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.

    Regards,

    Norbert

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