high base line due to surfactant or bleeding

[jawad]

I use GC for some oily chemcials in dichloromethane + non-ionic surfactant after extraction from water. Before use base line of GC at 300 C was 5,000uV and after 2days of use it has gone up to 30,000uV at 300C. Is it due to surfactant or bleeding of column.

[AICMM]

Jawad,

What GC and detector are you using? What does the zero look like at a lower temperature? What does the baseline look like if you don't do an injection and you ramp the oven up to 300? What does the noise level do? Did you just install this column or has it been in use for a while? It is often difficult to differentiate between bleed and contamination but these are some of the questions that will help.

[jawad]

AICMM, Thanks for posting a reply.

I am in a real trouble.

GC and column conditions are here.
I am using Schimadzu GC with FID detector. without any sample injection at 150C its really zero at 300 C it was getting to 5,000uV. then I injected some samples and after every sample injection is was going up, my program was like this; start at 150C for 0.5min then to 275C at 7C/min and maintain for 10min and then to 300 at 10C/min and maintain for 10min. To lower the baseline at 300C I did conditioning but during conditioning its going up and now its at 300C about 30,000uV. Its new column using only for 2 weeks and injected about 30 samples. I am not doing long time conditioning because during conditioning it keeps on going up and I fear that long conditioning will damage the column irreversibliy.

Now waiting for your reply. thanks

Jawad,

What GC and detector are you using? What does the zero look like at a lower temperature? What does the baseline look like if you don't do an injection and you ramp the oven up to 300? What does the noise level do? Did you just install this column or has it been in use for a while? It is often difficult to differentiate between bleed and contamination but these are some of the questions that will help.

[zokitano]

Dear Jawad,

What type of stationary phase (column) are you using? Did you check the column upper limit (operating) temperature, prescribed by manufacturer?
Is it lower or higher than 300C (temperature that you're using in your method)?

Best regards

[jawad]

Its upper limit is 320 C and I am conditioning 20 C below the limit. I am using DB-1 cappilary colulmn with non-polar surface made of 100% Dimethylpolysiloxane.

Dear Jawad,

What type of stationary phase (column) are you using? Did you check the column upper limit (operating) temperature, prescribed by manufacturer?
Is it lower or higher than 300C (temperature that you're using in your method)?

Best regards

[Consumer Products Guy]

I'd inject trimethylsilyl derivatizing agent several times and bake column at highest allowed temperature overnight.

[jawad]

Dear Consumer guy,

I have heard such an idea for the first time. can u please explain this?

Thanks

I'd inject trimethylsilyl derivatizing agent several times and bake column at highest allowed temperature overnight.

[Consumer Products Guy]

Nonionic surfactants typically have one (or more) hydroxy functional groups -C-O-H at an end which are very polar and exhibit strong hydrogen bonding. Using a trimethylsilyl derivatizing agent (such as Tri-Sil or BSTFA, from Pierce or Regis, etc.) converts those to -C-O-Si(CH3)3 which adds some weight, but reduces the polarity. If you inject some onto your column, it will derivatize in-situ any nonionic still on your column, to make it more volatile and allow it to elute at the high temperatures, to help clean out your column. Another alternative would be to remove your column (I'll assume it's a bonded phase, as I believe all DB-1 types have been for decades) and backflush with solvent such as CH2Cl2; there are small set-ups to do this commercially available from chromatography companies, but it may take a couple of hours. We typically assay nonionic surfactants dissolved in dimethylformamide, then add trimethylsilyl derivatizing agent, and straight to GC.

[AICMM]

Jawad,

I think you have a leak. A DB-1 is a very rugged column, not usually subject to much bleed at 300 C if the system is in good condition. I believe this to be the case mostly because you saw such a large rise in the baseline on an FID with no injection early in the game and the column is relatively new. I won't rule out contamination but I would look for a leak first , on the inlet side, before I did a column rinse or a silanizing treatment. The other possibility is that you have contaminated carrier gas, especially oxygen. This would also cause excessive baseline rise.

If you have another column, especially a new one, toss it in and see if it does the same high baseline.

Regarding rinse versus silane, I have had good luck with rinsing and poor long term luck with silane. If you need to go to silane, I would not expect it to last too long. If you rinse, I would suggest a polar solvent followed by a non-polar solvent. Supelco, Restek have column rinsing kits that will do the trick nicely.

Best regards.

[jawad]

Thanks for taking time to suggest some nice ideas

After checking these comments I find that better I first change the comlun with some new one and be sure that GC state is ok and there is no leakage. Then I will try to rinse the column with methanol followed by CH2Cl2.

Again thankyou very much.

Jawad,

I think you have a leak. A DB-1 is a very rugged column, not usually subject to much bleed at 300 C if the system is in good condition. I believe this to be the case mostly because you saw such a large rise in the baseline on an FID with no injection early in the game and the column is relatively new. I won't rule out contamination but I would look for a leak first , on the inlet side, before I did a column rinse or a silanizing treatment. The other possibility is that you have contaminated carrier gas, especially oxygen. This would also cause excessive baseline rise.

If you have another column, especially a new one, toss it in and see if it does the same high baseline.

Regarding rinse versus silane, I have had good luck with rinsing and poor long term luck with silane. If you need to go to silane, I would not expect it to last too long. If you rinse, I would suggest a polar solvent followed by a non-polar solvent. Supelco, Restek have column rinsing kits that will do the trick nicely.

Best regards.