Performance Qualification detector response intercept

[John]

Part of our UV detector PQ involves plotting AU/conc to determine linearity and intercept. Intercepts are usually positive and sometimes fail high, even when linearity is near perfect.
The remedy usually lies in replacing optical components. Can anyone explain to me why degraded optics cause a high intercept?

[Uwe Neue]

A possible explanation is the deterioration in signal. A positive intercept is just another way of saying that the response at high concentration is too small. For example, if your detector window goes blind, you get less light through the detector, and your detector departs from linearity at a lower sample concentration.

[tom jupille]

Uwe beat me to the response

Remember that UV detectors don't measure absorbance; they calculate absorbance. What they actually measure is transmittance (absorbance is the negative log of transmittance). That means that when you have low absorbance, the detector is actually working with a very small difference in two very large quantities. It doesn't take much crud on the optics to mess things up.

[DR]

Excellent question & answers.

[John]

Thanks!