Standard Area Degrading

Hi all, hoping you can offer some explanations and help please. 

We are doing trace level of MDEA (methyl diethanolamine) in water, using pulsed splitless injection. 40 psi, 1 uL sample, 0.2 uL internal standard using an autosampler with air gaps in between. 

Have injected 10 standards using autosampler with good area reproducibility for both MDEA and internal standard, Ethanediol. 

Proceeded to do a test run with some samples, did a calibration 0-20 ppm, followed by a QC check , 6 samples and QC check again. 

I realised that after those 6 samples, for the QC the area of the internal standard was consistent but area of analyte was significantly lower compared to the first QC. Repeated injections of the QC were the same. .Changing the liner returned the peak areas once again. This issue only seems to happen when i inject samples in between my standard. Injecting just standard by itself has no issues with peak area degradation. 

I have tried 2 different tapered liners: 

1. Glass wool in middle - 70% reduction in area
2. Small amount of glass wool at bottom - 50% reduction in area

Is it safe to assume that the glass wool maybe forming active sites causing lower MDEA area after a few sample injections ? 

Other instrument settings:

DB-WAX column (30m x 0.53mm x 1um)
Inlet Temp : 260 deg C
Oven Temp : 80C, hold 1 min, ramp to 200 @ 30c/min
pulsed splitless mode, 40 psi till 0.75 min, split vent at 0.5 min

  • Hi silencer2008,

    all ethanolamines always stuck to metal and/or glass/quartz surfaces, if they are not perfectly deactivated. Increase the injector temp to at least 270 °C (when water is your matrix, you might want to test temps up to 300 °C), use an UltraInert liner with or without glass wool, single taper. (80 °C oven start temp might be too low too.)

    Keep in mind splitless injections have to be slower than split injections, rule of thumb is 1 µL/s.

    A Wax column is also not the best choice, we have the CP-Sil 8 CB for amines, which is made for the analyses of ethanolamines or the CP-Volamines column.



  • Hi Norbert, 

    Thank you for the response and apologies for the late response. Just returned from holidays Slight smile

    I have left instructions for my team to try out the higher inlet temperature but that hasn't caused much improvement. The standard area still degrades after a few sample injections. 

    We do have the CP-Volamine as well as RTX-35 amine column on hand, but since we tend to swap between analysis often i was hoping we would be able to get away with using the same column without downtime from swapping between columns. 

    What did you mean regarding the 1 uL/s rule ? 

    With your CP-Sil column, are you looking at trace levels ? We normally look at under 10 ppm in solution MDEA which makes this method. 

    There is also always some slight carryover, we use MeOH/MillIQ for the wash. Is that suitable ?

  • Hi silencer2008,

    the 1 µL/s rule means the injection speed used for splitless injections, which is very slow compared to split injection where you want to inject as fast as possible.

    Two interesting articles would be:

    You use an FID therefore concentration do not make any sense, it's a rate-sensitive detector and the absolute amount of "carbon" plays a role. With low concentrated samples you just inject more and therefore need a retention gap (see above) for solvent-focussing effects.

    Your wash solvent is ok.

    Hope this helps,


  • Hi Norbert, 

    At the moment we are injecting 150 uL/s via the autosampler. Would you recommend reducing this down to 1 uL/s ? We do have a 10 m retention gap lying around that I could use as well. 

    You have also mentioned 80 deg start temp as too low. I've always assumed that start temp should be 20 deg lower than the solvent ? 

    Apologies for the questions as I am still learning as much as I can while using the instrument. 

  • Hi again,

    that depends on your injection method.

    For splitless you would need a retention gap and a slow injection, length depending on the solvent, but 2.5-3 m are normally fine. The slow injection gives time for solvent focussing and cryo-focussing, therefore your oven starting temp should be below the boiling point of your sample solvent.

    For split you would use no retention gap and a fast injection speed which evaporates your sample immediately (and all the vapour need to fit into the liner volume). The injector temp of 250 °C is good for almost every solvent except methanol and/or water. Their heat capacities are so high that their evaporation would cool down your liner. In case of water that can be 70 °C lower and might be below the boiling point of some analytes. So 270 - 300 °C would be better.

    Ethanolamine analysis is one of the trickiest ones out there, especially because of inertness issues in the whole flow path of your system.


  • Hi Norbert, 

    Thank you for the reply.  We are use pulsed splitless injection method with relatively high pressure in order to inject the 1 uL required for sufficient sensitivity. Would you then recommend slowing down to 1 uL/s injection using the ALS ? I assume if we slow down the injection speed, would need to increase the split time to compensate for the slow sample transfer ? At the moment its 50 mL/min at 0.5 min. 

    I will try using the retention gap we currently have as a start while waiting placing an order for a shorter one. 

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