has_metric_category has_units has_value_concept has value type precision single value ID based metric MS2 metric fragment ppm deviation sigma The standard deviation of the distribution of observed fragment mass accuracies (MS:4000072) [in ppm] of identified MS2 spectra after user-defined acceptance criteria (FDR) are applied Factors like temperature, airflow, or instrument calibration can cause mass shifts. See MS:4000179 for the MS1 equivalent. MS:4000028 MS parts per million