The present invention relates, e.g., to a method for pre-processing a sample for mass spectral analysis, comprising cleaving proteins in the sample to peptides and immunodepleting highly abundant and/or well-ionizing and/or proteotypic peptides from the sample. Also described are methods for identifying well-ionizing peptides for use in this and other methods; analytic (diagnostic) methods using antibodies against highly ionizable peptides from a protein target of interest; and compositions, kits and devices comprising antibodies of the invention.