Chromatography Forum

Hello,
I am struggling with my standard preparation for a while now, and I cannot put my finger on what I do wrong. I have aq. sol of 2.3 micro methilene chlorid and 30.9 micro ethanol 96% in 100 mL vol flask, and I keep on getting Rsd for MC greater than 15%. ( for ethanol its less than 2% all the time)
I have 7890 Gc with G1888 hss , using a db-624 column, and split of 20/1. Hss temp are 80, 85,and 90, (thats the written method, that I cannot change).
I tryed shaking the flask longer, trying to take from the same place with the pipette, but the RSD is still bad.
I could really use some advice on this.
Thank you so much

I presume by "micro" that you mean microliter.

Is the problem that you get poor repeatability when you analyse different portions of one standard, or that you get poor repeatability between different preparations of standards ?

What rsd do you get ? - your idea of "bad" might be close to what you can reasonably expect.

Please post full details of how you measure and dispense the liquids when making up the standards, and the precautions that you take to stop the methylene chloride from evaporating.

Peter

It is a challenge to pipette Meth. Chlor with good precision because it evaps rapidly and displaces in the pipette tip (assuming you are using a VWR MicroPipettor). It can be done and no doubt you are able to do it--even at the very low volumes you cited using (2.3 uL no less!). It appears your RSD (reproducibility) problem is not really a "problem" per se, but rather a manifestation of the limitations of the analytical procedure. Again, pipetting 2.3 uL of highly volatile solvent Meth Chlor is wrought with intrinsic error (not your fault!). May I suggest trying to scale up the sample prep by say 10 fold so that you are pipetting 23 uL of Meth Chlor instead. Draw up the solvent slowly and leave the tip submerged for a few seconds after filling the tip.

My good news is that I made that happen; I managed one close-to-limit 12% RSD for my standard and the only thing I did different was the way I shaked the flask; so i first put the ethanol,shaked for 10 min, then the methylen chlorid, and again shake for another 10-12 min.; and I tryed to get that 2 mL for the HSS vial from the center of the flask.

For Peter: it was a bad RSD between different portions of the same standard, and for the prep I inserted the needle in the aq. sol, shake for 3-5 min, and tried to take the 2 mL for the HSS vial as quick as possible, and between that, keeping the flask closed.

For Jumpshooter: 2,3 not less ,my "pippeting" term was bad chosen, did it with a micro syringe (Hamilton, 1-10 micro for methylen chlorid and 50 for ethanol), and by the looks of that RSD, it could have been more precise. The nice result could have been from the cucaracha dance, better dissolved probably.

I am a beginner in this field, so I appreciate your quick answers!

Thanks

Using A class 10mL volumetric flasks with stoppers and weighing the DCM into DMAc or DMF then filling to volume works well. Then add needed volume10 25, etc microliters of DMAc or DMF to your main solvent (200 microliters water or DMF?) should allow you to get less than 1% RSD.

Good luck,

Rod

For Peter: it was a bad RSD between different portions of the same standard, Thanks

Unless there is methylene chloride actually floating on the water the solution would have been completely homogeneous after even modest shaking (rather than shaking the flask try tipping it upside down, letting the bubbles rise into the body, tipping it back again, etc about 20 times), and this suggests that the analytical step itself is not repeatable for methylene chloride. That it is repeatable for ethanol points to the strong partitioning of methylene chloride into the headspace as at least part of the problem.

Peter