Tutorial: Correction of shifts in single-stage LC-MS(/MS) data

Label-free LC-MS(/MS) provides accurate quantitative profiling of proteins and metabolites in complex biological samples such as cell lines, tissues and body fluids. A label-free experiment consists of several LC-MS(/MS) chromatograms that might be acquired over several days, across multiple laboratories using different instruments. Single-stage part (MS1 map) of the LC-MS(/MS) contains quantitative information on all compounds that can be detected by LC-MS(/MS) and is the data of choice used by quantitative LC-MS(/MS) data pre-processing workflows. Differences in experimental conditions and fluctuation of analytical parameters influence the overall quality of the MS1 maps and are factors hampering comparative statistical analyses and data interpretation. The quality of the obtained MS1 maps can be assessed based on changes in the two separation dimensions (retention time, mass-to-charge ratio) and the readout (ion intensity) of MS1 maps. In this tutorial we discuss two types of changes, monotonic and non-monotonic shifts, which may occur in the two separation dimensions and the readout of MS1 map. Monotonic shifts of MS1 maps can be corrected, while non-monotonic ones can only be assessed but not corrected, since correction would require precise modelling of the underlying physicochemical effects, which would require additional parameters and analysis. We discuss reasons for monotonic and non-monotonic shifts in the two separation dimensions and readout of MS1 maps, as well as algorithms that can be used to correct monotonic or to assess the extent non-monotonic shifts. Relation of non-monotonic shift with peak elution order inversion and orthogonality as defined in analytical chemistry is discussed. We aim this tutorial for data generator and evaluators scientists who aim to known the condition and approaches to produce and pre-processed comparable MS1 maps.