Chromatography Forum

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

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

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

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.