In the best case, a library MS/MS spectrum should be measured from only a single, pure reference compound. In practice, during large-scale spectral library generation efforts multiple reference compounds are measured simultaneously to minimize the data acquisition time that is needed. Although it is typically ensured that no near-isobaric compounds are simultaneously measured during such multiplexing of reference compounds to avoid potential confusion when annotating the library spectra, interference during MS data acquisition might still occur. Additionally, other typical quality considerations for mass spectrometry experiments , such as the presence of contaminants, carry-over, and other factors that can influence the data can impact spectral library generation. However, it is also important to be mindful of the biases associated with using pure reference compounds to generate spectral libraries. First, this requires a physical specimen of the pure compound, obtained from commercial sources or through laborious purification of biological samples. Unfortunately the majority of biological molecules whose structures have been elucidated are not readily available for purchase. An example of this bias is the disproportionately large number of unique matches to medicines and drugs when analyzing human fecal samples, while there are much fewer matches to microbial metabolites, which are not well represented in reference spectral libraries. A second type of bias is via the adduct that is chosen for fragmentation. For example, protonated and sodiated adducts are most frequently considered, with two thirds of positively charged MS/MS spectra in the MoNA and GNPS spectral libraries corresponding to protonated adducts (Figure ). However, many other adducts can be formed as well, especially during analysis of heterogeneous biological samples. Therefore, unless a complex background matrix is added to the pure standards, it is likely that an adduct that is observed in an experiment may not have been measured while generating library spectra from a reference compound. This is illustrated by the "ion identity molecular networking" approach, which was recently used to create a propagated spectral library that exhibits a broader coverage of different adducts, multimers, and in-source fragments (Figure ) . Nevertheless, because ion identity molecular networking can only find predefined ion forms, and we generally do not know the distribution and diversity of all ion forms that exist yet, several unanswered questions remain. For example, how many ions are protonated, sodiated, or acetonitrile-ammonia ion forms? How many ions are magnesium adducts, heterodimers, or other ion forms that are currently not considered? To alleviate these biases, although this is typically not performed, library spectra could be acquired by running pure reference compounds with a more representative background or in a biological matrix and unbiased searches need to be performed to find all ion forms of the standards. Alternatively, as is possible on the GNPS ecosystem, researchers that are experts in the biological systems under investigation can annotate experimental MS/MS spectra directly and add them to the reference libraries. Ion identity molecular networking was used to extract novel reference MS/MS spectra that exhibit overall broader coverage of different adducts, multimers, and in-source fragments . Note that these ion forms are found with a predefined inclusion list, rather than a comprehensive search for all ion forms that might be present in untargeted metabolomics data of a biological sample.