GC detection methods — Which of the following is generally NOT used as a routine detector directly coupled to a gas chromatograph?

Introduction / Context:
Gas chromatography is paired with a variety of detectors. Common pairings include universal or selective GC detectors (FID, TCD, ECD), spectrometric detectors like MS and IR, and electrolytic conductivity detectors for specific heteroatoms. NMR, while powerful structurally, is rarely used as an online GC detector in routine analysis.

Given Data / Assumptions:

• FID is the most widely used GC detector for organics.
• GC–MS and GC–IR couplings exist and are well-established.
• Electrolytic conductivity detectors (e.g., halogen-specific/Hall, sulfur chemiluminescence variants) are used for targeted detection.
• NMR typically requires liquid-phase samples and longer acquisition times; specialized GC–NMR setups are rare and not routine.

Concept / Approach:
Eliminate commonly used GC detectors and keep the modality that is impractical as a routine inline detector. NMR’s hardware, sensitivity, and flow coupling limitations make it the outlier.

Step-by-Step Solution:

Verification / Alternative check:
Survey of analytical labs and instrument catalogs shows widespread GC–MS/FID, occasional GC–IR, but almost no GC–NMR systems outside research settings.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming IR is not used with GC; GC–IR interfaces exist (light-pipe, matrix isolation), though less common than GC–MS.

Final Answer:
Nuclear magnetic resonance (NMR).