Tailing problem with PAH analysis

[M-KW]

you need to look closely, there is a (x10.000) or (x100.000) in the upper part of the chromatogramm bit.

the 5ppb is about 2.5*10^4 and the 50 ppb is at about 4*10^5, not as linear as I want it, but since I changed the liner in between and I face a bit of tailing anyhow, its acceptable I guess.

My injection volume is 1 µL.

However I just noticed that I have now a large contamination peak at about 220°C oven temperature which is now decreasing in intensity...

[James_Ball]

I get decent results for PAHs even when my injector is as low as 250-275C, I wonder if maybe you are baking something out of the septa at the higher temperature that is coating the liner and possible the column. If so that could be causing the peak broadening.

If you change the guard column I would also remove about a meter of the analytical column and see if things improve.

One of the instruments here is having problems losing pentachlorophenol quickly, change liner and column and you have great response then with each injection you can see it drop until it disappears after about 20 injections. The only difference in that instrument and the others is it has an old Silcosteel treated injection port that has been scrubbed out many times. I am beginning to wonder if that is bleeding something off that quickly coats the head of the column. I come to this conclusion because if we trim the column the peak returns for a few injections then is gone, but if we take a meter off and put the column in another instrument it will work fine. If your problem is similar to ours, then maybe you have something in the system that is bleeding out and contaminating the inlet, liner and column causing your problem.

[Peter Apps]

Hi James

You might be on to something there wrt the septum bleed coating the retention gap or column. With an experimental set-up doing cryofocussing I could see septum bleed unless I increased the septum purge from its default of 3 ml/min up to 10 ml/min, and I could see further improvement all the way up to 30 ml/min. This was on a Varian 1079 inlet.

I suspect that the 2-4 ml/min guidance for septum purge is just another of those historical/cultural things that nobody bothers about.

Peter

[M-KW]

I got 5 ml/min purge at the moment.

I now reuse the old ZB35HT without guard column (running out of them) with 290°C Injection temperature splitless (without pressure pulse) and 300°C Ion Source temperature.

[M-KW]

Since I measured SIM all the time I didnt notice but I have some huge peak at about 210-220°C oven temp... it seems I do get some contamination out of somewhere. I noticed this peak also in SIM (at the HP5ms) but it wasnt that large...

Similarity search says diethylphthalate. (main peak at 149 m/z)
Is my column degrading? or is this only from septum bleeding and James was totally right? What can I do except for more septum purge? (Thermogreen septa of supelco are among the most temperature stable I have seen)

[Rndirk]

Since I measured SIM all the time I didnt notice but I have some huge peak at about 210-220°C oven temp... it seems I do get some contamination out of somewhere. I noticed this peak also in SIM (at the HP5ms) but it wasnt that large...

Similarity search says diethylphthalate. (main peak at 149 m/z)
Is my column degrading? or is this only from septum bleeding and James was totally right? What can I do except for more septum purge? (Thermogreen septa of supelco are among the most temperature stable I have seen)

According to this link: https://www.agilent.com/cs/library/Supp ... F05001.pdf

The phtalates are plasticizers coming from O-rings or vinyl gloves.

I would try to isolate the problem. Make a test method with a single ramp without injection (manual injection), and do a full MS scan (m/z 50-500 or something). Then repeat it without heating the inlet, see if there is a difference. Note that you can also manually check your MS signals without performing a GC run.

[Peter Apps]

Phthallates are everywhere, but they are volatile enough to get swept through the column under the conditions you are using, so I would not expect them to be the cause of a progressive decline in performance.

On the other hand, if your standards are contaminated with phthallates who knows what else might be in there ?

Peter

[M-KW]

OK I will try and isolate the problem.
I did 2 "injections" at 290°C Injection temperature: one with just n-hexane, it showed severe phthalate contamination.
second one without injecting anything: it also showed severe phthalate contamination.
So I get phthalate contamination without a real injection. And that about the same size as with a real injection.
At 100 °C injection temperature I also get the same contamination with only n-hexane injected. which is clearly not septum bleeding

Therefore I will clean the injection port...

Another thing:
I was told that septum bleeding is getting worse over time. And a fresh septum has least bleeding. However I though this through and I cant explain it to myself, if it is phthalates of the septum, it should get less over time since the content in the septum is lowered over time due to the bleeding...

however I dont plan on testing this, but did anyone test it and is it true that bleeding gets worse over time?

[Rndirk]

OK I will try and isolate the problem.
I did 2 "injections" at 290°C Injection temperature: one with just n-hexane, it showed severe phthalate contamination.
second one without injecting anything: it also showed severe phthalate contamination.
So I get phthalate contamination without a real injection. And that about the same size as with a real injection.
At 100 °C injection temperature I also get the same contamination with only n-hexane injected. which is clearly not septum bleeding

Therefore I will clean the injection port...

Another thing:
I was told that septum bleeding is getting worse over time. And a fresh septum has least bleeding. However I though this through and I cant explain it to myself, if it is phthalates of the septum, it should get less over time since the content in the septum is lowered over time due to the bleeding...

however I dont plan on testing this, but did anyone test it and is it true that bleeding gets worse over time?

