Chicken lysozyme was added into every sample at a constant amount before tryptic digestion to serve as an internal standard for quality assurance and quality control (QA/QC) and assess stability of the LC column and MS instrument. After tryptic digestion, 9 chicken lysozyme peptides were quantified. The quantile normalized log2 intensities of the peptides were averaged to obtain the log2 protein intensity for chicken lysozyme in each sample. The log2 protein intensities for chicken lysozyme for each sample (n = 9, 10/group) underwent a group comparison. There were no significant differences among the 6 groups in the intensity of the internal standard, suggesting that there was good procedural stability across samples and that no technical artifacts were contributing to differences between groups.
