Detector principles — Select the detector that provides a mass flow-dependent signal (signal proportional to the mass of analyte reaching the detector per unit time).

Introduction / Context:
GC detectors are often categorized as mass flow-dependent (signal tracks the mass rate delivered, relatively insensitive to carrier gas flow variations) or concentration-dependent (signal tracks analyte concentration in the carrier gas). Knowing which category a detector belongs to guides method transfer and quantitation strategies.

Given Data / Assumptions:

• Flame ionisation detector (FID) produces ions proportional to the mass of combustible carbon entering the flame per unit time.
• Thermal conductivity detector (TCD) and electron capture detector (ECD) generally measure concentration changes in the carrier gas stream.
• Photoionisation detector (PID) response is commonly treated as concentration dependent within normal operating ranges.

Concept / Approach:
Identify the detector whose physics tie signal to absolute mass delivery rather than concentration. The FID’s ion current scales with the number of carbon atoms burned per second, making it a classic mass flow-dependent detector widely used for hydrocarbons.

Step-by-Step Solution:

Verification / Alternative check:
During flow-rate changes at constant analyte mass delivery, FID signal is comparatively stable; TCD/ECD signals vary with concentration and flow dynamics.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming TCD is mass-sensitive; it measures thermal conductivity differences that depend on analyte concentration and carrier composition.

Final Answer:
Flame ionisation detector (FID) — mass flow-dependent.