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

Parents Reply
  • 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. 

Children
  • 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.

    Norbert

  • 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. 

  • Hi again,

    true, you need to adapt the splitless time a little by the t0 of the retention gap.

    Success

    Norbert

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