GC detectors — Which of the following detectors provide signals that are concentration-dependent (i.e., depend on analyte concentration in the detector cell rather than on total mass flow)?

Introduction / Context:
Detectors in GC are often categorized as concentration-sensitive (CSD) or mass-flow-sensitive (MFSD). Understanding this distinction helps with split/splitless decisions, column flow optimization, and quantitative method design.

Given Data / Assumptions:

• Concentration-dependent detectors respond to the analyte concentration within the detector volume.
• Mass-flow-dependent detectors respond to the total mass of analyte per unit time entering the detector.
• We are comparing ECD, TCD, and IR (NDIR) for their response mode.

Concept / Approach:
TCD measures changes in thermal conductivity of the carrier gas as a function of analyte concentration. ECD measures decreases in standing current caused by electron-capturing analytes and is also considered concentration-sensitive under typical conditions. NDIR measures absorbance proportional to the analyte concentration path product in the detector cell. Hence, all listed detectors are fundamentally concentration-dependent.

Step-by-Step Solution:

Verification / Alternative check:
Texts listing detector classifications place TCD and ECD under concentration-sensitive detectors; IR absorption inherently depends on concentration across a fixed cell pathlength.

Why Other Options Are Wrong:

• Any single-detector choice omits others that are also concentration-dependent.
• MFSD examples (e.g., FID under many conditions) are not in the provided correct set.

Common Pitfalls:
Assuming ECD is mass-flow sensitive because it is highly selective; selectivity does not dictate response mode.

Final Answer:
All of these.