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Abstract
A critical comparison of methods for correcting severely retention time shifted gas
chromatography-mass spectrometry (GC-MS) data is presented. The method reported herein
is an adaptation to the piecewise alignment algorithm to quickly align severely shifted
one-dimensional (1D) total ion current (TIC) data, then applying these shifts to broadly
align all mass channels throughout the separation, referred to as a TIC shift function
(SF). The maximum shift varied from (-) 5s in the beginning of the chromatographic
separation to (+) 20s toward the end of the separation, equivalent to a maximum shift
of over 5 peak widths. Implementing the TIC shift function (TIC SF) prior to Fisher
Ratio (F-Ratio) feature selection and then principal component analysis (PCA) was
found to be a viable approach to classify complex chromatograms, that in this study
were obtained from GC-MS separations of three gasoline samples serving as complex
test mixtures, referred to as types C, M and S. The reported alignment algorithm via
the TIC SF approach corrects for large dynamic shifting in the data as well as subtle
peak-to-peak shifts. The benefits of the overall TIC SF alignment and feature selection
approach were quantified using the degree-of-class separation (DCS) metric of the
PCA scores plots using the type C and M samples, since they were the most similar,
and thus the most challenging samples to properly classify. The DCS values showed
an increase from an initial value of essentially zero for the unaligned GC-TIC data
to a value of 7.9 following alignment; however, the DCS was unchanged by feature selection
using F-Ratios for the GC-TIC data. The full mass spectral data provided an increase
to a final DCS of 13.7 after alignment and two-dimensional (2D) F-Ratio feature selection.
(c) 2009 Elsevier B.V. All rights reserved.