Conditioning Column 3% OV225

[GaryR]

Hi All

I am conditioning a new packed column and am experiencing a lot of bleed.
The packing is 3% OV225 on acid washed, silanised diatomaceous earth (100-120#).
This is a fresh lot of packing, and after packing the column we did a slow ramp to 230°C and held overnight.

Normal operating temps are:
Inlet: 230°C
Oven: 210°C
Detector: 250°C (FID)
Carrier = N2 @30mL/min
GC = Agilent 6890

At low temps, the detector output is normal @ <50pA.
As soon as we ramp up the temperature the signal starts to climb, peaking at around 5000pA @ 210°C. After running all weekend at temperature it had only fallen to around 2500pA.
I have tried the column in another GC and have the same problem.

I have run a checkout injection to rule out GC/detector problems and all is good there.

We have also packed another column with an older lot of packing and are experiencing the same problem.

Does anyone have experience with conditioning this type of packing?

[chromatographer1]

It seems you believe that 2500 pA is too much bleed for a packed column using a phase at elevated temperatures.

I can't say either way as I don't have a packed column FID to compare. They seem to be a vanishing breed of hardware today.

You also say that an older lot of packing performs the same way. So it does not seem that the phase or the support is the cause of the excessive bleed.

How much bleed do you get when using another kind of packing like OV-1?
OV-17?

It is at the same order of magnitude?

If it is, then I would suggest the bleed you are seeing is appropriate unless you have previous history indicating it is not.

best wishes,

Rod

[GOM]

Hi,

Rod has the most likely explanation. You noted that it was coming down with conditioning at 230°C. I would suggest conditioning it at 250°C (it should still be below the MAOT) for a few days, then operating it at 210°C for your analysis.

Regards,

Ralph

[AICMM]

Gary,

Without wanting to sound pat, if you are runing isothermal, why do you care what the zero is? If you are running with Chemstation, the electrometer has plenty of available range and if you are running a strip chart or integrator, you can set the zero offset on the 6890 to eliminate most of the high zero. I do this all the time in helium ionization mode since you almost always have a little bit of air in your system and, thus, a high standing current. The only down side of my answer is that your detector is going to get dirty a little bit faster but FID's are not that hard to clean. Now, if it's noisy, that is a totally different matter.

Again, forgive me if I sound flippant, but I typically operate my detectors with standing currents between 750 and 14000 depending on the detector and application.

Best regards.

[GaryR]

Thanks all for yor suggestions.

When I use our 'checkout' column (10% OV101) I get a low baseline of around 25pA, which I call typical of our system. And 'normal' peak responses (retention, area).

Yes, I would have been happy to re-zero the detector at 2500pA if I was getting peaks.

The signal is not noisy.

I will condition at 250 and see what happens.

Regards
Gary

[Consumer Products Guy]

Gary - why are you still using packed-column for this type of assay? I could maybe understand if you were doing gases, but why not use capillary on your 6890? If you don't have split/splitless inlet, it can be added at your site. Move up to the 1980s.

[GaryR]

Hi CPG

We are using packed columns because that is what we have registered with the health authorities.

Basically these are pharmacopieal methods that we don't want to bother re-validating for capillary columns due to low turn-over.

Thankfully we don't have very many of these methods left anymore.

Until recently, we were doing manual injections on an old varian 3300GC (which also had a chart recorder when I first started here!!).
We thought it was a great improvement when we started to acquire into Empower.

[Consumer Products Guy]

OK, now I understand. Stuff like USP is full of antiquated and wet methods because the costs to modernize them are so high. Recently our QA said that since test methods on an acquired product were validated on a slightly different matrix for GLP, that we'd need to revalidate. So, having to start from scratch, we moved from packed-glass columns to HP-5 capillary and 10 micron Bondapak to a modern Type B, 3 micron particle, in narrow-bore. The supplier of the active still uses the old procedures so they don't have to revalidate, like in your case. So much for advances in science...

[Victor]

Gary,

I checked on the Ohio Valley website and the maximum operating temperature of this phase is 250 deg. I was taught that these temperatures were very optimistic and never to go closer than about 20 deg towards these (optimistic) limits. A capillary column has much less stationary phase, and therefore much less bleed, so the problem will be much worse with a packed column. I think basically you are just pushing the system too hard- but if you are forced to use a less than ideal method then you will just have to put up with it.

OV 101 has a much higher maximum operating temperature - I think in excess of 300 deg. It is a methylsilicone which is quite different from your cyanopropyl silicone phase. So I am not surprised that it is giving much less bleed than your column. I guess you are not allowed to change the loading of the stationary phase. If you were I might suggest trying a 2% loading which would enable you to use a lower temperature-this would give you less bleed and you would have less stationary phase there too which should also reduce the bleed. Of course you might also have worse peak shapes due to the reduced coverage of any active sites on the support.

[GOM]

Hi,

I think that you will be OK conditioning to 250°C - just don't have it connected to the FID whilst you are doing it.

Regards,

Ralph

[GaryR]

Thanks all for your suggestions.

We conditioned overnight at 250°C and this brought the baseline down to a more manageable 200pA, and we got our peaks.

Unfortunately we are not able to change any parameters as this is a registered method.

Like CPG we are in the process of updating and re-validating a lot of our old methods, but most of the GC methods are well down the list - HPLC related substances take precedence, particularly those updated from TLC methods.

99% of our GC methods are for residual solvents in API's by headspace/capillary column.
We only have a few packed column related substance methods, the test using the OV225 column is for isomeric ratio (2 peaks only - normalised) and we would be lucky to do it once per year.

Cheers All

[chromatographer1]

Gary,

Without getting into too many details, this phase like many other 60s and 70s technologies is a distribution of MW silicone polymers and may vary from batch to batch.

At one time the recommended high temperature limit (programmed oven temperature) was 275°C.

Of course folks were using packed column GCs and were used to 'high' bleed levels.

These polymers may last a long time after manufacture and can break down into smaller MW units which of course bleed at lower temperatures than the bulk of the MW distribution.

It is difficult to describe the proper expectation of bleed due to these factors. 'Back in the day' lower temperatures were used in GC analysis and as applications were developed higher and higher temperatures were utilized and more stable phases, then bonded phases were the desire of analysts.

Your conditioning removed most of these lower MW components of the phase. This can be a requirement if the temperatures you intend to use will elute them from the column and interfere with your analysis.

However, it is not always desirable as the lower MW fraction of the phase gives better separation of the more volatile analytes than the higher MW fractions. Thus more conditioning is not always a good thing.

It does depend upon your analysis and duplicating the history of your analysis is the desired goal.

I am glad to hear that you are satisfied with the bleed levels now. I hope you did not loose too much of the phase loading and that your analysis will be acceptable.

best wishes,

Rod

[GaryR]

Hi Rod

Thanks for the info. The evolution of the phases over the decades is interesting.

We were unsure of the MAOT for this phase so were working by the 'rule of thumb' to condition by slow ramp up to 20°C above our maximum working temperature. This is obviously not hot enough, or long enough for this phase.

Thankfully we are only looking for the ratio of the two peaks, so a high baseline is not really a big problem.

Regards
Gary