I am using a method to analyse impurities in a pharmaceutical preparation. The method runs successfully but it has a drawback: there are negative peaks near the void volume that sometimes interfere in the determination of the first eluting impurity. I understand what vacancy chromatography is and I know that the source of the problem are the impurities of tetrabutylammonium hydroxide (Fluka purum >98%) used in the preparation of buffer (TBA-OH 65 mM pH 6.7 adjusted with H3PO4). I am wondering if there is any brand of reagent considered to be better. I know that there is tetrabutylammonium fosfate in the market. Is it considered to be of higher quality as ion-pairing reagent?
The method was fully validated in the past when such negative peaks did not interfere for unknown reasons (using the same TBA brand). In case of shifting to tetrabutylammonium fosfate the final buffer would be equivalent after adjusting the pH?
Cromatographic conditions:
buffer : acetonitrile : methanol 58:21:21
215 nm, 1.5 mL/min, 10 µL
Kromasil C18 150*4.6 5µm
solvent: buffer : acetonitrile 30:70 (eluent cannot be matched for stability and solubility reasons)

I use TBAOH Fluka, puriss. p.a. (cat. n. 86854). I see no peak near the void volume (UV 254 nm). For better results I advise to clean TBAOH with a SPE cartridge. In gradient run you will obtain much better baseline.

I use TBAOH Fluka, puriss. p.a. (cat. n. 86854). I see no peak near the void volume (UV 254 nm). For better results I advise to clean TBAOH with a SPE cartridge. In gradient run you will obtain much better baseline.

Since nobody seems to know..... here is a guess:
the phosphate should have a longer shelf life(less de-alkylation and subsequent oxidation?), so if you stick with the hydroxide, convert it as soon as possible.
If your final solution has the same conc. of all components it will be the same no matter how you prepare it. See older chains on buffers.
Vojtech Z.: Did you inject your samples in mobile phase or something dirtier?
Antoni T.: You really didn´t do anything different in the earlier, better, runs? You had a diff. batch of TBA-OH?

Actually, while the phosphates salt has the advantage of being in the most chemically stable form, the phosphates salt is often not made from the highest purity grade. The process by which tetrabutylammonium hydroxide is manufactured permits preparation of the highest purity reagent. When in doubt I would recommend buying the hydroxide (the best quality reagent is available from Sachem http://www.sacheminc.com/ their electronic grade is the highest purity but Fluka appears to be selling the same grade albeit at a higher price: catalog number 86832). The main decomposition products of the hydroxide are tributlyamine and 1-butene. This reaction is slow at room temperature but occurs only under alkaline conditions so of course the phosphate salt should be substantially more stable.

Dear Moderator,

I'm finding difficulty in overcoming from tailing with one of our compound which has a Abs.max at 338nm.The compound is well eluting with 0.05M NDOP Buffer :ACN ::50:50 without any shift in RT in Hichrome RPB column. But there's a prominent tailing even at lesser pH with PO4 Buffer , TEA as an additive & 0.1% TFA :ACN Combinations.I would be highly thankful If I can be had any suggestion...

Thanks in Advance

Regards,

Syed Mustafa


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