Different terms are used to standardize the question, "How much of chemical X does this sample contain?" The smallest concentration at which a chemical can be feasibly measured is known as the limit of detection, or LOD. The limit of quantification (LOQ) is the LOD plus any adjustment for known measurement bias or imprecision. LOQ can be thought of as the lowest LOD that can be made confidently.
Because of different instruments and protocols, LOQs vary across studies, which may make comparison difficult. (See definition for "quantification thresholds" above.)