Retention time are shifted between days even if using same analytical conditions (column, mobile phase, sample, system, and etc.).
->The composition may have changed due to volatilization of the mobile phase or sample. Reprepare and check again.
->Ensure that the column is aged sufficiently.
->Depending on the storage method, the column may have deteriorated. In this case, replace the column.
->For the recommended column storage method, see the instruction manual.