salt_exchange

Incomplete salt exchange leaves two counter-ions in the cake and only one gets measured. That is the commonest way a real balance acquires a genuine remainder.

An acetate lot and a trifluoroacetate lot of the same peptide at the same claim contain measurably different peptide masses. Salt form belongs in any recovery comparison.

TFA-to-acetate exchange is rarely quantitative in one pass. A residual TFA figure alongside the acetate figure would say how complete the exchange was, and it is not on this record.

Lyophilising from dilute acetic acid drives residual TFA down over successive cycles. It is slow, it costs yield, and it is the reason acetate material often costs more.

Residual reagent from the cleavage step and counter-ion from the salt form are the same chemical and different origins. Only a process description distinguishes them, and certificates do not carry one.

Where the label states a peptide-content basis, the counter-ion is already excluded and the arithmetic is simple. Where it does not, this figure is how much of the difference it accounts for.

The index's mass balance on this record closes to two decimal places with 13.22 % acetate in it. The number is not an anomaly; it is a component of a coherent gravimetric picture.