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Strange wandering baseline GC-MS

Question asked by s.khatuntsev on Feb 24, 2019
Latest reply on Dec 17, 2019 by james_jenkins

Hi, guys!

I have a strange wandering baseline once I run a same calibration sample (orange) after a previous (purple) one immideately.

It is m/z 181 and 290 C isotermal part of an oven program. Low level of cypermetrin, fenvalerate and deltamethrin are present peaks on a chromatogram.

If I let an instrument to stand for a while and run sample it runs fine. Previous (lower temperature) SIM parts of a chromatogram are fine. In SCAN it looks the same.

It's 30m 0.25 0.25 HP-5MS UI column, running 150 kPa constant pressure, 15 kPa (250 kPa CFT and 15 kPa inlet ) postcolumn backflushing on EPC with a CFT device with 1m 0.1mm restrictor to transferline. 7890B+5977B with a turbopump (not a performance one), EI 350 Inert Source.

If I run it in a constant flow 1.4 ml, it gets much worse.

What is it?

Should I run it 120-130 kPa constant pressure or 0.6-0.8 ml/min constant flow? It all helps, but affects efficiency of a column. Why does it happen?