Benzoic Acid impurity

Hello, I've been using the UPLC systems for a while with a MS system with good luck, but recently have been required to switch to a PDA.  I'm noticing some benzoic acid in my solvent a low level (~10 ng/mL) in each injection that is detected by the PDA.  I'm wondering if this is a common problem with a solvent of citric acid and sodium chloride, and I would appreciate any help.Thanks.


  • I did some research, and there is no evidence that citric acid and sodium chloride may react to form benzoic acid. It is likely that the contamination is coming from citric acid. I have checked the bottle we have here (citric acid monhydrate crystalline extra pure from Merck), and there is no benzoic acid in the specifications. Are you sure that it is benzoic acid only?

  • I'll second mukhi on this. Common impurities for citric acid are going to be sulphur compounds (from sulphuric acid in manuf.), Fe, and Pb. NaCl impurities are typically other salts.
    Assuming you've run a solvent blank (without citric and NaCl) and don't see the benzoic acid in that spectrum, consider running your citric acid and NaCl individually so you can narrow your search to one of the two. Once you confirm the source, it should make dealing with it much more simple.

  • What kind of LC vials are you using? There are a lot of brands of vials that are cleaned with Benzoic acid, but never get rinsed very well. I had this problem on some methods I ran on Waters Alliance and Agilent 1200 HPLC systems.

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