Air peak, SPME & Merlin Microseal

[k.schmidt]

Hi all,

I have a problem with an air peak using SPME. The air peak appears at the beginning of the chromatogram. It’s air as I checked it with scan range 18-100 m/z.

The peak doesn’t appear due to the leak in the injector port as when a GC programme is run (with no fiber introduction), there is no air peak. It does appear when the SPME fiber is introduced into the injector port. The longer the SPME fiber in the injector port, the more intense is the air peak. The SPME fibers are the right size (23 ga) and I used a few (the peak is always there).

My SPME Merlin Microseal is new. But I also checked it with another, older (~ 200 injections) SPME Merlin Microseal and it doesn’t make any difference. I thought as Merlin claims that ‘septum leaks are acceptable as long as system pressure is maintained’, I will check it. I have CP-3800 Varian GC equipped with the 1079 injector port and EFC Type 1. I have calibrated the septum purge but the peak is still there. When I change the gas flow from 1 ml/min to 2 ml/min, it does not make any difference.

What is more, I checked some old SPME experiments (March 2013, I had a break with it for a while) and this peak WAS present then, however it wasn’t very intense and I ignored it as a fiber contamination. It looks like the problem is getting worse. I cannot ignore it now as the air is stripping off the siloxanes from my fibers. They may last shorter and the siloxanes are making my chromatograms messy.

If it’s not a leak in the injector port, a problem with septum purge or a faulty Microseal, I am running out of ideas. I have calibrated the septum purge at 4.5 ml/min (column pressure 4.8 psi for 1 ml/min and oven turned off).

Any advice will be very appreciated,

Thank you,

Kamila

[Peter Apps]

When you say that you have a problem with the air peak, is it getting in the way of other peaks, or are you worried that it will lead to column deterioration ?

I would be very surprised indeed if you did not get some air diffusing in while a SPME needle was penetrating a Microseal (or an ordinary septum for that matter).

Have you tried increasing the septum purge to sweep the space between the seal and the inlet liner ?

Peter

[k.schmidt]

The peak is quite substantial [img][img]http://s25.postimg.org/n6clfpzcr/Air_peak_SPME.jpg[/img][/img]. It coelutes with some peaks from my sample (this is not shown in the picture). So that is a problem.

I more worry about the fibres than the column really. I don't think this amount of air can deteriorate the column as there are no siloxanes present when the fiber is in the injector port but is not exposed. The siloxanes are stripped off of the fibres and although with time they are getting smaller, I worry about their coating being deteriorated slowly. Before (when there was no air peak), conditioning the fiber was enough to get a nice blank. Now to get the siloxanes smaller takes many analyses. It does not help that it's a trace analysis and my peaks are not very intense. As I said the Microseal is new so I wouldn't expect the leak yet.

I will try whether increasing the septum purge will make any difference. It is only that the GC manual says to set it in the range of 3-5 ml/min so I didn't try higher.

Thank you Peter

k.

[Peter Apps]

Good point about the fiber suffering because it is exposed to air while it is hot.

For special applications with silica transfer lines running through modified septa I have run septum purge flows of above 20 ml/min with no problems that I could see, and this was on the same Varian 1079 inlet you have. Bear in mind that if you are setting constant carrier flow the inlet pressure will increase during the temperature programme and the septum purge flow will increase with it, so you gas flow consumption will go up. Gas is cheaper than SPME fibers though.

Peter

[CE Instruments]

Dumb question ? You have tried a new SPME fibre ?

[k.schmidt]

I tried a few fibres, a new one as well.

Increasing the septum purge flow to ~10, 25 and 50 ml/min didn't change the intensity of the air peak:(

k.

[Peter Apps]

From you original post;

"The longer the SPME fiber in the injector port, the more intense is the air peak."

But that is not what the PIC shows - you have a decrease in intensity with time. If the air was due to a leak around the SPME needle I would expect it to rise to a fairly flat plateau at the dead time of the column, than stay fairly flat, then drop at the dead time after the fiber was removed.

The sudden rise and then gradual decay is consistent with a leak as the needle opens the valve, then the air being exponentially diluted by carrier gas. I doubt that there is a way to completely avoid some air getting in.

If you have peaks eluting on top of the air they are coming out very fast indeed, with nearly no thermal focussing at the head of the column, and so under quite sub-optimal chromatographic conditions in any case.

I presume that you are using the 1079 in temperature programmed mode ?. If not do so, and set an initial temp of 50C for 10s with a high split ratio to flush out the air plug, then close the split and heat the 1079 at maximum rate.

Peter

[MSCHemist]

I usually see an air peak actially a CO2 peak (m/z 44) as I start scanning at 40 m/z with SP and just use a normal septum S/Sl inlet.

[k.schmidt]

Sorry Peter. I didn't expressed what I mean very clearly about SPME fiber, time of its introduction in the injector port and the intensity of the air peak. Here is the picture explaining it better. From the top: 1) no fiber, 2) fiber in for ~ 5 s 3) fiber in for 0.5 min 4) fiber in for 1 min .

The initial temperature of my programe is actually 30C. Sorry, I didn't think about the dead time of the column. The peaks that I see coeluting with air must be then siloxanes stripped off the column, not my analytes. I am not sure why there are coeluting peaks only when I analyse my sample. They not present with blank (I mean here SPME blank in the injection port).

Anyway, had to change mode now to injection. I will try to play with the split in a month and see if it helps.

MSCHemist, if you see the air peak, do you also have problems with siloxanes being stripped off of the fibres?

Thank you.

k.