Because one good GCMS hit can still fool you. Even with a clean library match, reference standard, and nice ions, you’re still relying on one separation and one ionization in a complex matrix. Coelution, in-source fragments, near-isomers, or matrix additives can mimic the target. An orthogonal check changes the rules of the game to see if the ID survives. Run the reference and sample on a different column and confirm identical retention and RI, verify exact mass and fragments with HRMS/MS, and for polymer additives add FTIR on an extract. If it’s suspected as a leachable, extract the closure or filter and see if the same signal appears. If the call holds across those moves, you’ve got a robust identification rather than a good-looking coincidence.