GC-QQQ contamination problem

[Jasn]

Dear all,

I've been having a contaminationproblem for about a week now and I can't seem to get rid of it.
Instrument Agilent 7000C QQQ with 7890B GC and AS.

Method:
Injection: 25µL (hexane) solvent vent PTV
Injection speed: 40µL/min
Washes 1 : 3 pre and 3 post acetone
Washes 2 : 2 pre and 2 post hexane
1 Sample wash

starting temp: 40°C (0.85min)
900°C/min to 285°C (12min)
900°C/min to 350°C (2min) cleaning

Venting 75ml/min @ 5psi for 0.75min
splitless transfer between 0.75min and 1.85min
Inlet purge 20mL/min @ 1.85min

Temperature program/Flows:

1.2mL/min
Start 45°C (2.5min)
50°C/min to 280°C (5min)
35°C to 350°C (2min)

column: Agilent PAH Select 15m*0.15mm*0.1µm

I get 6-8 peaks that elute around 200-220°C that all have about the same spectrum. I added the spectrum underneath. If I check with NIST the best match is for di-isopropylnaphthalene, but I don't know where that could be coming from, since it is not something we analyze or is in our standards/samples. The system is used for determination of PAH, PCB and CLB.

The contamination is 'worst' when the system hasn't been used for a while (e.g. overnight or weekend). After the first injection the peaks are still present though at a lower intensity (+-1/10). The peaks also show when injecting air.

What I already cleaned/replaced: Septum, syringe, other type of vials and septa, liner, cleaned inlet base, flushed purge/gas/split lines with MeOH and hexane. Cleaned the split vent trap. Nothing so far has solved my problem.

I know that in the past, while experimenting, I might have had backflash into some lines (25µL injection solvent vent in narrow liner), that's why I cleaned all of the lines.

Any more tips or causes for these peaks are welcome. I'm kinda stuck...



thanks in advance,

Jasn

[scottythree]

Have you tried to cut off a bit of your column on the inlet side? Your start temp of 40C is less then the boiling point of your solvent (Hexane). Just a guess that maybe it is condensing on/in your column and has built up enough to be noticeable.

[Jasn]

Have you tried to cut off a bit of your column on the inlet side? Your start temp of 40C is less then the boiling point of your solvent (Hexane). Just a guess that maybe it is condensing on/in your column and has built up enough to be noticeable.

Scotty,


yes I already trimmed the column at the inlet side. The start temperature is that low because I need the 'solvent effect' to refocus the components with lower boiling points. The fact is that these peaks are even noticable when i don't inject anything at all, so must be coming from the system? What I did not mention in my first post is that those peaks are high (as high as standard 1ppb (25pg on column)).

Thanks for the suggestion though.

Regards,

Jasn

[kubowicz.tomasz]

Hello

Are you using any gloves when working with GC/MS (changing columns, septa, liners etc)? I know that sometimes vinyl or nitrile rubber gloves can cause funny problems with sensitive GC/MSD systems.
I'd clean column tip (inlet, detector) with acetone.

Regards

Tomasz Kubowicz

[C.raposo]

The compound seems to be diisopropylnaphthalene.

Is this somthing you know you have in your lab?

[Jasn]

Hello

Are you using any gloves when working with GC/MS (changing columns, septa, liners etc)? I know that sometimes vinyl or nitrile rubber gloves can cause funny problems with sensitive GC/MSD systems.
I'd clean column tip (inlet, detector) with acetone.

Regards

Tomasz Kubowicz

Tomasz,

yes I use gloves when working with GC/MS. They are nitrile. Might be a good idea to always clean the column ends with acetone before use... Not something I do for now. I know that not wearing gloves can produce 'peaks', but was not aware of the same with gloves.

Regards,

Jasn

[Jasn]

The compound seems to be diisopropylnaphthalene.

Is this somthing you know you have in your lab?

