RSD% failure - PAHs

   I please ask if you can verify what I am doing wrong. My analysis is 8270 PAHs using a 7890A/5975C inert XL MSD. I currently use 30m x 0.250mm x 1.00µm DB-5MSUI (Part # 122-5533UI) Agilent column. The attachment below shows my RSD% for three of my four groups of analytes over 15% with noticeable peak deviation in the final CC concentrations. I have made the calibration curve with the desired concentrations more than once with the same instrument results. I will greatly appreciate your help.


Current column settings:

1) 50°C (1min)

2) 15 °C/min to 300° - hold 4.5min

3) 25 °C/min to 320° - hold 12.5

Total time 35.467min


Current GC settings:

MS Source 230°C, MS Quad 150°C, Aux Temp 310°C, HiVac 1.14e-05

Inlet:  Temp=280°C, Pressure=10.052psi, Flow=24.20mL/min, Constant Flow

Splitless injection at 50mL/min at 1min

  • Hi jerivera,

    I could not tell from your attachment whether the PAH curves were curving "up" with higher concentration or flattening out.  Flattening out is easiest to fix as it is almost always due to too much material in the source or too high of a gain.  Either increase the split ration or lower the gain.  Curving up is a bit more difficult to fix.  It has the appearance of more response than expected at the higher concentrations but it is really lower response than expected at the lower concentrations.  This has been studied extensively and a good summation can be found in Agilent publication

    5991-3003EN.pdf.  I am not suggesting that you try the hydrogen addition as detailed in that note.  The problem is about 30% inlet related and 70% source related.  At a minimum try these steps: Use a 9mm drawout lens, part# G3440-20022. Ron suggested the 6mm earlier, which will work if you have already purchased it.  The 9 will give a bit better linearity with a slight S/N loss vs the 6.  Source temp to 350C.  MMI to 330C.  Pulsed injection either split or splitess. Liner with wool is a must for either injection to transfer heat to the heaviest PAHs that stick to everything.

    Best regards

  • Hello trifecta,

    Yes my curve is curving upwards. I have not used a source temp over 230C or an MMI temp over 280C as I will give it go. I will order the drawout lens as Ron and you suggest. I really appreciate your time to ease my concerns.

    Warmest regards

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