Internal Tannin: Rather than using quebracho as a reference standard it may be preferable to make a standard from the samples being studied. We refer to this as an internal tannin. Thus, one can express results in two ways: as “quebracho equivalents” and as “internal tannin equivalents” (Giner-Chavez et al., 1997). Some authors refer to this as an internal standard; it is not.
Giner-Chavez, B.I., Van Soest, P.J., Robertson, J.B., Lascano, C., Reed, J.D., Pell, A.N., 1997. A method for isolating condensed tannins from crude plant extracts with trivalent ytterbium. J. Sci. Food. Agric. 74, 359-368.