Citric by C18 not stable

[thohry]

Hi all,
Can anyone help me with this project.
We start to analyze citric acid by C18 column, pH 2.6, isocratic with no organic solvent. UV = 216nm.The retention time is not stable, and if the concentration of sample is increased, the retention time seems to be shorter.
Thanks

[Gerhard Kratz]

Why do you use C18??????? A Special column for organic acids will do it much better. A refractive index RI detector would be also better rather than using UV at 216nm. What Matrix and what sample preparation do you use?

[HPLCaddict]

As Gerhard already mentioned, using a C18 column is not the best thing to do here .
Citric acid is something like the worst case for pure reversed phase chromatography - small, highly polar, and even when it's protonated at low pH it will still be highly hydrophilic, so you're not going to see much interaction with a hydrophobic column. You should be able to get decent retention on C18 under ion-pairing conditions using a cationic ion-pairing reagent at higher pH. But still, I'd prefer HILIC, ion-exchange or a suitable mixed-mode column.
The problems you're seeing (unstable retention time) might be the consequence of phase dewetting. Are you using a C18 column suitable for 100% aqueous mobile phases (some sort of "AQ" or EPG column)?
I'd strongly suggest to switch to a different chromatographic mode...

[thohry]

Thank you both.
I just referred to some application notes. The problem is I have not a column that's compatible to 100% aqueous at the moment.

[Consumer Products Guy]

Why do you use C18??????? A Special column for organic acids will do it much better. A refractive index RI detector would be also better rather than using UV at 216nm.

We use a specialty "organic acids" for citric acid; we've even validated the procedure in two completely different types of products. One of those validations used conductivity detector only, the other was validated to use either conductivity or low wavelength UV. We never tried RID for citric acid.

[Vlad Orlovsky]

Here are four different approaches to analyze citric acid: HILIC, mixed-mode anion-exchange, mixed-mode anion-exclusion and hydrogen bonding:
http://www.sielc.com/Compound-Citric-Acid.html

Contact me if you have questions

[Arne]

I have seen reasonable retention for citric acid on a C18 column. If you have to use a C18 column, make sure you add at least 2% organic modifier to avoid collapse of the stationary phase. But I agree with the other posters that other methods are better. We use anion exchange with good success in our lab. We have found HILIC to be temperamental and require long column re-equilibration times.