Analyte transformation post-sample collection can introduce uncertainty and reduce the reproducibility and reliability of results during the non-targeted analysis (NTA) of environmental samples. Understanding the type and degree of information lost is critical for accurate interpretation of the experimental results. Changes in the NTA feature profile of a sample were assessed over the immediate-toshort term (i.e., the first 24 and subsequent 120 h) to determine how much information is lost or changed. A wastewater sample was analyzed within 90 min of collection using liquid chromatography coupled to a high-resolution mass spectrometer. The sample remained in the cooled autosampler and was re-analyzed 47 times over 24 h and daily over the following 5 days (n = 15 data points). Detected features were classified as "formed," "increased," "unchanged," "decreased," or "eliminated." During the first 24 h, 7.4% of detected features underwent transformation, increasing to 58% over the following 5 days. Simultaneously analyzed aliquots kept at ambient temperature showed changes for 67% of the features. 86 features were (tentatively) identified, including one formed after sample collection. This shows that even on a short-term scale, considerable amounts of information can change or be lost while conducting NTA, potentially affecting reproducibility and reliability.