Abstract: The very higher and unstable background phenomena at 45 U on ICP-MS can be found in many laboratories. In this paper， two form mechanism of the interference were described: one was the normal memory effect on sampling system from the solution matrix (e.g., C, Ca, Al matrix)， which could form the polyatomic interference ions at 45 u; the other one was due to the carbon contamination in the mass system. The carbon deposited on ion lens would react with the hydrogen oxygen ions in the ion beam and then formed the interference species ¹²C¹⁶O₂¹H⁺. The character of this polyatomic interference ion could be recognized by the kinetic energy discrimination (KED). The most main difference of two mechanisms was the signal of latter, which decayed very slowly with continues washing or sampling process. There was no desired effect by the sampling system cleaning. Benefit from the KED mode，the polyatomic ions interference were suppressed, and the best results could be achieved. The detection limit of ⁴⁵Sc is less than 1.2 ng/L, the background equivalent concentration （BEC） of ⁴⁵Sc is less than 16 ng/L. And also, the variability of the data is improved. The Sc analysis results of China national sediment and rock standard reference material （SRM）enter the permitted error range. The internal precision of results is less than 1.5% (RSD)（n=3）. The external precision of parallel sample results (n=4) come back to normal level（within 2.3%—5.23%, RSD）. A suggestion has be made in this paper，the background signal at 45 U can be used as an indicator of carbon contamination on ion lens.