Fig. 5.

(a) The labeled traces show the measured absorbance from a cuvette filled with PBS buffer and placed in the three holders H1, H2, and H3. Trace H2 is slightly smaller than trace H1 due to the additional reflection caused by the cuvette placed in front of the IS entrance port. The measured absorbance at wavelengths close to 200 nm is due to molecular absorption by the buffer. For wavelengths greater than 250 nm the absorbance recorded with PBS was identical to that recorded for distilled water. (b) The labeled traces show the measured absorbance from a cuvette filled with microspheres suspended in PBS buffer and places in the three holders H1, H2, and H3. In all cases the buffer contribution was subtracted. The three traces are dramatically different reflecting the different instrument acceptance angles as discussed in the text. The measured absorbance in H3 is due to molecular absorption for wavelengths less than 300 nm, and backward scattered light escaping through the entrance aperture of the IS detector for wavelengths greater than 300 nm.