Chromatography Forum

Hi all!

We have been quantifying MDMA with GC-FID since the beggining of the year; in our routine sequences we add some controls solutions, prepared with reference material standards.
Basic preventive maintenance (change of liner and septum) used to be monthly or once every six weeks, and there were no problems with the values of these controls (we obtained the expected results).

We analyse other drugs with the same chromatograph and we are not having any problem. However, "suddenly", one month ago, we have observed that a change of liner is necessary prior to our MDMA sequences if we want the expected values of our prepared MDMA standard solutions (even if we have made maintenance one week ago and the liner is apparently clean); otherwise, we will have lower values than the lower limit of acceptance.

The liner temp with this method is 230 ºC; with the other drugs is higher, and at the end of the sequences we run a standby method, where inlet temp goes to 150 ºC. We use split glass wool deactivated liners.
I dont'k know if these changes of temperature can affect the performance of the liner, but it was being our way of working until one month ago, without any problem, and now I'm questioning even the sense of living .

If anyone has experience with these kind of analytes or liners, it would be very helpful for me if you have some idea.

Maybe it doesn't have nothing to do with inlet but with FID cleaning? (it is done annually and it will be done soon, it's been nearly 12 months since the last one). Maybe a cut of column is necessary? We should clean the syringe more often? (Injection volume 0,2 uL, syringe volume 10 uL).

Thank you for reading

L

Not much to go on here...

I'd trim some off the inlet end of the column and see if that helps. Next would be the FID cleaning. One or the other should help.

That said, who is "we"?
What is your sample prep routine? Are these biological samples that are being extracted? Does "we" include any new users who may not yet have a real grasp on the sample prep procedure and the consequences of getting it wrong?
Have you changed anything about your wool and/or liners, how they're handled or sourced?

Fixing the issue will do you little good unless you figure out the cause of the issue. Poor extraction techniques resulting in "wet" or dirty samples could be the problem.

0.2 ul from a 10ul syringe is too little, and syringe wear could cause exactly the symptoms you see. Why not dilute the samples more an dinject 1uL?

Peter

Not much to go on here...

I'd trim some off the inlet end of the column and see if that helps. Next would be the FID cleaning. One or the other should help.

That said, who is "we"?
What is your sample prep routine? Are these biological samples that are being extracted? Does "we" include any new users who may not yet have a real grasp on the sample prep procedure and the consequences of getting it wrong?
Have you changed anything about your wool and/or liners, how they're handled or sourced?

Fixing the issue will do you little good unless you figure out the cause of the issue. Poor extraction techniques resulting in "wet" or dirty samples could be the problem.

Hi and thanks for answering so quickly;

"We" is simply me and my colleagues who perform this analysis. We all have been working on this since more or less the same days (I started a little before since I was in charge of the validation process), but the procedure is analogue to other substances (more stable I guess, they don't show this problem) we have been analysing since many years ago, so I don't think inexperience of analists could be the cause of the problem. We are talking about drugs in solid matrix (powder) that are solved in a internal standard solution, it's not rocket science.
Plus, the problem has been detected with a control solution that is prepared once (once in a few months, I mean), from a reference standard, and kept in the fridge, used to check the status of the calibration curve (linear). Obviously, the first thing we thought was that the control solution had been degradated and we prepared a new one, but we still have the same problems.
Anual maintenance, which includes FID cleaning and column trimming, is about to be done this week, so I'm expecting to see if the problem is solved.


Thank you again,

L

0.2 ul from a 10ul syringe is too little, and syringe wear could cause exactly the symptoms you see. Why not dilute the samples more an dinject 1uL?

Peter

Hi Peter,

I have never liked 0,2 ul in a 10 uL syringe, but during the tuning of the method I had to do juggling acts to fit the FID analysis into routine methods already in process in the lab, characteristics of the equipments and other facts. It didn't (doesn't, except for the motivation of this post) give as bad results as expected.

With syringe wear you mean the durability of it? A weekly cleaning could help? It's being a headache bcs it was working properly with a basic maintenance and suddenly it feels as if any thing could be the cause of the problem. I'll keep doing research to figure oout the source of the problem.

Thank you again for your suggestion, I'll keep it in mind.

L

So it returns back to the validated state when you replace with a brand new liner?

In our lab we had to change columns between different analysis and the depth of the column in both injection and detector port effected the sensitivity. Also a factor was constantly installing new ferrules and seeing movement in the depth if the ferrule was not installed correctly, as the GC temp cycled.

I would be looking at the period of time when the change occurred to see what if anything was done to the instrument around that time. Also, with the constant replacement of liners, looking at the condition of the column in the injection port.

Syringe plungers slowly lose their seal to the barrel over time. The smaller the injection volume, the more significant the loss. Trying a new syringe (better yet a new 5ul syringe) is quick and easy.