n-hexane and vial septa

[rivina]

We use n-hexane as solvent to inject FAMEs into our Rtx 2330 column and I wonder if the solvent is affecting the (good quality; Thermogreen-Lb2) septa of our vials, since there appears to carry an increasing amount of contaminants in succesive injections, even when running n-hexane alone (no analytes). Has anyone had similar experiences?. Regards to all.

[chromatographer1]

Hexane should not affect the septa.

I suspect you may be washing out deposits left by previous injections from your injection port.

Your injection volume may be too large or you may be injecting too quickly or into too small an injection port volume which is backflashing sample into cooler parts of the pneumatics. Your samples may also be leaving deposits from oxidation in your sample solutions which are dissolved by the hexane from later injections and deposited in your column.

best wishes,

Rodney George
consultant

[Don_Hilton]

You can extract silanes from the septa. You may also be baking out silanes from crumbs in the bottom of the inlet. Be sure that you do not have septum fragments lying on the seal plate or on the packing of the liner, if you are using a packed liner.

I see this increasing level in MS work when I am doing splitless injections and looking for things at part per billion level.

[chromatographer1]

Don is correct but to extract detectable amount of silanes from septa with hexane one would be required to have really really bad septa or a MS.

Since the poster did not mention any special detector to measure FAMEs (usually a FID is used) my answer reflected the assumed conditions of analysis.

Cleaning the injection port and replacing the injection liner would be an excellent start to trouble shooting his contamination issues.

Of course, washing several of the cap liners with hexane or methylene chloride might be a way and then determine if the contamination is noticed might be another trouble shooting step to consider.

Plasticizers from the storage bag of the vial caps might be another source.

Did anyone contaminate the vials or the hexane in storage?

best wishes,

Rodney George
consultant

[Peter Apps]

For contaminants to increase in successive injections of "clean" solvent is highly unusual. Where on the chromatogram do the contamiants elute, and are they sharp peaks or baseline humps ? Posting a chromatogram would help (instructions in a sticky on the LC page). What are your analytical conditions ?. Presuambly the contaminants go away between injection sequences ? Areyou making repeat injections from a single vial, or from a series of vials ?

Peter

[Don_Hilton]

The other thing that is not clear is how long it is between injections. When I see increases in junk from the vial septum, the second injection is a day or two after the first - not just a few minutes.

[HW Mueller]

This isw from the early 1980´s so it could be that the septa at that time were "very bad". Someone wanted to save money, they also thought to get a better seal, by using silicon rubber septa which had the teflon liner not cover the entire area of the septum. This way some of the rubber came in direct contact with the FAME in hexane, producing enormous extraeneous peks right in the FA. The problem was gone when I replaced these septa with completely teflon covered ones. Now the problem reappeared when I wanted to rerun samples, because the hole made in the septum by the autosampler cannulae exposed the silicon rubber. Aggravating, since I usually wanted to do the reruns due to failures in the autosampler over night. Exposure times invoved: Half a day and more. Also, there must have been some refluxing going on in those vials, . . . the only way I could explain some of the unwanted extractions.

[Ron]

One common source of siloxanes is the vial on the septum if it is punctured multiple times. Each injection from the same vial will increase the concentration of the siloxanes in the clean solvent. To see if this is the cause set up a vial and cap without a septum and one with a septum and do 8 to 10 replicates from each vial to see if the vial septum is the source of the siloxanes.

[pueblito]

When I run a tuning check standard for the 8270 method ( each 12hours shift) the chromatogram looks fine when I work with a fresh solution, but little by little four o more peaks appear ( m/z 73, 341, 147 ). The library tool shows cyclic siloxanes. So, they only disappear when I prepare a fresh solution.

bye.

[Ron]

Those are masses I would expect to see from vial septa.

[rivina]

Thanks to all for the replies/suggestions.
I learnt from the septa makers that hexane should not be in contact with the septa, but as some of you noted, this is practically impossible if each vial is sampled several times. We have decided to discard septa after a day of work or so, and that seems to do the trick. Also frequent dry runs with the gas mixture help in eliminating phantom peaks.
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