A strong correlation between molecules with similar structure and molecules with similar spectra is essential for this identification strategy. This is also the basis of recently developed scoring approaches for metabolite identification by tandem MS . To further quantify the ability of the unsupervised IRIS spectral search to find structurally similar metabolites, the structural similarity between each deprotonated metabolite and [DOPAC-H] -was calculated using the Dice similarity score of their bit vectors. For each metabolite in our library, the grey dots in Figure mark the structural similarity to [DOPAC-H] -plotted versus the spectral similarity to its IRIS spectrum. For a large majority of structures with intermediate spectral similarity to [DOPAC-H] -, the structural similarity fluctuates greatly. This arises from the fact that coincidental overlap between absorption bands due to different functional groups is common in vibrational spectra. However, the moving average (bin size of 15) shows a steep increase as the spectral score approaches its maximum value of 1000. This indicates that good spectral similarity indeed correlates well with similarity in the type, number and relative positioning of functional groups within a molecule. This is clearly reflected in the top-9 retrieved structures all being very similar to DOPAC (Figure ).