Yes, I also noticed that it was diisopropylnaphthalene. It is NOT something we use in the lab, so it's very strange. The instrument is a demo instrument though and I do not know what applications it ran before we bought it. We've bought the instrument 9 months ago though, so any remaining 'contamination' is ought to be removed when we bought it + 9 months later is quite a time.

Regards,

Jasn

[kubowicz.tomasz]

Hello

Diisopropylnaphthalene is used as susbstitute for PCB in transformers (see link below)

http://apps.kemi.se/flodessok/floden/ke ... en_eng.htm

Perhaps it comes from samples...

Regards

Tomasz Kubowicz

[Jasn]

Hello

Diisopropylnaphthalene is used as susbstitute for PCB in transformers (see link below)

http://apps.kemi.se/flodessok/floden/ke ... en_eng.htm

Perhaps it comes from samples...

Regards

Tomasz Kubowicz

It is also used as pesticide or an monomer for fabrication of some plastic. If it was in a sample I would have expected to have seen huge peaks in the sample itself, but I have not seen that. The funny thing is that the system has only seen a 20 samples since it arrived. The rest has been experiments with standards for narrow bore chromatography, PTV, backflush...

Could it come from the EPC?

regards,

Jasn

[kubowicz.tomasz]

Hello

If you have another inlet (S/SL) you can check it. If you won't see contamination perhaps it is EPC module.

Regards

Tomasz Kubowicz

[Jasn]

Hmm, i sure hope it is not EPC module, since I presume that an EPC is not easily cleaned.
Thanks for all the ideas, keep 'em coming

regards,

Jasn

[cleh]

Jasn,

Does your carrier gas regulator have a stainless steel diaphragm? If not, that may be the problem (some are neoprene). You said it's worse after it hasn't been run for a while so the DIPN has time to build up in the lines, etc. You may not have seen this until now if you have a gas purifier between the regulator and GC. The purifier (when new) could do it's job holding onto the DIPN and now it is saturated so the DIPN is getting to your instrument.

Regards,
Cathy

[Jasn]

Jasn,

Does your carrier gas regulator have a stainless steel diaphragm? If not, that may be the problem (some are neoprene). You said it's worse after it hasn't been run for a while so the DIPN has time to build up in the lines, etc. You may not have seen this until now if you have a gas purifier between the regulator and GC. The purifier (when new) could do it's job holding onto the DIPN and now it is saturated so the DIPN is getting to your instrument.

Regards,
Cathy

Cathy,

I have no idea about the diaphragm. I will check it out. We have no gas purifier between the regulator and the instrument though (not really needed since very high purity He is used and MS shows almost no H2O or oxygen (both <1% of PFTBA)).

Kind regards,

Jasn

[Rocknroll99]

Hello everyone, We are seeing a similar issue, Is there any fix found?

MAtt

[James_Ball]

Hello everyone, We are seeing a similar issue, Is there any fix found?

MAtt

If the contamination is worse on the first injection after sitting overnight then I will have to go with what Cathy posted above, contamination from either the carrier gas itself or the regulator. I found that using one of the capillary grade hydrocarbon traps https://www.restek.com/en/products/acce ... ation/945/ right before the EPC helps remove a lot of background from these systems. We always used Ultra High Purity Helium as our gas but over time the lines become contaminated with hydrocarbons, which when using one of these traps drops very noticeably. Just scanning in manual tune with the PFTBA valve closes you can see a reduction in the number of peaks detected as background.

Since most helium is obtained from natural gas, it is understandable that there could be some very trace amounts of hydrocarbons present. High purity helium will have these at less than ppm levels, but over time they can accumulate in lines and cause problems. When they analyze these carrier gas tanks they look for total hydrocarbon, which if it is 100 different hydrocarbons at a total of 1ppm would not be noticeable, but if it is just one or two at 1ppm that becomes much more obvious in the background.

One note on the hydrocarbon traps, I have found they have a lot of CO2 adsorbed in the carbon and it takes a few days for that to completely bleed out so you see it in the background when you first install them. After that it is completely gone.