Detector characteristics in HPLC Which statement about a refractive index (RI) detector in HPLC is correct under routine analytical conditions?

Introduction / Context:
Detector choice in HPLC determines sensitivity, selectivity, and compatibility with elution modes. Refractive index (RI) detectors are universal but have specific limitations compared with UV, fluorescence, or MS detectors.

Given Data / Assumptions:

• RI detectors measure differences in refractive index between sample and reference streams.
• RI signal drifts with temperature and composition changes.
• Gradient elution changes mobile phase composition during a run.

Concept / Approach:
Because RI signal is highly sensitive to mobile-phase composition, gradient elution causes large baseline shifts that obscure peaks. Therefore, RI is typically restricted to isocratic runs. UV detectors, in contrast, handle gradients well and are usually more sensitive for chromophoric analytes.

Step-by-Step Solution:
Identify the operational limitation: composition sensitivity.Relate gradients to composition changes → unstable baseline.Select the option stating isocratic-only compatibility.

Verification / Alternative check:
Method development guides advise using RI for sugars, polymers, and other non-UV-absorbing solutes in isocratic mode with precise temperature control.

Why Other Options Are Wrong:
Option a: RI is generally less sensitive than UV/fluorescence.Option c: RI is broadly responsive (near-universal) rather than highly selective; its limitation is sensitivity, not lack of response.Option d: incorrect because option b is true.

Common Pitfalls:
Attempting gradient methods with RI; neglecting temperature stabilization leading to drift and noise.

Final Answer:
It can only be used for isocratic elutions (not gradients).