Poor Precision in MeOH

[mtnshawn]

I apologize in advance for the novel. . .

1 month ago, I developed a method for the quant of a tri-halogenated benzene and associated regio-isomers. Great peak shape, resolution, etc. However instrument precision was a problem. Of 10 injections, 2-3 rogue injections would occur. I classified these as rogue, because when all injection areas were used to calc RSD, the result was ~15-20%. RSD w/o rogue injections would be ~ 1-2%. After extensive evaluation of periferals (liner, flows, T ramps), the hardware was then evaluated. New needle, different Tower, rebuilt detector (cleaned/new jet), new gold seal, etc. . When the rogue injection problem was not resolved, I went with an IS method (everything was wonderful) and the method was qualified. The whole rogue injection thing still bothered me, so I tried using DMSO as the diluent and WHAMO, 20 injections and RSD was always < 1.75%.

I now have a new set of compounds (Starting material through 3 process steps). 3 are short chain amines (RT 4.5 - 9 min), one is a derivatized compound and elutes ~ 12 min. Decent retention, great peak shape and rogue injections again (when using MeOH). In the end I can find a compound that would be satisfactory for an IS method, but I just don't understand what may be occurring. DMSO (high boilers) as a solvent isn't an option as it would interfere w/ 2 of the compounds, and all of the compounds have poor peak shape in DMSO. I have also evaluted other solvents (IPA and MTBE) but they also have the rogue injections.

$10,000 dollar question: Has anyone else seen this rogue injection problem when using MeOH as the diluent?

PS - According to the liner volume calculator that we use, the liners that I have evaluated should have satisfactory volume and flashback shouldn't be a problem.

Thank you for your time.
shawn

[Peter Apps]

Hi Shawn

I have seen this with both methanol and ethanol on an Agilent 6890. Try increasing the needle dwell time before injection, and slowing down the injection speed - the Agilent default settings are way too fast for these solvents.

Flash evaporation as a way of introducing sample to capillary columns is very vulnerable to all sorts of problems - take the $10 000 and buy yourself a PTV inlet !

Peter

[chromatographer1]

It is commonly assumed that if a big company like Agilent offers an autosampler that has a limited choice of injection speed and other variables then it is not needed. Unfortunately, this is not true as I believe they offer what they believe the common QA lab requires and not what is necessarily needed in the research laboratory. I developed original methods and I have never appreciated the Agilent autosampler for method development, but a generic and limited while certainly good autosampler.

I have used many autosamplers and have seen the need for many variables in getting a useful injection of samples.

I still dearly love the Varian GC autosamplers over many of their competition although many may have duplicated its features and its great flexibility by now in the marketplace.

I don't work for Varian but after many years of working with other company's products I have to confess I prefer their hardware for the research environment.

Check out other vendors of autosamplers is the bottom line here.

best wishes,

Rod

[AICMM]

mtnshawn,

What is your starting temperature? Are you injecting split or splitless. What column are you using? Are all you peaks ugly or just earlier eluters?
Poor re-focus, insufficient flow, poor wettability are some things that come to mind.

Agilent does really fast injections because they did a study long ago showing fast injection works best. So be it, I guess. However, I thought they had a dip switch setting that allowed for slower injections.

Best regards.