Figure 4. False-positive optical density readings (absorbance of 620 nm) from biological solutes.

(a–c) Absorbance of biological solutes from the brain (a), blood (b), and plasma (c) samples (diluted to 500 µl in saline), with or without protein-precipitation by 1:1–1:3 of 50% trichloroacetic acid (TCA). [TCA _final] following the addition of 1:1, 1:2, and 1:3 volume ratios of 50% TCA were 25, 33, and 37.5%, respectively.