Tailing problem with PAH analysis

[lpnian]

Good discussion.

peak tailing and sensitive lose may caused by contamination. what i do is cut the column (should >50cm) and change inlet , split pan (glod pan) may be also need change if you do trace analysis.

[James_Ball]

Viton o-rings would be the best even at 270-280C inlet temperatures. These are usually brown or black in color. If they are red, those are for low temperature use I believe.

[M-KW]

guess what, the contamination came back... Honestly I dont know what to do right now, peaks are still in good shape. I will measure some samples and see how bad the tailing gets. Although I dont have any hopes since I aint using a guard column at the moment ;(

[lpnian]

So, as the PAH concentration is very low 5ppb, maybe we can discuss the sample preparation and cleanup method?

[Rndirk]

guess what, the contamination came back... Honestly I dont know what to do right now, peaks are still in good shape. I will measure some samples and see how bad the tailing gets. Although I dont have any hopes since I aint using a guard column at the moment ;(

Can you show us some chromatograms over the full retention time range? Preferably a good one and one with the problem present. Do you see this contamination in SIM? In standards and samples or only samples?

[Peter Apps]

guess what, the contamination came back... Honestly I dont know what to do right now, peaks are still in good shape. I will measure some samples and see how bad the tailing gets. Although I dont have any hopes since I aint using a guard column at the moment ;(

Don't bother running the samples, the performance will not be stable if you have major contamination.

Step 1 - do a series of blank temperature programmes, no injection at all. does the contamination peak get smaller from run to run, and is that decrease maintained form run to run ? . If it does the contaminants are being introduced by the injections.

Step 2 - do a blank temperature programme after letting the instrument sit for a few hours. If the contaminants increase they are coming from the instrument or the carrier gas, possibly in addition to the injections.

Step 3 - with a short wait since the last blank run, do a zero volume injection using a brand new syringe that has never been exposed to any solvents. If your autoinjector insists on having a vial to play with, use one without a septum. If the contaminants increase they are coming from the injections.

Peter

[M-KW]

I will do what you suggested Peter, I will later upload a few chromatograms. I dont have any time at the moment.. so I guess I can do it next week.

[Hornet]

I've found this:

https://www.agilent.com/cs/library/Supp ... F05001.pdf

do you still run your inlet at 320°C?

Davide

[M-KW]

I am still working on that problem, I did what Peter suggested although I aint really sure what is the outcome.

I did some blank injections, it did decrease VERY little over 10 injections. I also saw already that the first injection is a lot more contaminated than the following here. I was able to verify that indeed after letting the instrument sit for a few hours I get a higher contamination peak. Then I did the 0-Volumen injection (which was a 0.1µL air injection). However the signal was a little increased, it was not much, I would say little higher than the blank runs.

Since the contamination decreased now by a factor of 10 since 2 weeks I suspect that it is indeed from the instrument, being slowly evaporated into the column over time. To be honest the only remaining part I did not clean/change is some steel tubing now. I could heat it up to see if the contamination increases.

[James_Ball]

I am still working on that problem, I did what Peter suggested although I aint really sure what is the outcome.

I did some blank injections, it did decrease VERY little over 10 injections. I also saw already that the first injection is a lot more contaminated than the following here. I was able to verify that indeed after letting the instrument sit for a few hours I get a higher contamination peak. Then I did the 0-Volumen injection (which was a 0.1µL air injection). However the signal was a little increased, it was not much, I would say little higher than the blank runs.

Since the contamination decreased now by a factor of 10 since 2 weeks I suspect that it is indeed from the instrument, being slowly evaporated into the column over time. To be honest the only remaining part I did not clean/change is some steel tubing now. I could heat it up to see if the contamination increases.

If you happened to get one bad tank of gas it could have contaminated the lines and is now slowly bleeding through the system. I have had that happen before and it is a real pain to get cleaned out.

If you suspect the lines are the source of the problem, you could put and old column in the instrument and run hydrogen carrier through it for a few days, which will scrub out the gas flow pathways. I experimented with hydrogen carrier on one of our systems for a few months, the background was terrible at first then after about a month it dropped to normal levels. When I switched the system back to helium, I had the lowest background levels I had ever experienced and it lasted for several months before slowly rising again. Seems any type of high boiling contamination will drop out of the helium and adhere to the flow path, while hydrogen will keep contaminates "in solution" and sweep them on through.

[Bigbear]

I'll chime in late. In the past I chased endrin breakdown issues that was caused by phthalates in my green septa. I switched to Agilent BTO reds and have no further issues.

I run a series of PAH's in my 525 method and only run my injection port at 250.