What method are you using? As I recall the reagent (as listed by APHA or AOAC) was TBA or Thiobarbituric Acid. Why do you want the extinction coefficient?
A colormetric assay kit for detection of Malondialdehyde (MDA) or Thiobarbituric Acid Reactive Substances (TBARS) in multiple species and sample types. Designed for simplicity and affordability, this assay utilizes various improvements to provide the most dependable data among commercial assays of this type.
I agree fully with the answer and question posted from Bruce:
What do you want to measure with the extinction coefficient? (ipse dixit Bruce)