Noise signal at the first point in TDI

[Changkun]

Is there anyone who has seen a noise signal at the first point in TDI?

Attached shows that high signal (20 cps/nA) only at the first point and normal signals (~0.3 cps/nA) at following points in TDI for Ti in melilite.
Since Ti is trace in melilite (generally less than 500 ppm as TiO2 in this sample), the first signals are definitely noise, probably due to hardware problem.

The noise signals appeared temporarily for ~3 hours in the spectrometer 5 (LiFH) and are not seen anymore yet.