tailing and rounded peaks

[alba]

Dear all,

we are setting up a GC to measure CH4, CO, CO2, SF6 and N2O according to this configuration: http://www.atmos-meas-tech-discuss.net/ ... 1-2009.pdf.

We have already done it before but now, with a similar but new equipment, we are having some problems we didn't have before. Basically, the peaks we are obtaining for CO and CH4 are not well-defined enough. You can see an example in the following link:

http://albabloc-fr.blogspot.com.es


What would you suggest us to do? We have already conditioned the columns but not the methanizer (there is one and all FID compounds [CH4 (5.8 min), CO, CH4 (9.8 min) and CO2] pass trough it. Do you think this would be fine to do?

We have also several union fittings that we didn't have in the GC previously set up, do you think that they could be acting as dead volumes?

We thank you in advance for your help.

[chromatographer1]

Your poor shape of the CO peak appears to be not a dead volume problem but over-conditioned or crushed mole sieve packing in its column. The other methane peak distortion appears to be a valving timing issue or more likely a flow imbalance between the two trains going to the FID, V6+V5 and V3 + V5. This causes an upset in the FID as the H2 flow pulses into the detector.
This may be due to the high restriction of a partially crushed MSieve column.

best wishes,

Rod

[alba]

Dear Rod,

thank you very much for your reply. So, your opinion is that our Molsieve column might be not working properly. It's strange for us because this column is a “new” one. The technician who came to install some parts of the setup conditioned it for several hours. We are not sure about which temperature he used but we think it was not higher than 300ºC; we did another conditioning of the columns approximately a month after the first one at a maximum temperature of 300ºC but before this last conditioning the CO peak was already tailing...

I have read again an old post you sent to me some time ago, where you said the following:
“ … I would not condition packings above 200C unless you intend to use them above that temperature. The higher temp you use for the polymer packings the more damage you will inflict on them. If water is present in MS packing you WILL DAMAGE the packing above 200C. It will take days of conditioning to remove all-most of the water. Longer conditioning times at lower temperatures will cause less damage to the MS packing. There is no advantage in using higher temperatures for the MS except quicker conditioning, and that comes with a cost of increased damage and poorer chromatography. Proceed at your own risk. ..”

Maybe we risked too much… Is there any solution for the over-conditioned/crushed column? Or should we buy a new one?

Thank you again for your help,

[chromatographer1]

Think of Mole Sieve as made of glass. Once the glass is broken, throw it away.

Yes, I would buy a new column and condition it SLOWLY starting at 100C and don't exceed 200C. A capillary column should condition rather quickly, less than 8 hours. A micropacked column I would condition in 20C steps from 100C and hold for 4 hours at each step. A packed column (2.1 or 4.6mm ID) I would do the same and hold each step for 8 hours. As always use the highest flow rate of carrier as you can afford. Water is effectively removed at temperatures up to 200C. Any additional temperature or quicker removal of water will only react the water with the crystalline sieve and destroy the passages through the packing.

best wishes,

Rod

[alba]

Dear Rod,

we got a new column from our supplier but unfortunately they conditioned it previously up to 350ºC, using a ramp of 2ºC/min and for 8 hours (!!!).

We did another conditioning only up to 200ºC (we believed it was not conditioned when we received it...) but probably, if the problem is the over-conditioning of the column, it didn't change anything...

We still get the bad tailing CO peak...

We have a question: why suppliers indicate that Molsieve columns can be conditioned up to 350ºC? Why do they proceed like this if this can be a problem?

Could there be any other problem, apart from the column, causing the tailing CO peak?

Thanks again for your attention,

[chromatographer1]

Alba wrote:

"We have a question: why suppliers indicate that Molsieve columns can be conditioned up to 350ºC? Why do they proceed like this if this can be a problem?"

One reason is no one who did the original research on Mole sieve is still working and later employees just took assumptions a bit too far. This requires explanations that might take too long here.

You CAN condition MS packing to over 400C (this might be necessary to remove hydrocarbons and other light gases from the pores)

BUT

the 'gotchas' have to do with the water content of the initial condition of the packing and how quickly you condition it.

If you don't know what you have and what you are doing (and that includes almost every vendor selling columns today) it is VERY easy to damage MS's crystalline structure. If you damage it you get TAILING peaks.

Now if you send a methane peak through your column does it also tail? If it does, then you have a void space in your flow path. But if your methane peak is sharp you can discount the problem is caused by a void space.

If the packing is blackish grey instead of a light yellow beige, that is also an indication that the packing may be damaged due to excessive heat.

While keeping a symmetrical peak for CO, 90% or so of the N2-CO peak separation distances are achieved by conditioning mole sieve packing at temperatures only up to 200C and conditioning it VERY SLOWLY. Why try for another 10% when this may ruin the column?

I learned this the HARD WAY back in the early 1990s. (yes I was ignorant back then) and until I read the original research in the natural minerals used for making packings before they were commercially prepared in bulk I did not understand why my peaks were tailing. I used MORE HEAT and MORE TIME. Things only got worse.

After 2000 I worked on improving MS5A commercially (my company made and sold columns) by making my own packings. I tried to recover the original performance of the packing that was documented in work performed in the 1960s and 1970s.

