Effect of storage temperature and duration on concentrations of 27 fungal secondary metabolites spiked into floor dust from an office building.

Fungi are ubiquitous in environments and produce secondary metabolites that are usually low-molecular-weight organic compounds during growth processes. Dust samples containing these fungal secondary metabolites collected from study sites are often stored in certain temperature conditions for an extended period until laboratory analysis resources are available. However, there is little information on how stable fungal secondary metabolites are over time at different storage temperatures. We examined the stability of 27 fungal secondary metabolites spiked into floor dust samples collected from a moisture-damaged office building. Ninety-five dust aliquots were made from the spiked dust; five replicates were randomly assigned to a baseline (time = 0) and each of the 18 combinations of three temperatures (room temperature, 4 °C, or -80 °C) and six time points (2, 12, 25, 56, 79, and 105 weeks). At the baseline and each subsequent time point, we extracted and analyzed the fungal secondary metabolites from the spiked dust using ultra-performance liquid chromatograph-tandem mass spectrometer. To estimate change in concentration over storage time at each temperature condition, we applied multiple linear regression models with interaction effect between storage temperature and duration. For 10 of the 27 fungal secondary metabolites, the effect of time was significantly (p-values <0.05) or marginally (p-values <0.1) modified by temperature, but not for the remaining 17 metabolites. Generally, for most fungal secondary metabolites, storage at room temperature was significantly (p-values <0.05) associated with a larger decline in concentration (up to 83% for 3-nitropropionic acid at about 11 months) than storing at 4 °C (up to 55% for emodin) or -80 °C (55% for asperglaucide). We did not observe significant differences between storage at 4 °C, or -80 °C. Storage temperature influenced degradation of fungal secondary metabolites more than storage time. Our study indicates that fungal secondary metabolites, including mycotoxins in floor dust, quickly degrade at room temperature. However, storing dust samples at 4 °C might be adequate given that storing them at -80 °C did not further reduce degradation of fungal secondary metabolites.