GC problems with PAH

[kartoffel]

Hi everybody,

i want to analyse PAHs, but i have a problem my GC-MS/MS (Agilent TQ 7000B, 7890A).
When i injected pure toluene, which i use for sample extraction, i was surprised to see huge signals in the scan chromatogramm. Then i injected the same toluene into an older system (single quad) where i didn't see any signals in scan mode. So it shouldn't be the toluene which is contaminated. Problem is that these peaks also appear in my MRM method and interfere with my PAH peaks, so i can't see low concentrations.
I checked my hole system after that (changed syringe, replaced Inlet and Septum, cleaned the MMI Injector, changed column, baked out the MS) but nothing helped. When i removed the column from the inlet, i saw that the part of the column which was in the inlet was dark black. Inlet Temp is 300°C, shouldn't be a problem for a restek Rxi5-MS, shouldn't it? Reducing Temperature to 250°C helped (peaks decreased about 2-10 fold), but i think this temp is to low for analysing low levels of the heavy PAHs.
It is my first method development on that TQ system, so does anyone have a hint where these peaks may come from, may it be the column end that degrades at 300°C?

[James_Ball]

I can easily get Benzo[ghi]perylene and indeno[cd]pyrene onto the column with 250C injection port. Depending on how accurate the temperature reading is you could be getting near the limit of the column when set to 300C.

With the 7000QQQ you are going to see much more than with the single quad, I use ours in single quad mode and can see probably a factor of ten lower concentrations without a problem versus our 5975/5973 instruments. Also when injecting DCM I see a long tail of the solvent out several minutes into the run even when running 1:10 initial split and increasing to 1:50 split after 0.5 minutes. It is just that much more sensitive.

[kartoffel]

First i thought also the device would be much more sensitive, but when injecting a PAh standard, the areas don't raise that much.

turned out that the column gets black only after injections of toluene, not when heating the injector to 300C only. When injecting n-hexane, the ghost peaks compared to toluene are much smaller (about 10-50 time).

I reduced the injector temp to 250C and then to 200C. The ghost peak areas (of toluene) decreased by factor 10 at each step. Now i can see my lowest standard, that's fine, but I've never seen that at the other devices.

i used the same column, same method and the same solvent on a other device at 250C and hadn't those problems with ghost peaks. There the column end didn't turned black after injecting toluene 3 times. Maybe active sides in the injector, leading to formation of some reactive molecules at high inlet temparatures?

[Yama001]

I occasionally resort to Toluene to clean the inlet - some times it seems to clean out material that other solvents will not. Perhaps the Toluene in this case is pulling out gunk from the inlet that has never been cleared out before.

[James_Ball]

First i thought also the device would be much more sensitive, but when injecting a PAh standard, the areas don't raise that much.

turned out that the column gets black only after injections of toluene, not when heating the injector to 300C only. When injecting n-hexane, the ghost peaks compared to toluene are much smaller (about 10-50 time).

I reduced the injector temp to 250C and then to 200C. The ghost peak areas (of toluene) decreased by factor 10 at each step. Now i can see my lowest standard, that's fine, but I've never seen that at the other devices.

i used the same column, same method and the same solvent on a other device at 250C and hadn't those problems with ghost peaks. There the column end didn't turned black after injecting toluene 3 times. Maybe active sides in the injector, leading to formation of some reactive molecules at high inlet temparatures?

If you have the quick open inlet tops, make sure that they are completely tight. I have had problems with leaks there. The top of the inlet should not rock side to side once it is tight. If you get a small air leak there you could get problems such as this.

With lower sensitivity, are you tuning with the "EI High sensitivity mode" box checked? That makes a large difference in the sensitivity of the instrument.