I (re)discovered the causes of the tailing CO peak and the lack of resolution of argon and oxygen at ambient temperatures. (These can be separated in large percents, not ppm levels, at room temperature) I saved a large US government organization from having to buy an expensive GC-MS or a liquid nitrogen cooled GC oven by providing them with a packed column to make this separation at ambient. Their title I will not state here but will only say that the organization is usually discussed by name with its letter abbreviations.

Test your methane peak first, and if it tails, rework your flow path and remove the void space within it.

good luck,

Rod

[Yama001]

I know it won't pay the bills, Rod, but you are truly a treasure on this board.

[chromatographer1]

well, thank you for your kind comment. Sometimes my grumpy responses are not so well received.

Just a FYI

Once a MS column gives a tailing peak for CO and there are no voids present, there is no way to improve the tailing. It is a lost cause.

Rodney George

[alba]

I also thank you for the comments, for us are very helpful!!

We are trying to get a new column...

[chromatographer1]

Many if not all manufacturers of MS 5A columns will condition the packing BEFORE it is packed as well as after the column is packed.

Just a FYI.

Rod

[alba]

Dear Rod,

we are still fighting against our CO peak problem. We have now some more...:

1. We have got a new column and we have conditioned it up to 200ºC only. We have done several tests with this new column and now we don't see any peak of CO at all ... Any idea about this result?

2. When removing the "old" column we have noticed that ferrules were somewhat "carbonized"... This is a new doubt that arises to us, because we didn't use temperatures higher than 300ºC, so we can't understand why this ferrules are that bad... Just bad quality? Any idea? I show you one picture here:
http://albabloc-fr.blogspot.com.es/

3. We are using a Molsieve 5A 60/80 column in a GC we have working in a station and the columns we are testing are also 60/80. However, we have noticed that in the paper from van der Laan et al. (2009) they were using a Molsieve 5A 60/60 column. Erroneously we ordered this 60/80 column... Do you think this is an important difference that could be worsening our results?

We hope you can give us some light to these doubts.

Thank you in advance. Best regards,

[chromatographer1]

First let me say that there is no such thing as a 60/60 mesh. That is a typo. 60/80 is probably correct. The numbers refer to the two different mesh pans which separated the particles by size. Packing of 60/80 mesh means that particles larger than 60 mesh in size were excluded and particles smaller than 80 mesh were permitted to leave this sample of particles.

I would guess that either your timing of your valves is now incorrect or your methanizer is not working. To quote from the article you supplied:

"This way, the FID first analyzes CO eluting from column 4 and sequentially analyzes CH4 and CO2 which will elute from column 5. At 3.98 min V5 is switched off and the FID is connected to column 5 just in time to detect CH4, followed by CO2. The exact switching time is chosen such that oxygen, which precedes CH4, will not enter and degrade the methanizer by oxidizing the nickel catalyst powder."

If you kept your methanizer heater on while you were changing columns and exposes its nickel packing to air you oxidized the nickel and ruined the catalyst.

Valve Timing is critical in this application. You should purchase some gas blends, perhaps individual blends to verify your peak location and the proper operation of your methanizer.

best wishes,

Rod

[alba]

Dear all,

we are still fighting against the CO problem. On the comment from Rod about the methanizer, if we still obtain the CO2 peak, do you really think that the methanizer could be the problem?

What do you think about the ferrules condition (http://albabloc-fr.blogspot.com.es/) we detected after disconnecting the first column? They look as if they were carbonized... Any idea about this?

Thanks again for your help.

[chromatographer1]

OK, I see that CO2 is passing through the methanizer. Since it does not seem to have any problems in its conversion to methane it appears the methanizer is working properly.

The problem seems to be either there is a void in the MS 5A column or too much dead space in the connecting tubing.

Are you using carbon ferrules to connect to the methanizer? The temperature of the methanizer should be above the temperature limits on an extended basis for the ferrules. The broadening of the CO peak indicates that the conversion of CO to methane is occurring slowly and possibly incompletely, or there is a void present. Since the methane peak coming off the MS5A column is not broadened a void possibility seems remote. The other issue could be that the CO peak is swamped by the amount of N2 present co-eluting with the CO peak. It appears from your diagram that you are using N2 carrier instead of H2 and are using a makeup H2 gas at the methanizer. If the amount of CO is much smaller than the amount of CO2 this could be the problem why CO looks so flat. The concentration of the CO2 might be high enough to minimize the problem for it, but it would give a smaller result for CO2 than it should.

In any case I would not use carbon ferrules to seal the connections to and from the methanizer. I would use SS ferrules.

best wishes,

Rod

[alba]

Dear all,

some weeks ago I initiated this post. After all this time I'd like to share with you our first conclusion.

For a period we were using three different columns from the same supplier (we checked ferules, conditioning procedure, etc.) and had always the same problematic result with CO. But finally we decided to order a new one from another supplier and this new one didn't show the tailing problem that was bothering us.

So, our first impression is that we had basically a problem with the supplier. They are going to test one of the columns in their premisses to cross-check conclusions.

Thanks for your help,