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. 2016 Nov 2;6(4):39. doi: 10.3390/metabo6040039

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© 2016 by the authors; licensee MDPI, Basel, Switzerland.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).

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Overlay of two histograms showing the distribution of the weighted average mass to charge ratio (m/z) shifts of all search frames of a target file (blue) relative to a reference file (grey). The baseline frequency of about 200 is caused by unrelated signals, which have a random m/z shift relative to their respective reference feature. The m/z shift of signals corresponding to reference features is, however, not random and shows a significant increase around the systematic m/z offset of the respective file relative to the reference. This causes a maximum around the aforementioned offset. For the illustrated target datafile, this m/z offset was determined to be −5 ppm.