Benzene contamination in my GC runs

[EnvChemist]

I need help in getting rid of a Benzene peak. I am using an Agilent 6890 with Combi Pal (CTC Analytics) Headspace system.

My routine maintenace before any runs includes changing the septum, cleaning the FID jet by sonication in Methanol, and changing the liner.

Since the appearance of the Benzene peak in my blank (DMF) runs I have one by one changed the diluent (DMF - opened a brand new bottle), column - brand new one, syringe, vials and caps from different manufacturers and now the gold seal.

Still I have a Benzene peak of about 0.400 area counts.

Please help.

[Jumpshooter]

0.40 of a 1 unit area count is very very very low---likely just background. Is this a significant problem that has confounded your experiment and is prventing you from doing more work? From most of the research I've published the baseline values were on the order of 10+2 at minimum.
What was your peak area before you saw the purported benzene contamination?
Only thing left to concern is the gases--but they would't contain benzene.

[chromatographer1]

It need not be benzene. It could be a detector upset due to injection pulse. It also could be an analyte eluting at benzene's RT.

best wishes,

Rod

[Peter Apps]

Do you actually see a peak on the chromatogram, or are you just getting a report from Chemstation that says that benzene is present ?

Peter

[krickos]

Hi

Also would like to add the possibility of quality issues with the DMF in general.
I am not the only one that have stumbled into that problem, we had to change supplier for our residual solvent testing of pharmaceuticals a few years ago.
This may also become more of an issue when you look for class 1 solvents such as benzene in the 1-20ppm area as you typically require a higher sample oven temperature to get a decent sensitivity and consequently trace impurities may start to show from the DMF. In worst case at elevated temperatures you may even get breakdown products from your sample.

There are "special" GC-HS DMF quality bottles around to buy, however typically a bit more expensive.
Personally I today use a "LC quality" of >99,9% which works fine and it is cheaper than the GC-HS one but we now and then see tendencies (slight hump) to a blank peak corresponding to about the RT of benzene on a DB-624 column when sample oven is set to 70-80°C.

[EnvChemist]

@Jumpshooter - whereas the area counts are very low, our system suitability requirements are that Benzene is not detected at all in our blank (DMF).

@chromatographer1 - I ran through the program by pressing start on the instrument and did not see an increase in signal, so perhaps that rules out the detector?

@Peter Apps - yes I am seeing a peak on the chromatogram in our Empower report.

@krickos - Thanks for the suggestions, this is not the first time we are having this problem with DMF (even though we are running on a DB 1701), the last time we had to try a buch of different manufactures (HPLC grade >99.5%) this time I have had no luck. I will search for GC-HS DMF, any manufacturer recomendations in particular?

Some additional troubleshooting;
-I ran an air blank and still have the peak.
-I ran another air blank taking out the septum in the headspace vial cap, still have the peak.

[Peter Apps]

The tiny peak area puzzled me - presumably Empower generates smaller numbers for peak areas than Chemstation - they are arbitrary units in any case.

If you have the peak in air blanks than it is either a) in the air, b) carryover, c) in the carrier gas, d) in the syringe.

What happens with a no-injection blank ? if you see the peak it is either carryover or in the carrier gas.

What happens if you do a series of no-injection blanks. If the peak gets smaller it is carryover, if it stays the same it is in the carrier gas.

You have changed the syringe and still see the peak, but the syringe might be getting contaminated by the purge gas, do you have a charcoal scrubber in that line ?

How are you cleaning your carrier gas, and if you have scrubbers when were they last changed ?

Try aninjection form a vial that has been flushed with clean nitrogen to elimiante air contamination - a little of what is in the air will be stuck to the inside of the vial of course.

You may need to change the split line filter.

Peter