Inconsistent GC-NPD results

Hello everyone,

I am analyzing organophosphates using GC-NPD instrument (6890 Series) and I am experiencing inconsistency in the obtained signals.
For my tests, I did several runs in splitless and split mode in triplicate. Each time the first injection gave the highest response and the second one lowest. The signals from the second run are 50% of the signals from the first one or less.

I do analysis with hexane as the solvent and I use DB-5 column.
I have already changed the syringe, the column (the peaks are fronting no matter what so I thought this and integration could be the culprit), the septum, and the liner (liner is not packed).
The NPD detector (Agilent) seems to give a flat and stable baseline. The bead is not completely new but with a voltage 3.15 and hydrogen 3.8 I obtain an output of around 13.9.

I am not very experienced with GC so my question is: what could be my next steps if I want to fix the issue?

Thank you in advance for all your suggestions.

  • hello

    did you analyze organophosphates using the same method/instrument before ? 

    so we can judge if this is method issue or hard ware issue


    also you can inject NPD standard solution to see the repeatability .

    if the repeatability of NPD STD went good , so you have an issue in your application method and you need to adjust it .

    if the repeatability of NPD STD is poor , so you may have a hardware problem .

    you need to check every part individually .

  • Hi Mohammad,

    Unfortunately, it is the first time when I run this analysis so I don't have any reference.

    Could you tell me what NPD standard solution do you mean exactly? I was googling but I didn't find any test kit for such type of detector. 

  • it is a mix of three compounds : malathion , azobenzene and n-octadecane :

    if they are available i can send you the chromatographic conditions or you can get them from web .

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