Chromatography Forum

Hello GCMS experts,

I am working with an Agilent 7890A/7000A in positive CI for the first time and am having an extremely high baseline throughout the entire run. I am fairly experienced with NCI and EI but this is my first time in PCI and I am lost. I'm using methane for the CI source.

The noise ions are 12 or 14 amus apart which means hydrocarbons to me, but I am analyzing a standard in DCM. I know it can come from the fore line pump, but the instrument was fine in EI a few days prior to installing the CI source.

I'm thinking my ionization parameters are too high? I have 80 µA running through the filament. This seems high to me (EI is usually 35) but 80 is the default setting and my manual suggests 150 µA.

Any suggestions would be greatly appreciated

Hello Physics,

What is your scan range ?

Regards,
JMB

Scan range is about 60-600. I don't have much experience with PCI so I went off Agilent's suggestions

Hello PhysisGC,

I think that your problem is due to the relatively low value (m/z 60) of the start mass.
In the region below m/z 100, you have all of the ions due to water/ air/ reagent gas (methane or isobutane) and their reaction products with each other.
Not having run PICI in 30 yrs I cannot remember the m/ z values, but generally start the scan at m/z 100 or even higher—-often the higher the better.
Can you give the structure of the analyte or at least the MW ?
Regards,
JMB