The black solid bars indicate histograms of coefficient of variations expressed as percentages (CVs) for all possible pairs of QC spectra for the variables of interest in the 2–4 kDa mass region (clockwise from top left, TIC, normalised TIC, total intensity of peaks and total number of peaks). The dotted line perpendicular to the abscissa indicates the critical value for these empirical significance tests based on the 95% quantile of these measures. The red hatched histograms shows the distribution of CVs calculated for the duplicate technical replicates. CVs which are greater than the critical value will be rejected at the 5% level. These QC fails will be indicated by crosses in Table 1.