FID hydrocarbon analysis, low response volatile compounds

[sjps220]

I'm running a few 6890 FIDs, dual on-column injectors with Hydrogen as a carrier and nitrogen makeup. I have an issue with a decreased response to the lighter end hydrocarbons on one injector. My standard has C10, C16 and C34 and the C10 has a much smaller response than the other two. Usually the peak areas for these 3 compounds are pretty close to identical, but now the C10 response has dropped off.

The method has been the same for a long time, so I don't think it's a method issue. Our solvent is toluene, and our initial oven temp is 60C, inlet temp 70C. The column was recently changed. The column didn't have this problem right away, but it did show up pretty soon after the change. Column is a 15m, 320um ID, .10um film ZB-1HT inferno.

A similar situation happened a few months ago on a different FID. I ended up checking the flows with our digital flow meter and found that the air flow was lower than it should be on one detector, and once I bumped that up, the responses increased across the board and the C10 fell in line with the other two. I have tried bumping up the air flow to the detector on this instrument and the issue remained.

I have swapped out the jet, cut the column at both ends, replaced the septa, replaced the autosampler syringe, replaced the ferrules and increased the air flow to the detector. I also tried an inlet leak test, but I've tried this before and never really had my inlets hold pressure. Not sure if I'm doing it wrong, or if my inlets are terrible. I capped the septum purge vent, and put a no hole ferrule in the inlet, set the inlet pressure up, then turned the pressure off and watched what happened. My pressure always drops faster than it should according to the documents I've read.

Any suggestions for what I should be checking here? I'm fairly new to chromatography but I'll do my best to answer any questions that you may have. Thanks for any help.

[chromatographer1]

Sounds like a leak.

Examine the diagram of the injector and see where the seals and sealing surfaces are located. Be sure to cap off the septum purge and put a new septum in your inlet.

The loss of your most volatile component is indicative of a leak.

Clean and replace seals and find where the gases are going as the pressure drops.

If the drop is too fast, find the leak and fix it.

best wishes,

Rod

[sjps220]

Thanks for the quick reply Rod.

I'll take a look at the inlet a little closer and do some more leak testing. We have an electronic leak detector around so I'll check all my connections.

[chromatographer1]

There is a tiny O-ring which many will ignore when it drops off. This caught me once.

Your comment that the pressure drops too quickly is a dead giveaway.

Thanks for doing the basic diagnostics.

good luck,

ROd

[sjps220]

Looked for leaks today. I had the cool on column inlet popped out of the instrument with a no hole ferrule inserted, so I could check for leaks all along the inlet tube. I didn't get any readings at all. I checked along the gas lines to the EPC and didn't get anything either. I did the pressure test again. New septum, septum purge vent capped tightly, and the pressure lost a full psi in about a minute.

[Peter Apps]

I am not familiar with the on-board leak tests, but shouldn't you be capping the split vent as well ? Are you checking for gas flow at the split vent ? Could be that the splitter plumbing is leaking.

Peter

[sjps220]

I have not tried capping the split vent. I assumed that since I was on-column, this wouldn't be doing anything. I'll check it for leaks next time I try this. Thank you for the suggestion.

[sjps220]

An update:
I accidently ran my method with the makeup gas off. When I looked at the results, the peaks had returned to the proper proportion, with each peak's area being very close to equal, although the response of all peaks was diminished. Any ideas on what could explain this?

I called agilent and they suggested cleaning the collectors of the FID with a nylon brush. I had already sonicated them, but tried it and my responses returned to where they should be, and everything looked great. It passed my calibration and I started running on it again. Now I'm finding that the results are starting to head towards failing again in the same way (decreased response on the volatile end).

Is it possible that I have contamination somewhere that is causing a buildup on my collector?

[Peter Apps]

I can't think of anything that can happen in an FID to give the kind of discrimination that you describe - it should be completely unselective wrt molecular weight for the molecules that you have - as soon as they come out of the end of the column they are completely fragmented and ionised.

Peter

[sjps220]

it should be completely unselective wrt molecular weight for the molecules that you have - as soon as they come out of the end of the column they are completely fragmented and ionised.

So if that's the case when the makeup gas is off, but not when it's on, do I look into my makeup gas?

There's a filter on the line that was replaced within the last few months, but we also had some gas contamination since that new filter was installed, so maybe the trap is contaminated?

Should collectors be replaced at all, or is cleaning generally enough?

[Peter Apps]

With the molecules that you have there is nothing that can go wrong with the detector that can cause discrimination on the basis of molecular weight - not makeup gas, not dirty collectors, nothing. Anything that affected the detector's response to one hydrocarbon would have an equal effect on the response to the others.

It is just conceivable that something is happening to the detector that causes a discrimination on the basis of retention time (which in you case coincides with differences in MW). There might be a slow fluctuation in collector voltage, makeup or fuel gas flows, or something in the electrometer that is slowly changing.

What happens if you change the oven temperature programme to change the retention times of the hydrocarbons - increase the temperature to get them all to come out quicker for example ?

Peter

[oiltest]

Producer and exporter of Trace Hydrocarbon Analyzer in China.
High quality and Competitive price.
For more information, please contact us through www.dgaequipment.com.