has_metric_category has value type n-tuple ID based metric retention time metric iRT calibration formula A polynomial formula to calibrate retention time based on iRT reference peptides. The order of the values corresponds to polynomial terms. I.e. a linear equation is represented by a two-tuple consisting of (slope, intercept). More general, the position in the n_tuple indicates the power of `x`: position `n → x^0`, position `n - 1 → x^1`, position `n - 2 → x^2`, etc. MS:4000149 MS