I am seeking advice from users.
I am looking into methods to quantitate some ciders for sorbic acid and benzoic acid by HPLC since the GC method requires derivatization and ether extraction. Has anyone done the LC method I am hoping to have it done by just filtering/centrifuging the cider and shooting as I have seen in vendor's applic notes, but I don't know if this will really work without sample prep. I don't want to go to SPE if not necessary since I could get sample loss.

I've analysed sorbic and benzoic acid on both C18 (Co-elution of peaks) and on a polar embedded phase (2 nice peaks, well separated).

If you don't use SPE then you take the risk of having interference from carbohydrates in the chromatogram. These carbohydrates may / may not damage your column.

I have also analysed acids on SDVB columns (HyperREZ XP organic Acids - ThermoQuest. You can inject the cider straight onto this column. I've injected apple juice, orange juice etc and the column behaves very well.

i've analyzed for sorbic and benzoic acids in aqueous food extracts after filtering; but i'm experiencing two problems: (1) i'm getting a benzoic acid peak from water? (2) standards injected before samples differ in response from standards injected after sample extracts? i'm using a Supelco ABZ column with acetonitrile-phosphate buffer at pH 2. The good news is that the peak shapes are symetrical and retention times are stable.

i do determination of sorbic acid in dairy product.
i use column of c-18 inersil ODS 3V (GL-SCIENCES)
mobile phase of :phosphate buffer ph 6.8,methanol
80:20.the extraction of sample is by hot water than methanol ,cetrifugation and filtration ,i get very nice peak and good recovery.
i think your buffer ph is too low.
Elana

Sorbic and benzoic acid can be well resolved under several conditions. Fast analysis can be achieved on a 300A C18 (BioBasic 18, Keystone Scientific) column using 10/90 MeOH/0.05M KH2PO4, pH 7.0. Benzoic acid should be at about 4 minutes and sorbic at 6 minutes. It is also possible to run under ion supresion conditions for greater retention using the same column, 10/80 MeOH/0.05M KH2PO4, pH 2.5 retention times are 18 min for benzoic and 22 min for sorbic.

With fruit juices, filtering is usually adequate. However, SPE w/ a WAX (weak anion exchange)media can be used.

For varying peak areas, make sure that the sample loop and syringe are thoroughly flushed b/w injections, sample carry over may be to be blame.