clusters in visual “KMD plots” (KMD vs. NKM or m/z). Regardless of its field of application (petroleums,9) biomolecules,3) synthetic / natural polymers10,11)), a KMD analysis is contingent upon the resolution of the mass spectral data – the term “resolution” covering both the resolving power of the analyzer and the accuracy of the mass measurement. KMD analyses have been
traditionally
conducted
using
ultrahigh-resolution
FT-ICR,9)
orbitrap,12)
high-resolution
spiralTOF5,7,8) and QTOF analyzers.6) A high mass accuracy is required for the points to be satisfactorily aligned in a KMD plot (errors in mass measurements = dispersion of points in the KMD plot) while a high resolving power is needed to separate isobaric components. The resolving power is a feature of the mass spectrometer and cannot be corrected by a data analysis. It dictates the expected amount of information to be extracted from either the mass spectrum or the KMD plot but also invariably impacts the accuracy of the mass measurement. A lack of accuracy has been recently overcome by using the new concept of “fractional base units”.13) A fraction of the mass of the repeating unit (noted R/X, X being an integer) used as the base unit enhances the visualization of complex data by separating ion series in a high and controlled extent.14) These “resolution-enhanced” defects (further noted RE-KMDs in contrast to the regular KMDs) allowed a world’s first informative RE-KMD analysis of low-resolution data recorded from a regular ion trap (resolution
