A 40-vial sequence for a volatile organic compound screen started at 06:10. By 14:40, the internal standard response in vial 36 had dropped 18% relative to vial 4, and the quantitation for benzene in that same vial read 12% low against the calibration curve. The run did not fail an audit. It failed a mass balance. The vials were not cracked, the caps were not loose by hand feel, and the autosampler never flagged a pressure error. The problem was analytical testing seal failure, evaporation ...