when we are quantifing DMF content in a particular compound with HSGC (with DB-624), we have the carryover of DMF, since we use the DMF as a diluent for the other experiments.How to avoid the carry over?
By Anonymous on Monday, February 23, 2004 - 06:09 am:
Shoot several injections of water and methanol through the column. This will clean your active sites and desorb the DMF. You still have to shoot blanks until your carryover is gone.
It may take several injections to achieve this. Be aware that these sites will re-adsorb DMF. This may affect your analytical results in measuring low levels of DMF. You may wish to use Dimethylacetamide instead of DMF for the other experiments. This will be of some assistance. Other solvents are possible, be sure they are basic, more basic than DMF.
By Jason Ellis on Monday, February 23, 2004 - 08:41 am:
For you to see DMF carryover peaks in subsequent blanks the DMF has to be retained somewhere in front of your analytical column (because it must focus and chromatograph on the column to elute as a nice peak at the correct RT). If there were DMF retained by active sites in the column itself, it would not show up on subsequent runs as a discrete chromatographic peak at the expected retention time of DMF. It is likely that you have some residual DMF in your headspace analyzer. What might work is to try heating the sample loop, valve and transfer line up to their maximum allowable temperature and then run several water blanks (vial temp at 80-90C) thru the headspace instrument to "steam clean" it. You might also want to try heating the inlet temperature on your GC up higher than normal during this procedure as well.
Agilent GC column/application support
By Anonymous on Monday, February 23, 2004 - 01:57 pm:
I agree with Mr. Ellis.
Most of the DMF carryover you see is from the rotor and bare non-deactivated metal parts (transfer lines and connectors) on valved headspace systems.
Another source is found in the deposits left behind from vapor plugs of DMF and other carrier solvents when they get diluted with carrier gas and chemical deposits 'rain' or 'dew out' from the gas plug injected down the transfer line.
These can trap residual amounts of DMF in their deposited filmy layer. This sounds silly but I used to clean a continuously operated system every few months or so to remove these deposits, and I did headspace research for 5 continuous years.
As your gas plug migrates down the transfer line and analytical column it carries water and the same carrier solvent vapor in concentrated amounts. This can free small amounts of DMF from the deposited coat of 'filmy junk' as it flows down the line and give a rather reproducible small peak of DMF, especially if you are not doing isothermal analyses.
'Steam cleaning' is recommended. But don't think it happens quickly. I found that 10 to 20 injections of water were necessary to get DMF out of one valved headspace analyzer flow path before I could run a sample. And that was an improved version using deactivated tubing and connectors.
Avoid all metal surfaces (yes, DMF will stick to it). You will also have adsorbed DMF to any salt deposits (carbonates, chlorides, etc) in the system lines as well.
From practical experience I found non-valved (timed injection) headspace analyzers a great improvement over valved (fixed volume loop) system.
Both have strengths and weaknesses. This DMF carryover is a weakness of the valved system transfer line contamination.
I think you may have to clean and configure your system to high temperatures to do DMF residual analysis. I would even recommend doing the analysis the traditional dilute and shoot way if possible.
By vijay on Thursday, February 26, 2004 - 02:14 am:
1. to make sure i suggest to change the column of the same type or to a different one.
2. remove head space sample carrier loop and run a blank with the same parameters and check the rt
3. if the problem persists heat your injector port to a elevated temperature to remove the contaminants.
4. if you are using water+dmf solvent system please evaluate the solvent system