Stearic acid TMS contamination GCMS

[KM-USA]

We've used BSTFA and BSTFA containing TMCS from several vendors, no difference. Of course, we have no idea if they all use same source.

[carlo.annaratone]

agree on cleaning the split vent line! sometimes Agilent machines have this problem with greasy material backventing

[kvirkler]

Ah, that would explain a lot since we have been puzzled why so many methods seemed to go downhill after tranferring from PE to Agilent. Thank you very much for this insight!

[mchem]

We change our liners/septum before the start of every new run. We did a thorough cleaning of the inlet using Agilent's procedure at the end of December. We use Agilent 4mm deactivated single taper liners with glass wool. The column on this same instrument was just changed on 1/14.

I know this post is 10 years old, but I did a search for "fatty acid contamination" and this post came up. After extensive testing, I narrowed the stearic/palmitic acid GC inlet contamination down to this exact same liner. We started having an issue with a new lot. When putting the old lot back in, the fatty acids went away. Did you ever find a resolution to this? Or find a way to remove them faster?

[kvirkler]

I haven't thought about this for 10 years! Imagine my surprise to have an email notification about the topic reply this morning. I had to re-read the whole thread. I don't think we did anything specific to get rid of the problem. We've actually moved majority of our methods to LC-MS/MS. A lot of my original concern was more theoretical as the stearic acid wasn't physically interfering with any of our target compounds, but I was concerned that our derivatization efficiency was affected. That probably wasn't the case since the contamination was on the instrument and not during the derivatization process. We do clean our split lines every 6 months and change the trap every year.

Did you reach out to Agilent? Maybe they can send you additional lots to compare.

[mchem]

Right! So perhaps you had some excess derivatization agent left over that found what it could in the inlet to derivatize. We'll see it (BSTFA + 0.1% TMCS) do that with residual methanol from the syringe/inlet sometimes.

Agilent was helpful, sent us a new lot but it unfortunately was plagued by the same issues. They stated the acids were coming from the glass wool and recommended switching to a "better quality controlled" line such as their ultra inerts and provided a bunch to try. While they didn't have any fatty acid contaminants, the chromatography wasn't sufficient for some of the compounds. One of the newer lots we purchased are better, but still ~ 20% of them are no good. We call it "liner roulette" - as you never know if you're going to pull a good or bad one lol. Many analysts still keep some secret hidden stashes of the good lot of liners for really bad days. For those unlucky, we ramp up the inlet temperature for 30+ minutes and run some IPA injections which helps to an extent. However, compounds containing -OH groups (like morphine) take a big sensitivity hit and thus must be adsorbing onto an active site in the glass wool somewhere. Would be nice to figure out how to reverse that, as once the fatty acids get burned off, chromatography of everything else is perfect.

Still a bit of a mystery as to why the glass wool of these liners have these fatty acids and why not every liner is affected. Initially, we thought it was us, but after 5+ analysts over multiple different GC-MS and FID systems were seeing this issue immediately after a liner change, we figured it couldn't be. Especially considering we went though an entire 100 pack of them with not a single issue. It was suggested by a tech that it arises from hand moisturizer permeating the gloves (or potentially no gloves) as an employee is hand rolling the glass wool. Apparently they've also seen it happen with steroid creams (for hand eczema) that caused puzzling steroid contamination problems on the instrument. As the liner and wool are deactivated separately, there isn't an additional step after assembling to remove the acids like in the Ultra Inert/Topaz lines. And as there are various factory employees rolling the wool, it explains why not every liner is affected by it. And why when we do come across a bad lot, there are many bad liners in that lot (not just like 1 or 2). The loss of morphine, and other hydroxyl containing compounds, is still a bit of a mystery though.

[kvirkler]

Wow, that's all very fascinating. And quite an investigation to figure out that random employees at the liner factory with or without certain hand cream are contaminating the wool. Reminds me of the time we kept getting SPE lots contaminated with codeine and had to start our column conditioning with elution solvent (we've thankfully gotten away from that over the years).

Thanks for all the additional information! I'll keep this is mind as we do still have a basic drug panel on GCMS that derivatizes with MTBSTFA, so if this starts becoming a problem as we add new compounds to the method, I'll know where to look first

[MonicaHSE]

I have noticed that our BSTFA derivatized blanks, standards, and samples all have a significant stearic acid TMS peak when run on the GCMS. This is present in both SPE extracted and non-extracted samples. The neat standards/blanks we prepare only involve drying down a standard solution, reconstituting in BSTFA and ethyl acetate, and then injecting after derivatization. The peak is still there. Does anyone know where the stearic acid would be coming from? Gloves are always worn and we use disposable test tubes. It seems like it could be preventing the actual analytes of interest from being fully derivatized and therefore reducing sensitivity.

Thank you very much for your recommendation.