Concept of sensitivity and detection limit

Sensitivity refers to how small a change in analyte concentration a method can reliably detect.

• Glass pH electrodes are accurate but typically detect changes on the order of 0.01 pH units; optical fluorescence methods can detect far lower concentrations of fluorescent species because emission is measured over a very low background.

Option-wise explanation

1. Absorption spectroscopy

   • Measures decrease in transmitted light intensity at specific wavelengths.

   • Sensitive, but still limited by background light and instrumental noise; generally less sensitive than fluorescence for trace analytes.

2. Refractive index

   • Detects changes in how light bends when passing through a solution.

   • Very small composition changes cause tiny refractive index shifts, making this method comparatively less sensitive and more prone to noise.

3. Circular dichroism (CD)

   • Measures differential absorption of left‑ and right‑circularly polarized light by chiral molecules.

   • Extremely useful for conformational analysis of proteins and nucleic acids, but not optimized for ultra‑low concentration detection; signals are often weak.

4. Fluorescence spectroscopy – more sensitive

   • Detects emitted light from excited fluorophores against a nearly dark background, yielding a very high signal‑to‑noise ratio.

   • Can routinely detect nanomolar to picomolar concentrations, far surpassing the detection limits of glass electrodes and the other listed optical methods.

Therefore, the method of detection that is more sensitive than a glass electrode used for pH measurement is fluorescence spectroscopy (option 4).