What is the most common source of loss of linearity in a spectrophotometer?

• A. Stray light
• B. Thermal drift
• C. Detector saturation
• D. Lamp instability

Answer: (A)

Linearity in spectrophotometry means that the measured absorbance increases proportionally with concentration within a chosen range, following the Beer-Lambert relationship. Stray light is the most common source of loss of this linearity because it adds a constant amount of light to what the detector sees, independent of how much the sample absorbs. This background light makes the detected signal not scale exactly with concentration, especially at higher absorbances where the true transmitted light is already very small. The stray light becomes a larger fraction of the detected signal, bending the calibration curve away from a straight line and producing a nonlinear response.

Other issues like detector saturation, lamp instability, or thermal drift can cause drift or local nonuniformities, but they typically affect the baseline or extreme ends of the range rather than causing the widespread nonlinearity seen across a series of standards.