Chromatography Forum

Hello,


I am injecting ethanol samples by direct injection which contain trace amounts of methanol and notice the methanol peak varies injection to injection on the same sample. I also injected pure air (empty vial injection) to find out if there is something eluting from the system and notice there is a peak with the same RT of methanol at the that varies by injection to injection.

I am wondering where this unknown peak is coming from, since it is interfering with my ability to quantitate trace levels of methanol.


I changed the liner, septum, o-ring, gold seal, column, syringe and the issue reappears.

Is there any methanol used in the lab?

If there is, it could be contamination from the air, since you saw it in the air injection.

Also Vinyl Acetate elutes very close to Methanol on that column.

Acetaldehyde comes out right before Methanol on a 60m DB-624 column at 35C isothermal for 5 minutes so under different conditions they might co-elute....and Acetaldehyde is often found in Ethanol as an impurity...if you don't have Acetaldehyde to inject try a lower starting temp like 35 and see if you get two peaks...or other methods to separate coleluters....

what is the column you used? what is the condition? with more detailed information, we can help you

... I also injected pure air (empty vial injection) to find out if there is something eluting from the system and notice there is a peak with the same RT of methanol at the that varies by injection to injection.
...
I changed the liner, septum, o-ring, gold seal, column, syringe and the issue reappears.

What if you execute blank run (no injection)?
Solvent in washing vials may be contaminated - try pure ethanol.

Acetaldehyde comes out right before Methanol on a 60m DB-624 column at 35C isothermal for 5 minutes so under different conditions they might co-elute....and Acetaldehyde is often found in Ethanol as an impurity...if you don't have Acetaldehyde to inject try a lower starting temp like 35 and see if you get two peaks...or other methods to separate coleluters....

I can separate Acetaldehyde and methanol. I will try 35C since my initial was 40C

what is the column you used? what is the condition? with more detailed information, we can help you

I'm following USP monograph for Alcohol, Organic impurities test:

DB-624, 30m x 0.32 mm, 1.8um

temp gradient: 40C hold 12 min, ramp 10C/min to 240C, hold 10 min, inlet: 200C, detector: 280C.

Is there any methanol used in the lab?

If there is, it could be contamination from the air, since you saw it in the air injection.

Also Vinyl Acetate elutes very close to Methanol on that column.

I was worried about this as well, but replicate injections form the same vial show inconsistent peak areas. Sometimes it gets larger than goes back down.

is it only MeOH problem? Does any other solvent have similar issue?

is it only MeOH problem? Does any other solvent have similar issue?

Only at the RT of MeoH

Check this

http://gs-tek.com/blog/application/gsbp ... -note.html

GS-Tek has end year promotion.

contact me： Mike_t@GS-Tek.com