By Anonymous on Thursday, January 2, 2003 - 12:00 pm:
Perhaps someone can shed some light on a perplexing situation for me. My analytes are THF, ethyl acetate, toluene, N,N-DMF, and DMSO. My internal standard is dodecane @ 25ug/mL in both samples and standards. GC conditions as follows:
Column = Restek RTX-200, 30m x 0.32mm, 1.5um df
Flow = 3mL/min
Inj = 250C, split 5:1, Inj vol = 2ul
Detector = FID @ 250C
Oven: Init=60C, hold 4 min, ramp 15C/min to 180C, hold 1 min, ramp 40C/min to 250C, hold 5.25 min.
My sample is a drug substance @ 50mg/mL in methylene chloride.
My problem: Six consecutive injections of STD give a dodecane peak area of 50000 with RSD <1%. Sample injections give dodecane area of 65000 with RSD <1%. My blank is clear of peaks and so is my control (drug substance without internal standard). What is happening here? Shouldn't dodecane be a fairly inert compound?
Many thanks in advance for your advice.
-Jeff
By Albert on Thursday, January 2, 2003 - 12:11 pm:
Is there any difference in peak(s) shape and performance between standard and sample?
By Anonymous on Thursday, January 2, 2003 - 01:22 pm:
Dear Anon,
Yes dodecane is nicely inert. What you may be seeing is sort of a salting-out effect where the hogh conc. of drug substance is causing a lower solubility of the volatiles in your solvent, hence increasing their conc. in the headspace. Standards should ideally be run in a matrix blank if possible (the drug without any residual solvent) This should give you the best match between standards and sample injections.
Regards,
Mark
By Rodney on Thursday, January 2, 2003 - 07:51 pm:
Jeff
Mark is correct.
If this Mark is from StL, Hi Mark.
Rodney George
Gas Separations Research
Supelco
Bellefonte, PA
By Anonymous on Friday, January 3, 2003 - 01:17 pm:
All-
Thanks for the info. Sorry, however, that I neglected to mention this is not a headpace method, it is direct injection. I use a 4mm ID inlet liner w/ glass wool plug in the injector. The high concentration of drug substance does not seem to be a problem as it all comes out in the final ramp and the next chromatogram starts with a normal baseline. The peak shape for dodecane in the standards and samples looks similar (no fronting or tailing in either). Any other advice would be well appreciated.
-Jeff
By Anonymous on Friday, January 3, 2003 - 09:47 pm:
Are you preparing and injecting the control EXACTLY as you are for the new sample drug? I suspect there is something you are not telling us or that is being overlooked.
You did run the new sample without the Istd EXACTLY as you are running it with the Istd?
Perhaps if you detail your preparation procedure for the samples and stds it might give us a clue in solving your problem.