I think it's a good idea to clean your inlet.

The bleeding gets worse because the needle damages the septum, and it gets more brittle with temperature cycles. You're not going to deplete the septum with a couple of runs, the concentrations you bleed are pretty low. The signal you get depends how sensitive your detector is for these kind of molecules.

[Hornet]

I don't think you need those extreme conditions in your injector to analyze your PAHs, unless you are forcing your instrument to very low concentrations.

I can run PAHs on a single quad with:
Inj T = 280°C
Splitless time 0.5 min pulsed, 25 psi
Restek Sky liner single gooseneck with wool
1 microL sample volume.

About the phthalate problems, i had the same issue with my old Saturn 2000. Could it be vial septum? When they are already pierced they can be a problem, are you injecting the solution from the same old vial?
Also what type of septums are you using? At those temperature you need high temperature septum and different liner o-rings (graphite ones which i hate for leaking reasons)

What is transferline temperature?

Do you get to lose sensitivity only on latest eluting PAHs or even on earlier? like fluoranthene for example...i usually know there is a problem if i see PeryleneD12 or ChryseneD10 IS areas decreased while earlier IS are still at the same level. Then i know it's time to change liner and cut 10 cm of column injector side.

Good luck

[M-KW]

Thanks for the replies so far by the way, I really appreciate your help!

These extreme conditions were tested and I found out that there is a noticeable increase in the response of the late PAHs. So I tested 250-340°C injection temperature and I found out that for the highest injection temperatures I get the best results. Given that they state I can go up to 350°C I thought 320°C was a good intermediate between what the septa can and life span. (Thermogreen LB-2 Septa, preconditioned)

However it seems that I misjudged and 320°C is too high... I will go down to 280°C and live more stable but with a bit smaller response of the higher PAHs.

The pressure pulse increased (now that I go without I can see it again) the performance of the naphthaline peak, so I will start using it again since that hasnt been the problem. (instead of 25 psi I use 300kPa (43 psi))

The septa of the vials dont seem to be the problem since the contamination was found also without injection, even in about the same concentration.
I have to contact my service partner to get to know if there are graphite o-rings to decrease septum bleed. I actually do use some standard o-ring (since they are delivered by the manufacturer, I thought they are for all purpose)

I am now at 290°C transferline temp and 300°C EI Source temp (was 280°C EI)

I get sensitivity loss first on last PAHs and then the earlier are getting worse too. Since this happened in about 6-10 injections I was able to see this decay.

If you found your liners to be perfect for this analysis I want them too , did you mean these: Sky 3.5 mm ID Uniliner Inlet Liner w/ Wool (Catalog # 23459.1)?

[M-KW]

So... I think I am done with that problem.

I changed the molecular sieve and the split trap and heated out some tubing, cleaned the insert and voilá the contamination has decreased and with cutting the ZB 35HT by 1 meter, no more excessive tailing.

In between I couldnt find a leak for 3 hours, it turned out it was on the other end of the insert.

I hope this is not reoccuring...

I want to thank all of you helping me with your ideas to find the problem.

[Hornet]

i use single gooseneck for agilent, they are a bit different than shimadzu's.
I also have a shimadzu GC2010Plus with ECD which i use for PCBs, i use the Restek 23336 which are fine to me so far.
Never tried uniliner yet, can't tell pros and cons.

i'm glad you solved the problem, but still i wouldn't keep the transferline at 290°C with the highest program temp at 340°C.
I keep the transferline at the same temperature as the highest of the over, with a 50°C less you could have some condensation problems with latest eluting PAHs.

In Italy we have interest in the very heavy PAHs like Dibenzo(a,h)pyrene, 302 m/z and i can calibrate those decently with my last oven temp at 320°C, the same temperature of the transferline.

I would stick with standard viton o-rings and lower temperature, graphite o-rings will bring other problems.

[Rndirk]

i use these ones http://www.restek.com/catalog/view/11042

i'm glad you solved the problem, but still i wouldn't keep the transferline at 290°C with the highest program temp at 340°C.
I keep the transferline at the same temperature as the highest of the over, with a 50°C less you could have some condensation problems with latest eluting PAHs.

In Italy we have interest in the very heavy PAHs like Dibenzo(a,h)pyrene, 302 m/z and i can calibrate those decently with my last oven temp at 320°C, the same temperature of the transferline.

I would stick with standard viton o-rings and lower temperature, graphite o-rings will bring other problems.

Keep in mind the PAHs in the transfer line are close to vacuum (MS). They will not condense there at ~300°C, even if they don't elute at that temperature.

Glad you solved the problem M-KW

[M-KW]

now I dont know what to do.

I am using the fluororubber o-rings from shimadzu P/N: 036-11203-84
Honestly they were never really worn when I took them out even after 500 injections. I even reused them when changing the liner, without hesitation. Using Graphite O-Rings has benefits, but with only 280°C injection temperatures I wont need them anymore, do I?

As Rndirk pointed out, the vacuum cleans my column end, although I thought about raising that temperature...

I aint satisfied right now, at the insert, the pressure is highest and the temperature is lowest in the whole system (at least at the end of the run). This does not sound like a good idea to me.