What specific chemical component were subsequent Viking tests searching for that led to the rejection of the LR experiment's positive reading?

Organic molecules, the chemical building blocks of life

While the Viking Labeled Release (LR) experiment suggested biological activity through gas release upon nutrient addition, the scientific community ultimately discounted this as proof of life. The critical counter-evidence came from the other biological experiments conducted concurrently during the Viking missions. These additional tests were specifically designed to search for organic molecules within the Martian soil samples. Organic molecules are universally recognized as the fundamental chemical building blocks necessary for terrestrial life. Because these subsequent tests failed to detect any such organic compounds, the scientific conclusion leaned toward the LR result being caused by abiotic chemical reactions occurring within the soil, rather than true metabolism.