N2 and O2 detection issue

[cmoral]

Hi all,

I am trying to get a consistent detection for N2 and O2 using a Mol Sieve 5A column, He carrier gas, and TCD detection.

I am finding that when I inject 100 ul of room air, I get a nice separation of the two peaks, 7.8 min for O2 with area count of 500000 or so and 10.8 min for N2 with area count of 2e6, But when I try a second injection, the area counts drop to around 1e4 for O2 and 2e4 for N2.

I don't recall having this problem before and wondering if someone can suggest what might be the issue. If I monitor the total flow out of the split/splitless injector port, it is around 32 mls/min. If I monitor the flow out of the TCD outlet port, it is 18 mls/min with the ref and make-up gas. I don't think it is a flow issue, else why would I get such nice peaks with each first injection. The same thing happens if I start the oven at 40 oC or 50oC. And if I start the sequence, the first injection is great, all subsequent ones really low. If I restart the sequence, same thing happens.

Column retention issue? All suggestions would be most appreciated.

[CE Instruments]

If the retention times remain the same you can ignore the flow rates. If you get bigger peaks you must be getting more onto the column. If it happens every time you re-start the sequence then you should look to the injection method etc. Is it possible the first injection has gas save or similar on , is the injector in splitless mode ? The difference must be in the conditions of the first injection. Are you using a syringe or loop ? Any part that is unpurged for the first injection ?

[Consumer Products Guy]

Septum leak, maybe from damaged needle tip of syringe?

[cmoral]

Thanks guys for your suggestions. It's weird. I am pretty sure it is not the injection method, which is exactly the same...by me using a syringe...my technique is pretty consistent but this happens every time I restart exactly the same sequence using the same vol. I don't think the needle gets plugged because I check it immediately after by just pushing air into a vial of water to make sure of bubbles.

I'll check things again but it is pretty weird.

[cmoral]

I have removed the column, changed the inlet liner and septum, checked all column ends and made sure everything is tight. I am wondering if when inserting the column into the inlet, I knocked the gold piece out of alignment. Anyway, I made sure everything was seated properly.

Now, I can say I have two well separated broad peaks, more like blobs, which I am considering progress since they are consistent over three injections. So, my question is now, does this suggest that the TCD is now saturated? Or other reasons?

Thanks.

[AICMM]

cmoral,

You have a mol-sieve at 50C and 18 mL's/min out the TCD and your nitrogen elutes at 10 minutes? Based on your flow rate, I would say you may not have enough reference flow/column flow. The lack of flow would certainly explain the blob. 50C on a ShinCarbon (column I am more familiar with) at about 8-10 mL's/min yields an O2 peak around 1.5 min and N2 around 2.5 or so. A sieve should be similar. BTW, sounds to me like you are using a PLOT column which would still want at least 8-10 mL's/min if it is a 0.53.

So, turn off your TCD filament and measure flow out TCD without reference. If a 5890, your reference should be about 2-3 times your column flow.

Best regards,

AICMM

[cmoral]

Thanks AICMM for your response and suggestions.

I have actually tried different flow combinations and I completely lost all peaks until I went back to the original flow combo where the peaks started coming out again at around 6.5 min for O2 and 9.9 min for N2.

I checked the column plate ID and it is definitely a MolSieve 5X (30 m x 0.53 um) not a plot column. We have tried a QPlot column and it doesn't separate out the O2, N2.

The blobs I discovered were actually related to the length of column sitting in the injector. It was too little (< 2 mm), so I pushed it up into the column more (closer to 6 mm) and I obtained the nice peaks.

I tried increasing the column flow (TCD off) and then the ref flow (2 to 3 times the column flow) and that is where I ran into the problems.

Perhaps you can figure out where my problem lies

First, I am a bit confused as to what the actual capillary column rate should be...kind of getting a couple of different answers...I have read 2mls/min but with make-up it should be 5 mls / minute and then I have read that it should be 5 mls/min by itself:

I upped the tank output pressure to 40 PSI, which slightly increased the col flow to around 15 mls/min when measured at the TCD output (too high I think) but to follow the ref = 3 x col flow, I increased the ref flow so that the total flow was somewhere around 45 mls/min. That is where I completely lost the peaks. I also had the split/splitless flow at around 32 mls/min with the septum purge at around 2 mls/min.

I am not quite sure about the split/splitless control knob which says "split/splitless total column flow" so if I increase that knob is it supposed to significantly change the col flow when measured at the TC output? Because there seemed to be a limit to what it actually did in terms of the total flow measured at the TCD output. But the flows are different when I measure at the TCD output and the split/splitless controller output.

Right now, I am happy to get anything out but am definitely willing to go back and try playing around with the flows again since I would like the peaks to come out much faster.

So any further help would be appreciated.