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C18 column problems due to ammonium bicarbonate buffer? [Copy URL]

Rank: 8Rank: 8

Post time 2013-1-1 11:26:12 |Show all posts

Hi all

I am using the following mobile phase:

Mobile Phase A: 10 mM aqueous ammonium bicarbonate containing 0.2% triethylamine, pH 7.5 (adjusted with formic acid)
Mobile Phase B: Acetonitrile
Gradient: 90% A (0 min) - 90% A (6 min) - 70% A (21 min) - 70% A (50 min) - 90% A (55 min) - 90% A (65 min)

We are using Waters Xterra C18 and Xbridge C18 columns (4.6mm, 150mm). After less than a 100 runs we start experiencing peak splitting or peak-shoulders on the high RT end of the peak and increase in column pressure, rendering the column useless. We've gone through 3 columns in so many months.

Anyone have any insight? Is there something wrong with this buffer?

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Rank: 8Rank: 8

Post time 2013-1-1 11:26:15 |Show all posts

My first suggestion is to measure the pH again after a few hours. It may have crept into the alkaline range, and you are not really using a buffer. I had believed myself that it is possible to make a stable carbonate buffer at pH 7, but it did not work (how do you degas, for example).

The other thing that could happen is the adsorption of contaminants in your sample matrix.

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Rank: 8Rank: 8

Post time 2013-1-1 11:26:17 |Show all posts

I have worked on methods using a filtered sodium bicarbonate solution where, on preparation, a specific pH (8.2 ?

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Rank: 8Rank: 8

Post time 2013-1-1 11:26:19 |Show all posts

Uwe: We degas the aqueous buffer and also use an in-line degasser on our HPLC.
Yes, it is true that we do get pH drift but I would think that this would only effect RT, if anything, and RTs are relatively stable.
What I am concerned about is the short lifetime of our columns. We've busted 3 columns in three months and each one shows the same characteristics - peak shouldering, followed by pressure increase. In one case, we've had to replace a column after only one week of use. Since then we've switched from Xterra to Xbridge and the situation is still problematic. I can only conclude that it is the buffer or maybe incompatibility between buffer and analyte.

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